CN1692546A - Synchronous rectification switching power supply device - Google Patents

Abstract

A synchronous rectification type switching power supply device is provided, which can reliably cut off a rectification element before a main switch is turned on even when the on time of the main switch element is prolonged due to the surge of a load. The synchronous rectification type switching power supply device includes: a synchronous rectifier element (Q2) connected between the output terminals (13, 14) in series with the secondary winding (N2) of the transformer (T); and a drive means composed of an auxiliary coil (N3) or the like for complementarily turning on the synchronous rectifying element (Q2) and the main switching element (Q1). A transistor (Tr1) as a cut-off means for cutting off the synchronous rectifying element (Q2) is provided between the gate and the source of the synchronous rectifying element (Q2). The turn-off timing at which the synchronous rectification element (Q2) is turned off by the turn-off means (Tr1) is set to a fixed time after the main switching element (Q1) is turned on, the fixed time being a timing within a range as close as possible to a fixed drive period of the main switching element (Q1).

Description

Synchronous rectification switching power supply device

technical field

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

Background technique

In the existing switching power supply device-flyback converter equipped with a synchronous rectification rectification circuit, for example, as disclosed in (Japanese) Unexamined Patent Publication No. 2000-116122, a DC power supply and a main switching element are connected to the primary coil end of the transformer. A series circuit composed of a synchronous rectification element is arranged in series on the secondary coil of the transformer, and then connected to the output terminal through the rectification circuit. The flyback converter performs switching control on the main switching element of the MOS-FET. When the main switching element is turned off, the synchronous rectification element-MOS-FET of the secondary side circuit of the transformer is turned on, and the feedback generated on the secondary coil is used. The sweep voltage charges the output capacitor of the rectifier circuit. Thereafter, the synchronous rectification element is turned off before the main switching element is turned on, and this operation is repeated to supply power to the output terminal.

In the case of this synchronous rectification flyback converter, if the off timing of the synchronous rectification element is shifted, and the main switch is turned on while the synchronous rectification element is turned on, the circuit on the secondary side of the power supply device will be short-circuited. A large inrush current flows through the main switching element, which may damage the main switching element or synchronous rectification element.

Therefore, (Japanese) Unexamined Patent Publication No. 2000-116122 discloses a switching power supply circuit in which, in order to prevent the simultaneous conduction state of the main switching element and the synchronous rectifying element, after the synchronous rectifying element is turned on due to the main switching element being turned off, The timing capacitor is charged with the current determined by the voltage induced in the auxiliary coil and the timing resistor, and after a certain period of time, the auxiliary transistor is turned on and the rectifier element is turned off.

In the case of the synchronous rectification switching power supply device of the prior art, as shown in FIG. 1, in order to make the rectifying element must be turned off before the main switching element is turned on, there is a dead time (deadtime) td, so that it is determined by the timing capacitor The certain time Tc of is before the certain time of the turn-on timing of the main switching element. The dead time td is set by the time constant of the timing capacitor, so that in a steady state, the rectifier element is turned off within the turn-off time of the main switching element determined by the input and output voltages and the turns ratio of the transformer.

However, in the case of the synchronous rectification flyback converter of the prior art described above, sometimes the load current increases sharply, and the main switching element may be turned on longer than the main switching element determined by the input-output voltage and the turn ratio of the transformer. conduction for a period of time. In this case, as shown by the dotted line in FIG. 1 , sometimes the voltage of the timing capacitor does not reach the threshold voltage of the auxiliary transistor element that turns off the synchronous rectifier element within a certain on-off period of the main switching element. In this case, there is a problem that the main switching element is turned on when the rectifying element is not turned off, and a very large inrush current flows through the main switching element, and the circuit on the secondary side of the power supply unit becomes short-circuited. The through current flows through the stage side circuit, causing damage to the main switching element or synchronous rectification element, etc.

On the other hand, during the above dead time td, a diode connected in parallel to the synchronous rectification element or a body diode of a MOS-FET as a synchronous rectification element performs a rectification operation. The loss during the rectification period of this diode is larger than that during the conduction period of the rectification element of the MOS-FET. Therefore, there is a problem that although it is desirable to shorten the dead time as much as possible, the dead time td cannot be shortened in order to reliably turn off the synchronous rectifier element before the main switching element is turned on. Furthermore, since the dead time td cannot be shortened, the switching frequency cannot be increased, which hinders miniaturization and cost reduction of the device.

