JPH0965650A - DC-DC converter - Google Patents

Abstract

(57) [PROBLEMS] To provide a DC-DC converter which has a very simple secondary side control circuit with a small number of parts and can be started and operated without adding a special power source. The purpose is to SOLUTION: An output control MOSFET 4 inserted in a direction to block a rectified current of a rectifying element 5, and an output control MO.
A first switch 11 inserted between the gate of the SFET 4 and the secondary winding of the transformer 2 and an output control MOSF
The second switch 12 inserted between the gate and the source of the ET4 and the voltage of the secondary winding of the transformer 2 become the rectifying element 5
The first switch 1
An edge detection circuit 10 for turning on 1 and an output voltage detection circuit 8 for turning on the second switch 12 when the output voltage of the DC-DC converter exceeds a certain set value are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter having a circuit for controlling an output voltage on the secondary side of a transformer.

[0002]

2. Description of the Related Art FIG. 4 shows a configuration example of a conventional DC-DC converter.

In FIG. 4, 1 is a main switch, 2 is a transformer, 3 is a main switch control circuit, and 4 is an output control MOS.
FET, 5 is a rectifying element, 6 is a commutation element, 7 is a smoothing circuit,
8 is an output voltage detection circuit, 30 is a primary-secondary synchronization signal transmission means, 31 is a pulse width modulation circuit, 32 is a drive circuit, 33
Is a secondary side starting circuit.

The circuit operation of this DC-DC converter is as follows. The main switch 1 on the primary side is repeatedly turned on and off at a constant cycle to generate an AC voltage, which is a voltage converted from an input voltage, on the secondary side of the transformer 2. When the output control MOSFET 4 is on, a rectified current flows through the rectifying element 5. A current alternately flows through the rectifying element 5 and the commutation element 6 depending on whether the main switch 1 is turned on or off, and is smoothed by the smoothing circuit 7 to output the current. The output voltage of the converter can be controlled by limiting the rectification current by adjusting the ON period of the output control MOSFET 4 while the main switch 1 is ON. In order to realize this control, the output voltage detection circuit 8 monitors the output voltage, the pulse width modulation circuit 31 and the drive circuit 32 modulate the ON period of the output control MOSFET 4, and the output voltage is kept constant. In order to synchronize with the main switch 1 on the primary side, a synchronization signal is input to the pulse width modulation circuit 31 using the primary-secondary synchronization signal transmission means 30. Further, since no voltage is generated on the secondary side when the converter is started, the secondary side starting circuit 33 causes the pulse width modulation circuit 3 to operate.
1. A voltage for operating the secondary side control circuit such as the drive circuit 32 is supplied to start up the secondary side control circuit.

[0005]

However, in such a conventional DC-DC converter, the circuit size of the secondary side control circuit becomes large, the number of parts increases, and the current consumption of the control circuit increases. In addition, it is difficult to secure the operating voltage of the secondary side control circuit at the time of startup.

The present invention has been made in order to solve the above-mentioned problems, and constitutes a very simple secondary side control circuit with a small number of parts and can be started and operated without adding a special power source. The purpose is to do so.

[0007]

A main switch, a main switch control circuit, a transformer, a rectifying element, a commutation element,
In a DC-DC converter including a smoothing circuit,
An output control MOSFET inserted in a direction to block the rectified current of the rectifying element, a first switch inserted between the gate of the output control MOSFET and the secondary winding of the transformer, and the output The second switch inserted between the gate and the source of the control MOSFET and the first switch when the secondary winding voltage of the transformer is changed to a polarity that allows a rectified current to flow through the rectifying element. An edge detection circuit that turns on and an output voltage detection circuit that turns on the second switch when the output voltage of the DC-DC converter exceeds a certain set value are provided.

[0008]

1 is a diagram showing the configuration of a DC-DC converter according to the present invention. In FIG. 1, 1 is a main switch, 2 is a transformer, 3 is a main switch control circuit, 4
Is an output control MOSFET inserted in a direction to block the rectified current of the rectifying element 5, 6 is a commutation element, 7 is a smoothing circuit,
Reference numeral 8 is an output voltage detection circuit, 10 is an edge detection circuit for detecting a change in the secondary winding voltage of the transformer 2, 11 is inserted between the gate of the output control MOSFET 4 and the secondary winding of the transformer 2. 1st switch, 12 is M for output control
The second inserted between the gate and the source of the OSFET4
Switch.

