JP2007068278A - Switching power supply circuit and transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply circuit for rapidly supplying a current required for a steep current rise in a load during turnon and turnoff periods. <P>SOLUTION: The circuit has a primary coil, a secondary coils loosely coupled, a transformer provided with a connection tap, a switching element for switching turnon and turnoff of a DC voltage to the other end of the primary coil, a primary diode for carrying a current to the other end of the primary coil, and a secondary diode for carrying a current to the other end of the secondary coil. During turnon, a first current is output and carried from the primary coil to the tap, and a second current is output by a magnetic induction generated in the secondary coil and carried to the tap. During turnon, a third current is output and carried from the primary diode to the tap through the primary coil, and a fourth current is output and carried to the tap by the magnetic induction generated in the secondary coil. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a switching power supply circuit using a tapped inductor and a transformer used in the circuit.

  The step-down switching power supply circuit is required to output a large current, a low voltage, and a high-speed response in many applications. FIG. 12 shows a basic example of a “tapped inductor step-down converter” described in Patent Document 1, Non-Patent Document 1, etc., which is one of step-down switching power supply circuits. The step-down switching power supply circuit of FIG. 12 has a transformer T10 having a tap at a connection point between a coil L11 and a coil L12 coupled in series with a choke coil, and the switching element Q21 has a DC voltage from the DC power supply Vin to the coil L11. Turn on / off the application of. The cathode of the diode D31 is connected to the tap, and the anode is connected to the ground point. A smoothing capacitor C is connected between the output side of the coil L12 and the ground point, and a voltage Vo is output. A pulse width modulation signal is input to the control terminal of the switching element Q21, and feedback control is performed to keep the output voltage of the power supply circuit constant by adjusting the ON period in response to load fluctuations. The diode D32 indicated by a broken line is shown for the purpose of explaining a problem described later, but is not actually connected.

  When the switching element Q21 is on, a DC voltage is output to the output terminal Vo through the coils L11 and L12. When Q21 is turned off, the voltage supply to the coils L11 and L12 is stopped, but since the diode D31 is connected to the tap, the counter electromotive force (flyback voltage) generated in the coil L12 causes the diode D31 to pass through the coil L12. A current flows and a DC voltage is output to the output terminal Vo.

  First, in order to explain the role of the coil L11, the problem of the circuit when the coil L11 is not provided (only the coil L12 and the diode D31) in the circuit of FIG. 12 will be described. If the inductance of the coil L12 is reduced in order to obtain a sufficient output current, the peak value of the current flowing through the coil L12 at the moment when Q21 is turned on is large, and the noise level emitted as electromagnetic waves from the circuit wiring or the like is large. The peak current value at the time of ON cannot be lowered no matter how narrow the pulse width during the ON period.

  Further, since the inductance of the coil L12 is small, the pulse width during the ON period cannot be expanded. Increasing the pulse width brings about the same effect as when a DC voltage is directly applied. As the DC voltage application time becomes longer, the coil L12 works only as a DC resistance component, so that the DC voltage is directly applied to the load. The load with low voltage specification is destroyed. In order to avoid this, if switching control is performed with a very narrow pulse width, the response characteristic of the switching element Q21 is limited.

In order to avoid the above problems, it is necessary to insert a coil L11 having a larger inductance than the coil L12 in series as shown in FIG. As a result, the on-state peak current value flowing through the coil L11 and the coil L12 is limited, and an on period having a length that can be easily controlled by the switching element Q21 can be taken.
Japanese Patent Laying-Open No. 2005-101406 (FIG. 5, 0026) Cosel Corporation, “About Power Supply”, p36-37, search on July 20, 2005, <URL: http://www.cosel.co.jp/jp/products/img/technotes.pdf>

However, insertion of the coil L11 having a large inductance as in the circuit of FIG.
One of them is that when the switching element Q21 is turned on, the current value of the coil L11 having a large inductance gradually increases, so that it cannot cope with the steep rise of the load current. That is, there arises a problem that the responsiveness to a load that requires a large current, a low voltage, and a rise of current in a short time during the ON period is not good.

  On the other hand, in the off period, all of the magnetic energy accumulated in the coil L11 and the coil L12 in the on period is supplied to the coil L12, so the maximum current value supplied from the coil L12 to the load is large. At this time, a current flows through the closed circuit of the coil L12 → the load and C → the diode D31 → the coil L12, and the current is supplied to the load. That is, the coil L11 is open at both ends, and only the coil L12 contributes to the output current and the coil L12 has a small inductance, so that a current can be supplied to the load in a large current, a low voltage, and in a short time. However, it can be said that the current supply to the load is delayed in terms of the operation only during the off period.

  Yet another problem is that when the switching element Q21 is turned off, a large spike voltage is generated at the connection point between the Q21 and the coil L11 (source for FET, emitter for bipolar transistor), so the switching element Q21 needs to have a high breakdown voltage. Is that there is. As a countermeasure, if a diode D32 (broken line) is inserted at the connection point between the switching element Q21 and the coil L11, the spike voltage can be suppressed, but the large combined inductance of the coils L11 and L12 becomes a current source even in the off period. Therefore, a small current, a high voltage, and a current are supplied to the load in a long time. This is because a current flows through a closed circuit of the coil L11 → the coil L12 → the load and C → the diode D32 → the coil L11 → the coil L12 during the off period. As a result, the load voltage cannot be maintained for a heavy load, resulting in a voltage drop, and since it is an emission current source for a long time, the magnetic flux of the magnetic circuits of the coils L11 and L12 cannot be reset during the off period and magnetic saturation occurs. As a result, sufficient energy cannot be supplied to the magnetic circuit. If the inductance of the coil L11 is reduced to avoid this, an excessive voltage is generated for a light load. Therefore, the method of connecting the diode D32 as a countermeasure against the spike voltage at the time of OFF cannot be adopted.

  As a countermeasure against the spike voltage at the OFF time, it is known that a snubber circuit is used instead of the diode D32 in the actual circuit of FIG. The snubber circuit is generally composed of a diode, a series circuit of a diode and a resistor, a series circuit of a capacitor and a resistor, or a series circuit of a capacitor and a diode, and absorbs or clamps a spike voltage. However, since the snubber circuit is not suppressed by the spike voltage but is absorbed by an element such as a diode or a capacitor, power loss occurs and the power conversion efficiency of the switching power supply circuit is reduced.

In view of the problems of the switching power supply circuit using the conventional tapped inductor described above, the present invention has the following objects.
In the present invention, even when a primary coil with a large inductance is connected to reduce a peak current value when the secondary coil with a small inductance is turned on, it can be turned on or off with respect to a steep current rise of the load. Another object of the present invention is to provide a switching power supply circuit that can supply a necessary current immediately.
In addition to the above, an object of the present invention is to provide a switching power supply circuit in which an excessive current does not flow from a secondary coil having a small inductance to a load even if the pulse width of the ON period is expanded.
It is another object of the present invention to provide a switching power supply circuit that can eliminate a spike voltage generated in a primary coil at the time of off without using a high breakdown voltage switching element or a snubber circuit.

(1) A switching power supply circuit according to claim 1 includes a primary coil, a secondary coil loosely magnetically coupled to the primary coil, and a connection point between one end of the primary coil and one end of the secondary coil. A transformer having a provided tap;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side diode that conducts a current toward the other end of the primary coil that is at a negative potential when the switching element is off;
A secondary diode that conducts a current toward the other end of the secondary coil when the other end of the secondary coil has a negative potential;
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the primary diode to the tap through the primary coil by the back electromotive force generated in the primary coil when the switching element is turned off, and the tap side is caused to have a positive potential due to the third current. A fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil is output from the tap.

(2) A switching power supply circuit according to claim 2 includes a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a connection point between one end of the primary coil and one end of the secondary coil. A transformer having a provided tap;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side switching element that has a current path that is controlled in the opposite phase to the on-off of the switching element, is interrupted when the switching element is on, and conducts current toward the other end of the primary coil when the switching element is off;
Controlled in the same phase as the on / off state of the switching element. When the switching element is on, the current toward the other end of the secondary coil is conducted. When the switching element is off, the current toward the other end of the secondary coil is conducted. A secondary-side switching element having a current path for blocking a current flowing out from the other end of the secondary coil when a positive potential is applied to the other end of the secondary coil when the switching element is off. ,
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the current path of the primary side switching element through the primary coil to the tap due to a counter electromotive force generated in the primary coil when the switching element is off, and the tap due to the third current A fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil with the side being a positive potential is output from the tap.

(3) The switching power supply circuit according to claim 3 includes a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a connection point between one end of the primary coil and one end of the secondary coil. A transformer having a provided tap;
A switching element for switching on and off the voltage applied to the other end of the primary coil;
A primary-side diode that conducts a current toward the other end of the primary coil that is at a negative potential when the switching element is off;
Controlled in the same phase as the on / off state of the switching element. When the switching element is on, the current toward the other end of the secondary coil is conducted. When the switching element is off, the current toward the other end of the secondary coil is conducted. A secondary-side switching element having a current path for blocking a current flowing out from the other end of the secondary coil when a positive potential is applied to the other end of the secondary coil when the switching element is off. ,
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the primary diode to the tap through the primary coil by the back electromotive force generated in the primary coil when the switching element is turned off, and the tap side is caused to have a positive potential due to the third current. A fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil is output from the tap.

