CN107819399A - Excess voltage protection - Google Patents

Abstract

An overvoltage protection circuit that can distinguish whether the overvoltage state of the output voltage of a switchable power supply device is caused by internal feedback failure or an external power supply, and can use the time difference between the startup of the overvoltage protection to avoid the short-term and harmless influence of the external power supply The switching power supply device works normally, but when the internal feedback of the switching power supply device fails, it can stop the operation of the switching power supply device in real time to achieve protection.

Description

Overvoltage Protection Circuit

technical field

The present invention relates to an overvoltage protection circuit, in particular to an overvoltage protection circuit suitable for a switching power supply device.

Background technique

A general switching power supply device (Switching power supply) will use an overvoltage protection circuit to control the pulse width modulation (PWM) in the switching power supply device when the output terminal of the switching power supply device has an overvoltage. Modulation, PWM) control circuit stops outputting the pulse width modulation signal, and then reduces the output voltage of the output terminal, so as to protect the internal circuit of the switching power supply device and the internal circuit of the external system of the output terminal from overvoltage, avoiding the two Either one of them is damaged.

The output end will exhibit overvoltage due to two situations. One is caused by the failure of the internal circuit of the switching power supply device, such as when the feedback circuit fails; the other is caused by an external system, such as a motor with a motor. The back EMF feedback caused by the external system during deceleration. However, the traditional overvoltage protection circuit cannot distinguish between the two, so that when the user operates the external system normally and causes back electromotive force feedback, the traditional overvoltage protection circuit will still control the switching power supply device to immediately reduce its output voltage The size of the overvoltage protection is implemented, which will cause the external system to shut down and cannot be used normally, causing users a lot of trouble.

Contents of the invention

The purpose of the present invention is to provide an overvoltage protection circuit, which can distinguish the overvoltage caused by the two situations, and adopt two different methods to perform overvoltage protection.

The invention proposes an overvoltage protection circuit, which is suitable for a switching power supply device. It can distinguish whether the output voltage overvoltage state of the switching power supply device is caused by internal feedback failure or external power supply, and can use the time difference of starting overvoltage protection to avoid short-term and harmless external power supply from affecting the normal operation of the switching power supply device , but when the internal feedback of the switching power supply device fails, the switching power supply device can be stopped in real time to achieve protection.

In order to make the above and other objects, features and advantages of the present invention more comprehensible, preferred embodiments are described below in detail with accompanying drawings.

Description of drawings

FIG. 1 is a schematic diagram of a coupling relationship of an overvoltage protection circuit.

Detailed ways

In FIG. 1, the switching power supply device 100 includes a noise filter 110, an AC-DC conversion circuit 120, a power stage (Power stage) 130, a transformer 140, a rectifier circuit 150, an energy storage unit 160, a feedback circuit 170 and A signal isolation unit 180 . In this example, the transformer 140 has two primary windings and one secondary winding, and one of the primary windings of the transformer 140 can pass through the power stage 130, The AC-DC conversion circuit 120 and the noise filter 110 are coupled to the input voltage VIN. The noise filter 110 can be an EMI filter, and users can decide whether to use the noise filter 110 .

In addition, the AC-DC conversion circuit 120 can be a bridge rectifier (Bridge rectifier). As for the power stage 130, it has a PWM control circuit 132 and a power transistor 134 used as a switch. The power transistor 134 can be connected in series with the primary winding of the power stage 130 in the transformer 140 , and whether the current is allowed to pass through the primary winding can be determined by controlling the on/off state of the power transistor 134 . The PWM control circuit 132 is used to generate a PWM signal and output the PWM signal to the control terminal of the power transistor 134 to control the switching frequency of the power transistor 134 between on and off states.

After the output of the secondary side coil of the transformer 140 is rectified and filtered by the rectifier circuit 150 and the energy storage unit 160, it can be used as the output voltage VOUT of the switching power supply device 100 and provided to the external system ( not shown). In addition, the signal isolation unit 180 can be a photo coupler, which is used to transmit the feedback signal generated by the feedback circuit 170 to the PWM control circuit 132, so that the PWM control circuit 132 can adjust the PWM signal accordingly. Duty cycle. Therefore, when the output terminal 190 of the switching power supply device 100 has an overvoltage, the PWM control circuit 132 can reduce the output voltage VOUT according to the feedback signal from the signal isolation unit 180 for overvoltage protection.

