Short circuit detection circuit of H-bridge two-line output LED dimming power supply

Abstract

The utility model discloses a short circuit detection circuit of an H-bridge two-wire output LED dimming power supply. The short circuit detection circuit comprises an EM I filter circuit, an AC-DC circuit, an H-bridge two-wire output dimming circuit and a short circuit protection circuit. The input end of the EM I filter circuit is connected with an external power supply; the input end of the AC-DC circuit is connected with the output end of the EM I filter circuit; the H-bridge two-line output dimming circuit is connected between the output end of the AC-DC circuit and the lamp; the short-circuit protection circuit is connected between the H-bridge two-line output dimming circuit and the lamp; the H-bridge two-line output dimming circuit can cut off a dimming signal output to the lamp when the short-circuit protection circuit detects that the lamp is short-circuited; through the structure, the H bridge is quickly protected, abnormal output work due to long-time short circuit is avoided, meanwhile, the H bridge circuit is prevented from being damaged, an MCU circuit system can be reported to be subjected to next-step processing, and the H bridge protection circuit has the advantages of being simple in circuit connection, small in device number, low in cost, quick in response and stable in performance.

Description

Technical Field

[0001] The utility model relates to the technical field of LED dimming driving power supply and switching power supply, in particular to a short circuit detection circuit of an H-bridge two-wire output LED dimming power supply. Background Art

[0002] At present, H-bridge two-wire output LED dimming power supply is widely used in the market. This LED dimming power supply has diversified dimming and color temperature adjustment modes, can achieve minimum brightness, and has the advantages of smooth dimming. For the H-bridge two-wire output dimming and color temperature adjustment power supply, its H-bridge circuit is composed of 4 MOS tubes as main components, and the output only needs two lines to dim and adjust the color temperature. When the output (load) is short-circuited, the H-bridge will not work properly, and in severe cases, it will cause damage to the H-bridge and make the H-bridge circuit unable to work normally. Therefore, a short-circuit detection circuit of the H-bridge two-wire output LED dimming power supply is urgently needed to solve the above problems. Utility Model Content

[0003] The utility model aims to solve at least one of the technical problems existing in the prior art. To this end, the utility model proposes a short circuit detection circuit of an H-bridge two-wire output LED dimming power supply.

[0004] The technical solution adopted by an embodiment of the utility model to solve the technical problem is: a short-circuit detection circuit of an H-bridge two-wire output LED dimming power supply, including an EM I filter circuit, an AC-DC circuit, an H-bridge two-wire output dimming circuit and a short-circuit protection circuit;

[0005] The input end of the EM I filter circuit is connected to an external power supply;

[0006] The input end of the AC-DC circuit is connected to the output end of the EM I filter circuit;

[0007] The H-bridge two-wire output dimming circuit is connected between the output end of the AC-DC circuit and the lamp;

[0008] The short-circuit protection circuit is connected between the H-bridge two-wire output dimming circuit and the lamp;

[0009] The H-bridge two-wire output dimming circuit can cut off the dimming signal output to the lamp when the short-circuit protection circuit detects that the lamp is short-circuited.

[0010] As one of the preferred embodiments of the utility model, the short circuit protection circuit includes a differential ratio operation module, a comparison module and a control module;

[0011] The first input end of the differential ratio operation module is connected to one end of the lamp, and the second output end is connected to the other end of the lamp, so as to generate a differential signal at both ends of the lamp;

[0012] The input end of the comparison module is connected to the output end of the differential ratio operation module;

[0013] The control module is connected between the H-bridge two-line output dimming circuit and the output end of the comparison module;

[0014] The control module can control the short-circuit protection circuit to conduct or cut off the dimming signal output to the lamp according to the signal output by the comparison module.

[0015] As one of the preferred embodiments of the utility model, the differential ratio operation module includes resistors R2-3, resistors R6-7 and an operational amplifier U1 B, the resistor R3 is respectively connected to one end of the lamp and the reverse input end of the operational amplifier U1 B, one end of the resistor R6 is connected to the other end of the lamp, the other end of the resistor R6 is connected to the positive input end of the operational amplifier U1 B and connected to the GND end through the resistor R7, the resistor R2 is connected between the reverse input end of the operational amplifier U1 B and the output end of the operational amplifier U1 B, and the output end of the operational amplifier U1 B is connected to the input end of the comparison module.