In addition, when a voltage equal to or higher than the set voltage is applied from the outside between the output voltage terminals, or when the power supply is stopped when an external large-capacity capacitor is added between the output terminals, there is a problem that the secondary The synchronous rectification element on the secondary side is turned off and a through current flows, or the circuit on the secondary side self-oscillates due to the power of a large-capacity external capacitor connected between the output terminals.

Contents of the invention

The purpose of the present invention is to provide a flyback type synchronous rectification switching power supply device, which can make the synchronous rectification element reliable before the main switch is turned on even when the conduction time of the main switching element becomes longer due to a sudden change in the load. due.

Another object of the present invention is to provide a flyback type synchronous rectification switching power supply device which prevents through current and self-oscillation of the switching power supply circuit regardless of sudden changes in load or external devices connected between output terminals.

The present invention is a synchronous rectification switching power supply device, which has: a control circuit, a primary coil of a transformer and a main switching element are connected in series between input terminals, and PWM control is performed on the main switching element within a certain period; a synchronous rectifying element, and The secondary coil of the above-mentioned transformer is connected in series between the output terminals; and a driving part, which makes the above-mentioned synchronous rectification element and the above-mentioned main switching element conduction complementary, is characterized in that another power supply and cut-off part is provided, and the other The power supply is charged by the pulse voltage generated in the coil of the secondary side of the transformer due to the on-off of the above-mentioned main switching element. The rectifier element is turned off, and the cut-off timing of the above-mentioned cut-off part to turn off the above-mentioned synchronous rectifier element is set to a certain time set by the current from the other power supply after the above-mentioned main switching element is turned on, and the certain time is as close as possible to the above-mentioned main switching element. timing within a certain range of drive cycles.

The above-mentioned cut-off means is composed of a transistor and a timing capacitor connected to the signal input terminal of the transistor, the above-mentioned timing capacitor is charged by the above-mentioned other power source, discharged at the instant when the above-mentioned main transistor is turned on, and from this instant the above-mentioned timing capacitor starts For charging, the time until the voltage of the timing capacitor exceeds the threshold value of the signal input terminal of the transistor is set to a time within the range of a certain driving cycle of the main switching element.

In addition, the above-mentioned another power source is a constant voltage source or a constant current source connected to the secondary side of the above-mentioned transformer. Furthermore, another power supply for charging the timing capacitor can also be used as a snubber circuit for absorbing surge energy when the synchronous rectification element is turned off, and the timing capacitor can be charged with the energy absorbed by the snubber circuit.

In the synchronous rectification switching power supply of the present invention, the synchronous rectification element is reliably turned off within a constant period from the timing when the main switching element is turned on, so even if the load current suddenly changes, the main switching element and the synchronous rectification element are not turned on at the same time. Accordingly, the dead time from when the synchronous rectifying element is turned off to when the main switching element is turned on can be shortened as much as possible, the rectification period of the diode can be shortened to suppress loss, and the switching frequency can also be increased. Furthermore, it also contributes to miniaturization and cost reduction.

In addition, another invention of the present application is a synchronous rectification switching power supply device, which has: a control circuit, the primary coil of the transformer and the main switching element are connected in series between the input terminals, and PWM control is performed on the above-mentioned main switching element in a certain period. a synchronous rectification element connected in series with the secondary coil of the above-mentioned transformer between the output terminals; and a driving part that makes the above-mentioned synchronous rectification element and the above-mentioned main switching element conduct in a complementary manner; it is characterized in that another power supply and a cut-off component, the other power supply is charged by the pulse voltage generated in the coil of the secondary side of the transformer due to the on-off of the above-mentioned main switching element, and the cut-off component is provided at the gate-source of the above-mentioned synchronous rectification element and cut off the above-mentioned synchronous rectification element, including such a control element: comparing the output voltage of the above-mentioned another power supply with the output voltage of the output terminal of the above-mentioned switching power supply device, when the output voltage of the above-mentioned other power supply drops to less than or equal to a certain value In the case of , the above-mentioned synchronous rectification element is cut off by the above-mentioned cut-off member.