(4) A switching power supply circuit according to claim 4 includes a primary coil, a secondary coil loosely magnetically coupled to the primary coil, and a connection point between one end of the primary coil and one end of the secondary coil. A transformer having a provided tap;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side switching element that has a current path that is controlled in the opposite phase to the on-off of the switching element, is interrupted when the switching element is on, and conducts current toward the other end of the primary coil when the switching element is off;
A secondary diode that conducts a current toward the other end of the secondary coil when the other end of the secondary coil has a negative potential;
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
When the switching element is turned off, a third current that flows from the primary side switching element to the tap through the primary coil due to a counter electromotive force generated in the primary coil, and the tap side is positively caused by the third current. A fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil as a potential is output from the tap.

(5) A switching power supply circuit according to a fifth aspect of the present invention is the switching power supply circuit according to any one of the first to fourth aspects, wherein the transformer includes a pair of opposing yokes, a central leg that connects the central portions of both yokes, and both yokes. Having a core composed of a pair of outer legs extending between the first ends and the second ends facing each other;
The primary coil is wound around the central leg, and the secondary coil is wound around each of the pair of outer legs;
A portion of the magnetic flux from the central leg to the outer leg is configured to pass through a gap between the primary coil and the secondary coil;
Magnetic flux flowing from the central leg to the pair of outer legs due to the first current flowing through the primary coil increases in each outer leg to generate a reverse magnetic flux to generate a reverse magnetic flux. And the second current flows through
Magnetic fluxes respectively flowing from the central leg to the pair of outer legs due to the third current flowing through the primary coil are in the same direction as the magnetic flux due to the first current and increase in each outer leg. Thus, the fourth current flows through the secondary coil so as to generate a reverse magnetic flux.

(6) A switching power supply circuit according to a sixth aspect of the present invention is the switching power supply circuit according to any one of the first to fourth aspects, wherein the transformer includes a pair of opposing yokes, a central leg that connects the central portions of both yokes, and both yokes. Having a core composed of a pair of outer legs extending between the first ends and the second ends facing each other;
The primary coil is wound around the central leg, the secondary coil is wound around one of the pair of outer legs, and at least a magnetic gap at the middle position of the outer leg on which the secondary coil is not wound Is provided,
A part of the magnetic flux from the central leg to the outer leg passes through the gap between the primary coil and the secondary coil and the outer leg not wound with the secondary coil;
A magnetic flux flowing from the central leg to the outer leg around which the secondary coil is wound due to the first current flowing through the primary coil increases in the outer leg, so that a reverse magnetic flux is generated. The second current flows through the secondary coil; and
The magnetic flux flowing from the central leg to the outer leg on which the secondary coil is wound due to the third current flowing through the primary coil is in the same direction as the magnetic flux resulting from the first current and the outer leg. The fourth current flows through the secondary coil so as to generate a reverse magnetic flux.

(7) A switching power supply circuit according to a seventh aspect of the present invention is the switching power supply circuit according to any one of the first to fourth aspects, wherein the transformer includes a pair of opposing yokes, a central leg that connects the central portions of both yokes, Having a core composed of a pair of outer legs extending between the first ends and the second ends facing each other;
The primary coil is wound around the central leg, the secondary coil is spaced apart from the primary coil and concentrically with the primary coil, wound around the pair of outer legs,
A portion of the magnetic flux from the central leg to the outer leg is configured to pass through a gap between the primary coil and the secondary coil;
The second current flows through the secondary coil to generate a reverse magnetic flux by increasing the magnetic flux flowing through the central leg due to the first current flowing through the primary coil; and
The magnetic flux flowing through the central leg due to the third current flowing through the primary coil is in the same direction as the magnetic flux due to the first current and increases, thereby generating the secondary magnetic flux to generate a reverse magnetic flux. The fourth current flows through the coil.

(8) A switching power supply circuit according to an eighth aspect of the present invention is the switching power supply circuit according to any one of the first to fourth aspects, wherein the transformer includes a pair of opposing yokes, a central leg that connects the central portions of both yokes, and both yokes. Having a core composed of a pair of outer legs extending between the first ends and the second ends facing each other;
The primary coil is wound around the central leg, and the secondary coil is wound concentrically with the primary coil via a magnetic piece disposed outside the primary coil,
A part of the magnetic flux from the central leg toward the outer leg passes through the magnetic piece,
The second current flows through the secondary coil to generate a reverse magnetic flux by increasing the magnetic flux flowing through the central leg due to the first current flowing through the primary coil; and
The magnetic flux flowing through the central leg due to the third current flowing through the primary coil is in the same direction as the magnetic flux due to the first current and increases, thereby generating the secondary magnetic flux to generate a reverse magnetic flux. The fourth current flows through the coil.

(9) A power transformer according to claim 9 includes a pair of opposing yokes, a central leg that connects the central portions of both yokes, and between the first end portions and the second end portions that face each other. A core composed of a pair of outer legs each extending to
A primary coil wound around the central leg;
A secondary coil wound around each of the pair of outer legs,
The gap between the primary coil and the secondary coil is a leakage magnetic circuit.

(10) A power transformer according to claim 10 includes a pair of opposing yokes, a central leg connecting the central portions of both yokes, and between the first end portions and the second end portions facing each other. A core composed of a pair of outer legs each extending to
A primary coil wound around the central leg;
A secondary coil wound around one of the pair of outer legs,
A magnetic gap is provided at an intermediate position of at least the outer leg on which the secondary coil is not wound,
A gap between the primary coil and the secondary coil and an outer leg on which the secondary coil is not wound are used as a leakage magnetic circuit.

(11) A power transformer according to an eleventh aspect includes a pair of opposing yokes, a central leg connecting the central portions of both yokes, and between the first end portions and the second end portions facing each other. A core composed of a pair of outer legs each extending to
A primary coil wound around the central leg;
A secondary coil spaced apart from the primary coil inside the pair of outer legs and wound concentrically with the primary coil;
The gap between the primary coil and the secondary coil is a leakage magnetic circuit.

(12) According to a twelfth aspect of the present invention, a power transformer includes a pair of opposing yokes, a central leg connecting the central portions of both yokes, and between the first end portions and the second end portions facing each other. A core composed of a pair of outer legs each extending to
A primary coil wound around the central leg;
A secondary coil wound concentrically with the primary coil via a magnetic piece disposed outside the primary coil;
The magnetic piece is a leakage magnetic circuit.

(13) A power transformer according to claim 13 includes a primary coil having a first magnetic circuit, a secondary coil having a second magnetic circuit and loosely magnetically coupled to the primary coil, and generation of the primary coil. A part of the magnetic flux to be leaked without passing through the secondary coil, and a leakage magnetic circuit,
When a DC voltage is applied to the primary coil, a voltage is induced in the secondary coil, and the magnetic flux density of the magnetic flux existing in the first magnetic circuit of the primary coil is in the second magnetic circuit of the secondary coil. Hold as greater than the magnetic flux density of the magnetic flux present in the
When the application of the DC voltage to the primary coil is stopped and the current caused by the counter electromotive force generated in the primary coil flows to the primary coil, the same polarity as when the DC voltage is applied to the secondary coil The voltage is induced.

(14) In the power transformer according to claim 14, when the application of the DC voltage to the primary coil is stopped and a current caused by the counter electromotive force generated in the primary coil flows to the primary coil, the power transformer A magnetic flux flows into the second magnetic circuit.

In the switching power supply circuit according to any one of claims 1 to 4, a transformer including a sparsely magnetically coupled primary coil and secondary coil is used, and a DC voltage that is turned on and off by a switching element is applied to the primary coil. An output is obtained from a tap which is a connection point connecting one end of the primary coil and one end of the secondary coil. The other end of the primary coil (DC voltage application end) becomes a negative potential due to the back electromotive force when the switching element is off, but at this time the primary side diode (or the primary side) that conducts the current toward the other end of the primary coil Switching element). Further, a secondary side diode (or secondary side switching element) is connected that conducts current toward the other end of the secondary coil when the other end of the secondary coil has a negative potential. The effects of the present invention are as follows.

  During the on-period, when a DC voltage is applied to the primary coil (primary side diode has positive potential and tap side has negative potential), the current rises slowly through the tap when the primary coil inductance is increased sufficiently and sufficiently. Supplied to the load (first current).

  On the other hand, since the secondary coil is transformer-coupled with the primary coil, when a voltage is applied to the primary coil, the magnetic flux generated in the primary coil passes through the secondary coil and the magnetomotive force of mutual induction against it instantaneously becomes 2 A corresponding electromotive force (positive potential on the tap side and negative potential on the secondary diode side) is generated in the secondary coil. If the inductance of the secondary coil is made sufficiently small, the induced current due to the electromotive force is instantaneously supplied to the load through the tap (second current).

