Switching power supply starting protection circuit
The power difference is calculated by the input detection and status detection modules, the power difference is judged by the voltage comparison module, the power transmission is controlled by the start control module, and the high frequency output module performs high frequency adjustment. This solves the problem that the abnormality of the input rectifier and filter circuit cannot be judged before the switching power supply starts, thus improving the safety of power supply startup.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- SHENZHEN CHUXINZHIZHI TECHNOLOGY CO LTD
- Filing Date
- 2025-06-04
- Publication Date
- 2026-05-05
AI Technical Summary
In the prior art, switching power supplies cannot determine whether the input rectifier and filter circuit is abnormal before startup, which leads to abnormal startup power and reduces circuit safety.
The input detection module and the status detection module calculate the power difference, the voltage comparison module determines the relationship between the power difference and the set threshold, the start control module controls the power transmission and the high-frequency output module performs high-frequency adjustment, and the self-locking module stops the detection work.
It improves the safety of switching power supply startup, reduces the impact of power anomalies, and enhances circuit safety.
Smart Images
Figure CN224204989U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of switching power supply technology, specifically a switching power supply startup protection circuit. Background Technology
[0002] A switching power supply consists of an input rectifier and filter circuit, a power conversion circuit, an output rectifier and filter circuit, and a control circuit. It achieves high-frequency voltage regulation. In the existing technology, to avoid overcurrent during startup, current-limiting startup control is applied to the power output of the input rectifier and filter circuit. However, before the switching power supply starts up, it is impossible to know whether the abnormal power is caused by the input rectifier and filter circuit, and this can easily lead to abnormal startup power, reducing the safety of the circuit. Therefore, improvements are needed. Utility Model Content
[0003] This utility model provides a switching power supply startup protection circuit to solve the problems mentioned in the background art.
[0004] To achieve the above objectives, this utility model provides the following technical solution:
[0005] A startup protection circuit for a switching power supply includes: a power supply module, an input detection module, a status detection module, a voltage comparison module, a self-locking module, a startup control module, and a high-frequency output module.
[0006] The power module is used to transform, rectify, and filter the incoming AC power to output the first electrical energy.
[0007] The input detection module is connected to the power supply module and is used to step down, rectify, and divide AC power for sampling and outputting the first detection signal.
[0008] The status detection module, connected to the power supply module and the input detection module, is used to receive the first electrical energy, perform voltage division sampling on the first electrical energy, calculate the voltage difference between the first detection signal and the sampled signal, and output the second detection signal.
[0009] The voltage comparison module, connected to the status detection module, is used to output a first start signal when the second detection signal is greater than a set second voltage threshold and less than a set first voltage threshold.
[0010] The self-locking module is connected to the voltage comparison module, the input detection module, the power supply module and the status detection module. It is used to receive the first electrical energy and perform self-locking processing on the first start signal, output the second start signal and control the status detection module to stop receiving the first electrical energy, and control the input detection module to stop working.
[0011] The start control module is connected to the power supply module, the high-frequency output module and the self-locking module, and is used to transfer the first electrical energy to the high-frequency output module when the second start signal is received.
[0012] The high-frequency output module is used to perform overvoltage protection and high-frequency voltage regulation of the first input electrical energy and then output it.
[0013] As a further embodiment of this utility model: the power supply module includes a power interface, a power regulating device, and a first capacitor; the start-up control module includes an eighth resistor, a second power transistor, and a third switching transistor; the high-frequency output module includes a second capacitor, a tenth resistor, a third diode, a ninth resistor, a second diode, a first transformer, a power regulating device, a fourth diode, and an output port;
[0014] Preferably, the first and second terminals of the power interface are respectively connected to the first and second terminals of the power regulating device. The third terminal of the power regulating device is connected to one end of the first capacitor and the source of the second power transistor, and is connected to the gate of the second power transistor and the collector of the third switching transistor through an eighth resistor. The emitter of the third switching transistor is connected to the other end of the first capacitor, the fourth terminal of the power regulating device, the ground terminal of the power regulating device, and the ground terminal. The drain of the second power transistor is connected to one end of the second capacitor and the first end of the primary side of the first transformer, and is connected to the other end of the second capacitor, the cathode of the third diode, and one end of the ninth resistor through a tenth resistor. The other end of the ninth resistor is connected to the cathode of the second diode. The anode of the third diode is connected to the second end of the primary side of the first transformer and the control terminal of the power regulating device. The first end of the secondary side of the first transformer is connected to the anode of the fourth diode. The cathode of the fourth diode is connected to the feedback terminal of the power regulating device and one end of the output port. The second end of the secondary side of the first transformer is connected to the other end of the output port.