In addition, in FIG. 1 , the overvoltage protection circuit 200 includes an overvoltage detection unit 210 , a half-wave rectification unit 220 , a switch unit 230 , a switch unit 240 , a voltage sampling unit 250 , an energy storage unit 260 and an impedance 270 . The overvoltage detection unit 210 is coupled between the output terminal 190 of the switching power supply device 100 and the reference potential SGND for receiving the output voltage VOUT on the output terminal 190 and judging whether the output voltage VOUT exceeds a first default value, When the judgment is yes, a coupling signal is generated. The input of the half-wave rectification unit 220 is coupled to the other primary coil 140 - 2 of the transformer 140 and the reference potential PGND.

The switch unit 230 has a first end 230-1, a second end 230-2, a third end 230-3 and a control end 230-4, and the first end 230-1 and the second end 230-2 are both coupled to the half The output of the wave rectification unit 220 , and its third terminal 230 - 3 is coupled to the PWM control circuit 132 . The switch unit 230 determines whether to provide an electrical path between its second terminal 230-2 and its third terminal 230-3 according to the voltage of its control terminal 230-4, so as to output the current through its third terminal 230-3. The voltage protection trigger signal TRI is used to control the PWM control circuit 132 to stop outputting the pulse width modulation signal, and when the switch unit 230 receives the above-mentioned coupling signal, it provides the power between its first terminal 230-1 and its control terminal 230-4. sexual path.

In addition, one end of the energy storage unit 260 and one end of the impedance 270 are both coupled to the control end 230 - 4 of the switch unit 230 . The switch unit 240 has a fourth end 240-4 and a fifth end 240-5, and the fourth end 240-4 is coupled to the other end of the energy storage unit 260 and the other end of the impedance 270, and the fifth end 240-5 is coupled to Reference potential PGND. The voltage sampling unit 250 is coupled to the output of the half-wave rectification unit 220, the reference potential PGND, and the switch unit 240. When the output voltage of the half-wave rectification unit 220 exceeds a second default value, the voltage sampling unit 250 controls the switch unit 240 An electrical path is provided between the fourth end 240-4 and the fifth end 240-5.

In this example, the impedance 270 and the energy storage unit 260 can be resistors and capacitors, respectively. Next, the detailed implementation of the rest of the overvoltage protection circuit 200 will be introduced first. In this example, the overvoltage detection unit 210 includes a voltage dividing circuit 212 , a switch unit 214 , a signal transmission part 216 - 1 , an impedance 218 and a Zener diode 219 . The voltage dividing circuit 212 is coupled between the output terminal 190 and the reference potential SGND, and generates a voltage dividing signal according to the voltage of the output terminal 190 . One end of the signal transmission part 216 - 1 is coupled to the output terminal 190 through the impedance 218 and the Zener diode 219 in sequence, the anode of the Zener diode 219 is coupled to the impedance 218 , and the cathode of the Zener diode 219 is coupled to the output terminal 190 . The switch unit 214 has a sixth terminal 214-6 and a seventh terminal 214-7, and the sixth terminal 214-6 is coupled to the other terminal of the signal transmission part 216-1, and the seventh terminal 214-7 is coupled to the reference potential SGND. The switch unit 214 is used to receive the voltage division signal generated by the voltage divider circuit 212, and when the voltage of the voltage division signal reaches the default reference potential, the switch unit 214 provides a voltage between the sixth terminal 214-6 and the seventh terminal 214-7. The electrical path is used to further enable the signal transmission part 216-1 to generate a coupling signal.

In this example, the voltage dividing circuit 212 can be impedances 212-1 and 212-2. One end of the impedance 212-1 is coupled to the output terminal 190, one end of the impedance 212-2 is coupled to the other end of the impedance 212-1 for generating a voltage division signal, and the other end of the impedance 212-2 is coupled to the reference potential SGND. In addition, the signal transmitting part 216-1 can be a light emitting part of an optical coupler, and the light emitting part is used to generate a light source as a coupling signal. The switch unit 214 can be a switch component of the TL431, and the user can use the reference terminal of the switch component to receive the voltage-divided signal generated by the voltage-dividing circuit 212 . In addition, the impedances 212 - 1 , 212 - 2 and 218 can all be resistors, and the position of the impedance 218 and the position of the Zener diode 219 can be reversed, and the user can decide whether to use the impedance 218 and the Zener diode 219 according to requirements.