  Therefore, in the present invention, even when a primary coil of a large inductance is connected in series, a current can be quickly supplied to a load at the time of on-state due to an induced current from a transformer-coupled small-inductance secondary coil. It is possible to cope with a sudden rise in current at the load.

  Further, the primary coil and the secondary coil are loosely coupled even if they are transformer coupled. “Loose coupling” means that all of the magnetic flux generated in the magnetic circuit of the primary coil is not passed through the magnetic circuit of the secondary coil, but part of it is leaked to the leakage magnetic circuit that is intentionally provided. It means that the transformer is configured to reduce the magnetic flux passing through the circuit. Thereby, since a violent peak current is not generated in the secondary coil at the time of ON, the intensive peak current to the load can be suppressed. As a result, the noise level emitted as electromagnetic waves from the circuit wiring or the like is reduced. Further, the choke coil that has been necessary in the past is not necessary.

  Furthermore, as described above, since a rapid current supply at the time of ON is ensured by the secondary coil, the inductance of the primary coil can be made sufficiently large. As a result, even if the pulse width of the ON period is widened, it is possible to avoid a direct voltage being directly applied to the load. Further, by expanding the pulse width, the demand for the response of the switching element is relaxed and the control becomes easy.

  Furthermore, since the primary coil and the secondary coil are transformer-coupled, even if a wide pulse voltage is applied to the primary coil, the magnetic circuit is magnetically saturated with the lapse of application time, and the electromotive force of the secondary coil disappears. Therefore, the current from the secondary coil to the load is limited to a constant value, and an excessive current is prevented from flowing.

  In addition, the magnetic circuit of the primary coil does not pass all the magnetic flux generated in the magnetic circuit of the primary coil through the magnetic circuit of the secondary coil but leaks a part of it. While the magnetic flux is accumulated and the magnetic flux density is high, the magnetic flux density of the magnetic circuit of the secondary coil remains low, and the magnetic flux density of the magnetic circuits of both coils can be realized to be in an unbalanced state. This unbalanced state of magnetic flux density causes a current flowing in the secondary coil during the off period described below.

  Next, in the off period, the application of the DC voltage to the primary coil is stopped, so that a counter electromotive force is generated in the primary coil due to self-induction. In this circuit, since a closed circuit of primary coil → load → primary side diode (or primary side switching element) → primary coil is formed at this time, a current flows to the load through the tap by the back electromotive force of the primary coil (third Current).

  The third current flowing through the primary coil has a function of holding (caulking) the magnetic flux accumulated in the magnetic circuit of the primary coil during the ON period. For this reason, the magnetic flux of the magnetic circuit of the primary coil decreases gradually without disappearing instantaneously. As a result, the imbalance state of the magnetic flux density between the primary coil and the secondary coil continues for a while even during the off period. Due to the magnetic flux imbalance state in the magnetic circuit of both coils, the magnetic flux generated from the primary coil and passing through the secondary coil continues to increase even though the magnetic flux in the primary coil magnetic circuit tends to decrease. .

  A magnetomotive force that resists this increasing magnetic flux is generated in the magnetic circuit of the secondary coil. That is, in response to this magnetomotive force, an electromotive force (positive potential on the tap side and negative potential on the secondary diode side) is generated in the secondary coil in the same direction as the ON period. In this circuit, a secondary coil → load → secondary side diode (or secondary side switching element) → secondary coil closed circuit is formed at this time, so current flows through the tap through the tap due to the electromotive force of the secondary coil. (4th current).

  Although a counter electromotive force (flyback voltage) is also generated in the secondary coil, the electromotive force generated in the secondary coil is more dominant due to the magnetic flux caulked in the primary coil, so the counter electromotive force is offset and becomes apparent. do not do. Therefore, in the secondary coil in this circuit, a current flows in the same direction, that is, in the forward direction during the on period and the off period, and can be supplied to the load.

  Further, if the inductance of the secondary coil is made sufficiently small, current can be supplied quickly for the off-period current in response to a sharp rise in the load current.

  In addition, when the magnetic flux of the magnetic circuit of the primary coil decreases in the off period, while the magnetic flux of the magnetic circuit of the secondary coil increases, the flow of the magnetic flux from the primary coil to the secondary coil at the time when both balance each other. The magnetic flux is reset (zero magnetic flux) in both coils. At this time, a counter electromotive force is generated in the secondary coil in which the magnetic flux rapidly decreases, but no reverse current flows because of the secondary diode (or the secondary switching element).

  Further, since the third current flows through the primary coil and the magnetic flux is caulked, no spike voltage is generated when the primary coil is turned off. As a result, the withstand voltage against the spike voltage of the switching element is not required and a low withstand voltage can be used. Naturally, the snubber circuit is not required, the circuit is simplified, and the problem of power loss due to the snubber circuit is solved.

In claim 2 and 3, when a secondary side switching element is connected instead of a secondary side diode that conducts current toward the other end of the secondary coil, switching synchronized with the on / off of the applied voltage to the primary coil in the same phase Control can be performed. As a result, in the on period, the current path can be reliably turned on to allow a sufficient second current to flow. In the off period, the reverse current from the smoothing capacitor in parallel with the load to the other end of the secondary coil is blocked, the reverse voltage at the time of resetting the magnetic flux of the secondary coil is blocked, and the secondary switching element is parasitic. The fourth current can flow in the same direction as the second current during the ON period by using the diode element. In addition, the output voltage to the load increases as much as there is no forward voltage drop of the diode as compared with the case where the diode is used.

In claim 2 and 4, when a primary side switching element is connected instead of a primary side diode that conducts current toward the other end of the primary coil, switching control synchronized with the on / off of the applied voltage to the primary coil in reverse phase is performed. It can be carried out. Therefore, during the ON period in which the DC voltage is applied to the primary coil, the DC voltage is reliably prevented from being grounded by turning off the primary side switching element. In this case as well, the output voltage to the load increases as much as there is no forward voltage drop of the diode compared to the case where the diode is used.

-In Claims 5-8, the core of the transformer in the switching power supply circuit of Claims 1-4 comprises a pair of opposing yokes, a central leg connecting them, and a pair of outer legs. A primary coil is wound around the central leg. The secondary coil is wound on one or both of the outer legs, is wound on the inner side of both outer legs concentrically with the primary coil, or is wound on the primary coil via a magnetic piece. It is disguised. In this transformer, the primary coil is wound by winding the secondary coil so that a gap is formed between the primary coil and the secondary coil, or by winding the secondary coil via a magnetic piece. And the secondary coil are separated from each other.

  In such a transformer, when the switching element is turned on, the magnetic flux generated in the central leg by the primary coil flows into the magnetic circuit of the secondary coil, so that a magnetomotive force is instantaneously generated in the secondary coil. A current due to the corresponding electromotive force flows through the secondary coil.

  In such a transformer, since the primary coil and the secondary coil are separated by a gap or a magnetic piece, a part of the magnetic flux generated in the primary coil from the central leg to the outer leg is separated from the leakage magnetic circuit. As a result, a magnetic flux leaks through the gap between the two coils or the magnetic piece, and the magnetic flux interlinking with the secondary coil decreases. Since there is almost no energy loss in the magnetic flux passing through the leakage magnetic circuit, it functions to promote an increase in the magnetic flux of the primary coil. On the other hand, the increase in the magnetic flux of the secondary coil is delayed by reducing the magnetic flux passing through the secondary coil. Thereby, a big difference can be produced in the magnetic flux density of both coils.

  Therefore, at the end of the ON period, magnetic flux is accumulated in the magnetic circuit of the primary coil and the magnetic flux density is high, while the magnetic flux density of the secondary coil magnetic circuit remains low, and the magnetic flux of both coils Density is in an unbalanced state. When the flyback current flows through the primary coil in this off-balanced state, the magnetic flux of the primary coil magnetic circuit is caulked and the decrease is slow, while the magnetic flux of the secondary coil magnetic circuit continues to increase. . In order to resist this increasing magnetic flux, a magnetomotive force in the same direction as the on period is generated in the magnetic circuit of the secondary coil, and a current due to the electromotive force corresponding thereto flows in the same direction as the on period.

  In claim 6, when the secondary coil is wound only on one side of the outer leg, a magnetic gap is provided on at least the outer leg that is not wound. This prevents excessive magnetic flux from flowing to the outer leg that is not wound, and this outer leg also becomes a leakage magnetic circuit. Thus, magnetic saturation can be prevented and a sufficiently large secondary coil output current can be secured. It is advantageous in terms of manufacturing cost to wind the secondary coil only on one side.

The transformer according to claims 9 to 12 is composed of a pair of opposing yokes, a central leg and a pair of outer legs connecting them, and a window space is formed between the central leg and each outer leg. A primary coil is wound around the central leg. The secondary coil is wound on one or both of the outer legs, is wound concentrically with the primary coil, is wound inside the outer legs, or is placed on the primary coil via a magnetic piece. It is wrapped in layers. In this transformer, the primary coil is wound by winding the secondary coil so that a gap is formed between the primary coil and the secondary coil, or by winding the secondary coil via a magnetic piece. And the secondary coil are separated from each other.

