LED short circuit protection circuit
The LED short-circuit protection circuit, built using discrete components, detects the LED short-circuit state and directly shuts off the drive power supply, solving the problems of high cost of shunt resistors and easy damage to power circuits, and achieving fast and high-efficiency LED short-circuit protection.
Patent Information
- Application Number
- CN202423154018.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-19
- Publication Date
- 2025-11-25
- Estimated Expiration
- 2034-12-19
AI Technical Summary
Existing LED lights suffer from high shunt resistor costs and reduced product efficiency when short-circuited, and the power circuit is susceptible to damage from the switching stress of the drive circuit.
The LED short-circuit protection circuit, built using discrete components, detects the instantaneous short-circuit state of the LED, triggers short-circuit protection, and directly shuts off the drive power supply, avoiding the use of shunt resistors. It utilizes switching devices and drive circuits to control the conduction and shutdown of the power transistor.
It reduces costs, decreases heat generation, improves product efficiency, quickly protects the power circuit from damage, and prevents power transistors from being damaged by the switching stress of the drive circuit.
Smart Images
Figure CN223600067U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The utility model relates to circuit design technical field, especially relate to a LED short circuit protection circuit.
BACKGROUND TECHNIQUE
[0002] Current LED lamp when using, extremely easy to occur LED short circuit, in order to protect power circuit not to be damaged, LED power circuit's short circuit protection is more and more important. The LED short circuit protection on the market mainly is the current on the shunt resistance of LED power circuit is detected and the drive signal of LED is turned off, and the shunt resistance cost is high and can reduce product efficiency.
[0003] Therefore, it is necessary to provide a new technical scheme to solve the above problems.
UTILITY MODEL CONTENTS
[0004] One of the purposes of the utility model is to provide a kind of LED short circuit protection circuit, it is built by discrete device, circuit is simple and reliable, by detecting LED short circuit transient state, will trigger LED short circuit protection, to shut off drive power supply. Like this, not only shunt resistance cost is saved, reduce heat and improve the efficiency of product, and power tube will not be subjected to the switch stress damage risk caused by drive circuit constantly on-off. In addition, by directly shutting off drive power supply, response time is relatively fast, and power circuit will be protected more quickly not to be damaged.
[0005] According to one aspect of the utility model, the utility model provides a kind of LED short circuit protection circuit, it includes LED lamp string, power tube Q1, short circuit detection circuit and drive circuit, the input end of the LED lamp string is connected with power supply Vbat;The first connecting end of the power tube Q1 is connected with the output end of the LED lamp string, and its second connecting end is grounded;The short circuit detection circuit includes input end A, detection end B and output end C, the input end A is connected with the power supply Vbat, the detection end B is connected with the output end of the LED lamp string, the output end C is connected with the power end D of the drive circuit, when the short circuit detection circuit detects that the LED lamp string is in short circuit state based on the voltage of the output end of the LED lamp string, the short circuit detection circuit makes its input end A and output end C communicate, to make the power supply Vbat power supply end D of the drive circuit with power;When the short circuit detection circuit does not detect that the LED lamp string is in short circuit state based on the voltage of the output end of the LED lamp string, the short circuit detection circuit cuts off the connection of its input end A and output end C, to cut off the power supply of the power supply Vbat to the power end of the drive circuit;The input end E of the drive circuit receives PWM signal, and its output end F is connected with the control end of the power tube Q1, when the power supply Vbat power supply end D of the drive circuit with power, the drive circuit generates periodic drive signal based on the PWM signal, and the periodic drive signal is output by the output end F, to drive the power tube Q1 periodic conduction and shut off;When cutting off the power supply of the power supply Vbat to the power supply end D of the drive circuit, the drive circuit outputs constant drive signal by its output end F, to shut off the power tube Q1.
[0006] Further, the short circuit detection circuit includes diode D1, resistor R6 and switching device Q4, the first connecting end of the switching device Q4 is connected with the input end A of the short circuit detection circuit, and its second connecting end is connected with the output end C of the short circuit detection circuit, and its control end is grounded through the resistor R6;The negative pole of the diode D1 is connected with the control end of the switching device Q4, and its positive pole is connected with the detection end B of the short circuit detection circuit.
