JP4191577B2 - Synchronous rectification switching power supply - Google Patents

Description

  The present invention relates to a switching power supply that converts a DC input voltage into a desired DC voltage and supplies it to an electronic device, and particularly relates to a flyback type synchronous rectification switching power supply apparatus.

  As a flyback type switching power supply device 10 having a conventional synchronous rectification type rectifier circuit, for example, there is a power supply device as shown in FIG. This switching power supply device 10 is provided with an input capacitor C1 in parallel with DC power supply input terminals 11 and 12, and an inverter transistor such as a primary winding N1 of a transformer T and a MOS-FET at both ends of the input capacitor C1. The main switch element Q1 is connected to the series circuit. The primary winding N1 of the transformer T has a dot connected to the input terminal 11, and the one without a dot connected to the main switch element Q1.

  The secondary winding N2 of the transformer T has a terminal without a dot connected to the output terminal 13, a terminal with a dot connected to the drain of a synchronous rectifier TR1 such as a MOS-FET, and the source thereof It is connected to the output terminal 14 on the reference potential side. Further, a smoothing output capacitor C2 is provided between the output terminals 13 and 14.

  A voltage source 16 provided on the secondary side is provided between the drain and source of the synchronous rectifier element TR1. The voltage source 16 includes a capacitor C5 having one end connected to the drain of the synchronous rectifier element TR1, the other end of the capacitor C5 is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to the cathode of the Zener diode ZD1. The anode of the Zener diode ZD1 is connected to the reference potential on the output terminal 14 side. Further, the other end of the resistor R3 is connected to the anode of the diode D3, and the cathode of the diode D3 is connected to the reference potential on the output terminal 14 side via the capacitor C6. The middle point of the cathode of the diode D3 and the capacitor C6 is the output of the voltage source 16.

  Further, an auxiliary winding N3 for turning on the synchronous rectifying element TR1 is provided on the secondary side of the transformer T, and a terminal on the side of the auxiliary winding N3 with a dot is connected to a reference potential, and a side without a dot Is connected to the gate of the synchronous rectifier element TR1 via a speed-up capacitor C4. Furthermore, the terminal on the non-dot side of the auxiliary winding N3 is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the reference potential on the output terminal 14 side via the series circuit of the resistor R1 and the timing capacitor C3. Yes. The middle point of the resistor R1 and the timing capacitor C3 is connected to the cathode of the diode D3 of the voltage source 16 and the middle point of the capacitor C6 via the resistor R2.

  Further, the middle point of the resistor R1 and the timing capacitor C3 is connected to the base of an npn-type transistor TR2 that is a switch element. The collector of the transistor TR2 is connected to the gate of the synchronous rectifier element TR1, and the emitter is connected to the reference potential on the output terminal 14 side. Further, the cathode of the diode D2 is connected to the gate of the synchronous rectifier element TR1, and the anode of the diode D2 is connected to the reference potential.

  The operation of the switching power supply 10 is controlled by PWM (Pulse Width Modulation) by turning on and off the main switch element Q1 of the MOS-FET by a control circuit (not shown). During the on period of the main switch element Q1, the synchronous rectifier element TR1 is off and no current flows, and the capacitor C6 of the voltage source 16 is charged via the capacitor C5. The charging voltage is a voltage set by the Zener diode ZD1. Further, during the ON period of the main switch element Q1, the capacitor C4 is charged with the cathode side of the diode D2 being positive by the auxiliary winding N3.

  Next, when the main switch element Q1 is turned off, the gate of the synchronous rectifier element TR1 is charged by adding the charging voltage of the capacitor C4 to the voltage of the terminal on the non-dot side of the auxiliary winding N3, thereby synchronizing the synchronous rectifier element TR1. Turns on. At the same time, the energy stored in the secondary winding N2 is charged in the output capacitor C2 by the flyback voltage generated in the secondary winding N2.