  In such a transformer, the magnetic circuit of the primary coil and the magnetic circuit of the secondary coil are separated by a gap or a magnetic piece, so that a part of the magnetic flux generated in the primary coil is both coils that become a leakage magnetic circuit. Leakage magnetic flux passing through the gap or magnetic piece between them, and the magnetic flux interlinking with the secondary coil decreases. Thereby, a transformer in which the primary coil and the secondary coil are loosely coupled can be realized. In claim 10, when the secondary coil is wound only on one side of the outer leg and a magnetic gap is provided on at least the outer leg that is not wound, excessively to the outer leg that is not wound. While preventing the flow of magnetic flux, this outer leg also becomes a leakage magnetic circuit.

  In the transformer according to claims 13 and 14, since a leakage magnetic circuit is provided in which a part of the magnetic flux generated in the primary coil leaks without passing through the secondary coil, all of the magnetic flux generated when a voltage is applied to the primary coil. The magnetic flux density of the primary coil is stored in the primary coil without being given to the secondary coil, and the magnetic flux density of the primary coil is relatively increased with respect to the magnetic flux density of the secondary coil. When voltage application to the primary coil is stopped in a state where such a difference in magnetic flux density occurs, a current due to the counter electromotive force flows through the primary coil. As a result, an electromotive force is also generated in the secondary coil, and the electromotive force is generated so that a current flows in the same direction as when a voltage is applied to the primary coil due to the difference in magnetic flux density. Therefore, in this transformer, it is possible to obtain an output current in the same direction from the secondary coil both when the voltage is applied to the primary coil and when it is not applied.

(1) First Embodiment of Switching Power Supply Circuit (1-1) Circuit Configuration In the step-down switching power supply circuit using the tapped inductor according to the present invention, for example, the output is 1V, 100A with respect to the input DC voltage 12V. Therefore, a low voltage of 1 μS rise, a large current, and a short time response are required.

  FIG. 1 is a basic configuration circuit of a first embodiment of a step-down switching power supply circuit according to the present invention. This circuit includes a transformer T. The transformer T includes a primary coil L1 and a secondary coil L2 connected in series, and a tap is provided at the connection point X0. That is, the winding end terminal of the primary coil L1 (the opposite side of the terminal indicated by the black dot) and the winding start terminal (terminal indicated by the black dot) of the secondary coil L2 are connected. The node X0 is connected to the output terminal Vo, from which an output voltage is obtained. The primary coil L1 and the secondary coil L2 are transformer-coupled, but are loosely coupled due to the characteristic configuration of the transformer T (described later with reference to FIGS. 6 and 9 to 11). That is, it is transformer coupling that actively leaks a part of the magnetic flux from the primary coil to the secondary coil to the leakage magnetic circuit. Therefore, unlike the conventional transformer, in the transformer T of the present invention, the output voltage is not determined only by the turn ratio but also depends on the amount of magnetic flux leaked. However, like the general setting, in order to achieve the step-down type, the number of turns is usually set so that the inductance of the primary coil L1 is relatively large and the inductance of the secondary coil is relatively small.

  The winding start terminal (terminal indicated by a black dot) X1 of the primary coil L1 is connected to the source of an N-channel FET (field effect transistor) which is the switching element Q1. The drain of this FET is connected to a DC voltage input terminal Vin. A pulse voltage signal, which is a control signal, is input to the gate of the FET.

  As another example of the switching element Q1, when using a P-channel FET, the source is connected to the DC voltage Vin, and the drain is connected to the winding start terminal X1 of the primary coil L1. Similarly, a pulse voltage signal which is a control signal (however, the negative voltage is on) is input to the gate. Also in other embodiments of the present invention described below, an N-channel FET and a P-channel FET can be similarly used as the switching element Q1 of the DC voltage Vin.

Further, the cathode of the primary diode D1 is connected to the winding start terminal X1 of the primary coil L1, and the anode is connected to the ground point.
The cathode of the secondary diode D2 is connected to the winding end X2 of the secondary coil L2, and the anode is connected to the ground point. Since the supply voltage to the load is reduced by the forward voltage drop (usually about 0.6V) of the diode D2, it is preferable to use a Schottky barrier diode having a voltage drop of about 0.2V as the diode D2.

  A smoothing capacitor C is connected between the output terminal Vo and the ground point. Although not shown, a load is also connected between the output terminal Vo and the ground point.

  The switching power supply circuit shown in FIG. 1 is a circuit that obtains a positive output voltage from a positive input voltage. To obtain a negative output voltage from a negative input voltage, the circuit configuration of FIG. It is obvious to those skilled in the art that only different circuits having the same configuration may be used. In that case, the polarities of the constituent elements (diodes and switching elements) and the on / off control signal may be switched as necessary, and substantially the same operation is realized only by reversing the positive and negative polarities.

(1-2) Explanation of operation <Operation during ON period>
The pulse voltage signal input to the gate of the switching element Q1 performs feedback control to keep the output voltage of the power supply circuit constant by adjusting the ON period of the input DC voltage with respect to the load variation.

  When the pulse voltage signal applied to the gate of the switching element Q1 is turned on, the current path between the drain and the source is conducted, and the DC voltage Vin is applied to the winding start terminal X1 of the primary coil L1. At this time, the primary coil L1 functions as a load of the DC voltage Vin. The winding start terminal X1 of the primary coil L1 has a positive potential, and the winding end terminal (tap X0 side) has a negative potential. Since the primary diode D1 is reverse-biased, no current flows. Therefore, a current flows through the path of the DC power source Vin → the switching element Q1 → the primary coil L1 → the load and C. The current output from the tap X0 through the primary coil L1 during this ON period is referred to as “first current (i1)”.

  On the other hand, when the first current flows through the primary coil L1, a magnetic flux is generated in the magnetic circuit of the primary coil L1. Then, when this magnetic flux passes through the magnetic circuit of the secondary coil L2 coupled to the transformer, a magnetomotive force due to mutual induction against this is instantaneously generated in the magnetic circuit of the secondary coil L2. At this time, the winding start terminal (tap X0 side) of the secondary coil L2 is a positive potential, the winding end terminal X2 is a negative potential, and the secondary diode D2 is forward biased. Due to the electromotive force due to the mutual induction, a current flows through a path of the secondary coil L2 → load and C → secondary diode D2. The current output from the tap X0 through the secondary coil L2 during this ON period is referred to as “second current (i2)”.

  The rise of the first current is relatively slow because the inductance of the primary coil L1 is large, but the rise of the second current is quick because the inductance of the secondary coil L2 is small and is supplied to the load instantly without delay. Further, since the primary coil L1 and the secondary coil L2 are loosely coupled, a violent peak current does not flow through the secondary coil L2 due to the magnetic flux generated in the primary coil L1 when it is turned on. Furthermore, since the inductance of the primary coil can be increased, it is possible to avoid a direct current voltage being directly applied to the load even if the pulse width is increased.

  Up to the required load voltage, the secondary coil L2 can be driven with low impedance by setting the turn ratio of the primary coil L1 and the secondary coil L2 by the transformer coupling action. As a result, it is possible to cope with a steep rise of current in the load. For example, if the input DC voltage is 12V, the required load voltage is 1V, and the winding ratio of the primary coil L1 and the secondary coil L2 is set so that 1V is generated in the secondary coil L2, it is less than 1V (a voltage slightly lower than 1V) ) Until the current rises sharply. When the load voltage reaches 1V, the voltage generated by the secondary coil L2 balances with the load voltage (including the capacitor C), so that no current flows. On the other hand, in the conventional technology, if the inductances of the primary coil and the secondary coil are both reduced in order to make the current rise steeply, a situation occurs in which DC voltage is directly applied to the load, exceeding the required load voltage to the load. A voltage was to be applied.

  Since the primary coil L1 and the secondary coil L2 are loosely coupled due to the transformer structure, only a part of the magnetic flux generated in the primary coil L1 passes through the magnetic circuit of the secondary coil L2. As a result, at the end of the ON period, a large amount of magnetic flux is accumulated in the magnetic circuit of the primary coil L1 and the magnetic flux density is high, while the magnetic flux density of the magnetic circuit of the secondary coil L2 remains low. Yes, the magnetic flux density of the magnetic circuit of both coils is in an unbalanced state. This unbalanced state causes a current in the following off period.

<Operation during off period>
When the pulse voltage signal applied to the gate of the switching element Q1 is turned off, the current path between the drain and the source is immediately cut off, and the application of the DC voltage Vin to the primary coil L1 is stopped. When the applied voltage is suddenly stopped, a counter electromotive force (flyback voltage) based on self-induction is generated in the primary coil L1. At this time, the primary coil L1 functions as a power source, and the winding start terminal X1 has a negative potential and the winding end terminal (tap X0 side) has a positive potential. Therefore, the primary diode D1 is forward biased. As a result, a current flows through a closed circuit of primary coil L1 → load and C → primary side diode D1 → primary coil L1. This is a quasi-short circuit condition. The current output from the tap X0 through the primary coil L1 during this off period is referred to as “third current (i3)”.