[0015] As a further embodiment of this utility model: the input detection module includes a first relay switch, a first resistor, a third capacitor, a first rectifier, a second resistor, and a third resistor;
[0016] Preferably, the first stationary terminal and the second stationary terminal of the first relay switch are respectively connected to the first terminal and the second terminal of the power interface; the first moving terminal of the first relay switch is connected to one terminal of the third capacitor and connected to the other terminal of the third capacitor and the second terminal of the first rectifier through the first resistor; the second moving terminal of the first relay switch is connected to the first terminal of the first rectifier; the third terminal of the first rectifier is connected to the first terminal of the third resistor through the second resistor; and the second terminal of the third resistor is connected to the fourth terminal of the first rectifier and the ground terminal.
[0017] As a further embodiment of this utility model: the state detection module includes a fourth resistor, a fifth resistor, a subtraction device, a first power transistor, a sixth resistor, and a first switching transistor;
[0018] Preferably, one end of the fourth resistor is connected to the drain of the first power transistor and the third terminal of the power regulating device, and is connected to the gate of the first power transistor and the collector of the first switching transistor through the sixth resistor. The other end of the fourth resistor is connected to the second input terminal of the subtraction device, and is connected to the emitter of the first switching transistor and the ground terminal through the fifth resistor. The first input terminal of the subtraction device is connected to the first terminal of the third resistor. The power supply terminal of the subtraction device is connected to the source of the first power transistor. The output terminal of the subtraction device is connected to the voltage comparison module.
[0019] As a further embodiment of this utility model: the self-locking module includes a seventh resistor, a first diode, and a first logic chip; the input detection module also includes a first relay and a second switching transistor;
[0020] Preferably, the B terminal of the first logic chip is connected to one end of the first relay and the third terminal of the power regulating device through a seventh resistor. The other end of the first relay is connected to the collector of the second switching transistor. The emitter of the second switching transistor is grounded. The base of the second switching transistor is connected to the Y terminal of the first logic chip, the base of the third switching transistor, the anode of the first diode, and the base of the first switching transistor. The A terminal of the first logic chip is connected to the cathode of the first diode and the voltage comparison module.
[0021] As a further embodiment of this utility model: the voltage comparison module includes a first comparator, a second comparator, a first reference power supply, a second reference power supply, and a second logic chip;
[0022] Preferably, the inverting input of the first comparator is connected to the non-inverting input of the second comparator and the output of the subtraction device; the non-inverting input of the first comparator and the inverting input of the second comparator are respectively connected to the first reference power supply and the second reference power supply; the output of the first comparator and the output of the second comparator are respectively connected to the A and B terminals of the second logic chip; and the Y terminal of the second logic chip is connected to the A terminal of the first logic chip.
[0023] Compared with the prior art, the beneficial effects of this utility model are as follows: Before startup, the switching power supply startup protection circuit of this utility model can calculate the difference between the electrical energy input to the power supply module and the output of the power supply module by the input detection module and the status detection module. The voltage comparison module determines whether the power supply module is processing electrical energy normally by comparing the detected electrical energy difference with the voltage magnitude of the set voltage threshold. When the power supply module is normal, the startup control module transmits electrical energy and the high-frequency output module performs high-frequency regulation processing, controlling the input detection module and the status detection module to stop detection work, reducing the impact on electrical energy and improving startup safety. Attached Figure Description
[0024] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the description of the embodiments of this utility model will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0025] Figure 1 This is a schematic block diagram of a switching power supply startup protection circuit provided for an example of this utility model.
[0026] Figure 2 A circuit diagram of a switching power supply startup protection circuit provided for this utility model embodiment.