[0016] As one of the preferred embodiments of the utility model, the comparison module includes a comparator U2A, a comparator U2B, a resistor R1, a resistor R4, a resistor R8 and transistors Q5-6. The negative end of the comparator U2A is respectively connected to the positive end of the comparator U2B and the output end of the differential ratio operation module, the positive end of the comparator U2A is respectively connected to the negative end of the comparator U2B and the GND end, the power supply end of the comparator U2A is connected to the VCC end, the output end of the comparator U2A is connected to the base of the transistor Q5 and is connected to the VCC end through the resistor R4, the emitter of the transistor Q5 is respectively connected to one end of the resistor R1, the emitter of the transistor Q6, the input end of the control module and the H-bridge two-wire output dimming circuit, the other end of the resistor R1 is connected to the VDD end, the collector of the transistor Q5 and the collector of the transistor Q6 are connected to the GND end, and the output end of the comparator U2B is connected to the base of the transistor Q6 and is connected to the VCC end through the resistor R8.

[0017] As one of the preferred embodiments of the utility model, the control module includes a transistor Q7 and a transistor Q8, the collector of the transistor Q7 and the collector of the transistor Q8 are respectively connected to the H-bridge two-wire output dimming circuit, the base of the transistor Q7 and the base of the transistor Q8 are connected to the output end of the comparison module, and the emitter of the transistor Q7 and the emitter of the transistor Q8 are connected to the GND end.

[0018] The beneficial effects of the utility model include: a short-circuit detection circuit of an H-bridge two-wire output LED dimming power supply, comprising an EMI filtering circuit, an AC-DC circuit, an H-bridge two-wire output dimming circuit and a short-circuit protection circuit; the input end of the EMI filtering circuit is connected to an external power supply; the input end of the AC-DC circuit is connected to the output end of the EMI filtering circuit; the H-bridge two-wire output dimming circuit is connected between the output end of the AC-DC circuit and the lamp; the short-circuit protection circuit is connected between the H-bridge two-wire output dimming circuit and the lamp; the H-bridge two-wire output dimming circuit can cut off the dimming signal output to the lamp when the short-circuit protection circuit detects a short circuit in the lamp; the H-bridge is quickly protected by the above structure to avoid long-term short-circuit output malfunction, and also to avoid damage to the H-bridge circuit, and can also report to the MCU circuit system for the next step of processing, with the advantages of simple circuit connection, small number of components, low cost, fast response and stable performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0020] Figure 1 It is a principle block diagram of a short-circuit detection circuit of an H-bridge two-wire output LED dimming power supply;

[0021] Figure 2 The circuit schematic diagram is a short circuit detection circuit of an H-bridge two-wire output LED dimming power supply;

[0022] Figure 3 This is the circuit schematic diagram of the differential proportional operation module;

[0023] Figure 4 This is the circuit schematic diagram of the comparison module;

[0024] Figure 5 This is the circuit schematic diagram of the control module. DETAILED DESCRIPTION

[0025] This section will describe in detail the specific embodiments of the utility model. The preferred embodiments of the utility model are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the utility model, but it cannot be understood as a limitation on the protection scope of the utility model.

[0026] In the description of the present utility model, the meaning of "more than" is more than two, "greater than", "less than", "exceed" etc. are understood to exclude the number itself, and "above", "below", "within" etc. are understood to include the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0027] In the description of the present invention, it should be understood that descriptions involving orientation, such as up, down, front, back, left, right, etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0028] In the present invention, unless otherwise clearly defined, the words "set", "install", "connect" and the like should be understood in a broad sense, for example, they can be directly connected or indirectly connected through an intermediate medium; they can be fixedly connected or detachably connected or integrally formed; they can be mechanically connected; they can be the internal connection of two elements or the interaction relationship between two elements. Those skilled in the art can reasonably determine the specific meanings of the above words in the present invention in combination with the specific content of the technical solution.