The transistor of the cut-off part is an npn transistor for turning off the synchronous rectification element, the control element is a pnp transistor whose emitter is connected to the output terminal, and the collector is connected to the base of the npn transistor. The base of the above-mentioned another power supply is connected to the output terminal. In addition, the output voltage of the other power supply may be divided and input to the base of the pnp transistor.

According to another invention of the present application, it is possible to reliably prevent the through current caused by sudden changes in the load, self-excited oscillation when the power supply is stopped or when an external voltage is applied, and the components of the device can be miniaturized, which greatly contributes to the stability of the entire device. Miniaturization and cost reduction.

Description of drawings

FIG. 1 is a working sequence diagram of a conventional flyback synchronous rectification switching power supply device.

2 is a schematic circuit diagram of a synchronous rectification switching power supply device according to a first embodiment of the present invention.

3 is an operation timing chart (A) and a timing chart (B) when the duty ratio of the main switching element of the synchronous rectification switching power supply device according to the embodiment is wide.

4 is a schematic circuit diagram of a synchronous rectification switching power supply device according to a second embodiment of the present invention.

5 is a schematic circuit diagram of a synchronous rectification switching power supply device according to a third embodiment of the present invention.

6 is a schematic circuit diagram of a synchronous rectification switching power supply device according to a fourth embodiment of the present invention.

7 is a schematic circuit diagram of a synchronous rectification switching power supply device according to a fifth embodiment of the present invention.

8 is a schematic circuit diagram of another example of the synchronous rectification switching power supply device according to the fifth embodiment of the present invention.

Detailed ways

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a circuit of the flyback type synchronous rectification switching power supply device according to the first embodiment of the present invention. In this switching power supply circuit, a DC power supply 10 is connected between input terminals 11 and 12, and a primary winding N1 of a transformer T and a main switching element Q1 of a MOS-FET are connected in series. On the input terminal 11 of the positive end of the DC power supply 10, a terminal generating a positive voltage when the main switching element Q1 is turned on is connected to the terminal on the dotted side of the primary coil N1, and the terminal on the non-dot side of the transformer T. is connected to the drain of the main switching element Q1. In addition, the source of the main switching element Q1 is connected to the input terminal 12 of the negative terminal of the DC power supply 10, and the gate of the main switching element Q1 is connected to the drive signal output terminal of the control circuit 18. PWM (Pulse Width Modulation, pulse width modulation) control is performed on the main switching element Q1 in a certain period according to the input and output conditions.

The terminal on the non-dot side of the secondary coil N2 of the transformer T is connected to one end of the output capacitor C2, and the terminal on the dot side of the secondary coil N2 of the transformer T is connected to the drain of the synchronous rectification element Q2 of the MOS-FET. top notch. The source of the synchronous rectification element Q2 is connected to the other end of the output capacitor C2, which is the reference potential end. Both ends of the output capacitor C2 are connected to the output terminals 13 and 14 . A diode D4 is connected in parallel between the drain and the source of the synchronous rectification element Q2. The anode of the diode D4 is connected to the source of the synchronous rectification element Q2, and the cathode is connected to the drain. Therefore, the diode D4 can also be replaced with the body diode of the synchronous rectification element Q2 of the MOS-FET.

Furthermore, at the secondary end of the transformer T, an auxiliary coil N3 is provided as a driving part of the synchronous rectification element Q2. The terminal on the side with the dot of the auxiliary coil N3 is connected to the reference potential, and the terminal on the side without the dot is connected to the reference potential. The terminal is connected to one end of a capacitor C4 for accelerating operation via a resistor R1. The other end of the capacitor C4 is connected to the cathode of the diode D1, and the anode of the diode D1 is connected to the reference potential. Between the cathode of the diode D1 and the other end of the capacitor C4, the gate of the synchronous rectification element Q2 is connected.