  At the same time as the main switch element Q1 is turned off, charging of the timing capacitor C3 by the voltage source 16 is started via the resistor R2. When the potential of the timing capacitor C3 gradually rises and reaches the threshold potential at which the transistor TR2 is turned on, the transistor TR2 is turned on to discharge the gate charge of the synchronous rectifier element TR1 and turn off the synchronous rectifier element TR1. The timing at which the transistor TR2 is turned on is set to the timing immediately before the main switch element Q1 is turned on. When the main switch element Q1 is turned on, charging of the capacitor C6 of the voltage source 16 starts again, and the charge of the timing capacitor C3 is discharged through the resistor R1 and the diode D1. By repeating the above operation, the switching power supply device 10 performs a synchronous rectification operation without supplying the main switch element Q1 and the synchronous rectification element TR1 at the same time, and supplies power to the output side.

In this type of power supply circuit, when the primary main switch element Q1 is turned on, the timing capacitor C3 that determines the on-time of the secondary side synchronous rectifier element TR1 is discharged, and then the main switch element Q1 is turned off. The timing capacitor C3 is charged by the secondary winding or the voltage source during the period when the secondary side synchronous rectifier element TR1 is turned on. When the voltage of the timing capacitor C3 reaches the threshold value for turning on the transistor TR2 for turning off the synchronous rectifier element TR1 by this charging current, the synchronous rectifier element TR1 is turned off. In this way, the through current of the synchronous rectifier element TR1 is prevented by charging the timing capacitor C3 and turning off the synchronous rectifier element TR1 before the main switch element Q1 is turned on. Therefore, this type of circuit has an advantage that a synchronous rectification circuit that easily prevents a through current can be easily configured by simply adjusting the resistance value of the charging / discharging path of the timing capacitor C3, thereby improving efficiency.
JP 2002-101656 A

  In the case of the above prior art circuit, when the output voltage is lowered by the overcurrent protection circuit at the time of overcurrent, the on-time of the main switch element Q1 is shortened and the timing capacitor C3 is not easily discharged. For this reason, the on-time of the synchronous rectifying element TR1 is shortened as the output voltage is reduced during an overcurrent, and the rectifying operation is performed by the body diode of the synchronous rectifying element TR1 or the diode D5 connected in parallel to the synchronous rectifying element TR1. A large current flows through the diode D5 and the like, and the loss is remarkably increased as compared with the case where the synchronous rectifying element TR1 is turned on.

  Conventionally, in order to withstand such a large loss, a complicated circuit in which the transistor of the synchronous rectifier element TR1 and the diode D5 connected in parallel with the transistor are large, or the inverter operation is intermittent in an overcurrent, is configured on the primary side. It is necessary to reduce the size and cost of the power supply device.

  The present invention has been made in view of the above-described problems of the prior art, and can maintain a long synchronous rectification period even when an output voltage is lowered due to an output overcurrent, thereby downsizing an electronic device. An object of the present invention is to provide a flyback type synchronous rectification switching power supply device that can contribute to downsizing of the product.

In the present invention, a main switch element such as a MOS-FET that is turned on and off within a certain period is connected in series with a primary winding of a transformer and provided between input terminals. A synchronous rectifying element such as a MOS-FET connected in series with the terminal, an auxiliary winding for complementarily turning on the synchronous rectifying element with respect to the main switch element, and turning off the synchronous rectifying element The switching element to be controlled, the timing capacitor connected to the control terminal of the switching element, and the on-time of the main switching element is controlled so as to decrease the output voltage when an overcurrent flows between the output terminals. An overcurrent protection circuit, and the timing capacitor has a charging current that flows when the main switch element is off to increase the voltage at both ends, and when the main switch element is on. Voltage across descends discharge current flows, the switching element is connected Synchronous flyback such that the voltage across the timing capacitor is turned off when turning on, the synchronous rectifier exceeds a predetermined threshold potential In the switching power supply device, when the on-time of the main switch element is shortened due to the operation of the overcurrent protection circuit and the voltage between the output terminals is reduced to a predetermined voltage or less , the charging current flowing through the timing capacitor is reduced. It is a synchronous rectification switching power supply device provided with a current adjustment circuit to be reduced.