In FIG. 1, the secondary side voltage of the transformer 2 charges and discharges the gate capacitance of the output control MOSFET 4. When the gate capacitance is discharged, the output control MOSFET 4 is off, and no rectified current flows through the rectifying element 5. On the contrary, when it is charged, the output control MO
The SFET 4 is turned on, and a rectification current flows through the rectification element 5. The current that charges the gate capacitance in order to turn on the output control MOSFET 4 is changed when the voltage of the secondary winding of the transformer 2 changes in the direction in which the rectified current flows.
Since the first switch 11 is turned on by the edge detection circuit 10 and flows from the secondary winding through the first switch 11, the turn-on of the output control MOSFET 4 is synchronized with the change in the secondary winding current. Is done. On the other hand, the turn-off of the output control MOSFET 4 changes depending on the output voltage of the DC-DC converter. When the output voltage of the DC-DC converter is low, including when the DC-DC converter is activated, the second switch 12 remains off and the output control MOSFET 4 maintains the on state without discharging the gate capacitance. DC is not blocked by rectified current
-The output voltage of the DC converter rises. The output voltage of the DC-DC converter increases, and the output voltage detection circuit 8
2 exceeds the detection voltage value, the second switch 12 is turned on, the gate capacitance of the output control MOSFET 4 is discharged, and the output control MOSFET 4 is turned off. The output control MOSFET 4 is turned off while the rectified current is flowing, so that the rectified current is blocked and the output voltage changes from rising to decreasing. After that, when the polarity of the secondary winding voltage of the transformer 2 is reversed and the secondary winding voltage of the transformer 2 is changed again to the direction in which the rectified current flows, the edge detection circuit 10 turns on the first switch 11. Then, the gate capacitance of the output control MOSFET 4 is charged by the voltage of the secondary winding, and the output control MOSFET 4 is turned on. Then, a rectified current flows and the output voltage of the DC-DC converter rises. And the output voltage detection circuit 8
When the voltage exceeds the detection voltage, the output control MOSF
ET4 blocks the rectified current. In this way, the time ratio of the rectified current (the ratio of the current flowing per cycle) is controlled,
Constant voltage control is performed so that the output voltage of the DC-DC converter is almost the detected voltage value.

FIG. 2 is a diagram for explaining the outline of the operation of the circuit. While the main switch 1 is on, a voltage having a positive polarity on the side of the terminal indicated by the black circle in FIG. 1 is generated in the secondary winding of the transformer 2, and while the main switch 1 is off, the secondary voltage is generated. A negative voltage of opposite polarity is generated in the winding. During a period in which a positive voltage is generated in the secondary winding, if the output control MOSFET 4 is on, a rectified current flows through the rectifying element 5 and the output control MOSFET 4. While the rectified current does not flow, a current flows as a commutation current in the commutation element 6. That is, current flows through the rectifying element 5 and the commutation element 6 alternately. The current enters the smoothing circuit 7, and the smoothed voltage is output.

In the embodiment, the output control MOSFET 4 is used.
Since the n-channel MOSFET is used for the output control MOSFET 4, the output control MOSFET 4 is turned off when the gate voltage with respect to the source of the output control MOSFET 4 is equal to or lower than the threshold voltage, and the output control MOSFET 4 when the gate voltage is equal to or higher than the threshold voltage.
Turns on. Since there is a gate capacitance between the gate and the source of the output control MOSFET 4, the output control MO
To turn on the SFET 4, it is necessary to charge the gate capacitance and to turn off the SFET 4, it is necessary to discharge the gate capacitance. When the secondary winding voltage changes positively, this change is detected by the edge detection circuit 10, the first switch 11 is turned on as shown in FIG. 2, and output control is performed through the first switch 11. By charging the gate capacitance of the MOSFET, the output control MOSFET 4 can be turned on in synchronization with the turn-on of the main switch 1. On the other hand, the turn-off of the output control MOSFET 4 is
It is performed when the output voltage of the DC-DC converter exceeds the detection voltage value of the output voltage detection circuit 8. That is, when the output voltage detection circuit 8 detects that the DC-DC converter output voltage exceeds the detection voltage, the second switch 12 is turned on as shown in FIG.
The gate capacitance of SFET4 is discharged and turned off.