The gate of the synchronous rectification element Q2 is connected to the collector of the npn transistor Tr1, and the emitter of the transistor Tr1 is connected to the reference potential. One end of a timing capacitor C3 is connected to the base which is a signal input terminal of the transistor Tr1, and the other end of the timing capacitor C3 is connected to a reference potential. The base of the transistor Tr1 is also connected to the output terminal of another power source——the constant voltage source 16 via the resistor R2, and is also connected to the collector of the npn transistor Tr2. The emitter of the transistor Tr2 is connected to the reference potential, and the base is connected to the terminal on the dotted side of the secondary coil N2 via the capacitor C7. Between the base and the emitter of the transistor Tr2, a resistor R3 and a diode D2 are connected in parallel. The cathode of the diode D2 is connected to the base, and the anode is connected to the reference potential.

The constant voltage source 16 is made up of the following parts: capacitor C5, one end of which is connected on the terminal on the side of the point attached to the secondary coil N2; diode D3, the other end of the capacitor C5 is connected on its anode; capacitor C6 , is connected between the cathode of diode D3 and the reference potential; and Zener diode ZD1 is connected between the anode of diode D3 and the reference potential. The cathode of the Zener diode ZD1 is connected to the anode of the diode D3, and the anode is connected to the reference potential. In addition, the constant voltage source 16 also serves as a snubber circuit for absorbing surge energy when the synchronous rectification element Q2 is turned off.

Next, the control method and operation of the synchronous rectification switching power supply device of this embodiment will be described with reference to FIGS. 2 and 3 . First of all, after the main switching element Q1 in the circuit of Figure 2 is turned on, the dot sides of the primary coil N1 and the secondary coil N2 are respectively positive, but as shown in Figure 3 (A) and (B), the synchronous rectification element Q2 When the gate-source potential Vgs is "low", the synchronous rectification element Q2 is turned off, and the current Id1 of the synchronous rectification element Q2 does not flow. In addition, at this time, in the constant voltage source 16, a current flows from one side of the secondary coil N2 to charge the capacitors C5 and C6, and one end of the capacitor C6 of the constant voltage source 16 is provided with a Zener diode ZD1. Set a certain voltage. The current flows from the output end of the constant voltage source 16—one end of the capacitor C6 through the resistor R2 to the timing capacitor C3 to charge it. Furthermore, during the period in which the main switching element Q1 is turned on, one side of the auxiliary winding N3 is "high", and the gate of the synchronous rectifying element Q2 is at the reference potential via the diode D1.

Thereafter, after the control circuit 18 turns off the main switching element Q1 through PWM control according to the input and output conditions, a flyback voltage is generated on the terminal on the non-dot side of the secondary coil N2, and at the same time, a terminal on the non-dot side of the auxiliary coil N3 A retrace voltage is also generated on the capacitor C4, and the gate capacitance Ciss of the synchronous rectification element Q2 is charged through the capacitor C4, the potential Vgs between the gate and the source becomes "high", and the synchronous rectification element Q2 is turned on. Accordingly, the current Id1 flows from the terminal on the non-dot side of the secondary coil to the terminal on the dot side via the output capacitor C2, thereby charging the output capacitor C2.

In addition, the timing capacitor C3 is charged by the current from the constant voltage source 16 immediately after the main switching element Q1 is turned on, and the potential of the timing capacitor C3 reaches the threshold value of the base of the transistor Tr1 after a certain time elapses. As a result, the transistor Tr1 is turned on, the gate capacitance Ciss of the synchronous rectification element Q2 is discharged, and the synchronous rectification element Q2 is turned off. However, until the main switching element Q1 is turned on thereafter, the current Id2 also flows through the diode D4 provided in parallel with the synchronous rectifying element Q2. The current Id2 has losses caused by diodes, so it is smaller than the current Id1. The period during which the current flows through the diode D4 is a dead time dt for turning off the synchronous rectification element Q2 before the main switching element Q1 is turned on.

Thereafter, after the main switching element Q1 is turned on, the potential on the one side of the secondary coil N2 is applied to the base of the transistor Tr1 through the capacitor C7, and the transistor Tr1 becomes "low" at this moment, and the charge of the timing capacitor C3 is instantaneous. was discharged. Since the capacitance of the capacitor C7 is relatively sufficiently small during this period, this period is completed within an instant that is sufficiently shorter than the switching frequency of the main switching element Q1. From this moment on, charging of the timing capacitor C3 starts again as described above.