The switch unit 230 includes a signal receiving part 216 - 2 , an impedance 232 , an impedance 234 and an NPN transistor 236 . The signal receiving part 216 - 2 has an eighth end 216 - 28 and a ninth end 216 - 29 , and the eighth end 216 - 28 is coupled to the output of the half-wave rectification unit 220 . When the signal receiving part 216-2 receives the coupled signal, it provides an electrical path between the eighth end 216-28 and the ninth end 216-29. In this example, the signal receiving part 216-2 can be the light receiving part in the same optical coupler as mentioned above. In addition, the impedance 232 is coupled between the ninth end 216 - 29 and the control end 230 - 4 of the switch unit 230 , and one end of the impedance 234 is coupled to the output of the half-wave rectification unit 220 . The collector of the NPN transistor 236 is coupled to the other end of the impedance 234 , its emitter is coupled to the third terminal 230 - 3 of the switch unit 230 , and its base is coupled to the control terminal 230 - 4 of the switch unit 230 .

The switch unit 240 includes a PNP transistor 242 and a resistor 244 . The emitter of the PNP transistor 242 is coupled to the other end of the energy storage unit 260 and the other end of the impedance 270 , its collector is coupled to the reference potential PGND, and its base is coupled to the voltage sampling unit 250 through the impedance 244 . The voltage sampling unit 250 includes a Zener diode 252 and an impedance 254 . The cathode of the Zener diode 252 is coupled to the output of the half-wave rectification unit 220 . One end of the impedance 254 is coupled to the anode of the Zener diode 252 and the switch unit 240 , and the other end is coupled to the reference potential PGND. Both the above-mentioned impedances 244 and 254 can be resistors, and the user can decide whether to use the impedance 244 according to requirements. In addition, the half-wave rectification unit 220 includes a diode 222 and an energy storage unit 224 . The anode of the diode 222 is coupled to one end of the primary coil 140-2. One end of the energy storage unit 224 is coupled to the cathode of the diode 222 and used as the output of the half-wave rectification unit 220 , and the other end thereof is coupled to the other end of the primary detection coil 140 - 2 and the reference potential PGND. As mentioned earlier, the energy storage unit 224 can be a capacitor.

Next, the detailed operation of the overvoltage protection circuit 200 will be introduced. Firstly, the operation of the voltage protection circuit 200 will be described when the internal circuit of the switching power supply device 100 fails and the output terminal 190 of the switching power supply device 100 exhibits overvoltage. Please refer to FIG. 1 again. When the output terminal 190 presents an overvoltage, that is, the output voltage VOUT on the output terminal 190 exceeds the first default value, so that the voltage of the divided signal generated by the voltage dividing circuit 212 reaches the default reference potential, the switch The unit 214 provides an electrical path between the sixth terminal 214 - 6 and the seventh terminal 214 - 7 , so that the Zener diode 219 collapses and conducts, and then the signal transmission part 216 - 1 generates a coupled signal. When the signal receiving part 216-2 receives the coupling signal, it provides an electrical path between the eighth terminal 216-28 and the ninth terminal 216-29, so that the potential of the base of the NPN transistor 236 is pulled high.

Based on the above, since the overvoltage at the output terminal 190 is caused by the failure of the internal circuit of the switching power supply device 100, for example, the failure of the feedback circuit 170, the feedback circuit 170 will not be able to generate the feedback signal. . Therefore, the PWM control circuit 132 cannot adjust the duty cycle of the PWM signal according to the feedback signal from the signal isolation unit 180 , so the output voltage VOUT cannot be reduced, and the voltage output by the primary side coil 140 - 2 will continue to increase.

When the voltage output by the primary side coil 140-2 continues to increase, so that the output voltage of the half-wave rectifier unit 220 exceeds the second default value, the Zener diode 252 will collapse and turn on, thereby pulling up the PNP The potential of the base of the type transistor 242 causes the PNP type transistor 242 to be in an off state (Off state). And because the PNP transistor 242 is in the off state, the current flowing through the impedance 232 will not flow into the delay circuit formed by the impedance 270 and the energy storage unit 260, but will flow into the NPN transistor 236, so that The potential of the base of the NPN transistor 236 is rapidly pulled up, so that the NPN transistor 236 can be quickly turned on, so as to quickly generate the overvoltage protection trigger signal TRI to the PWM control circuit 132, so that the PWM control circuit 132 can be transparent. The output voltage VOUT is immediately reduced by controlling the operation of the power transistor 134 for over-voltage protection.