[0027] Figure 3 A connection circuit diagram of the voltage comparison module provided for this utility model embodiment. Detailed Implementation
[0028] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.
[0029] In one embodiment, see Figure 1 A switching power supply startup protection circuit includes: a power module 1, an input detection module 2, a status detection module 3, a voltage comparison module 4, a self-locking module 5, a startup control module 6, and a high-frequency output module 7.
[0030] Specifically, power module 1 is used to transform, rectify and filter the incoming AC power and output the first power.
[0031] Input detection module 2, connected to power module 1, is used to step down, rectify and divide AC power for sampling and outputting the first detection signal;
[0032] The status detection module 3 is connected to the power supply module 1 and the input detection module 2. It is used to receive the first electrical energy, perform voltage division sampling on the first electrical energy, calculate the voltage difference between the first detection signal and the sampled signal, and output the second detection signal.
[0033] The voltage comparison module 4 is connected to the status detection module 3 and is used to output a first start signal when the second detection signal is greater than the set second voltage threshold and less than the set first voltage threshold.
[0034] The self-locking module 5 is connected to the voltage comparison module 4, the input detection module 2, the power supply module 1, and the status detection module 3. It is used to receive the first electrical energy and perform self-locking processing on the first start signal, output the second start signal and control the status detection module 3 to stop receiving the first electrical energy, and control the input detection module 2 to stop working.
[0035] The start control module 6 is connected to the power supply module 1, the high-frequency output module 7 and the self-locking module 5, and is used to transmit the first electrical energy to the high-frequency output module 7 when the second start signal is received.
[0036] The high-frequency output module 7 is used to perform overvoltage protection and high-frequency voltage regulation of the input first electrical energy and then output it.
[0037] In a specific embodiment, the power supply module 1 can be a power circuit composed of a power interface, a power regulating device, and a capacitor, which can perform voltage transformation, rectification, and filtering on the input AC power; the input detection module 2 can be an input detection circuit composed of relays, resistors, rectifiers, etc., which can perform voltage reduction, rectification, and voltage division on the input AC power, and control the detection state; the state detection module 3 can be a state detection circuit composed of resistors, subtraction devices, field-effect transistors, etc., which can perform voltage division, subtraction calculation, and subtraction operation control; the voltage comparison module 4 can be a voltage comparison circuit composed of a comparator, a reference power supply, and a diode, which can set a first voltage threshold and a second voltage threshold, and compare and subtract... The operating state of power module 1 is determined by comparing the calculated signal with the voltage magnitudes of the first and second voltage thresholds, where the first voltage threshold is the maximum voltage difference between the input and output of power module 1, and the second voltage threshold is the minimum voltage difference between the input and output of power module 1. The self-locking module 5 can be a self-locking circuit composed of logic chips, diodes, and resistors to perform self-locking processing on the input high-level signal. The startup control module 6 can be a startup control circuit composed of field-effect transistors, resistors, and transistors to control the power transmission state. The high-frequency output module 7 can be a high-frequency output circuit composed of transformers, power conditioning devices, diodes, etc., to perform high-frequency voltage regulation and overvoltage protection.
[0038] In another embodiment, please refer to Figure 1 , Figure 2 and Figure 3 The power module 1 includes a power interface, a power regulating device, and a first capacitor C1; the start control module 6 includes an eighth resistor R8, a second power transistor Q2, and a third switching transistor V3; the high-frequency output module 7 includes a second capacitor C2, a tenth resistor R10, a third diode D3, a ninth resistor R9, a second diode D2, a first transformer B1, a power regulating device, a fourth diode D4, and an output port.