[0007] Further, when the LED lamp string is in short circuit state, the voltage of the output end of the LED lamp string makes the switching device Q4 conduction, to make the input end A and output end C of the short circuit detection circuit communicate by the switching device Q4;When the LED lamp string is in non-short circuit state, the voltage of the output end of the LED lamp string makes the switching device Q4 shut off, to cut off the connection of the input end A and output end C of the short circuit detection circuit by the switching device Q4.
[0008] Further, the driving circuit comprises a resistor R3, a resistor R4, a resistor R5, a switching device Q2 and a switching device Q3, a power supply end D of the driving circuit is grounded through the resistor R5, and the power supply end D of the driving circuit is connected with a node G through the resistor R3; the node G is connected with an output end F of the driving circuit through the resistor R4; a first connection end of the switching device Q2 is connected with the node G, a second connection end thereof is grounded, and a control end thereof receives the PWM signal; a first connection end of the switching device Q3 is connected with the output end F of the driving circuit, a control end thereof is connected with the node G, and a second connection end thereof is grounded.
[0009] Further, the driving circuit further comprises a resistor R1 and a resistor R2, one end of the resistor R1 is connected with an input end E of the driving circuit, and the other end thereof is connected with a control end of the switching device Q2; one end of the resistor R2 is connected with the control end of the switching device Q2, and the other end thereof is grounded.
[0010] Further, resistance values of the resistor R1 and the resistor R2 are selected to satisfy that when the switching device Q4 is turned on and the PWM signal is a first logic level, a voltage division voltage on the resistor R2 is greater than a turn-on voltage threshold of the switching device Q2, so that the switching device Q2 is in a turn-on state.
[0011] Further, when the power supply Vbat supplies power to the power supply end D of the driving circuit, if the PWM signal is the first logic level, the switching device Q2 and the switching device Q3 are both turned on, so that the output end F of the driving circuit outputs a second logic level of the periodic driving signal, so that the power tube Q1 is turned off; if the PWM signal is a second logic level, the switching device Q2 and the switching device Q3 are both turned off, so that the output end F of the driving circuit outputs a first logic level of the periodic driving signal, so that the power tube Q1 is turned on; when the power supply Vbat supplies power to the power supply end D of the driving circuit is cut off, the output end F of the driving circuit outputs the constant driving signal, so that the power tube Q1 is turned off.
[0012] Further, the power tube Q1 is an NMOS tube, the first connection end, the second connection end and the control end of the power tube Q1 are the drain, the source and the gate of the NMOS tube respectively; the switching device Q2 is an NPN type triode, the first connection end, the second connection end and the control end of the switching device Q2 are the collector, the emitter and the base of the NPN type triode respectively; the switching device Q3 is a PNP type triode, the first connection end, the second connection end and the control end of the switching device Q3 are the emitter, the collector and the base of the PNP type triode respectively; the switching device Q4 is a PNP type triode, the first connection end, the second connection end and the control end of the switching device Q4 are the emitter, the collector and the base of the PNP type triode respectively.
[0013] Compared with the prior art, the utility model discloses by discrete device builds, and the circuit is simple and reliable, and through the detection LED short circuit instantaneous state, will trigger LED short circuit protection, to shut off the drive power supply. In this way, not only the cost of shunt resistance is saved, the heat is reduced and the efficiency of product is improved, and the power tube will not be damaged by the switching stress of the driving circuit. In addition, by shutting off the drive power supply directly, the response time is relatively fast, and the power loop can be protected from damage more quickly. BRIEF DESCRIPTION OF DRAWINGS
[0014] In order to more clearly illustrate the technical scheme of the embodiments of the utility model, the following will be needed to use the drawings in the embodiment description briefly introduced, obviously, the following description in the drawings is only some embodiments of the utility model, for those skilled in the art, under the premise of not paying the creative labor, can also obtain other drawings according to these drawings. Wherein:
[0015] Figure 1 It is the circuit schematic diagram of the LED short circuit protection circuit of the utility model in an embodiment. DETAILED DESCRIPTION
[0016] In order to make the above purpose, features and advantages of the utility model more apparent and easy to understand, the following will be further detailed to the utility model with the drawings and specific implementation.
[0017] The "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the utility model. The "in one embodiment" appearing in different places in the specification is not all referring to the same embodiment, and is not an independent or selective embodiment mutually exclusive with other embodiments. Unless specifically stated, the coupling, connection, connection, connection of the words in this paper means that the electrical connection is directly or indirectly connected, such as A and B are connected, which includes that A and B are directly electrically connected, and A is connected with B through electrical components or circuit.
