JPH0357711B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a DC-DC converter (DC-DC converter) used in a stabilized DC power supply.

[Conventional technology and its problems]

A DC- DC converters are already known. By the way, the conventional typical on-off type DC-DC converter (FCC) is
It uses an oscillator to create a pulse width modulated wave (PWM wave), and is configured as a separately excited type that controls the conversion switching transistor on and off. Therefore, the circuit has become complex and expensive. As a solution to this kind of drawback, the special public
No. 18364 discloses a self-excited on/off type DC-DC converter in which a transformer is provided with a control winding, and the output voltage is adjusted by controlling the control winding. However, in this method, a control winding must be provided, making it difficult to significantly downsize and simplify the device.

As a DC-DC converter with a simple circuit configuration,
There is an on-off type self-excited DC-DC converter (RCC), but it has the disadvantage that the oscillation frequency increases under light loads, increasing the number of switching operations and increasing switching loss. Another disadvantage is that it is difficult to use field effect transistors as switching elements.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a DC-DC converter that can self-oscillate with a relatively simple circuit and easily achieve voltage control.

[Means for solving problems]

The first invention of the present application to solve the above problems and achieve the above objects turns on and off the voltage supplied from the DC power supply, and the voltage at the control terminal reaches the threshold voltage. a conversion switch configured to turn on when
A voltage conversion winding for obtaining an output voltage corresponding to on/off of the conversion switch; and a voltage conversion winding that is electromagnetically coupled to the voltage conversion winding and connected to the control terminal to turn on the conversion switch. a transformer having a switch drive winding; a rectifier and smoothing circuit connected to the output side of the voltage conversion winding;
a first capacitor connected to the conversion switch so as to turn on the conversion switch with a charging voltage; a first charging circuit for charging the first capacitor; and a first charging circuit for charging the first capacitor; a second capacitor for determining when the switch starts to turn off; and a second capacitor for starting charging the second capacitor in response to the start of turning on the conversion switch.
a charging circuit connected to the conversion switch and the first and second capacitors, and in response to the second capacitor being charged to a predetermined value, the conversion switch according to the voltage of the drive winding; a control circuit that cuts off the on-drive of the converter switch, turns the conversion switch into an off state, and puts the first and second capacitors in a discharge state; and a control circuit that changes the charging current of the second capacitor. The present invention relates to a DC-DC converter comprising a voltage control circuit that controls the output voltage of the rectifying and smoothing circuit by changing the time width until the voltage of the second capacitor reaches the predetermined value.

The conversion switch in the above invention is, for example, an element such as the FET 5 in the embodiment, and the voltage conversion winding is a device for voltage conversion, such as the primary winding 3 and the secondary winding 6 in the embodiment. The related windings, the switch drive winding being, for example, the tertiary winding 14 of the embodiment, the control circuit being, for example, a circuit including the control transistor 19 of the embodiment, and the voltage control circuit being, for example, the phototransistor 25 This is a circuit that includes

The second invention of the present application is to provide a circuit that detects the current flowing through the conversion switch, and to control the conversion switch off based on the sum of the voltage obtained from this current detection circuit and the charging voltage of the second capacitor. This is how it was done.

[Effect]

In the above invention, when the first capacitor is charged to a predetermined value, the off period of the conversion switch ends and the on period begins. Also, when the second capacitor is charged to a predetermined value, the on period ends;
Entering an off period. Therefore, the conversion switch operates on and off based on the charging and discharging of the first and second capacitors. Changing the charging current of the second capacitor changes the on-time width of the conversion switch.

Furthermore, in the second invention, when the current detection voltage increases due to overcurrent, the on-time width of the conversion switch becomes narrower than in normal times.