The output voltage is controlled as follows. When the output voltage is lower than the predetermined detection voltage value, the second switch 12 is off and the output control MOSFET 4 is on. Therefore, the rectified current flows in synchronization with the turning on of the main switch 1. In this way, energy is transmitted to the output using the entire ON period of the main switch 1, and the output voltage rises. When the output voltage becomes high, the DC-DC converter output voltage exceeds the detection voltage value of the output voltage detection circuit 8 during the period when the rectified current is flowing. Then, the output control MOSFET 4 is turned off, and the rectified current is blocked in the middle of the ON period of the main switch 1, as shown in the rectified current waveform of FIG. As shown in the output voltage waveform of FIG. 2, during the period when the rectified current flows, DC-DC
The output voltage of the converter rises, and the output voltage of the DC-DC converter falls while the rectified current is not flowing,
The output voltage is maintained at the detected voltage value.

As described above, the secondary side control circuit of the DC-DC converter, which has conventionally required a control circuit having a large circuit scale, can be constructed with a simple circuit without requiring a special starting circuit. .

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 shows a concrete embodiment of a DC-DC converter according to the present invention. In FIG. 3, 1 is a main switch, 2 is a transformer, 3 is a main switch control circuit, 4 is an output control MOSFET, 5 is a rectifying element, 6 is a commutation element, and 7
Is a smoothing circuit, 8 is an output voltage detection circuit, 10 is an edge detection circuit, 11 is a first switch, 12 is a second switch,
21 is a resistor, 22 is a capacitor, 23 is a diode, 2
Reference numeral 4 is a shunt regulator, 25 and 26 are resistors, and 27 is a transistor.

The first switch 11 is realized by a pnp transistor, and the second switch 12 is realized by an npn transistor. Diodes are used for the rectifying element 5 and the commutation element 6.

The output voltage detection circuit 8 comprises a shunt regulator 24, a resistor 25, a resistor 26, and a transistor 27. The shunt regulator 24 compares its input terminal voltage with a reference voltage built into the shunt regulator 24, amplifies the difference voltage, and operates so that a larger input terminal voltage draws a larger current from its output terminal. It is an element. A voltage obtained by dividing the output voltage by the resistor 25 and the resistor 26 is input to the shunt regulator 24. This resistance-divided voltage value is compared and amplified with the reference voltage of the shunt regulator 24, and becomes the output current of the shunt regulator 24. For example, when the reference voltage of the shunt regulator 24 is 2.5V, in order to set the detection voltage value of the output voltage detection circuit 8 to 5V, the resistor 25 and the resistor 2 are used.
The resistance value ratio of 6 may be 1: 1. The output of the shunt regulator 24 becomes the base current of the transistor 27. This current is further amplified by the transistor 27 and becomes the base current of the npn transistor which is the second switch 12. While the rectifying current is flowing through the rectifying element 5, the output voltage of the DC-DC converter is the resistance 25,
When the detected voltage value set by 26 is exceeded, the output current of the shunt regulator 24 increases, the transistor 27 current-amplifies, the npn transistor which is the second switch 12 becomes conductive, and the gate capacitance of the output control MOSFET 4 is increased. Discharge and turn off. In this way, the rectified current is blocked while flowing.

The edge detection circuit 10 is composed of a resistor 21, a capacitor 22 and a diode 23, and the base of a pnp transistor which is the first switch 11 is connected to the edge detection circuit 10. The capacitor 22 is charged and discharged according to the secondary winding voltage of the transformer 2.
When the secondary winding voltage changes so that the terminal indicated by the black circle becomes high potential, the charging current of the capacitor 22 flows between the emitter and base of the pnp transistor which is the first switch 11 and the resistor 21. The current stops when the capacitor is charged. This current is the first switch 1
It becomes the base current of the pnp transistor which is 1, the first switch 11 is turned on while the base current is flowing, a current for charging the gate capacitance of the output controlling MOSFET 4 flows, and the output controlling MOSFET 4 is turned on. When the polarity of the secondary winding voltage of the transformer 2 is reversed,
The capacitor 22 is quickly discharged through the diode 23. At this time, the first switch 11 is off.

The operation of the circuit of FIG. 3 is as follows.

As described above, the first switch 11 is turned on every time the secondary winding voltage of the transformer 2 changes, and if the output control MOSFET 4 is in the off state at that time, it is turned on and in the on state. If so, maintain that state.