In the flyback type synchronous rectification switching power supply of this embodiment, the control circuit 18 makes the switching cycle T of the main switching element Q1 constant, and as shown in FIGS. 3(A) and (B), the conduction period of the main switching element Q1 is —The duty ratio changes according to the input and output conditions. However, the constant voltage source 16 makes the potential of the timing capacitor C3 applied to the base of the transistor Tr1 in this embodiment reach the threshold value of the base of the transistor Tr1 within a certain period of time from the timing of turning on the main switching element Q1. When the load current increases rapidly and the output voltage decreases transiently, in order to increase the output voltage, even if the conduction period of the main switching element Q1 is temporarily lengthened, the synchronous rectification element Q2 is turned on for a certain period of time from the timing of the main switching element Q1 being turned on. Inner reliable cutoff. Accordingly, the dead time td from when the synchronous rectifying element Q2 is turned off to when the main switching element Q1 is turned on can be shortened as much as possible, and the rectification period of the diode D4 can be shortened to suppress losses and increase the switching frequency.

In addition, the constant voltage source 16, which is the charging circuit for the timing capacitor C3, constitutes a snubber circuit that absorbs the surge energy when the synchronous rectification element Q2 is turned off. high power supply.

Next, a synchronous rectification switching power supply device according to a second embodiment of the present invention is shown in FIG. 4 . Here, the same symbols are assigned to the same configurations as those in the above-mentioned embodiment, and description thereof will be omitted. In this embodiment, different from the first embodiment, the output terminal of the constant current source 20 composed of a constant current circuit is connected to the timing capacitor C3.

The constant current source 20 includes: a diode D5, the anode of which is connected to the terminal on one side of the secondary coil N2 of the transformer T; and a capacitor C8, the cathode of the diode D5 is connected to one end and the other end is connected to the reference potential. Furthermore, the cathode of the diode D5 is connected to the emitter of the pnp transistor Tr3 via the resistor R4, and the collector of the transistor Tr3 is connected to one end of the timing capacitor C3 as the output terminal of the constant current source 20. Furthermore, the cathode of the Zener diode ZD2 is connected to the cathode of the diode D5, and the anode of the Zener diode ZD2 is connected to the base of the transistor Tr3 and to the reference potential via the resistor R5. The constant current is set by a certain voltage set by Zener diode ZD2 and resistor R4.

In the flyback type synchronous rectification switching power supply device of this embodiment, the timing capacitor C3 can be charged with a constant current from the constant current circuit 20, and the voltage of the timing capacitor C3 rises linearly.

Also in the synchronous rectification switching power supply device of this embodiment, the same effect as the above-mentioned embodiment can be obtained, especially in this case, the voltage rise of the timing capacitor C3 is linear, and it is easy to set the turn-off timing of the synchronous rectification element Q2. A buffer circuit may also be provided on the constant current source 20 . Thereby, energy efficiency can be made higher.

Next, a synchronous rectification switching power supply device according to a third embodiment of the present invention is shown in FIG. 5 . Here, the same structures as those in the above-mentioned embodiment are assigned the same symbols, and description thereof will be omitted. In this embodiment, unlike the first embodiment, one end of the timing capacitor C3 is connected via a diode D6 between terminals connected to the operation accelerating capacitor C4 and the collector of the transistor Tr1. The anode of diode D6 is connected to timing capacitor C3 and the cathode is connected to the terminal of capacitor C4.

The operation of the synchronous rectification switching power supply device of this embodiment is the same as the circuit of the above-mentioned embodiment, and discharges the timing capacitor C3 by turning off the main switching element Q1 and making the dot side of the auxiliary coil N3 a positive potential. At this time, the current flows from the terminal on the dot side of the auxiliary coil N3 to the terminal on the non-dot side of the auxiliary coil N3 through the electrode at the reference potential end and the electrode at the opposite end of the timing capacitor C3, via the diode D6 and capacitor C4, and then flows to the terminal on the non-dot side of the auxiliary coil N3. discharge.