  The magnetic flux of the magnetic circuit of the primary coil L1 is maximum when the switching element Q1 is turned off. The third current, which is a flyback current flowing in the primary coil L1, has a function of holding (caulking) the magnetic flux accumulated in the primary coil L1 during the ON period, and slows the decrease of the magnetic flux. That is, the primary coil L1 has a relatively higher magnetic flux density than the secondary coil L2, while the magnetic flux of the primary coil L1 decreases during the off period. As long as the unbalanced state of the magnetic flux densities of the two coils continues, the magnetic flux passing through the magnetic circuit of the secondary coil L2 continues to increase due to the magnetic flux caulked by the primary coil L1, and the magnetomotive force that resists this increases. It occurs in the magnetic circuit of L2. Therefore, the direction of the magnetomotive force is the same as the on period. Therefore, an electromotive force in the same direction as the on period is generated in the secondary coil L2, and a forward operation is performed. That is, even in the off period, the primary coil L1 serves as a magnetic flux generation source and the secondary coil L2 serves as a magnetic flux reception source, the secondary coil L2 continues to receive the magnetic flux of the primary coil L1, and the rate of increase is positive. Accordingly, in the secondary coil L2, the winding start terminal (tap X0 side) is at a positive potential, the winding end terminal X2 is at a negative potential, and the secondary diode D2 is forward biased as in the on period. As a result, a current flows through a closed circuit of secondary coil L2 → load and C → secondary diode D2 → secondary coil L2. This is also a quasi-short circuit state. The current output from the tap X0 through the secondary coil L2 during this off period is referred to as “fourth current (i4)”.

  Note that the counter electromotive force (flyback voltage) is also generated in the secondary coil L2 due to the counter electromotive force generated in the primary coil L1 when the switch is turned off (the winding start terminal (tap X0 side) is at a negative potential as in the primary coil. However, since the electromotive force generated in the secondary coil L2 by the magnetic flux caulked by the primary coil L1 by the above-described third current is more dominant, the counter electromotive force is canceled and becomes apparent. (On the other hand, in the conventional circuit of FIG. 12, only the counter electromotive force is generated in the secondary coil in the off state). As described above, in the secondary coil L2 in this circuit, a current flows in the same direction (forward direction) during the ON period and the OFF period, and can be supplied to the load. Further, since the secondary coil L2 has a small inductance, a current can be quickly supplied to the load with a low impedance even during the off period.

  Thereafter, with the passage of time in the off period, the amount of magnetic flux retained in the primary coil L1, which is a magnetic flux generation source, decreases, and when the magnetic flux density decreases, a point is reached that balances with the magnetic flux density of the magnetic circuit of the secondary coil L2. . When the magnetic flux density of both coils is balanced, the flow of magnetic flux is lost and the magnetic flux of both coils is reset to zero. At this time, the third current flowing through the primary coil L1 and the fourth current flowing through the secondary coil L2 become zero. Although a counter electromotive force is generated due to abrupt reset of the magnetic flux of the secondary coil L2, no reverse current flows because the secondary diode D2 is reverse biased. Then, the on period of the next cycle is reached. These operations will be described in more detail later together with the relationship with the configuration of the transformer T in FIGS.

(1-3) Waveform Measurement Results FIGS. 3 to 5 are current or voltage measurement waveforms at the measurement points M1 to M5 in the circuit diagram shown in FIG. The horizontal axis is the time axis (s), and the vertical axis is the current or voltage (A or V).

  FIG. 3A shows a current waveform at the measurement point M1 on the source side of the switching element Q1. The first current i1 gradually increases from zero when the switching element Q1 is turned on, and is immediately cut off when Q1 is turned off.

  FIG. 3B shows a current measurement waveform at the measurement point M2 on the cathode side of the diode D1. The third current i3 does not flow during the ON period of the switching element Q1, and immediately rises at the maximum current and gradually decreases as Q1 turns OFF.

  FIG. 3C shows a current waveform at the measurement point M3 on the winding end terminal side of the primary coil L1. At the measurement point M3, the current flowing through only the primary coil L1 is measured. The first current i1 gradually increases from zero during the ON period of the switching element Q1, and switches to the third current i3 during the OFF period and gradually decreases. The current in FIG. 3 (C) has a waveform obtained by adding the currents in FIG. 3 (A) and FIG. 3 (B).

  Referring now to FIG. 3 (A), immediately after the first current i1 is cut off, an oscillating transient waveform is observed. Referring to FIG. 3 (B), the third current i3 is turned off. A transient waveform can be seen immediately after getting up. However, these transients disappear in the waveform of FIG. Thus, it can be seen that the effect of canceling out the transient at the time of switching between the first current i1 and the third current i3 can be obtained by overlapping the waveforms.

  FIG. 3D shows a current waveform at the measurement point M4 on the output terminal Vo side from the tap X0. That is, FIG. 3D shows a current waveform obtained by adding the current flowing through the primary coil L1 of FIGS. 3A and 3B to the current flowing through the secondary coil L2. It gradually increases from zero during the on period and gradually decreases from the maximum current during the off period.

FIG. 4A shows a source voltage waveform of the switching element Q1.
FIG. 4B shows the same current waveform that flows through the primary coil L1 as in FIG. 3C.
FIG. 4C shows a current waveform at the measurement point M5 on the winding start terminal side of the secondary coil L2. During the ON period of the switching element Q1, the second current i2 gradually increases from zero and flows, and during the OFF period, the fourth current i4 immediately rises at the maximum current and gradually decreases.

  FIG. 4D shows a current waveform at the measurement point M4 as in FIG. The current in FIG. 4D is a current waveform obtained by adding the currents in FIGS. 4B and 4C. During the ON period of the switching element Q1, the current obtained by adding the first current i1 and the second current i2 gradually increases from zero and flows, and during the OFF period, the third current i3 and the fourth current i4 are added. The maximum current immediately rises and flows while gradually decreasing.

FIG. 5A shows the source voltage waveform of the switching element Q1 as in FIG.
FIG. 5B shows the current waveforms of FIGS. 4B, 4C, and 4D on the same baseline.

(2) Second Embodiment of Switching Power Supply Circuit (2-1) Circuit Configuration FIG. 2 is a basic configuration circuit of a second embodiment of a step-down switching power supply circuit according to the present invention. This circuit is different from the first embodiment in that a secondary side switching element Q2 is connected instead of the secondary side diode D2, and a primary side switching element Q3 is connected instead of the primary side diode D1. . Other components are the same as those in the first embodiment.

  An FET is used as the secondary side switching element Q2, a drain is connected to the winding end terminal X2 of the secondary coil L2, and a source is connected to the ground point. The same signal as the pulse voltage signal input to the gate of the switching element Q1 is input to the gate which is the control terminal of the secondary side switching element Q2. Therefore, in the secondary side switching element Q2, the current path between the drain and the source is turned on and off at the same timing as when the switching element Q1 is turned on and off.

  In the FET of the secondary side switching element Q2, a current can flow in either direction between the drain and the source during the ON period. In the off period, the current from the drain to the source is cut off, but the current can flow from the source to the drain by the parasitic diode. Since the secondary side switching element Q2 operates in the same manner as the diode D2 of the first embodiment with respect to the current flow, the current flows from the source to the drain in both the on and off periods. Since this circuit is intended to output a large current, it is preferable that the FET of the secondary side switching element Q2 is for a larger current than the FET of the switching element Q1.

  The FET used as the primary side switching element Q3 has a drain connected to the winding start terminal X1 of the primary coil L1, and a source connected to the ground point. A signal having a phase opposite to that of the pulse voltage signal input to the gate of the switching element Q1 is input to the gate of the primary side switching element Q3. For this purpose, a pulse voltage signal is input to the gate of Q3 via the inversion circuit INV. Therefore, in the primary side switching element Q3, the drain and the source are cut off and conducted at the timing opposite to the ON and OFF of the switching element Q1.

(2-2) Description of operation <Operation during ON period>
The circuit operation of the second embodiment is the same as the circuit operation of the first embodiment except for the operations of the secondary side switching element Q2 and the primary side switching element Q3. Since the generation principles of the two currents and the third and fourth currents flowing during the off period are the same, a brief description will be given.

  When the pulse voltage signal applied to the gate of the switching element Q1 is turned on, the current path between the drain and source is conducted, and the DC voltage Vin is applied to the winding start terminal of the primary coil L1. At this time, the winding start terminal X1 of the primary coil L1 has a positive potential, and the winding end terminal (tap X0 side) has a negative potential. At this time, the primary side switching element Q3 is turned off, the drain and source are cut off, and no current flows. Accordingly, as in the first embodiment, the first current i1 flows through the path of the DC power source Vin → the switching element Q1 → the primary coil L1 → the load and C.

  On the other hand, the secondary coil L2 → load and C → secondary switching element Q2 due to the mutual electromotive force generated in the transformer-coupled secondary coil L2 due to the magnetic flux generated by the first current i1 flowing in the primary coil L1. , The second current i2 flows. At this time, the secondary side switching element Q2 is on and the drain and the source are conductive.