[0029] Reference Figures 1 to 5 , a short circuit detection circuit of an H-bridge two-wire output LED dimming power supply, comprising an EM I filter circuit 10, an AC-DC circuit 20, an H-bridge two-wire output dimming circuit 30 and a short circuit protection circuit 40;

[0030] The input end of the EM I filter circuit 10 is connected to an external power supply;

[0031] The input end of the AC-DC circuit 20 is connected to the output end of the EM I filter circuit 10;

[0032] The H-bridge two-wire output dimming circuit 30 is connected between the output end of the AC-DC circuit 20 and the lamp;

[0033] The short circuit protection circuit 40 is connected between the H-bridge two-line output dimming circuit 30 and the lamp;

[0034] The H-bridge two-line output dimming circuit 30 can cut off the dimming signal output to the lamp when the short-circuit protection circuit 40 detects a short circuit in the lamp.

[0035] As a preferred embodiment of the short-circuit protection circuit 40, the short-circuit protection circuit 40 includes a differential ratio operation module 41, a comparison module 42 and a control module 43;

[0036] The first input end of the differential ratio operation module 41 is connected to one end of the lamp, and the second output end is connected to the other end of the lamp, so as to generate a differential signal between the two ends of the lamp;

[0037] The input end of the comparison module 42 is connected to the output end of the differential ratio operation module 41;

[0038] The control module 43 is connected between the H-bridge two-line output dimming circuit 30 and the output end of the comparison module 42;

[0039] The control module 43 can control the short circuit protection circuit 40 to turn on or off the dimming signal output to the lamp according to the signal output by the comparison module 42 .

[0040] The working principle of the utility model is as follows:

[0041] ① When the system is powered on, the AC power passes through the EM I filter circuit 10 and is rectified into DC power through the rectifier bridge. When the AC-DC circuit works stably, due to the voltage outer loop feedback and the fixed turns ratio of the voltage regulator, the output stable voltages are V+, VCC and VDD respectively. V+ and VCC are then supplied to the H-bridge two-line output dimming circuit 30, and VCC is supplied to the MCU. The MCU outputs CW_PWM to the driver chip U1 and outputs WW_PWM to the driver chip U2. The driver chip U1 outputs CW_H to the gate of the MOS tube Q1, and the driver chip U1 outputs CW_L to the gate of the MOS tube Q3. The driver chip U2 outputs WW_H to the gate of the MOS tube Q2, and the driver chip U2 outputs WW_L to the gate of the MOS tube Q4. Among them, CW_PWM and WW_PWM are complementary duty ratios output by the MCU;

[0042] When the cold light and warm light are the brightest, CW_PWM and WW_PWM are complementary duty cycles. At the same time, VCC supplies power to driver chip U1 and driver chip U2. When CW_PWM is high and WW_PWM is low, CW_H is high, CW_L is low, WW_H is low, and WW_L is high. Therefore, V+ passes through MOS tube Q1, enters from the CW end of the lamp, flows through the cold light of the lamp, and then comes out from the WW end of the lamp, passes through Q4, passes through the R9 current limiting resistor, and then returns to to GND, at this time, the cold light of the lamp is bright; when CW_PWM is low and WW_PWM is high, at this time, CW_H is low, CW_L is high, WW_H is high, and WW_L is low, so V+ passes through MOS tube Q2, then enters from the WW end of the lamp and flows through the warm light of the lamp, and then comes out from the CW end of the lamp through MOS tube Q3, through the current limiting resistor R9 and returns to the GND end, at this time, the warm light of the lamp is bright; the duty cycle is controlled by the frequency, and at this time, the cold and warm light is the brightest;

[0043] When the cold light is brightest, CW_PWM is high and WW_PWM is low. At this time, CW_H is high, CW_L is low, WW_H is low, and WW_L is high. Therefore, V+ passes through MOS tube Q1, enters from the CW end of the lamp, flows through the cold light of the lamp, and then comes out from the WW end of the lamp, passes through MOS tube Q4, passes through the current limiting resistor R9, and returns to the GND end. At this time, the cold light of the lamp is the brightest;