In this embodiment, the same effects as those of the first embodiment can be obtained. Furthermore, the circuit configuration for discharging the timing capacitor C3 can be simplified, the number of electronic components can be reduced, and the miniaturization and cost reduction of the device can be further promoted.

Next, a synchronous rectification switching power supply device according to a fourth embodiment of the present invention is shown in FIG. 6 . Here, the same symbols are assigned to the same configurations as those in the above-mentioned embodiment, and description thereof will be omitted. In this embodiment, the constant voltage source 16 of the third embodiment is replaced by a constant current source 20 . According to this embodiment, the same effect as that of the above-mentioned second embodiment can be obtained. Moreover, like the above-mentioned third embodiment, the circuit configuration for discharging the timing capacitor C3 can be simplified, and the miniaturization and reduction of the device can be further promoted. cost.

Next, a synchronous rectification switching power supply device according to a fifth embodiment of the present invention is shown in FIG. 7 . Here, the same symbols are assigned to the same structures as those in the above-mentioned embodiment, and description thereof will be omitted. In the synchronous rectification switching power supply device of this embodiment, an input capacitor C1 is provided in parallel with the input terminals 11 and 12 of the DC power supply, and the two ends of the input capacitor C1 are connected to the primary coil N1 of the transformer T and the main switching element Q1. series circuit. The dot side of the primary coil N1 of the transformer T is connected to the input terminal 11, and the non-dot side is connected to the main switching element Q1. The main switching element Q1 is composed of semiconductor switching elements such as MOS-FETs. The terminal on the non-dot side of the secondary coil N2 of the transformer T is connected to the output terminal 13, and a synchronous rectification element Q2 such as a MOS-FET is arranged in series on the terminal on the dot side, and is connected to the output terminal 14 of the reference potential end. connect. Furthermore, between the output terminals 13 and 14, an output capacitor C2 for smoothing is provided.

Between the drain and the source of the synchronous rectification element Q2, another power supply, a constant voltage source 16, provided on the secondary side of the power supply circuit is provided. The constant voltage source 16 includes a capacitor C5 with one end connected to the drain of a synchronous rectification element Q2 such as a MOS-FET, the other end of the capacitor C5 being connected to one end of a resistor R6, and the other end of the resistor R6 being connected to a Zener diode On the cathode of ZD1, the anode of Zener diode ZD1 is connected to the reference potential. Furthermore, the other end of the resistor R6 is connected to the anode of the diode D3, and the cathode of the diode D3 is connected to the reference potential via the capacitor C6. The point between the cathode of the diode D3 and the capacitor C6 becomes the output terminal of the constant voltage source 16 .

Furthermore, at the secondary end of the transformer, an auxiliary coil N3 is provided as a driving part of the synchronous rectification element Q2, and the terminal on the side with the dot of the auxiliary coil N3 is connected to the reference potential, and the terminal on the side without the dot is connected to the reference potential. The capacitor C4 for acceleration is connected to the gate of the synchronous rectification element Q2. Furthermore, the terminal on the non-dot side of the auxiliary coil 3 is connected to the cathode of the diode D6, and the anode of the diode D6 is connected to the reference potential via a series circuit of the resistor R7 and the timing capacitor C3. The point between resistor R7 and timing capacitor C3 is connected to the point between the cathode of diode D3 of constant voltage source 16 and capacitor C6 via resistor R2. Furthermore, the point between the resistor R7 and the capacitor C3 is connected to the base of the npn transistor Tr1. The collector of the transistor Tr1 is connected to the gate of the synchronous rectification element Q2, and the emitter is connected to the reference potential. In addition, the cathode of the diode D1 is connected to the gate of the synchronous rectification element Q2, and the anode of the diode D1 is connected to the reference potential.

Between the output terminal 13 and the base of the transistor Tr1, a pnp transistor Tr4, which is a switching element control unit, is connected. The emitter of the transistor Tr4 is connected to the output terminal 13, and the collector is connected to the base of the transistor Tr1 via the resistor R8. The base of the transistor Tr4 is connected to the output terminal of the constant voltage source 16, the cathode of the diode D3, and the capacitor C6.