[0039] Specifically, the first and second terminals of the power interface are connected to the first and second terminals of the power regulating device, respectively. The third terminal of the power regulating device is connected to one end of the first capacitor C1 and the source of the second power transistor Q2, and is connected to the gate of the second power transistor Q2 and the collector of the third switching transistor V3 through the eighth resistor R8. The emitter of the third switching transistor V3 is connected to the other end of the first capacitor C1, the fourth terminal of the power regulating device, the ground terminal of the power regulating device, and the ground terminal. The drain of the second power transistor Q2 is connected to one end of the second capacitor C2 and the first end of the primary side of the first transformer B1, and is connected to the other end of the second capacitor C2, the cathode of the third diode D3, and one end of the ninth resistor R9 through the tenth resistor R10. The other end of the ninth resistor R9 is connected to the cathode of the second diode D2. The anode of the third diode D3 is connected to the second end of the primary side of the first transformer B1 and the control terminal of the power regulating device. The first end of the secondary side of the first transformer B1 is connected to the anode of the fourth diode D4. The cathode of the fourth diode D4 is connected to the feedback terminal of the power regulating device and one end of the output port. The second end of the secondary side of the first transformer B1 is connected to the other end of the output port.
[0040] In a specific embodiment, the power regulation device can be composed of a transformer, a rectifier, and a filter; the second power transistor Q2 can be a P-channel MOSFET; the third switching transistor V3 can be an NPN transistor; the ninth resistor R9 and the second diode D2 provide overvoltage protection; the second capacitor C2, the tenth resistor R10, and the third diode D3 provide peak voltage absorption; the power regulation device can be composed of a PWM power drive chip and a feedback circuit composed of an optocoupler and a TL431.
[0041] Furthermore, the input detection module 2 includes a first relay switch K1-1, a first resistor R1, a third capacitor C3, a first rectifier T1, a second resistor R2, and a third resistor R3;
[0042] Specifically, the first stationary terminal and the second stationary terminal of the first relay switch K1-1 are respectively connected to the first and second terminals of the power interface. The first moving terminal of the first relay switch K1-1 is connected to one end of the third capacitor C3 and, through the first resistor R1, to the other end of the third capacitor C3 and the second terminal of the first rectifier T1. The second moving terminal of the first relay switch K1-1 is connected to the first terminal of the first rectifier T1. The third terminal of the first rectifier T1 is connected to the first terminal of the third resistor R3 through the second resistor R2. The second terminal of the third resistor R3 is connected to the fourth terminal of the first rectifier T1 and the ground terminal.
[0043] In a specific embodiment, the first relay switch K1-1 can be a normally closed double-pole single-throw switch; the third capacitor C3 and the first resistor R1 are used for voltage reduction.
[0044] Furthermore, the status detection module 3 includes a fourth resistor R4, a fifth resistor R5, a subtraction device, a first power transistor Q1, a sixth resistor R6, and a first switching transistor V1;
[0045] Specifically, one end of the fourth resistor R4 is connected to the drain of the first power transistor Q1 and the third terminal of the power regulating device, and is connected to the gate of the first power transistor Q1 and the collector of the first switching transistor V1 through the sixth resistor R6. The other end of the fourth resistor R4 is connected to the second input terminal of the subtraction device, and is connected to the emitter of the first switching transistor V1 and the ground terminal through the fifth resistor R5. The first input terminal of the subtraction device is connected to the first terminal of the third resistor R3. The power supply terminal of the subtraction device is connected to the source of the first power transistor Q1. The output terminal of the subtraction device is connected to the voltage comparison module 4.
[0046] In a specific embodiment, the subtraction device may consist of an operational amplifier and a resistor; the first power transistor Q1 may be an N-channel MOSFET; and the first switching transistor V1 may be an NPN transistor.
[0047] Furthermore, the self-locking module 5 includes a seventh resistor R7, a first diode D1, and a first logic chip J1; the input detection module 2 also includes a first relay K1 and a second switch V2;
[0048] Specifically, the B terminal of the first logic chip J1 is connected to one end of the first relay K1 and the third terminal of the power regulating device through the seventh resistor R7. The other end of the first relay K1 is connected to the collector of the second switching transistor V2. The emitter of the second switching transistor V2 is grounded. The base of the second switching transistor V2 is connected to the Y terminal of the first logic chip J1, the base of the third switching transistor V3, the anode of the first diode D1, and the base of the first switching transistor V1. The A terminal of the first logic chip J1 is connected to the cathode of the first diode D1 and the voltage comparison module 4.