[First Embodiment] Next, referring to FIGS. 1 and 2, the first embodiment of the present invention will be described.
A DC-DC converter according to an embodiment will be explained.
In FIG. 1, one end of a primary winding 3 of a transformer 2 is connected to one DC power supply terminal 1. 1
An N-channel insulated gate type (MOS type) field effect transistor or FET 5 is connected between the other end of the next winding 3 and the other DC power supply terminal 4 as a conversion switching element. FET5 has a drain as the first terminal, a source as the second terminal,
It has a gate as a control terminal, and when a voltage equal to or higher than a threshold voltage V _TH is applied to the gate, the drain and source become conductive (on state).

The secondary winding 6 electromagnetically coupled to the primary winding 3 has
A rectifying and smoothing circuit 11 consisting of two diodes 7 and 8, a reactor 9, and a capacitor 10 is connected. Output terminals 12 and 13 connected to the rectifying and smoothing circuit 11 are parts that supply a stabilized output voltage to a load.

14 is the tertiary winding of the transformer, and the primary winding 3
and is electromagnetically coupled to the secondary winding 6. One end of the tertiary winding 14 is connected to the gate of the FET 5 via the first capacitor 15, and the other end is connected to the source. A first resistor 16 is connected between one power supply terminal 1 and the gate as a first charging circuit. This resistor 16 is provided to obtain a charging time constant for the first capacitor 15.

The on-time end control circuit according to the present invention has a second
17 and a second resistor 18 serving as a second charging circuit for charging the capacitor 17. A second capacitor 17 is connected in parallel to the tertiary winding 14 via a second resistor 18 for determining this charging time constant. Note that a diode D ₂ is connected in parallel to the second capacitor 17 to keep this reverse charging voltage constant.

The control transistor 19 is connected between the gate and source of the FET 5 via a reverse blocking diode _D1 , and is connected to the voltage V _C2 of the second capacitor 17.
controlled by. That is, the second capacitor 17 is connected between the base and emitter of the control transistor 19, and when the voltage of the second capacitor 17 exceeds a predetermined value, the control transistor 1
9 is turned on.

As a constant voltage control circuit, voltage detection resistors 20 and 21 are connected between output terminals 12 and 13, one input terminal is connected to a voltage dividing point of these resistors 20 and 21, and the other input terminal is connected to a reference voltage. An error amplifier 23 connected to the source 22, a light emitting diode 24 connected to the output terminal of the error amplifier 23, and a phototransistor 25 optically coupled to the light emitting diode 24 are provided. The phototransistor 25 is provided as a variable impedance element for changing the charging time constant of the second capacitor 15, and is connected in parallel to the second resistor 18. Therefore, the charging circuit for the second capacitor 17 includes the second resistor 18 and the phototransistor 25.
It consists of a parallel circuit with

In order to reset the residual magnetism of the transformer 2 by the flyback voltage, a resistor 27 is connected in parallel to the primary winding 3 via a diode 26. Further, a capacitor 28 is connected in parallel to the resistor 27 in order to suppress flyback voltage.

(Operation) When DC voltage is supplied from the pair of power supply terminals 1 and 4, the first resistor 16 and the first capacitor 15 and 3
Charging of the first capacitor 15 is started by the circuit consisting of the secondary winding 14 . first capacitor 15
is charged according to a charging time constant R ₁ C ₁ determined by this capacitance C ₁ and the value R ₁ of the first resistor 16,
This voltage V _C1 between both ends gradually increases as shown from t ₁ to _{t 4} in FIG. 2E. FET5 is the threshold voltage
Since it has V _TH (rise voltage), it does not turn on immediately in synchronization with power-on, but turns on at time t ₃ when the first capacitor 15 is charged to the threshold voltage V _TH or higher. When FET5 turns on,
As shown in FIG. 2C, the drain-source voltage V _DS of the FET 5 becomes low, and conversely, since the power supply voltage is applied to the primary winding 3, the voltage increases across the tertiary winding 14 as shown in FIG. 2D. V ₃ is generated, and this voltage V ₃ of the tertiary winding 14 becomes a positive feedback voltage and is applied to the gate of the FET 5. The voltage at the power supply terminal 1 is the voltage at the tertiary winding 14.
Since the voltage of V is set higher than ₃ , the FET
5 is on during the period t ₃ to _{t 4} as well, and the sum of the capacitor voltage V _C1 and the tertiary winding voltage V ₃ becomes the gate voltage V _G. Note that a current flows through the source-gate capacitance of the FET 5 based on the positive voltage obtained in the tertiary winding 14. As a result, the positive voltage in the tertiary winding 14 allows the capacitor 15 to discharge and then charge the capacitor 15 to the opposite polarity. When the FET 5 is turned on, a drain current _ID flows as shown in FIG. 2B, and a voltage that turns on the diode 7 is generated in the secondary winding 6 of the transformer, which is smoothed and becomes the output voltage.