The operation of this switching power supply device is to perform PWM control by using the control circuit 18 to turn on/off the main switching element Q1. During the conduction period of the main switching element Q1, the synchronous rectifying element Q2 is turned off, no current flows, and the capacitor C6 of the constant voltage source 16 is charged via the capacitor C5. Capacitor C5 is used to limit the charge amount of capacitor C6. The charging voltage of capacitor C6 is the voltage set by Zener diode ZD1. In addition, during the conduction period of the main switching element Q1, the auxiliary winding N3 charges the capacitor C4 with the cathode side of the diode D1 being positive.

After the main switching element Q1 is turned off, the gate of the synchronous rectifier Q2 is charged by applying the voltage of the terminal on the non-dot side of the auxiliary coil N3 and the charging voltage of the capacitor C4, and the synchronous rectifier Q2 is turned on. At the same time, the flyback voltage generated in the secondary coil N2 charges the energy accumulated in the secondary coil N2 into the output capacitor C2.

Further, at the same time as the main switching element Q1 is turned off, the constant voltage source 16 starts charging the timing capacitor C3 via the resistor R2. The potential of the timing capacitor C3 rises gradually, and when the potential of the transistor TR1 is turned on, the transistor TR1 is turned on and discharges the charge on the gate of the synchronous rectification element Q2 to turn off the synchronous rectification element Q2. The timing at which the transistor TR1 is turned on is set to the timing immediately before the main switching element Q1 is turned on. After the main switch element Q1 is turned on, the constant voltage source 16 starts charging the capacitor C6 again, and the timing capacitor C3 is discharged through the resistor R7 and the diode D6.

Here, the potential of the capacitor C6 of the constant voltage source 16 is applied to the base of the transistor Tr4, the output voltage of the constant voltage source 16 is compared with the output voltage of the output terminal 13, the main switching element Q1 stops, and the capacitor C6 of the constant voltage source 16 When the voltage drops and becomes less than or equal to the predetermined potential for turning on the transistor Tr4, the pnp transistor Tr4 is turned on, and the timing capacitor C3 is charged through the resistor R8, so that the transistor Tr1 is turned on. As a result, the gate charge of the synchronous rectification element Q2 is discharged, and the synchronous rectification element Q2 is turned off. While the transistor Tr4 is on, that is, while the potential of the capacitor C6 of the constant voltage source 16 is equal to or lower than a predetermined potential due to the stop of the main switching element Q1, etc., the transistor Tr1 is on and the synchronous rectifying element Q2 is off. . After the main switching element Q1 starts switching during this period, the synchronous rectifying element Q2 performs rectification with its body diode. In addition, the constant voltage source 16 is also charged via the transistor Tr4, and the constant voltage source 16 is charged more quickly.

According to the switching power supply device of this embodiment, even when the main switching element Q1 is stopped due to a sudden change in the load and then restarts switching, the synchronous rectifying element Q2 is reliably turned off, and the output voltage of the constant voltage source 16 reaches a predetermined value or more. 1. The synchronous rectification element Q2 is not turned on until the synchronous rectification element Q2 can be normally and reliably driven by the timing capacitor C3 and the transistor Tr1. Accordingly, a through current does not flow through the power supply circuit, and it is possible to reliably prevent damage to circuit elements and the like.

Also, when a voltage higher than the set voltage is applied between the output terminals 13 and 14 by an external device, the main switching element Q1 also stops, and the output voltage of the constant voltage source 16 decreases. In this case as well, the lowering of the potential of the constant voltage source 16 turns on the transistor Tr4 , and the transistor Tr1 turns off the synchronous rectification element Q2 , thereby preventing self-oscillation.

Furthermore, in the state where a large-capacity capacitor is connected as an external device between the output terminals 13 and 14, when the main switching element Q1 is stopped by remote control or by cutting off the input voltage, the constant voltage source 16 on the secondary side The voltage of is also reduced, so that the synchronous rectification transistor Q2 is cut off. This prevents self-oscillation caused by the energy stored in the external bulk capacitor between the output terminals 13 and 14. Furthermore, the resistor R4 consumes the energy stored in the bulk capacitor to rapidly lower the output voltage.