[0049] In a specific embodiment, the first logic chip J1 can be an AND gate chip, which works with the first diode D1 and the seventh resistor R7 to perform high-level self-locking; the first relay K1 controls the first relay switch K1-1 to open by magnetic attraction; the second switch V2 can be an NPN transistor.
[0050] Furthermore, the voltage comparison module 4 includes a first comparator A1, a second comparator A2, a first reference power supply VREF1, a second reference power supply VREF2, and a second logic chip J2;
[0051] Specifically, the inverting input of the first comparator A1 is connected to the non-inverting input of the second comparator A2 and the output of the subtraction device. The non-inverting input of the first comparator A1 and the inverting input of the second comparator A2 are respectively connected to the first reference power supply VREF1 and the second reference power supply VREF2. The output of the first comparator A1 and the output of the second comparator A2 are respectively connected to the A and B terminals of the second logic chip J2. The Y terminal of the second logic chip J2 is connected to the A terminal of the first logic chip J1.
[0052] In a specific embodiment, both the first comparator A1 and the second comparator A2 can be selected as LM358 comparators; the first reference power supply VREF1 and the second reference power supply VREF2 are respectively set to a first voltage threshold and a second voltage threshold; the second logic chip J2 can be selected as an AND gate chip.
[0053] In this embodiment of a switching power supply startup protection circuit, AC power is connected through a power interface. A power regulating device transforms, rectifies, and filters the AC power and outputs a first power signal. A third capacitor C3, a first resistor R1, a first rectifier T1, a second resistor R2, and a third resistor R3 step down, rectify, and divide the AC power and output a first detection signal. A fourth resistor R4 and a fifth resistor R5 divide the first power signal. A subtraction device performs subtraction and outputs a second detection signal. The second detection signal is greater than a second voltage threshold set by the second reference power supply VREF2 and less than the first reference power supply VR. When the first voltage threshold set by EF1 is reached, the second logic chip J2 outputs a high level, i.e., the first start signal. After being latched by the first logic chip J1, the first diode D1, and the seventh diode, the second switch V2 is triggered to turn on. The first relay K1 is energized and controls the first relay switch K1-1 to open, which in turn controls the first switch V1 to turn on. The first power transistor Q1 is turned off, and the subtraction device stops working. The third switch V3 turns on and triggers the second power transistor Q2 to turn on, thus transmitting electrical energy. The power regulation device samples the electrical energy input to the output port and controls the first transformer B1 to perform high-frequency voltage regulation to supply power to the output port.
[0054] It will be apparent to those skilled in the art that this invention is not limited to the details of the exemplary embodiments described above, and that it can be implemented in other specific forms without departing from the spirit or essential characteristics of this invention. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of this invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within this invention. No reference numerals in the claims should be construed as limiting the scope of the claims.
[0055] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.
Claims
1. A startup protection circuit for a switching power supply, characterized in that, The startup protection circuit of this switching power supply includes: a power supply module, an input detection module, a status detection module, a voltage comparison module, a self-locking module, a startup control module, and a high-frequency output module; The power module is used to transform, rectify, and filter the incoming AC power to output the first electrical energy. The input detection module is connected to the power supply module and is used to step down, rectify, and divide AC power for sampling and outputting the first detection signal. The status detection module, connected to the power supply module and the input detection module, is used to receive the first electrical energy, perform voltage division sampling on the first electrical energy, calculate the voltage difference between the first detection signal and the sampled signal, and output the second detection signal. The voltage comparison module, connected to the status detection module, is used to output a first start signal when the second detection signal is greater than a set second voltage threshold and less than a set first voltage threshold. The self-locking module is connected to the voltage comparison module, the input detection module, the power supply module and the status detection module. It is used to receive the first electrical energy and perform self-locking processing on the first start signal, output the second start signal and control the status detection module to stop receiving the first electrical energy, and control the input detection module to stop working. The start control module is connected to the power supply module, the high-frequency output module and the self-locking module, and is used to transfer the first electrical energy to the high-frequency output module when the second start signal is received. The high-frequency output module is used to perform overvoltage protection and high-frequency voltage regulation of the first input electrical energy and then output it.