[0044] When the warm light is brightest, CW_PWM is low and WW_PWM is high. At this time, CW_H is low, CW_L is high, WW_H is high, and WW_L is low. Therefore, V+ passes through MOS tube Q2, then enters from the WW end of the lamp and flows through the warm light of the lamp, and then comes out from the CW end of the lamp through MOS tube Q3, through the current limiting resistor R9 and returns to the GND end. At this time, the warm light of the lamp is the brightest;

[0045] ② In the short-circuit protection circuit 40, the differential ratio operation module 41 includes resistors R2-3, resistors R6-7 and an operational amplifier U1 B, the resistor R3 is respectively connected to one end of the lamp and the reverse input end of the operational amplifier U1 B, one end of the resistor R6 is connected to the other end of the lamp, the other end of the resistor R6 is connected to the positive input end of the operational amplifier U1 B and connected to the GND end through the resistor R7, the resistor R2 is connected between the reverse input end of the operational amplifier U1 B and the output end of the operational amplifier U1 B, and the output end of the operational amplifier U1 B is connected to the input end of the comparison module 42; the resistor R3, the resistor R2, the resistor R6, the resistor R7 and the operational amplifier U1 B form a differential ratio operation circuit, the calculation formula of the circuit is: (Vcw-Vww)*(R2 / R3)=U1 B output terminal 7 pin level (hereinafter referred to as V U1Bout Indicated), where Vcw-Vww is the voltage across the lamp;

[0046] When the output (load) of the H-bridge two-wire output dimming circuit 30 is not short-circuited, when the CW end of the H-bridge two-wire output dimming circuit 30 flows to the WW end, since the CW end is connected to the positive input end of the operational amplifier U1 B through the resistor R6, and the WW end is connected to the reverse input end of the operational amplifier U1 B through the resistor R3, the voltage difference between the two ends of the lamp is a positive voltage, that is, Vcw-Vww>0, and substituting it into the above formula yields: V U1Bout>0; and the comparison module 42 includes a comparator U2A, a comparator U2B, a resistor R1, a resistor R4, a resistor R8 and transistors Q5-6, the negative end of the comparator U2A is respectively connected to the positive end of the comparator U2B and the output end of the differential ratio operation module 41, the positive end of the comparator U2A is respectively connected to the negative end of the comparator U2B and the GND end, the power supply end of the comparator U2A is connected to the VCC end, and the output end of the comparator U2A is connected to the base of the transistor Q5 The emitter of transistor Q5 is connected to one end of resistor R1, the emitter of transistor Q6, the input end of control module 43 and H-bridge two-wire output dimming circuit 30 respectively, the other end of resistor R1 is connected to VDD, the collector of transistor Q5 and the collector of transistor Q6 are connected to GND, the output end of comparator U2B is connected to the base of transistor Q6 and connected to VCC through resistor R8; therefore, V U1Bout The negative terminal of comparator U2A and the positive terminal of comparator U2B are connected to the GND terminal through resistor R5, that is, the voltage of the positive terminal of comparator U2A = 0, the voltage of the negative terminal of comparator U2B = 0. Since the voltage of the negative terminal of comparator U2A> 0, the voltage of the positive terminal of comparator U2B> 0, the voltage of the negative terminal of comparator U2A> the voltage of the positive terminal of comparator U2A, the voltage of the positive terminal of comparator U2B> the voltage of the negative terminal of comparator U2B, since the VCC terminal is connected to the output terminal of comparator U2A through resistor R4, and the VCC terminal is connected to the output terminal of comparator U2B through resistor R8, the output terminal of comparator U2A (hereinafter referred to as V U2Aout The output of the comparator U2B (hereinafter referred to as V U2Bout =Indicates) is a high level, at this time, transistor Q5 is turned on, transistor Q6 is not turned on, because transistor Q5, transistor Q6 and signal AD are connected together, VDD terminal enters signal AD through resistor R1, so signal AD is a low level, signal AD enters control module 43 through resistor R11, and control module 43 includes transistor Q7 and transistor Q8, the collector of transistor Q7 and the collector of transistor Q8 are respectively connected to H bridge two-line output dimming circuit 30, transistor The base of Q7 and the base of transistor Q8 are connected to the output end of the comparison module 42, and the emitter of transistor Q7 and the emitter of transistor Q8 are connected to the GND end; that is, the signal AD enters the gate of transistor Q7 and the gate of transistor Q8 respectively, and the signal AD enters the MCU. At this time, transistor Q7 and transistor Q8 are not turned on, and the MCU receives the signal AD as a low level and does not process it. CW_PWM and WW_PWM are output normally, the H bridge works normally, and the lamp lights up normally;