When the output voltage is sufficiently lower than the detection value of the output voltage detection circuit 8, no current flows in the output of the shunt regulator 24, and no current flows in the transistor 27.
The second switch 12 remains off. Therefore, the output control MOSFET 4 is maintained in the ON state without being turned off, and the rectifying operation is performed by the rectifying element 5 in synchronization with the ON / OFF of the main switch. Even when the converter output voltage is 0 when the DC-DC converter is started, the above operation causes a rectified current to flow through almost the entire ON period of the main switch 1, and energy can be transmitted to the converter output. The output voltage can rise.
In this way, a circuit for starting is not particularly required.

When the output voltage exceeds the detection voltage value of the output voltage detection circuit 8, the shunt regulator 24 supplies the base current of the transistor 27, and the amplified collector current turns on the second switch 12. Second switch 1
2 discharges the gate capacitance of the output control MOSFET 4,
The output control MOSFET 4 is turned off. When the output control MOSFET 4 is turned off, the rectified current is blocked and the converter output voltage starts to drop.

When the main switch 1 is turned off, the polarity of the secondary winding voltage of the transformer 2 becomes negative, and when the main switch 1 is turned on thereafter, the polarity of the secondary winding voltage of the transformer 2 changes to positive. To do. Then, the first switch 11 is turned on, the output control MOSFET 4 is turned on, and a rectified current flows. When the rectified current flows, the converter output voltage rises, and when it exceeds the detected voltage value, the output control MOSFET 4 is turned off to block the rectified current, as already described. With the above operation, the converter output voltage is maintained at a constant value.

With such a circuit configuration, it is possible to control the output voltage of the DC-DC converter on the secondary side of the transformer with a simple circuit without requiring a special starting circuit. As a result, there are effects such as simplification of the circuit, speedup of the control system, and easy realization of a multi-output converter using a plurality of secondary side circuits.

The secondary side control circuit of the DC-DC converter according to the present invention described above performs the same operation even in a circuit in which the polarity of the semiconductor element is inverted, such as an n-channel MOSFET being a p-channel type. be able to. further,
Even when the rectifying element 5 and the commutation element 6 are replaced by MOSFETs that can reduce conduction loss instead of diodes, similar circuits are formed and the effect can be improved. Also,
The circuit on the primary side is not limited to the circuit configuration of one main switch shown in the embodiment, and a push-pull method, a half-bridge method, a soft switch, an active clamp, or another switching method using a plurality of switches is used. The present invention can be similarly implemented and effects can be obtained in other circuit forms used.

[0025]

The DC-DC converter according to the present invention is
The output control MOSFET is equipped with two switches for charging and discharging the gate capacitance of the output control MOSFET.
It is configured to turn on in synchronization with the change of the secondary winding voltage of the transformer, turn off when the output voltage of the DC-DC converter exceeds the detection voltage value, and modulate the pulse width of the rectified current to control the output voltage. As a result, a very simple secondary side control circuit is constructed with a small number of parts, and there is a great advantage that no special starting circuit is required for starting the converter.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a DC-DC converter according to the present invention.

FIG. 2 is a schematic diagram illustrating a circuit operation.

FIG. 3 is an embodiment of a DC-DC converter according to the present invention.

FIG. 4 is a configuration example of a conventional DC-DC converter.

[Explanation of symbols]

1 ... Main switch 2 ... Transformer 3 ... Main switch control circuit 4 ... Output control MOS
FET 5 ... Rectifying element 6 ... Commutation element 7 ... Smoothing circuit 8 ... Output voltage detection circuit 10 ... Edge detection circuit 11 ... First switch 12 ... Second switch 21 ... Resistor 22 ... Capacitor 23 ... Diode 24 ... Shunt regulator 25 ... Resistor 26 ... Resistor 27 ... Transistor 30 ... Primary-Secondary Synchronous Signal Transmission Means 31 ... Pulse Width Modulation Circuit 32 ... Drive Circuit 33 ... Secondary Side Starting Circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H03K 17/732 9561-5K H03K 17/73 B

Claims (1)

[Claims] 1. A DC-DC converter including a main switch, a main switch control circuit, a transformer, a rectifying element, a commutation element, and a smoothing circuit, in a direction in which a rectifying current of the rectifying element is blocked. Output control MOSF inserted
ET, a first switch inserted between the gate of the output control MOSFET and the secondary winding of the transformer, and a second switch inserted between the gate and the source of the output control MOSFET. Switch, an edge detection circuit that turns on the first switch when the secondary winding voltage of the transformer changes to a polarity that allows a rectifying current to flow through the rectifying element, and an output voltage of the DC-DC converter. The DC-DC converter comprises an output voltage detection circuit for turning on the second switch when the voltage exceeds a predetermined set value.