Next, the operation of the voltage protection circuit 200 will be described under the condition that the external system causes the output terminal 190 to exhibit overvoltage. Please refer to FIG. 1 again, when the output terminal 190 presents an overvoltage, as long as the output voltage VOUT on the output terminal 190 exceeds the first default value, so that the voltage of the divided signal generated by the voltage dividing circuit 212 reaches the default reference potential, The switch unit 214 provides an electrical path between the sixth terminal 214-6 and the seventh terminal 214-7, so that the Zener diode 219 collapses and conducts, and then the signal transmission part 216-1 generates a coupled signal. When the signal receiving part 216-2 receives the coupling signal, it provides an electrical path between the eighth terminal 216-28 and the ninth terminal 216-29, so that the potential of the base of the NPN transistor 236 is pulled high.

Based on the above, the overvoltage at the output terminal 190 is caused by the external system, for example, the back electromotive force feedback caused by the external system with a motor during deceleration, rather than the failure of the feedback circuit 170 or the switching power supply device Due to the failure of other internal circuits, the feedback circuit 170 will normally generate a feedback signal, so that the PWM control circuit 132 can adjust the duty cycle of the PWM signal according to the feedback signal from the signal isolation unit 180, thereby reducing the output The voltage VOUT, and the output voltage of the primary side coil 140-2 will drop.

When the voltage output by the above-mentioned primary side coil 140-2 keeps dropping, so that the output voltage of the half-wave rectifier unit 220 is lower than the second default value, the Zener diode 252 will be turned off (Turned off), so that the PNP type The potential of the base of the transistor 242 is pulled down, so the PNP transistor 242 is in an ON state. Therefore, in addition to a part of the current flowing through the impedance 232 flowing into the NPN transistor 236 , a part also flows into the delay circuit formed by the impedance 270 and the energy storage unit 260 to charge the energy storage unit 260 . In this way, the potential of the base of the NPN transistor 236 can only be pulled up slowly, thereby delaying the turn-on time of the NPN transistor 236 , thereby delaying the generation of the overvoltage protection trigger signal TRI to the PWM control circuit 132 . In other words, in this case, the switching power supply device 100 will not perform the overvoltage protection immediately to cause the external system to fail to operate normally, but will delay the overvoltage protection for a period of time. Of course, those skilled in the art should know that the above-mentioned delay time can be adjusted by changing the resistance value of the impedance 270 or by changing the capacitance value of the energy storage unit 260 .

To sum up, in the overvoltage protection circuit of the present invention, the judgment result of the voltage sampling unit can be used to reflect the overvoltage at the output terminal of the switching power supply device, which is caused by the failure of the internal circuit of the switching power supply device. Yes, it is still caused by the external system. Therefore, when the output terminal of the switching power supply device presents an overvoltage, the overvoltage detection unit will control the first switch unit to start generating an overvoltage protection trigger signal, and the voltage sampling unit can decide whether to control the first switch unit according to the judgment result. The second switch unit couples the other end of the impedance to the reference potential to further determine whether to allow the energy storage unit and the impedance to delay the generation time of the overvoltage protection trigger signal. Therefore, under the above two different overvoltage conditions, the time for the overvoltage protection circuit of the present invention to generate the overvoltage protection trigger signal is also different. With such control, the overvoltage protection circuit of the present invention can immediately generate an overvoltage protection trigger signal to control the switchover power supply device to perform overvoltage protection when the internal circuit of the switchover power supply device fails, and cause damage to the external system The timing of generating the trigger signal for over-voltage protection is delayed in case of over-voltage, so that the switching power supply device will not perform over-voltage protection immediately and cause the external system to fail to operate normally.

Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, this The protection scope of the invention shall be defined by the claims.