[0047] When the H-bridge output (load) is not short-circuited, when the H-bridge circuit WW flows to CW, since the CW end is connected to the positive input end of the operational amplifier U1 B through the resistor R6, and the WW end is connected to the negative input end of the operational amplifier U1 B through the resistor R3, the voltage difference between the two ends of the lamp is a negative voltage, that is, Vcw-Vww<0. Substituting into the above formula, we get: V U1Bout <0; due to V U1Bout The negative terminal of comparator U2A and the positive terminal of comparator U2B are connected to the GND terminal through resistor R5, that is, the voltage of the positive terminal of comparator U2A = 0, the voltage of the negative terminal of U2B = 0. Since the voltage of the negative terminal of comparator U2A < 0, the voltage of the positive terminal of comparator U2B < 0, the voltage of the negative terminal of comparator U2A < the voltage of the positive terminal of comparator U2A, the voltage of the positive terminal of comparator U2B < the voltage of the negative terminal of comparator U2B. Since the VCC terminal is connected to the output terminal of comparator U2A through resistor R4, and the VCC terminal is connected to the output terminal of comparator U2B through resistor R8, the output terminal V of comparator U2A is U2Aout is high level, the comparator U2B output terminal V U2Bout It is low level, at this time, transistor Q5 is not conducting, transistor Q6 is conducting. Since transistor Q5, transistor Q6 and signal AD are connected together, VDD terminal enters signal AD through resistor R1, so signal AD is low level, signal AD passes through resistor R11 and enters the gate of transistor Q7 and the gate of transistor Q8 respectively, and signal AD enters MCU. At this time, transistor Q7 and transistor Q8 are not conducting, MCU receives signal AD as low level, and does not process it. CW_PWM and WW_PWM output normally, H bridge works normally, and lamps light up normally;

[0048] When the H-bridge output (load) is short-circuited, the H-bridge circuit Vcw = Vww, so the voltage difference across the lamp = 0V, that is, Vcw-Vww = 0. Substituting into the above formula, we get: V U1Bout =0; due to V U1Bout The negative terminal of comparator U2A and the positive terminal of comparator U2B are connected to the GND terminal through resistor R5, that is, the voltage of the positive terminal of comparator U2A = 0, the voltage of the negative terminal of comparator U2B = 0. Since the voltage of the negative terminal of comparator U2A = 0, the voltage of the positive terminal of comparator U2B = 0, the voltage of the negative terminal of comparator U2A < the voltage of the positive terminal of comparator U2A, the voltage of the positive terminal of comparator U2B < the voltage of the negative terminal of comparator U2B. Since the VCC terminal is connected to the output terminal of comparator U2A through resistor R4, and the VCC terminal is connected to the output terminal of comparator U2B through resistor R8, the output terminal V of comparator U2A is U2Aout is high level, the output terminal V U2BoutIt is high level, at this time, transistor Q5 is not conducting, transistor Q6 is not conducting. Since transistor Q5, transistor Q6 and signal AD are connected together, VDD terminal enters signal AD through resistor R1, so signal AD is high level, signal AD passes through resistor R11 and enters the gate of transistor Q7 and the gate of transistor Q8 respectively, and signal AD enters MCU. At this time, transistor Q7 and transistor Q8 are turned on, and CW_PWM and WW_PWM are pulled down at the same time, so that the H bridge stops working and the lamp does not light up; MCU receives signal AD as high level, stops outputting CW_PWM and WW_PWM, and reports to the system for processing;

[0049] ③The advantages of the utility model are: the above structure can realize rapid protection of the H-bridge, avoid long-term short-circuit output malfunction, and also avoid damage to the H-bridge circuit. It can also report to the MCU circuit system for further processing, and has the advantages of simple circuit connection, small number of components, low cost, fast response and stable performance.