2. The switching power supply startup protection circuit according to claim 1, characterized in that, The power module includes a power interface, a power regulating device, and a first capacitor; the start-up control module includes an eighth resistor, a second power transistor, and a third switching transistor; the high-frequency output module includes a second capacitor, a tenth resistor, a third diode, a ninth resistor, a second diode, a first transformer, a power regulating device, a fourth diode, and an output port. The first and second terminals of the power interface are respectively connected to the first and second terminals of the power regulating device. The third terminal of the power regulating device is connected to one end of the first capacitor and the source of the second power transistor, and is connected to the gate of the second power transistor and the collector of the third switching transistor through the eighth resistor. The emitter of the third switching transistor is connected to the other end of the first capacitor, the fourth terminal of the power regulating device, the ground terminal of the power regulating device, and the ground terminal. The drain of the second power transistor is connected to one end of the second capacitor and the first end of the primary side of the first transformer, and is connected to the other end of the second capacitor, the cathode of the third diode, and one end of the ninth resistor through the tenth resistor. The other end of the ninth resistor is connected to the cathode of the second diode. The anode of the third diode is connected to the second end of the primary side of the first transformer and the control terminal of the power regulating device. The first end of the secondary side of the first transformer is connected to the anode of the fourth diode. The cathode of the fourth diode is connected to the feedback terminal of the power regulating device and one end of the output port. The second end of the secondary side of the first transformer is connected to the other end of the output port.
3. The switching power supply startup protection circuit according to claim 2, characterized in that, The input detection module includes a first relay switch, a first resistor, a third capacitor, a first rectifier, a second resistor, and a third resistor; The first stationary terminal and the second stationary terminal of the first relay switch are respectively connected to the first terminal and the second terminal of the power interface. The first moving terminal of the first relay switch is connected to one terminal of the third capacitor and connected to the other terminal of the third capacitor and the second terminal of the first rectifier through the first resistor. The second moving terminal of the first relay switch is connected to the first terminal of the first rectifier. The third terminal of the first rectifier is connected to the first terminal of the third resistor through the second resistor. The second terminal of the third resistor is connected to the fourth terminal of the first rectifier and the ground terminal.
4. The switching power supply startup protection circuit according to claim 3, characterized in that, The status detection module includes a fourth resistor, a fifth resistor, a subtraction device, a first power transistor, a sixth resistor, and a first switching transistor; One end of the fourth resistor is connected to the drain of the first power transistor and the third terminal of the power regulating device, and is connected to the gate of the first power transistor and the collector of the first switching transistor through the sixth resistor. The other end of the fourth resistor is connected to the second input terminal of the subtraction device, and is connected to the emitter of the first switching transistor and the ground terminal through the fifth resistor. The first input terminal of the subtraction device is connected to the first terminal of the third resistor. The power supply terminal of the subtraction device is connected to the source of the first power transistor. The output terminal of the subtraction device is connected to the voltage comparison module.
5. The switching power supply startup protection circuit according to claim 4, characterized in that, The self-locking module includes a seventh resistor, a first diode, and a first logic chip; the input detection module also includes a first relay and a second switching transistor. The B terminal of the first logic chip is connected to one end of the first relay and the third terminal of the power regulating device through the seventh resistor. The other end of the first relay is connected to the collector of the second switching transistor. The emitter of the second switching transistor is grounded. The base of the second switching transistor is connected to the Y terminal of the first logic chip, the base of the third switching transistor, the anode of the first diode, and the base of the first switching transistor. The A terminal of the first logic chip is connected to the cathode of the first diode and the voltage comparison module.
6. The switching power supply startup protection circuit according to claim 5, characterized in that, The voltage comparison module includes a first comparator, a second comparator, a first reference power supply, a second reference power supply, and a second logic chip; The inverting input of the first comparator is connected to the non-inverting input of the second comparator and the output of the subtraction device. The non-inverting input of the first comparator and the inverting input of the second comparator are respectively connected to the first reference power supply and the second reference power supply. The outputs of the first comparator and the second comparator are respectively connected to the A and B terminals of the second logic chip. The Y terminal of the second logic chip is connected to the A terminal of the first logic chip.