Claims (9)

1. An overvoltage protection circuit, suitable for a switching power supply device, is characterized in that it comprises: An overvoltage detection unit, coupled between an output terminal of the switching power supply device and a first reference potential, is used to receive an output voltage on the output terminal and determine whether the output voltage exceeds a first reference potential A default value, when it is judged to be yes, a coupling signal will be generated; a half-wave rectifier unit, the input of which is coupled to a primary side coil of a transformer and a second reference potential; A first switch unit, which has a first terminal, a second terminal, a third terminal and a control terminal, the first terminal and the second terminal are both coupled to the output of the half-wave rectifier unit, the third The terminal is coupled to a pulse width modulation control circuit, and the first switch unit decides whether to provide an electrical path between the second terminal and the third terminal according to the voltage of the control terminal, so as to output a signal through the third terminal. The over-voltage protection trigger signal is used to control the pulse width modulation control circuit to stop outputting a pulse width modulation signal, and when the first switch unit receives the coupling signal, it provides a connection between the first terminal and the control terminal electrical path; a first energy storage unit, one end of which is coupled to the control end; a first impedance, one end of which is coupled to the control end; A second switch unit, which has a fourth end and a fifth end, the fourth end is coupled to the other end of the first energy storage unit and the other end of the first impedance, and the fifth end is coupled to the a second reference potential; and A voltage sampling unit, coupled to the output of the half-wave rectification unit, the second reference potential and the second switch unit, when the voltage output by the output of the half-wave rectification unit exceeds a second default value, the voltage sampling unit The unit controls the second switch unit to provide an electrical path between the fourth terminal and the fifth terminal. 2. The overvoltage protection circuit according to claim 1, wherein the overvoltage detection unit comprises: a voltage divider circuit, coupled between the output end of the switching power supply device and the first reference potential, and generates a voltage divider signal according to the voltage of the output end; a signal transmission part, one end of which is coupled to the output end of the switching power supply device; and A third switch unit, which has a sixth end and a seventh end, the sixth end is coupled to the other end of the signal transmission part, the seventh end is coupled to the first reference potential, the third switch unit uses to receive the divided voltage signal, and when the voltage of the divided voltage signal reaches a third reference potential, the third switch unit provides an electrical path between the sixth terminal and the seventh terminal, so as to further enable the The signal transmission part generates the coupling signal. 3. The overvoltage protection circuit according to claim 2, wherein the overvoltage detection unit further comprises a second impedance, the second impedance is coupled between the signal transmission part and the switching power supply device between the output terminals. 4. The overvoltage protection circuit according to claim 2, wherein the overvoltage detection unit further comprises a Zener diode, and the Zener diode is coupled between the signal transmission part and the switching power supply device. Between the output ends, the anode of the Zener diode is coupled to the signal transmission part, and the cathode of the Zener diode is coupled to the output end. 5. The overvoltage protection circuit according to claim 2, wherein the first switch unit comprises: A signal receiving part, which has an eighth end and a ninth end, the eighth end is coupled to the output of the half-wave rectifier unit, and when the signal receiving part receives the coupling signal, it provides the eighth end an electrical path with the ninth terminal; a second impedance coupled between the ninth terminal and the control terminal; a third impedance, one end of which is coupled to the output of the half-wave rectification unit; and An NPN type transistor, its collector is coupled to the other end of the third impedance, its emitter is coupled to the third end, and its base is coupled to the control end. 6. The overvoltage protection circuit according to claim 1, wherein the second switch unit comprises a PNP transistor, and the emitter of the PNP transistor is coupled to the other end of the first energy storage unit and the first energy storage unit. On the other end of the impedance, the collector is coupled to the second reference potential, and the base is coupled to the voltage sampling unit. 7. The overvoltage protection circuit according to claim 6, wherein the second switch unit further comprises a second impedance coupled between the base of the PNP transistor and the voltage sampling unit . 8. The overvoltage protection circuit according to claim 1, wherein the voltage sampling unit comprises: A zener diode, the cathode of which is coupled to the output of the half-wave rectifier unit; and a second impedance, one end of which is coupled to the anode of the zener diode and the second switch unit, and the other end is coupled to the second reference potential . 9. The overvoltage protection circuit according to claim 1, wherein the half-wave rectification unit comprises: a diode, the anode of which is coupled to one end of the primary coil; and A second energy storage unit, one end of which is coupled to the cathode of the diode and used as the output of the half-wave rectification unit, and the other end of which is coupled to the other end of the primary measuring coil and the second reference potential.