[0050] Of course, the present invention is not limited to the above-mentioned embodiments, and technicians familiar with the field may make equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications and substitutions are all included in the scope defined by the claims of this application.

Claims

1. A short circuit detection circuit for an H-bridge two-wire output LED dimming power supply, characterized in that: It comprises an EMI filter circuit (10), an AC-DC circuit (20), an H-bridge two-line output dimming circuit (30) and a short-circuit protection circuit (40); The input end of the EMI filter circuit (10) is connected to an external power supply; The input end of the AC-DC circuit (20) is connected to the output end of the EMI filter circuit (10); The H-bridge two-line output dimming circuit (30) is connected between the output end of the AC-DC circuit (20) and the lamp; The short-circuit protection circuit (40) is connected between the H-bridge two-line output dimming circuit (30) and the lamp; The H-bridge two-line output dimming circuit (30) can cut off the dimming signal output to the lamp when the short-circuit protection circuit (40) detects that the lamp is short-circuited.

2. The short-circuit detection circuit of an H-bridge two-wire output LED dimming power supply according to claim 1, characterized in that: The short-circuit protection circuit (40) comprises a differential ratio operation module (41), a comparison module (42) and a control module (43); The first input end of the differential ratio operation module (41) is connected to one end of the lamp, and the second output end is connected to the other end of the lamp, so as to generate a differential signal at both ends of the lamp; The input end of the comparison module (42) is connected to the output end of the differential ratio operation module (41); The control module (43) is connected between the H-bridge two-line output dimming circuit (30) and the output end of the comparison module (42); The control module (43) can control the short-circuit protection circuit (40) to turn on or off the dimming signal output to the lamp according to the signal output by the comparison module (42).

3. The short-circuit detection circuit of the H-bridge two-wire output LED dimming power supply according to claim 2, characterized in that: The differential ratio operation module (41) comprises resistors R2-3, resistors R6-7 and an operational amplifier U1B, wherein the resistor R3 is respectively connected to one end of the lamp and the reverse input end of the operational amplifier U1B, one end of the resistor R6 is connected to the other end of the lamp, the other end of the resistor R6 is connected to the forward input end of the operational amplifier U1B and is connected to the GND end via the resistor R7, the resistor R2 is connected between the reverse input end of the operational amplifier U1B and the output end of the operational amplifier U1B, and the output end of the operational amplifier U1B is connected to the input end of the comparison module (42).

4. The short-circuit detection circuit of the H-bridge two-wire output LED dimming power supply according to claim 2, characterized in that: The comparison module (42) comprises a comparator U2A, a comparator U2B, a resistor R1, a resistor R4, a resistor R8 and transistors Q5-6, the negative end of the comparator U2A is respectively connected to the positive end of the comparator U2B and the output end of the differential ratio operation module (41), the positive end of the comparator U2A is respectively connected to the negative end of the comparator U2B and the GND end, the power supply end of the comparator U2A is connected to the VCC end, and the output end of the comparator U2A is connected to the base of the transistor Q5. The output terminal of the comparator U2B is connected to the base of the transistor Q6 and connected to the VCC terminal via the resistor R4; the emitter of the transistor Q5 is respectively connected to one end of the resistor R1, the emitter of the transistor Q6, the input end of the control module (43) and the H-bridge two-line output dimming circuit (30); the other end of the resistor R1 is connected to the VDD terminal; the collector of the transistor Q5 and the collector of the transistor Q6 are connected to the GND terminal; the output end of the comparator U2B is connected to the base of the transistor Q6 and connected to the VCC terminal via the resistor R8.

5. The short-circuit detection circuit of the H-bridge two-wire output LED dimming power supply according to claim 2, characterized in that: The control module (43) comprises a transistor Q7 and a transistor Q8, the collector of the transistor Q7 and the collector of the transistor Q8 are respectively connected to the H-bridge two-line output dimming circuit (30), the base of the transistor Q7 and the base of the transistor Q8 are connected to the output end of the comparison module (42), and the emitter of the transistor Q7 and the emitter of the transistor Q8 are connected to the GND end.

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