<Operation during off period>
When the pulse voltage signal applied to the gate of the switching element Q1 is turned off, the current path between the drain and the source is immediately cut off, and the application of the DC voltage Vin to the primary coil L1 is stopped. A counter electromotive force based on self-induction is generated in the primary coil L1. At this time, since the primary side switching element Q3 is turned on and the drain and source are made conductive, the third current i3 flows through a closed circuit of the primary coil L1 → load and C → primary side switching element Q3 → primary coil L1.

  The third current i3 flowing through the primary coil L1 caulks the magnetic flux accumulated in the primary coil L1 during the ON period. When the magnetic flux caulked by the primary coil L1 passes through the magnetic circuit of the secondary coil L2 while increasing, an electromotive force in the same direction as the ON period is generated in the secondary coil L2. The pulse voltage signal to the gate of the secondary side switching element Q2 is turned off simultaneously with the gate of the switching element Q1, but the secondary side switching element Q2 is turned off with a slight delay because it is for large current. Even if the secondary side switching element Q2 is turned off, a current can flow from the source to the drain by the parasitic diode. Therefore, a current flows through a closed circuit of secondary coil L2 → load and C → secondary side switching element Q2 → secondary coil L2.

(3) Third Embodiment of Switching Power Supply Circuit Although the third embodiment of the step-down switching power supply circuit according to the present invention is not shown, the secondary side diode D2 in the circuit of the first embodiment is replaced with the second This is a form in which the secondary side switching element Q2 of the embodiment is replaced. The connection and operation of the secondary side switching element Q2 in the third embodiment are the same as those in the second embodiment.

(4) Fourth Embodiment of Switching Power Supply Circuit Although the fourth embodiment of the step-down switching power supply circuit according to the present invention is not shown, the primary side diode D1 in the circuit of the first embodiment is replaced with the second embodiment. This is a form in which the primary side switching element Q3 is replaced. The connection and operation of the primary side switching element Q3 in the fourth embodiment are the same as those in the second embodiment.

(5) First Embodiment of Transformer (5-1) Configuration of Transformer FIG. 6 shows the first embodiment of the transformer T that is preferably used in the first to fourth embodiments of the switching power supply circuit described above. It is sectional drawing which shows a structure typically.

  The core of the transformer T includes a pair of opposing yokes 21a and 21b, a central leg 22 that connects the central portions of both yokes, and a first end portion and a second end portion that face each other. A pair of extending outer legs 23a and 23b is formed, and magnetic gaps 24a and 24b are provided on the outer legs 23a and 23b at intermediate positions between the yokes, respectively. Window spaces 25a and 25b are formed between the center leg 22 and the outer legs 23a and 23b, respectively.

  The primary coil L1 is wound around the center leg 22, and the winding start terminal X1 is drawn out. The X1 terminal is connected to the source of the switching element Q1 in FIG. 1 and the cathode of the primary side diode D1. The secondary coil L2 is divided into a first secondary coil L2a and a second secondary coil L2b and is wound around both outer legs 23a and 23b, respectively. The winding end of the first secondary coil L2a and the winding start of the second secondary coil L2b are connected. The second secondary coil L2b has its winding end terminal X2 pulled out. The cathode of the secondary side diode D2 of FIG. 1 is connected to the X2 terminal. The winding end terminal of the primary coil L1 and the winding start terminal of the first secondary coil L2a are connected, and the tap X0, that is, the output terminal Vo is drawn from this connection point. The primary coil L1 and the secondary coils L2a and L2b are wound substantially apart from each other by the width of the window spaces 25a and 25b (of course, the thickness around which the coils are wound is reduced). Sparse transformer coupling is realized. Hereinafter, the first and second secondary coils L2a and L2b are collectively referred to as “secondary coil L2”.

(5-2) Description of Operation of Transformer The operation of the transformer T shown in FIG. 6 will be described with reference to FIGS. 7 and 8 including the relationship between the magnetic circuit and the electric circuit.
7A flows into the magnetic flux φ1 of the magnetic circuit of the central leg 22 (hereinafter referred to as “central magnetic pole magnetic circuit”) and the magnetic circuits of the outer legs 23a and 23b (hereinafter referred to as “double-leg magnetic circuit”). It is the figure which showed typically the time change of each magnetic flux density of magnetic flux (phi) 1a.
FIG. 7B is a time change of the second and fourth currents i2 and i4 of the secondary coil L2 aligned with the time axis of FIG. 7A, and the flyback current of the primary coil L1 for reference. It is the figure which showed typically the time change of the 3rd electric current i3.
FIG. 8A shows a source voltage waveform of the switching element Q1. FIG. 8B shows the cathode potential of the secondary diode D2, that is, the potential waveform of the winding end terminal X2 of the secondary coil L2. FIG. 8C shows a flyback current (third current) waveform of the primary coil L1. FIG. 8D shows a current waveform (second and fourth currents) of the secondary coil L2.

<Operation during ON period>
When a DC voltage is applied to the winding start terminal X1 of the primary coil L1 and a current (first current i1 in FIG. 1) flows, a magnetic flux φ1 (broken line) is generated in the central magnetic pole magnetic circuit. The first current i1 flows through the primary coil L1 clockwise as viewed from the bottom surface of the transformer T in FIG.

  When a magnetic flux φ1a (solid line) which is a part of the magnetic flux φ1 flows into the both leg magnetic circuit, a magnetic flux φ2 is generated in the secondary coil L2 due to a magnetomotive force against the magnetic flux φ1a by mutual induction. The current of the secondary coil L2 (second current i2 in FIG. 7B) flows in the direction in which the magnetic flux φ2 is generated. Thus, the second current i2 flows through the secondary coil L2 clockwise as viewed from the bottom surface of the transformer T. Both the first current i1 and the second current i2 flow to the tap X0 and are output.

  In the transformer T, a magnetic flux φ1b (a two-dot broken line) that is a part of the magnetic flux φ1 generated in the center magnetic pole magnetic circuit passes through a leakage magnetic circuit that is a gap between the primary coil L1 and the secondary coil L2, thereby The magnetic flux φ1a passing through the magnetic circuit is reduced. The leakage magnetic circuit is a bypass for the magnetic flux φ1.

  As shown in FIG. 7A, when a DC voltage is applied to the primary coil L1, the magnetic flux density of the center magnetic pole magnetic circuit increases rapidly. At this time, the magnetic flux density of both leg magnetic circuits also increases due to the influence of the magnetic flux φ1 of the primary coil L1, but this increase is less than that of the central magnetic pole magnetic circuit. This is explained as follows. The magnetic flux φ1 generated in the central magnetic pole magnetic circuit is inherently difficult to pass through the both leg magnetic circuit around which the secondary coil is wound, and it is ideal that the conventional magnetic flux passes through the difficult magnetic flux without leakage as much as possible. On the other hand, in the transformer of the present invention, the magnetic flux φ1a interlinked with the secondary coil is reduced by actively diverting a part φ1b of the magnetic flux φ1 generated in the central magnetic pole magnetic circuit to the leakage magnetic circuit. The magnetic flux φ1b bypassed by the leakage magnetic circuit contributes to promoting an increase in the magnetic flux density of the central magnetic pole magnetic circuit because there is almost no energy loss. On the other hand, an increase in the magnetic flux density of the both leg magnetic circuit in which the interlinkage magnetic flux φ1a is relatively reduced is suppressed to a low level. As a result, there is a large difference in magnetic flux density between the center magnetic pole magnetic circuit and the both leg magnetic circuit.

  In the case of a conventional transformer, the magnetic flux to be given from the central magnetic pole magnetic circuit to the two-leg magnetic circuit is not given to the two-leg magnetic circuit and is accumulated in the central magnetic pole magnetic circuit, but is accumulated in the central magnetic pole magnetic circuit. Since the energy is released to the both-leg magnetic circuit after an off period described later, there is no energy loss.

  Although the increase in the magnetic flux density of the both leg magnetic circuit itself is small, the rate of change of the magnetomotive force φ2 generated against this is large enough to allow current to flow instantaneously through the secondary coil L2. A sufficient second current i2 flows as shown in FIG.

  Thus, as shown in FIG. 7A, at the end time t1 of the ON period, a large amount of magnetic flux is accumulated in the center magnetic pole magnetic circuit, and the magnetic flux density becomes maximum, while the magnetic flux density of both leg magnetic circuits is relatively low. It is in equilibrium.

  The reason why the magnetic gaps 24a and 24b are provided on the outer legs 23a and 23b is that it is suitable for increasing the magnetic resistance and preventing magnetic saturation, and may or may not be provided as necessary.

<Operation during off period>
When the application of the DC voltage to the primary coil L1 is stopped and the first current i1 is cut off, the magnetic flux φ1 disappears instantaneously if it is normal, but it is immediately closed by the back electromotive force generated in the primary coil L1. A current flows in (L1 is a quasi-short circuit state) (third current i3 in FIG. 7B). Then, as the third current i3 starts to flow through the primary coil L1, the magnetic flux φ1 is firmly held (caulked), and as shown in FIG. 7A, the magnetic flux density of φ1 decreases relatively slowly from the maximum value. . Therefore, even in the off period, the magnetic flux density unbalanced state of the center magnetic pole magnetic circuit and the both leg magnetic circuit continues, and as long as this unbalanced state persists, the magnetic flux φ1a flowing from the center magnetic pole magnetic circuit to the both leg magnetic circuit is Maintain an increasing trend.