[0018] In the description of the utility model, need understanding is, the terms "upper", "lower", "front", "back", "positive", "back", "left", "right", "vertical", "vertical", "horizontal", "top", "bottom", "internal", "external" and so on indicate the orientation or positional relation based on the orientation or positional relation shown in the drawing, just for the convenience of describing the utility model and simplifying the description, and not indicate or imply that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, therefore can not be understood as the limitation of the utility model.
[0019] Please refer to Figure 1 As shown in the figure, it is the circuit schematic diagram of the LED short circuit protection circuit in one embodiment of the utility model. Figure 1 The LED short circuit protection circuit shown in the figure includes LED lamp string LED1, power tube (Metal Oxide Semiconductor Field Effect Transistor, Mosfet) Q1, short circuit detection circuit 110 and driving circuit 120.
[0020] Among them, the input end (or anode) of LED lamp string LED1 is connected with power supply Vbat;The first connecting end of power tube Q1 is connected with the output end (or cathode) of LED lamp string LED1, and the second connecting end is grounded.
[0021] Short circuit detection circuit 110 includes input end A, detection end B and output end C, wherein, input end A is connected with power supply Vbat, detection end B is connected with the output end of LED lamp string LED1, and output end C is connected with power supply end D of driving circuit 120, when short circuit detection circuit 110 detects that LED lamp string LED1 is in short circuit state based on the voltage of the output end of LED lamp string LED1, short circuit detection circuit 110 makes its input end A and output end C communicate, so that power supply Vbat supplies power to power supply end D of driving circuit 120;When short circuit detection circuit 110 does not detect that LED lamp string LED1 is in short circuit state based on the voltage of the output end of LED lamp string LED1, short circuit detection circuit 110 cuts off the connection of its input end A and output end C, so as to cut off the power supply of power supply Vbat to power supply end D of driving circuit 120.
[0022] The input end E of the driving circuit 120 receives a PWM (Pulse Width Modulation) signal, and the output end F is connected with the control end of the power tube Q1. When the power supply Vbat supplies power to the power supply end D of the driving circuit 120, the LED lamp string LED1 is in a non-short-circuit state, the driving circuit 120 generates a periodic driving signal based on the PWM signal, and the periodic driving signal is output through the output end F to drive the power tube Q1 to be periodically turned on and turned off. When the power supply Vbat is cut off to supply power to the power supply end D of the driving circuit 120, the LED lamp string LED1 is in a short-circuit state, and the driving circuit 120 outputs a constant driving signal through the output end F to turn off the power tube Q1, thereby protecting the power tube Q1 from overheating and damage. The PWM (Pulse Width Modulation) driving signal is a periodic high-low square wave signal, and the duty cycle of the PWM driving signal can be changed to control the power tube Q1, thereby adjusting the luminous brightness of the LED lamp string LED1.
[0023] The short-circuit detection circuit 110 includes a diode D1, a resistor R6 and a switching device Q4. The first connection end of the switching device Q4 is connected with the input end A (or the power supply Vbat) of the short-circuit detection circuit 110, the second connection end is connected with the output end C (or the power supply end D of the driving circuit 120) of the short-circuit detection circuit 110, and the control end is grounded through the resistor R6. The negative electrode of the diode D1 is connected with the control end of the switching device Q4, and the positive electrode is connected with the detection end B (or the output end of the LED lamp string LED1) of the short-circuit detection circuit 110.
[0024] When the LED lamp string LED1 is in a short-circuit state, the voltage at the output end of the LED lamp string LED1 makes the switching device Q4 conduct, thereby connecting the input end A (and the output end C) of the short-circuit detection circuit 110 through the switching device Q4. When the LED lamp string LED1 is in a non-short-circuit state, the voltage at the output end of the LED lamp string LED1 makes the switching device Q4 turn off, thereby cutting off the connection between the input end A and the output end C of the short-circuit detection circuit 110 through the switching device Q4.