A voltage source charged to a predetermined potential by a pulse voltage generated in the secondary winding or the auxiliary winding in accordance with the switching operation of the main switch element is provided, and the output of the voltage source is connected to the timing capacitor And connected to the current adjusting circuit. Further, the auxiliary winding may be connected to the gate of the synchronous rectifier element, one terminal of the timing capacitor may be connected, and the current adjustment circuit may be connected to the timing capacitor.

  Furthermore, in the synchronous rectification switching power supply device according to the present invention, the timing capacitor is connected between a base and an emitter of the transistor that is the switching element that turns off the synchronous rectification element, and the current adjustment circuit charges the timing capacitor. A plurality of resistors connected in series between the voltage source and the timing capacitor, and one end of a series circuit of the resistor and the diode is connected between the plurality of resistors, and the other end of the series circuit of the resistor and the diode is connected It is connected to the output terminal.

  The synchronous rectification switching power supply device of the present invention can maintain the synchronous rectification time for which the synchronous rectification element is turned on for a long time even when the output voltage is reduced due to an overcurrent, and is parallel to the synchronous rectification transistor and the synchronous rectification transistor. Loss of the connected diode can be kept small. Thereby, a small thing can be used for a synchronous rectification element and a diode connected in parallel to it. Moreover, since protection by intermittent overcurrent is unnecessary, the number of parts is reduced, which contributes to downsizing and cost reduction of the synchronous rectification switching power supply.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit of a flyback-type synchronous rectification switching power supply apparatus 20 according to an embodiment of the present invention. The same components as those shown in FIG. In this synchronous rectification switching power supply device 20, an input capacitor C1 is provided in parallel with input terminals 11 and 12 of a DC power supply, and a primary winding N1 of a transformer T and a main switch element Q1 are connected in series at both ends of the input capacitor C1. It is connected. The primary winding N1 of the transformer T has a dot connected to the input terminal 11, and the one without a dot connected to the main switch element Q1. The main switch element Q1 is composed of a semiconductor switch element such as a MOS-FET.

  The secondary winding N2 of the transformer T has a terminal without a dot connected to the output terminal 13, a terminal with a dot connected to the drain of a synchronous rectifier TR1 such as a MOS-FET, and the source thereof It is connected to the output terminal 14 on the reference potential side. Further, a smoothing output capacitor C2 is provided between the output terminals 13 and 14.

  Between the drain and source of the synchronous rectifier element TR1, a diode D5 having an anode connected to the source side and a cathode connected to the drain side is provided in the same direction as the body diode. Further, a voltage source 16 provided on the secondary side is provided between the drain and source of the synchronous rectifier element TR1. The voltage source 16 includes a capacitor C5 having one end connected to the drain of the synchronous rectifier element TR1, the other end of the capacitor C5 is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to the cathode of the Zener diode ZD1. The anode of the Zener diode ZD1 is connected to the reference potential on the output terminal 14 side. Further, the other end of the resistor R3 is connected to the anode of the diode D3, and the cathode of the diode D3 is connected to the reference potential on the output terminal 14 side via the capacitor C6. The middle point of the cathode of the diode D3 and the capacitor C6 is the output of the voltage source 16.

  Further, an auxiliary winding N3 for turning on the synchronous rectifier element TR1 is provided on the secondary side of the transformer T, and the terminal on the side of the auxiliary winding N3 with a dot is set to the reference potential on the output terminal 14 side. The terminal on the side where no dot is connected is connected to the gate of the synchronous rectifying element TR1 through the speed-up capacitor C4. Furthermore, the terminal on the non-dot side of the auxiliary winding N3 is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the reference potential on the output terminal 14 side via the series circuit of the resistor R1 and the timing capacitor C3. Yes. The middle point of the resistor R1 and the timing capacitor C3 is connected to the cathode of the diode D3 of the voltage source 16 and the middle point of the capacitor C6 via resistors R4 and R2.