The flyback type synchronous rectification switching power supply device of the present invention is not limited to the above-mentioned embodiment. For example, as shown in FIG. 8, resistors R9 and R10 can be used to divide the output potential of the constant voltage source 16 of the circuit shown in FIG. , input to the base of transistor Tr4. Accordingly, the resistors R9 and R10 can be appropriately set to arbitrarily set the on-potential of the transistor Tr4. In addition, other circuits may be combined as appropriate.

Furthermore, in the flyback type synchronous rectification switching power supply device of this embodiment, the auxiliary coil may be used to charge another power supply, and the circuit may be appropriately combined with other circuits.

Claims (8)

1. A synchronous rectification switching power supply device has: a control circuit, a primary coil of a transformer and a main switching element are connected in series between input terminals, and PWM control is carried out to the above-mentioned main switching element in a certain period; the synchronous rectifying element, and the above-mentioned The secondary coil of the transformer is connected in series between the output terminals; and a driving part, which makes the above-mentioned synchronous rectification element and the above-mentioned main switching element conduction complementary, is characterized in that another power supply and a cut-off part are provided, and the other power supply Charging is performed by the pulse voltage generated in the coil on the secondary side of the transformer due to the on-off of the main switching element. The element is turned off, and the cut-off timing of the above-mentioned cut-off part to turn off the above-mentioned synchronous rectification element is set to a certain time set by the current from the other power supply after the above-mentioned main switching element is turned on, and the certain time is as close as possible to the time of the above-mentioned main switching element. Timing within a certain range of drive cycles. 2. The synchronous rectification switching power supply device as claimed in claim 1, wherein the above-mentioned cut-off part is composed of a transistor and a timing capacitor connected to the signal input terminal of the transistor, and the timing capacitor is charged by the other power supply, Discharge at the moment when the main transistor is turned on, and the timing capacitor starts charging from the moment, and the time before the voltage of the timing capacitor exceeds the threshold value of the signal input terminal of the transistor is set as a certain driving period of the main switching element. time within the range. 3. The synchronous rectification switching power supply device according to claim 2, wherein the other power supply is a constant voltage source or a constant current source connected to the secondary end of the transformer. 4. The synchronous rectification switching power supply device as claimed in claim 3, it is characterized in that, on another power supply that above-mentioned timing capacitor is charged, be provided with the buffer circuit that absorbs the surge energy when above-mentioned synchronous rectification element cuts off, with The energy absorbed by the snubber circuit charges the timing capacitor described above. 5. A synchronous rectification switching power supply device has: a control circuit, the primary coil of the transformer and the main switching element are connected in series between the input terminals, and PWM control is carried out to the above-mentioned main switching element in a certain period; the synchronous rectifying element, and the above-mentioned The secondary coil of the transformer is connected in series between the output terminals; and the driving part makes the above-mentioned synchronous rectification element and the above-mentioned main switching element conduction complementary; Charging is performed by the pulse voltage generated in the coil on the secondary side of the transformer due to the on-off of the main switching element. The element is cut off, including such a control element: compare the output voltage of the other power supply with the output voltage of the output terminal of the switching power supply device, and when the output voltage of the other power supply drops to a certain value or less, use the above cut-off component disables the aforementioned synchronous rectification elements. 6. The synchronous rectification switching power supply device according to claim 5, wherein the transistor of the above-mentioned cut-off means is an npn transistor that turns off the above-mentioned synchronous rectification element, and the emitter of the above-mentioned control element is connected to the above-mentioned output terminal. A pnp transistor whose electrode is connected to the base of the npn transistor is connected to the output terminal of the other power supply. 7. The synchronous rectification switching power supply device as claimed in claim 6, wherein said another power supply is a constant voltage source connected to the secondary end of said transformer, and the output voltage of said other power supply is divided and input to base of the above pnp transistor. 8. The synchronous rectification switching power supply device as claimed in claim 7, is characterized in that, on another power supply that above-mentioned timing capacitor is charged, be provided with the buffer circuit that absorbs the surge energy when above-mentioned synchronous rectification element cuts off, with The energy absorbed by the snubber circuit charges the timing capacitor described above.

Images