As shown in FIG. 7A, the increase rate per unit time of the magnetic flux φ1a supplied from the center magnetic pole magnetic circuit to the both leg magnetic circuit is smaller than that in the on period, but is positive as in the on period. is there. As a result, even in the off period, the direction of the magnetomotive force φ2 that opposes the magnetic flux φ1a is the same as that in the on period. Therefore, an electromotive force is continuously induced in the secondary coil L2 in the same direction as the on period. Flows (fourth current i4 in FIG. 7B). Thus, both the third current i3 and the fourth current i4 flow to the tap X0 and are output.
Incidentally, in the case of a conventional transformer, since there is no magnetic flux density imbalance between the center magnetic pole magnetic circuit and both leg magnetic circuits when the transformer is turned off, the magnetic flux density of both leg magnetic circuits immediately decreases, and the secondary coil Only a flyback voltage is generated in L2, and an electric current is caused to flow in the direction opposite to the ON period. In the present invention, a completely opposite operation is realized.

  Eventually, at time t2 in FIG. 7A, the magnetic flux density of φ1 of the central magnetic pole magnetic circuit and the magnetic flux density of φ1a of both leg magnetic circuits are balanced, and the flow of magnetic flux stops. As a result, as shown in FIG. 7B, the currents of the primary coil L1 and the secondary coil L2 become zero, and both coils are in an open state at this moment.

  Here, as shown in FIG. 7A, since the magnetic flux φ1a applied to the both leg magnetic circuits suddenly becomes zero at the time point t2, the rate of change in the magnetic flux is a negative maximum value. It becomes. As a result, a large magnetomotive force is generated in the opposite direction, and a counter electromotive force is generated in the secondary coil L2. Since the secondary diode D2 is reverse-biased, no current flows due to this counter electromotive force.

  Referring to the waveform in FIG. 8, the back electromotive force generated in the secondary coil L2 appears in the potential waveform at the winding end terminal of the secondary coil L2 in FIG. 8B. The time point at which a large back electromotive force is observed in the waveform of FIG. 8B is the time point when the flyback current (third current) of the primary coil L1 of FIG. 8C disappears, and the time point of FIG. It can be seen that this coincides with the time when the forward current (fourth current) of the secondary coil L2 disappears. From this waveform observation result, it can be proved that the supply of magnetic flux from the center magnetic pole magnetic circuit to the both leg magnetic circuit maintains an increasing tendency even in the off period.

(6) Second Embodiment of Transformer FIG. 9 is a cross-sectional view schematically showing the configuration of the second embodiment of the transformer T. The difference from FIG. 6 is that the secondary coil L2 is wound only on one outer leg 23b. About another structure, it is the same as that of embodiment of FIG.

  In the transformer T of FIG. 9, the magnetic flux φ1 generated in the primary coil L1 does not easily pass through the outer leg 23b wound with the secondary coil L2, and easily passes through the outer leg 23a that is not wound. As a result, an output current having a required magnitude cannot be obtained from the secondary coil L2. Therefore, in order to obtain a sufficient output current from the secondary coil L2, it is effective to widen the magnetic gap 24a of the outer leg 23a that is not wound, or not to provide the magnetic gap 24b on the outer leg 23b that is wound. is there. The outer leg 23a which is not wound acts as a leakage magnetic circuit serving as a bypass for the magnetic flux, like the gap between the primary coil L1 and the secondary coil L2. Thus, winding the secondary coil L2 only on one side has an advantage that the manufacturing cost can be reduced.

(7) Third Embodiment of Transformer FIG. 10 is a cross-sectional view schematically showing the configuration of the third embodiment of the transformer T. The difference from FIG. 6 is that the secondary coil L2 is wound concentrically with the primary coil L1. In FIG. 10, the secondary coil L <b> 2 is wound in close contact with the inner walls of both outer legs, but is not limited thereto, and may be separated from both outer legs as long as it is inside the outer legs. The transformer of the present invention is characterized in that the secondary coil L2 is not closely attached to the primary coil L1, and the primary coil L1 and the secondary coil L2 are separated and wound, and a leakage magnetic circuit is configured in this separation gap. It is not necessary to wind the secondary coil L2 in close contact with the inner walls of both outer legs. About another structure, it is the same as that of embodiment of FIG.

  In the transformer T of FIG. 10, since the secondary coil L2 is wound concentrically with the primary coil L1, a magnetomotive force φ2 against the magnetic flux φ1 generated in the primary coil L1 is applied to the magnetic circuit of the outer legs 23a and 23b. And a corresponding current flows through the secondary coil L2. In this case, when viewed from the bottom surface of the transformer T in FIG. 10, the first current i1 flows clockwise through the primary coil L1, and the second current i2 flows counterclockwise through the secondary coil L2. However, since the connection is the same, the operation of the electric circuit is the same. Similarly to the above, a part φ1b of the magnetic flux φ1 generated by the primary coil L1 leaks through the gap between the primary coil L1 and the secondary coil L2 as a leakage magnetic circuit.

(8) Fourth Embodiment of Transformer FIG. 11 is a cross-sectional view schematically showing the configuration of the fourth embodiment of the transformer T. The difference from FIG. 6 is that the secondary coil L2 is wound concentrically with the primary coil L1 via a pair of magnetic body pieces 26a, 26b arranged outside the primary coil L1. The magnetic pieces 26a and 26b each have an arcuate cross section when viewed from the bottom side of the transformer T. About another structure, it is the same as that of embodiment of FIG.

  In the transformer T of FIG. 11, since the secondary coil L2 is wound concentrically with the primary coil L1, the magnetomotive force φ2 against the magnetic flux φ1 generated in the primary coil L1 is applied to the magnetic circuit of the outer legs 23a and 23b. And a corresponding current flows through the secondary coil L2. In this case, when viewed from the bottom surface of the transformer T in FIG. 11, the first current i1 flows clockwise through the primary coil L1, and the second current i2 flows counterclockwise through the secondary coil L2. However, since the connection is the same, the operation of the electric circuit is the same. Further, a part φ1b of the magnetic flux φ1 generated by the primary coil L1 leaks through both magnetic pieces 26a and 26b. In this case, both magnetic pieces 26a, 26b act as a leakage magnetic circuit.

(9) Summary of characteristics of the transformer The transformer of the present invention is such that both coils are wound via a gap or a magnetic piece so as to form a leakage magnetic circuit between the primary coil L1 and the secondary coil L2. Yes. And the distance which separates the primary coil L1 and the secondary coil L2 is determined by how much the amount of leakage magnetic flux is made. In this respect, the transformer of the present invention is wound with the primary coil and the secondary coil in close contact so that the coupling ratio between the primary coil and the secondary coil is 100% (coupling degree = 1). It is very different from the point.

1 is a basic configuration circuit of a first embodiment of a step-down switching power supply circuit according to the present invention. 3 is a basic configuration circuit of a second embodiment of a step-down switching power supply circuit according to the present invention. FIG. 2 is a current or voltage measurement waveform at each measurement point in the circuit diagram shown in FIG. 1. FIG. FIG. 2 is a current or voltage measurement waveform at each measurement point in the circuit diagram shown in FIG. 1. FIG. FIG. 2 is a current or voltage measurement waveform at each measurement point in the circuit diagram shown in FIG. 1. FIG. It is sectional drawing which shows typically the structure of 1st Embodiment of the transformer T used suitably for the switching power supply circuit by this invention. (A) is the figure which showed typically the time change of the magnetic flux density of a center magnetic pole magnetic circuit and a both leg magnetic circuit. (B) is the figure which showed typically the time change of the electric current of the secondary coil L2 and the primary coil L1. FIG. 2 is a current or voltage measurement waveform at each measurement point in the circuit diagram shown in FIG. 1. FIG. It is sectional drawing which shows typically the structure of 2nd Embodiment of the transformer T used suitably for the switching power supply circuit by this invention. It is sectional drawing which shows typically the structure of 3rd Embodiment of the transformer T used suitably for the switching power supply circuit by this invention. It is sectional drawing which shows typically the structure of 4th Embodiment of the transformer T used suitably for the switching power supply circuit by this invention. It is a circuit diagram which shows the basic example of the conventional step-down switching power supply circuit.