From the time t ₃ when FET 5 turns on and a positive voltage is generated across the tertiary winding 14, the second capacitor 1
7 starts charging in the positive direction, and this charging voltage V _C2 gradually increases as shown in FIG. 2F. When the capacitor voltage V _C2 becomes equal to or higher than the rising voltage of the control transistor 19, the transistor 19 is turned on, the gate and source are short-circuited, the gate voltage V _G becomes zero, and the FET 5 is turned off.
While the transistor 19 is kept on due to the storage time, the charge in the first capacitor 15 is discharged by the circuit of the diode D ₁ , the transistor 19, and the tertiary winding 14, and
7 charges are also released.

Next, the operation after startup will be explained. At t ₁ in Figure 2
When FET5 turns off, the flyback voltage
V _F is generated, and the tertiary winding voltage is as shown in Figure 2D.
The direction of V ₃ is reversed. The reverse voltage of the tertiary winding 14 generated between t ₁ and t ₂ is blocked by the transistor 19 and the diode D ₁ , but since there is a capacitance between the source and gate of the FET 5, the first capacitor 15 is connected through this. Charge. Therefore, during the period t ₁ to _{t 2} during which the flyback voltage is generated, both the voltage of the tertiary winding 14 and the power supply voltage are
5 is charged. The reverse voltage of the tertiary winding 14 based on the flyback voltage is the on-time width of the FET 5.
The wider T _ON and the larger the drain current I _D , the higher it becomes.
Therefore, when the on-time width T _ON of FET5 becomes wider,
The slope of the rise in the charging voltage of the first capacitor 15 becomes large, and as a result, the off time width T _OFF of the FET 5 becomes narrow. Therefore, even if the load current fluctuates, the FET
The on/off cycle of No. 5 does not change much and remains almost constant.

Since the gate voltage V _G is the sum of the capacitor voltage V _C1 and the tertiary winding voltage V ₃ , it changes as shown in FIG. 2A, and during the period t ₁ to _{t 2} during which the flyback voltage V _F is generated. In this case, the voltage V _C1 −V _F becomes the gate voltage V _G. When the reset of transformer 2 is completed at t ₂ , the flyback voltage V _F also disappears, so
Capacitor voltage V _C1 becomes gate voltage V _G. 1st
The charging of capacitor 15 further progresses, and at t ₃ FET 5
When the threshold voltage V _TH is reached, FET 5 is turned on. The on-time width T _ON from t ₃ to _{t 4} after turning on is R, which is composed of the combined resistance value R ₂ of the resistor 18 and the phototransistor 25 and the value C ₂ of the second capacitor 17, as at the time of startup. ₂ C Determined by ₂ time constant circuit.

The output voltage is controlled by controlling the resistance value of the phototransistor 25. Now, if the output voltage becomes lower than a certain value, the output of the error amplifier 23 becomes low, the amount of light from the light emitting diode 24 decreases, and the resistance value of the phototransistor 25 increases.
Therefore, the charging current supplied by the charging circuit consisting of the resistor 18 and the phototransistor 25 decreases, and the charging speed of the second capacitor 17 becomes slower as shown by the dotted line in FIG. The time it takes for the voltage to reach the rising voltage of transistor 19 becomes longer. As a result, the on-time width T _ON from t ₃ to _{t 4} becomes longer, and the output voltage is returned to a predetermined value by increasing the duty ratio. When the output voltage becomes higher than the reference value, the operation is opposite to the above.