[0025] The driving circuit 120 includes a resistor R3, a resistor R4, a resistor R5, a switching device Q2 and a switching device Q3. The power supply end D of the driving circuit 120 is grounded through the resistor R5, and the power supply end D of the driving circuit 120 is connected with the node G through the resistor R3. The node G is connected with the output end F (or the control end of the power tube Q1) of the driving circuit 120 through the resistor R4. The first connection end of the switching device Q2 is connected with the node G, the second connection end is grounded, and the control end receives the PWM signal. The first connection end of the switching device Q3 is connected with the output end F of the driving circuit 120, the control end is connected with the node G, and the second connection end is grounded.
[0026] The driving circuit 120 further comprises a resistor R1 and a resistor R2, one end of the resistor R1 is connected with the input end E (or the PWM signal) of the driving circuit 120, and the other end is connected with the control end of the switching device Q2; one end of the resistor R2 is connected with the control end of the switching device Q2, and the other end is grounded.
[0027] In Figure 1 In the specific embodiment shown, the power tube Q1 is an NMOS tube, the first connection end, the second connection end and the control end of the power tube Q1 are the drain, the source and the gate of the NMOS tube respectively; the switching device Q2 is an NPN type triode, the first connection end, the second connection end and the control end of the switching device Q2 are the collector, the emitter and the base of the NPN type triode respectively; the switching device Q3 is a PNP type triode, the first connection end, the second connection end and the control end of the switching device Q3 are the emitter, the collector and the base of the PNP type triode respectively; the switching device Q4 is a PNP type triode, the first connection end, the second connection end and the control end of the switching device Q4 are the emitter, the collector and the base of the PNP type triode respectively.
[0028] When the power supply Vbat supplies power to the power supply end D of the driving circuit 120, if the PWM signal is high (which can be referred to as the first logic level of the PWM signal), then the switching device Q2 and the switching device Q3 are both turned on, so that the output end F of the driving circuit 120 outputs a low level (which can be referred to as the second logic level of the periodic driving signal), so that the power tube Q1 is turned off; if the PWM signal is low (which can be referred to as the second logic level of the PWM signal), then the switching device Q2 and the switching device Q3 are both turned off, so that the output end F of the driving circuit 120 outputs a high level (which can be referred to as the first logic level of the periodic driving signal), so that the power tube Q1 is turned on. In other words, the PWM signal, the resistor R1, the resistor R2, the resistor R3, the resistor R4, the resistor R5, the switching device Q2 and the switching device Q3 constitute a main power loop of the Mosfet, which realizes the dimming function of the LED in normal times.
[0029] When the power supply Vbat is cut off from the power supply end D of the driving circuit 120, the output end F of the driving circuit 120 is pulled low to the ground level by the resistor R5, so that the output end F of the driving circuit 120 outputs a low level (which can be referred to as a constant driving signal), so that the power tube Q1 is turned off.
[0030] Among them, the resistance values of the resistor R1 and the resistor R2 are selected to satisfy: when the switching device Q4 is turned on and the PWM signal is high (which can be referred to as the first logic level of the PWM signal), the voltage value of the voltage divider on the resistor R2 is greater than the turn-on voltage threshold of the switching device Q2, so that the switching device Q2 is in the on state.
[0031] The working principle of the LED short circuit protection circuit is specifically introduced as follows. Figure 1 When the PWM signal is high, the base electrode voltage of the switching device Q2 meets the saturation conduction condition of the switching device Q2 through the voltage division of the resistor R1 and the resistor R2, so that the switching device Q2 is turned on, and the collector electrode of the switching device Q2 is pulled to 0V. When the collector electrode of the switching device Q2 is 0V, the switching device Q3 is a PNP triode, so the switching device Q3 is saturated and turned on, so that the gate voltage of the power tube Q1 can be quickly discharged through the resistor R4 and the switching device Q2, thereby ensuring the quick turn-off of the power tube Q1, so that the gate drive of the power tube Q1 is pulled low, and the power tube Q1 is turned off, and then the LED lamp string LED1 is also in an off state.