Explanation of symbols

T transformer Q1 switching element Q2 secondary side switching element Q3 primary side switching element D1 primary side diode D2 secondary side diode L1 primary coil L2 secondary coil

Claims (14)

A transformer comprising a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a tap provided at a connection point between one end of the primary coil and one end of the secondary coil;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side diode that conducts a current toward the other end of the primary coil that is at a negative potential when the switching element is off;
A secondary diode that conducts a current toward the other end of the secondary coil when the other end of the secondary coil has a negative potential;
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the primary diode to the tap through the primary coil by the back electromotive force generated in the primary coil when the switching element is turned off, and the tap side is caused to have a positive potential due to the third current. A switching power supply circuit that outputs from the tap a fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil. A transformer comprising a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a tap provided at a connection point between one end of the primary coil and one end of the secondary coil;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side switching element that has a current path that is controlled in the opposite phase to the on-off of the switching element, is interrupted when the switching element is on, and conducts current toward the other end of the primary coil when the switching element is off;
Controlled in the same phase as the on / off state of the switching element. When the switching element is on, the current toward the other end of the secondary coil is conducted. When the switching element is off, the current toward the other end of the secondary coil is conducted. A secondary-side switching element having a current path for blocking a current flowing out from the other end of the secondary coil when a positive potential is applied to the other end of the secondary coil when the switching element is off. ,
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the current path of the primary side switching element through the primary coil to the tap due to a counter electromotive force generated in the primary coil when the switching element is off, and the tap due to the third current A switching power supply circuit that outputs from the tap a fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil with the side being a positive potential. A transformer comprising a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a tap provided at a connection point between one end of the primary coil and one end of the secondary coil;
A switching element for switching on and off the voltage applied to the other end of the primary coil;
A primary-side diode that conducts a current toward the other end of the primary coil that is at a negative potential when the switching element is off;
Controlled in the same phase as the on / off state of the switching element. When the switching element is on, the current toward the other end of the secondary coil is conducted. When the switching element is off, the current toward the other end of the secondary coil is conducted. A secondary-side switching element having a current path for blocking a current flowing out from the other end of the secondary coil when a positive potential is applied to the other end of the secondary coil when the switching element is off. ,
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
A third current that flows from the primary diode to the tap through the primary coil by the back electromotive force generated in the primary coil when the switching element is turned off, and the tap side is caused to have a positive potential due to the third current. A switching power supply circuit that outputs from the tap a fourth current that flows to the tap through the secondary coil by magnetic induction generated in the secondary coil. A transformer comprising a primary coil, a secondary coil that is loosely magnetically coupled to the primary coil, and a tap provided at a connection point between one end of the primary coil and one end of the secondary coil;
A switching element for switching on and off a DC voltage applied to the other end of the primary coil;
A primary-side switching element that has a current path that is controlled in the opposite phase to the on-off of the switching element, is interrupted when the switching element is on, and conducts current toward the other end of the primary coil when the switching element is off;
A secondary diode that conducts a current toward the other end of the secondary coil when the other end of the secondary coil has a negative potential;
When the switching element is turned on, a first current flowing through the primary coil to the tap by the DC voltage and a magnetic induction generated in the secondary coil due to the first current pass through the secondary coil by the tap. A second current flowing through the tap is output from the tap,
When the switching element is turned off, a third current that flows from the primary side switching element to the tap through the primary coil due to a counter electromotive force generated in the primary coil, and the tap side is positively caused by the third current. A switching power supply circuit that outputs, from the tap, a fourth current that flows as a potential to the tap through the secondary coil by magnetic induction generated in the secondary coil. The transformer includes a pair of opposing yokes, a central leg that connects the central portions of the yokes, and a pair of outer sides that extend between the opposing first ends and the second ends of the yokes. Having a core composed of legs,
The primary coil is wound around the central leg, and the secondary coil is wound around each of the pair of outer legs;
A portion of the magnetic flux from the central leg to the outer leg is configured to pass through a gap between the primary coil and the secondary coil;
Magnetic flux flowing from the central leg to the pair of outer legs due to the first current flowing through the primary coil increases in each outer leg to generate a reverse magnetic flux to generate a reverse magnetic flux. And the second current flows through
Magnetic fluxes respectively flowing from the central leg to the pair of outer legs due to the third current flowing through the primary coil are in the same direction as the magnetic flux due to the first current and increase in each outer leg. The switching power supply circuit according to claim 1, wherein the fourth current flows through the secondary coil so as to generate a reverse magnetic flux. The transformer includes a pair of opposing yokes, a central leg that connects the central portions of the yokes, and a pair of outer sides that extend between the opposing first ends and the second ends of the yokes. Having a core composed of legs,
The primary coil is wound around the central leg, the secondary coil is wound around one of the pair of outer legs, and at least a magnetic gap at the middle position of the outer leg on which the secondary coil is not wound Is provided,
A part of the magnetic flux from the central leg to the outer leg passes through the gap between the primary coil and the secondary coil and the outer leg not wound with the secondary coil;
A magnetic flux flowing from the central leg to the outer leg around which the secondary coil is wound due to the first current flowing through the primary coil increases in the outer leg, so that a reverse magnetic flux is generated. The second current flows through the secondary coil; and
The magnetic flux flowing from the central leg to the outer leg on which the secondary coil is wound due to the third current flowing through the primary coil is in the same direction as the magnetic flux resulting from the first current and the outer leg. 5. The switching power supply circuit according to claim 1, wherein the fourth current flows through the secondary coil so as to generate a reverse magnetic flux. The transformer includes a pair of opposing yokes, a central leg that connects the central portions of the yokes, and a pair of outer sides that extend between the opposing first ends and the second ends of the yokes. Having a core composed of legs,
The primary coil is wound around the central leg, the secondary coil is spaced apart from the primary coil and concentrically with the primary coil, wound around the pair of outer legs,
A portion of the magnetic flux from the central leg to the outer leg is configured to pass through a gap between the primary coil and the secondary coil;
The second current flows through the secondary coil to generate a reverse magnetic flux by increasing the magnetic flux flowing through the central leg due to the first current flowing through the primary coil; and
The magnetic flux flowing through the central leg due to the third current flowing through the primary coil is in the same direction as the magnetic flux due to the first current and increases, thereby generating the secondary magnetic flux to generate a reverse magnetic flux. The switching power supply circuit according to claim 1, wherein the fourth current flows through the coil. The transformer includes a pair of opposing yokes, a central leg that connects the central portions of the yokes, and a pair of outer sides that extend between the opposing first ends and the second ends of the yokes. Having a core composed of legs,
The primary coil is wound around the central leg, and the secondary coil is wound concentrically with the primary coil via a magnetic piece disposed outside the primary coil,
A part of the magnetic flux from the central leg toward the outer leg passes through the magnetic piece,
The second current flows through the secondary coil to generate a reverse magnetic flux by increasing the magnetic flux flowing through the central leg due to the first current flowing through the primary coil; and
The magnetic flux flowing through the central leg due to the third current flowing through the primary coil is in the same direction as the magnetic flux due to the first current and increases, thereby generating the secondary magnetic flux to generate a reverse magnetic flux. The switching power supply circuit according to claim 1, wherein the fourth current flows through the coil. Consists of a pair of opposing yokes, a central leg that connects the central parts of both yokes, and a pair of outer legs that respectively extend between the opposing first and second ends of both yokes Core to be
A primary coil wound around the central leg;
A secondary coil wound around each of the pair of outer legs,
A power transformer, wherein a gap between the primary coil and the secondary coil is a leakage magnetic circuit. Consists of a pair of opposing yokes, a central leg that connects the central parts of both yokes, and a pair of outer legs that respectively extend between the opposing first and second ends of both yokes Core to be
A primary coil wound around the central leg;
A secondary coil wound around one of the pair of outer legs,
A magnetic gap is provided at an intermediate position of at least the outer leg on which the secondary coil is not wound,
A power transformer, wherein a gap between the primary coil and the secondary coil and an outer leg on which the secondary coil is not wound are a leakage magnetic circuit. Consists of a pair of opposing yokes, a central leg that connects the central parts of both yokes, and a pair of outer legs that respectively extend between the opposing first and second ends of both yokes Core to be
A primary coil wound around the central leg;
A secondary coil spaced apart from the primary coil and wound concentrically with the primary coil inside the pair of outer legs;
A power transformer, wherein a gap between the primary coil and the secondary coil is a leakage magnetic circuit. Consists of a pair of opposing yokes, a central leg that connects the central parts of both yokes, and a pair of outer legs that respectively extend between the opposing first and second ends of both yokes Core to be
A primary coil wound around the central leg;
A secondary coil wound concentrically with the primary coil via a magnetic piece disposed outside the primary coil;
A power transformer, wherein the magnetic piece is a leakage magnetic circuit. A primary coil having a first magnetic circuit, a secondary coil having a second magnetic circuit and loosely magnetically coupled to the primary coil, and a portion of the magnetic flux generated by the primary coil passes through the secondary coil Without leaking magnetic circuit,
When a DC voltage is applied to the primary coil, a voltage is induced in the secondary coil, and the magnetic flux density of the magnetic flux existing in the first magnetic circuit of the primary coil is in the second magnetic circuit of the secondary coil. Hold as greater than the magnetic flux density of the magnetic flux present in the
When the application of the DC voltage to the primary coil is stopped and the current caused by the counter electromotive force generated in the primary coil flows to the primary coil, the same polarity as when the DC voltage is applied to the secondary coil A power transformer characterized by inducing a voltage of.   Magnetic flux flows from the first magnetic circuit to the second magnetic circuit when the application of the DC voltage to the primary coil is stopped and a current due to the counter electromotive force generated in the primary coil flows to the primary coil. The power transformer according to claim 13.