As is clear from the above, since this circuit is not a self-oscillating circuit that utilizes the saturation of the switching transistor, the switching period does not become short when the load is light and is kept almost constant. Therefore, switching loss during light loads is reduced. Also, on
Self-sustained oscillation is possible even though it is an off type. Further, the circuit configuration can be simplified and miniaturized.

[Second Embodiment] Next, a second embodiment of the present invention shown in FIGS. 3 and 4 will be described. However, parts common to FIGS. 1 and 2 are designated by the same reference numerals and their explanations will be omitted. In this example, the first capacitor 1
A transistor 30 is connected in parallel to the tertiary winding 14 via a diode D ₃ , and the base of the transistor 30 is connected via a resistor 31 to the lower end of the tertiary winding 14 . Therefore _, if a flyback voltage is generated in the period t ₁ to _{t 2} in FIG. 4 and a reverse voltage is generated in the tertiary winding 14 as shown in FIG. is turned on, and the charge in the first capacitor 15 is discharged. For this reason, the fourth
As shown in FIG. E, charging of the first capacitor 15 starts at time _t2 after the transformer 2 is reset. This completely prevents the gate voltage V _G from reaching the threshold voltage V _TH before the transformer 2 is reset. Further, when the reset of the transformer 2 is completed, vibration (not shown) occurs in the primary winding 3 of the transformer 2, but since the first capacitor 15 is in an uncharged state, the positive feedback voltage due to the vibration and the capacitor voltage V _C1 There is no risk that the sum of V TH and FET 5 will not reach the threshold voltage V _TH of FET 5, and FET 5 will be turned on by mistake.

At time _t2 , the transistor 30 is turned off, so charging of the first capacitor 15 starts from this time, and at time _t3 , the FET 5 is turned on.

32 is an overcurrent detection resistor, which is connected in series to the source of FET5. transistor 19
The emitter of is not directly connected to the source but is connected to the resistor 3.
It is connected to the left end of 2. Therefore, the voltage V _C2 of the second capacitor 17 and the voltage V _O across the resistor 32
is applied between the base and emitter of transistor 19. Therefore, when an overcurrent flows, the transistor 19 is turned on while the charging voltage V _C2 of the second capacitor 17 is low, and the FET 5
is turned off, and the on period t ₃ to t ₄ of the FET 5 becomes shorter.

[Third Embodiment] Next, a DC-DC converter according to a third embodiment of the present invention shown in FIG. 5 will be described. However, parts common to FIGS. 1 and 3 are designated by the same reference numerals and their explanations will be omitted. In FIG. 5, diode 3 is used to bias phototransistor 25.
3 and a capacitor 34 are connected in parallel to the winding 14, and the collector of the phototransistor 25 is connected to the capacitor 34. Further, a resistor 35 is connected in parallel to the second capacitor 17. In this circuit, charging current flows into the second capacitor 17 from both the resistor 18 and the phototransistor 25.

[Modified example]

The present invention is not limited to the embodiments described above, and the following modifications are possible, for example.

(a) A backflow blocking diode _D1 may be moved between the gate and the capacitor 15.

(b) FET5 can be replaced with a bipolar transistor. In addition, phototransistor 2
Connect a bipolar transistor in place of 5,
This may be controlled by the output of the error amplifier 23. Also, transistor 19 is FET
It can be done.

(c) The conversion switching element may be composed of a plurality of switching elements in which FETs or bipolar transistors are connected in series or in parallel.