[0032] When the PWM signal is low, the base electrode of the switching device Q2 is also low, and the switching device Q2 is in an off state, which does not affect the gate drive of the power tube Q1. Because the emitter of the switching device Q4 is connected to the power supply Vbat, and the power supply Vbat is after the LED lamp string LED1, and the LED lamp string LED1 has a tube voltage drop value, the base electrode voltage of the switching device Q4 is greater than the negative electrode voltage of the LED lamp string LED1. Because D1 is a diode, the diode has unidirectional conductivity, so the negative electrode voltage of the LED lamp string LED1 does not affect the base electrode voltage of the switching device Q4. Because the switching device Q4 is a PNP triode, the base electrode of the switching device Q4 is pulled down to GND through the resistor R6, so the switching device Q4 is saturated and turned on, and the collector electrode voltage of the switching device Q4 is about the voltage of the power supply Vbat. The collector electrode voltage of the switching device Q4 can directly drive the conduction of the power tube Q1 after passing through the resistors R3 and R4, so the power tube Q1 can be driven to conduct when the PWM signal is low, so that the LED lamp string LED1 can be lit.
[0033] 2. When the PWM signal is high, if the LED lamp string LED1 appears a short circuit condition, because the power tube Q1 is in an off state, the voltage of the power supply Vbat is superimposed on the positive electrode of the diode D1. After the voltage drop of the diode D1, the base electrode voltage of the switching device Q4 and the emitter voltage thereof do not meet the positive bias conduction of the switching device Q4, so the switching device Q4 is also off. That is, when the PWM signal is high, the LED lamp string LED1 appears a short circuit, and the entire driving loop and power loop are not affected and damaged.
[0034] 3、When the PWM signal is low, because the power tube Q1 is in the on state, if the LED lamp string LED1 appears a short circuit condition, the voltage of the power supply Vbat will be suddenly applied to the anode of the diode D1, the voltage difference between the emitter and the base of the switching device Q4 does not meet the condition of its saturated conduction, the switching device Q4 is off, the collector of the switching device Q4 is pulled down to GND through the resistor R5, when the switching device Q4 is in the off state, the voltage on the resistor R5 is 0V, because the resistor R5 is connected to the gate of the power tube Q1 through the resistors R3 and R4, so the gate voltage on the power tube Q1 will be rapidly discharged through the resistor R4 and the switching device Q2, and the resistors R4, R3 and R5, the loop of the LED lamp string LED1 to the power tube Q1 is also off, so that the power tube Q1 will not be damaged due to the short circuit of the LED.
[0035] In summary, the LED short circuit protection circuit has the following beneficial effects:
[0036] 1、The utility model discloses a discrete device is built, and the circuit is simple and reliable, and through the detection of LED short circuit instantaneous state, will trigger LED short circuit protection, thereby shutting off the driving power supply.
[0037] 2、The utility model discloses a short circuit protection is carried out to the LED through shutting off the driving power supply, so that the power tube will not be damaged by the switching stress of the continuous switching of the driving circuit. In addition, by shutting off the driving power supply directly, the response time is relatively fast, and the power loop can be protected more quickly and not be damaged. 3、The utility model discloses a short circuit of LED is not needed through the shunt resistance detection, reduces the cost, reduces the heat quantity of shunt resistance and improves the efficiency of product.
[0038] It should be pointed out that any modification made by the skilled in the art to the specific embodiments of the utility model does not deviate from the scope of the claims of the utility model. Correspondingly, the scope of the claims of the utility model is also not limited to the foregoing specific embodiments.
Claims
1. An LED short-circuit protection circuit, characterized in that, It includes LED light strings, power transistor Q1, short-circuit detection circuit, and driver circuit. The input terminal of the LED light string is connected to the power supply Vbat; The first connection terminal of the power transistor Q1 is connected to the output terminal of the LED string, and its second connection terminal is grounded. The short-circuit detection circuit includes an input terminal A, a detection terminal B, and an output terminal C. Input terminal A is connected to the power supply Vbat, detection terminal B is connected to the output terminal of the LED string, and output terminal C is connected to the power supply terminal D of the driving circuit. When the short-circuit detection circuit detects that the LED string is in a short-circuit state based on the voltage at the output terminal of the LED string, it connects its input terminal A and output terminal C, thereby allowing the power supply Vbat to supply power to the power supply terminal D of the driving circuit. When the short-circuit detection circuit does not detect that the LED string is in a short-circuit state based on the voltage at the output terminal of the LED string, it disconnects the connection between its input terminal A and output terminal C, thereby cutting off the power supply Vbat to the power supply terminal D of the driving circuit. The input terminal E of the drive circuit receives a PWM signal, and its output terminal F is connected to the control terminal of the power transistor Q1. When the power supply Vbat supplies power to the power supply terminal D of the drive circuit, the drive circuit generates a periodic drive signal based on the PWM signal. The periodic drive signal is output through the output terminal F to drive the power transistor Q1 to periodically turn on and off. When the power supply Vbat is cut off from the power supply terminal D of the drive circuit, the drive circuit outputs a constant drive signal through its output terminal F to turn off the power transistor Q1.