  Further, the middle point of the resistor R1 and the capacitor C3 is connected to the base of an npn-type transistor TR2 that is a switch element. The collector of the transistor TR2 is connected to the gate of the synchronous rectifier element TR1, and the emitter is connected to the reference potential on the output terminal 14 side. The gate of the synchronous rectifier element TR1 is connected to the cathode of the diode D2, and the anode of the diode D2 is connected to the reference potential on the output terminal 14 side.

  The middle point of the resistors R2 and R4 is connected to the anode of the diode D4 via the resistor R5, and the cathode of the diode D4 is connected to the output terminal 13. The resistors R2, R4, R5 and the diode D4 form a current adjustment circuit 22 that adjusts the current flowing to the timing capacitor C3.

  In the operation of the switching power supply device 20, the main switch element Q1 is turned on / off by a control circuit (not shown) to perform PWM control. During the on period of the main switch element Q1, the synchronous rectifier element TR1 is off and no current flows, and the capacitor C6 of the voltage source 16 is charged via the capacitor C5. Capacitor C5 limits the amount of charge of capacitor C6. The charging voltage of the capacitor C6 is a voltage set by the Zener diode ZD1. Further, during the ON period of the main switch element Q1, the capacitor C4 is charged with the cathode side of the diode D2 being positive by the auxiliary winding N3.

  Next, when the main switch element Q1 is turned off, the gate of the synchronous rectifier element TR1 is charged by adding the charging voltage of the capacitor C4 to the voltage of the terminal on the non-dot side of the auxiliary winding N3, thereby synchronizing the synchronous rectifier element TR1. Turns on. At the same time, the energy stored in the secondary winding N2 is charged in the output capacitor C2 by the flyback voltage generated in the secondary winding N2.

  At the same time as the main switch element Q1 is turned off, the voltage source 16 starts charging the timing capacitor C3 via the resistor R2. When the potential of the timing capacitor C3 gradually rises and reaches the threshold potential at which the transistor TR2 is turned on, the transistor TR2 is turned on to discharge the gate charge of the synchronous rectifier element TR1 and turn off the synchronous rectifier element TR1. The timing at which the transistor TR2 is turned on is set to the timing immediately before the main switch element Q1 is turned on. When the main switch element Q1 is turned on, charging of the capacitor C6 of the voltage source 16 starts again, and the charge of the capacitor C3 is discharged through the resistor R1 and the diode D1.

  By the operation as described above, the main switch element Q1 and the secondary side synchronous rectification element TR1 perform the synchronous rectification operation without being simultaneously turned on. Here, when the output from the output terminals 13 and 14 enters an overcurrent state in which a current equal to or higher than the rated current flows, an overcurrent protection circuit (not shown) operates to lower the output voltage. When the output voltage decreases and the output voltage becomes lower than the voltage between the resistors R2 and R4, the charging current to the timing capacitor C3 flows to the output terminal 13 side through the resistor R5 and the diode D4. The current adjustment circuit 22 reduces the charging current to the timing capacitor C3 in the overcurrent state, and increases the on-time of the synchronous rectifying element TR1.

  With the above operation, even when the output is in an overcurrent state and the output voltage is lowered, the synchronous rectifier element TR1 is turned on for a predetermined period without being turned off in a short period. The output voltage for starting the operation of reducing the charging current to the timing capacitor C3 can be arbitrarily determined by setting the resistors R2 and R4, and the amount by which the charging current is reduced can also be arbitrarily set by the resistor R5.

  Since the synchronous rectification switching power supply device of this embodiment can keep the synchronous rectification time for which the synchronous rectification element TR1 is on long even when the output voltage is reduced due to an overcurrent, the body diode of the synchronous rectification element TR1 In addition, the loss during rectification by the diode D5 can be kept small. Thereby, a small thing can be used for synchronous rectification element TR1 and diode D4 connected in parallel, and size reduction of a power supply device is attained. In addition, unlike the prior art, it is not necessary to protect the synchronous rectifying transistor and the diode connected in parallel with the primary overcurrent circuit by intermittent operation, which leads to a reduction in the number of components. From the above, it contributes to miniaturization and cost reduction of the synchronous rectification switching power supply device.