(d) A fourth winding is provided in the transformer 2, and a resistor 1 is connected here.
8. A circuit of capacitor 17 may be connected.
Further, the first capacitor 15 may be moved to the lower end side of the winding 14.

(e) The resistor 18 may be omitted and the second charging circuit according to the present invention may be configured only with a variable impedance element such as the phototransistor 25.

(f) The upper end of the resistor 18 may be connected to the lower end of the primary winding 3.

(g) A backflow blocking diode may be connected directly to the resistor 18.

〔Effect of the invention〕

According to the above invention, a self-excited DC-DC converter with variable off-width control of the switch can be obtained with a simple circuit configuration.

[Brief explanation of drawings]

FIG. 1 shows a direct current according to the first embodiment of the present invention.
A block diagram showing the DC converter, Fig. 2 is a waveform diagram of each part of Fig. 1, Fig. 3 is a circuit diagram showing the DC-DC converter of the second embodiment, and Fig. 4 is a diagram of each part of Fig. 3. FIG. 5 is a circuit diagram showing the DC-DC converter of the third embodiment. 1...Power terminal, 3...Primary winding, 5...
FET, 6... Secondary winding, 11... Rectifier smoothing circuit,
14... Tertiary winding, 15... First capacitor,
16...Resistor, 17...Second capacitor, 18
...Resistor, 19...Control transistor, 25...
...Phototransistor, 32...Resistor for overcurrent detection.

Claims (1)

[Scope of Claims] 1. A conversion switch that turns on and off a voltage supplied from a DC power source and is configured to be turned on when the voltage at a control terminal reaches a threshold voltage; A voltage conversion winding for obtaining an output voltage corresponding to on/off of the conversion switch; and a voltage conversion winding that is electromagnetically coupled to the voltage conversion winding and connected to the control terminal to turn on the conversion switch. a rectifying and smoothing circuit connected to the output side of the voltage converting winding; and a rectifying and smoothing circuit connected in series with the switch driving winding so as to turn on the converting switch by a charging voltage. a first capacitor connected to the first capacitor; a first charging circuit for charging the first capacitor; a second capacitor for determining when the conversion switch starts to turn off; and a first capacitor connected to the conversion switch. a second charging circuit that starts charging the second capacitor in response to the start of turning on; In response to being charged, the on-drive of the conversion switch by the voltage of the drive winding is cut off, and the conversion switch is turned off;
and a control circuit for bringing the first and second capacitors into a discharge state, and controlling the time width until the voltage of the second capacitor reaches the predetermined value by changing the charging current of the second capacitor. and a voltage control circuit that controls the output voltage of the rectifying and smoothing circuit. 2. A conversion switch that turns on and off the voltage supplied from the DC power source and is configured to turn on when the voltage at the control terminal reaches a threshold voltage, and a conversion switch that turns on the voltage supplied from the DC power supply. - A voltage conversion winding for obtaining an output voltage corresponding to the OFF state, and a switch drive winding that is electromagnetically coupled to the voltage conversion winding and connected to the control terminal to turn on the conversion switch. a rectifying and smoothing circuit connected to the output side of the voltage conversion winding; and a first rectifying and smoothing circuit connected in series with the switch drive winding so as to turn on the conversion switch by the charging voltage. a first charging circuit for charging the first capacitor; a second capacitor for determining when the conversion switch starts to turn off; and a second capacitor that responds to the start of turning on the conversion switch. a second charging circuit that starts charging the second capacitor when the conversion switch is activated; a current detection circuit that detects the current flowing through the conversion switch; and a current detection circuit that is connected to the control terminal to turn off the conversion switch; and a control element connected to the second capacitor and the current detection circuit so as to be turned on by the sum of the charging voltage of the second capacitor and the current detection voltage obtained from the current detection circuit; and a voltage control circuit that controls the output voltage of the rectifying and smoothing circuit by changing the charging current of the second capacitor to change the time width until the voltage of the second capacitor reaches the predetermined value. DC-DC converter.