2. The LED short-circuit protection circuit according to claim 1, characterized in that, The short-circuit detection circuit includes diode D1, resistor R6, and switching device Q4. The first connection terminal of the switching device Q4 is connected to the input terminal A of the short-circuit detection circuit, and its second connection terminal is connected to the output terminal C of the short-circuit detection circuit. Its control terminal is grounded through the resistor R6. The cathode of the diode D1 is connected to the control terminal of the switching device Q4, and its anode is connected to the detection terminal B of the short-circuit detection circuit.
3. The LED short-circuit protection circuit according to claim 2, characterized in that, When the LED string is in a short-circuit state, the voltage at the output terminal of the LED string causes the switching device Q4 to conduct, thereby connecting the input terminal A and the output terminal C of the short-circuit detection circuit through the switching device Q4. When the LED string is in a non-short-circuit state, the voltage at the output terminal of the LED string causes the switching device Q4 to turn off, thereby disconnecting the connection between the input terminal A and the output terminal C of the short-circuit detection circuit through the switching device Q4.
4. The LED short-circuit protection circuit according to claim 3, characterized in that, The driving circuit includes resistors R3, R4, and R5, and switching devices Q2 and Q3. The power supply terminal D of the driving circuit is grounded through the resistor R5, and the power supply terminal D of the driving circuit is connected to node G through the resistor R3; node G is connected to the output terminal F of the driving circuit through the resistor R4; the first connection terminal of the switching device Q2 is connected to node G, its second connection terminal is grounded, and its control terminal receives the PWM signal; the first connection terminal of the switching device Q3 is connected to the output terminal F of the driving circuit, its control terminal is connected to node G, and its second connection terminal is grounded.
5. The LED short-circuit protection circuit according to claim 4, characterized in that, The driving circuit also includes resistors R1 and R2. One end of the resistor R1 is connected to the input terminal E of the driving circuit, and the other end is connected to the control terminal of the switching device Q2. One end of the resistor R2 is connected to the control terminal of the switching device Q2, and the other end is grounded.
6. The LED short-circuit protection circuit according to claim 5, characterized in that, The resistance values of resistors R1 and R2 are selected to satisfy the following condition: when the switching device Q4 is turned on and the PWM signal is at the first logic level, the voltage across resistor R2 is greater than the turn-on voltage threshold of the switching device Q2, so that the switching device Q2 is in the on state.
7. The LED short-circuit protection circuit according to any one of claims 4-6, characterized in that, When the power supply Vbat supplies power to the power supply terminal D of the drive circuit, if the PWM signal is at the first logic level, both switching devices Q2 and Q3 are turned on, thereby the output terminal F of the drive circuit outputs the second logic level of the periodic drive signal, causing the power transistor Q1 to turn off; if the PWM signal is at the second logic level, both switching devices Q2 and Q3 are turned off, thereby the output terminal F of the drive circuit outputs the first logic level of the periodic drive signal, causing the power transistor Q1 to turn on. When the power supply Vbat is cut off from the power supply terminal D of the drive circuit, the output terminal F of the drive circuit outputs the constant drive signal, causing the power transistor Q1 to turn off.
8. The LED short-circuit protection circuit according to claim 7, characterized in that, The power transistor Q1 is an NMOS transistor, and the first connection terminal, the second connection terminal, and the control terminal of the power transistor Q1 are the drain, source, and gate of the NMOS transistor, respectively. The switching device Q2 is an NPN transistor, and the first connection terminal, the second connection terminal, and the control terminal of the switching device Q2 are the collector, emitter, and base of the NPN transistor, respectively. The switching device Q3 is a PNP transistor, and the first connection terminal, the second connection terminal, and the control terminal of the switching device Q3 are the emitter, collector, and base of the PNP transistor, respectively. The switching device Q4 is a PNP transistor, and the first connection terminal, the second connection terminal, and the control terminal of the switching device Q4 are the emitter, collector, and base of the PNP transistor, respectively.