  The flyback type synchronous rectification switching power supply apparatus of the present invention is not limited to the above-described embodiment. For example, as shown in FIG. 2, the voltage source for charging the timing capacitor C3 without providing the voltage source 16 is used. As an alternative, the driving voltage of the synchronous rectifier element TR1 by the auxiliary winding N3 may be used. In this embodiment, one end of the resistor R4 is connected to the cathode of the diode D6, and the anode of the diode D3 is connected to the gate of the synchronous rectifier element TR1. As in the above embodiment, the other end of the resistor R4 and the midpoint of the resistor R2 are connected to the output terminal 13 via the resistor R5 and the diode D4.

  According to this embodiment, it is not necessary to provide a separate voltage source, the circuit is simple, and the apparatus can be reduced in size and cost.

  The switching power supply device of the present invention is not limited to the above embodiment, and the current adjustment circuit can use an appropriate resistor and other elements, and the transistor TR2 only needs to be able to turn off the synchronous rectification element TR1. The connection method of the transistor TR2 and the synchronous rectifier element TR1 is not limited to the power supply circuit of the above embodiment. Further, an amplification stage and a buffer may be provided between the transistor TR2 and the synchronous rectifier element TR1. Further, the voltage source may be charged by the output of the auxiliary winding, or other appropriate combination of other circuits may be used.

  In addition, the switching power supply device of the present invention can be applied to switching power supply devices of any input / output voltage.

1 is a schematic circuit diagram of a synchronous rectification switching power supply device according to an embodiment of the present invention. It is a schematic circuit diagram of the synchronous rectification switching power supply device of other embodiment of this invention. It is a schematic circuit diagram of the conventional flyback type synchronous rectification switching power supply device.

Explanation of symbols

11, 12 Input terminal 13, 14 Output terminal 16 Voltage source 20 Switching power supply 22 Current adjustment circuit C3 Timing capacitor D1, D2, D3, D4, D5, D6 Diode N1 Primary winding N2 Secondary winding N3 Auxiliary winding Q1 main switch element R1, R2, R3, R4, R5, R6 resistor T transformer TR1 synchronous rectifier element TR2 switch element

Claims (3)

A main switch element that is turned on and off within a certain period is connected in series with the primary winding of the transformer and provided between the input terminals, and is connected in series between the secondary winding and the output terminal of the transformer. A synchronous rectifier element, an auxiliary winding for turning on the synchronous rectifier element complementarily to the main switch element, a switch element for turning off the synchronous rectifier element, and a control terminal of the switch element. A timing capacitor, and an overcurrent protection circuit that controls the on-time of the main switch element to be shortened so as to lower the output voltage when an overcurrent flows between the output terminals. When the main switch element is off, the charging current flows and the voltage at both ends rises. When the main switch element is on, the discharge current flows and the voltage at both ends falls, and the switch Child, the voltage across is turned on exceeds a predetermined threshold potential synchronous rectification switching power supply device of the connected flyback so as to turn off the synchronous rectifier of the timing capacitor,
A current adjustment circuit for reducing a charging current flowing through the timing capacitor when the on-time of the main switch element is shortened due to the operation of the overcurrent protection circuit and the voltage between the output terminals decreases to a predetermined voltage or less; A synchronous rectification switching power supply device characterized by that. A voltage source charged to a predetermined potential by a pulse voltage generated in the secondary winding or the auxiliary winding in accordance with the switching operation of the main switch element, and an output of the voltage source is connected to the timing capacitor; 2. The synchronous rectification switching power supply device according to claim 1, wherein the synchronous rectification switching power supply device is connected to the current adjustment circuit.   The timing capacitor is connected between the base and emitter of the transistor that is the switching element that turns off the synchronous rectifying element, and the current adjustment circuit is connected in series between the voltage source that charges the timing capacitor and the timing capacitor. A plurality of resistors connected to each other, one end of a series circuit of resistors and diodes is connected between the plurality of resistors, and the other end of the series circuit of resistors and diodes is connected to the output terminal. The synchronous rectification switching power supply apparatus according to claim 1 or 2.

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