A control method of a switching circuit and a switching circuit

Abstract

This invention discloses a control method and a switching circuit for a switching circuit. During the shutdown process of the switching circuit, the control terminals of the power switching transistor and the synchronous switching transistor are controlled, and the orientation of the body diode of the synchronous switching transistor is controlled based on the relationship between the input and output voltages. This ensures that the inductor current always has a freewheeling path during shutdown and prevents voltage spikes. Furthermore, after the main circuit stops working, its input and output terminals are completely blocked, providing timely and effective protection even in the event of a circuit fault, resulting in high reliability.

Description

Technical Field

[0001] This invention relates to the field of switching power supplies, and more specifically, to a control method and a switching circuit for a switching circuit. Background Technology

[0002] The schematic diagram of the synchronous rectification boost circuit in the prior art is as follows: Figure 1 As shown, the power stage circuit of the boost circuit includes an inductor L1, a power switch M1 (taking an NMOS switch as an example) and a synchronous switch M2 (taking a PMOS switch as an example), an input capacitor C1 and an output capacitor C2. The two ends of the inductor L1 are connected to the positive terminal of the DC input voltage VIN and the drain of the NMOS switch M1, respectively. The source of the NMOS switch M1 is grounded. The drain of the PMOS switch M2 is connected to the drain of the NMOS switch M1. The source of the PMOS switch M2 is connected to the positive terminal of the output voltage VOUT. The negative terminal of the output voltage VOUT is grounded. The switching states of the switches M1 and M2 are controlled by PWM signals to control the output voltage VOUT.

[0003] However, the aforementioned boost circuit has the following drawbacks: First, at system power-on, the voltage difference between the input voltage VIN and the output voltage VOUT is relatively large. The input voltage VIN charges the output capacitor C2 through inductor L1 and the body diode of the PMOS switch M2. This large input voltage can generate inrush current, potentially damaging circuit components and reducing system reliability. Second, the circuit can only control the output voltage VOUT to be higher than the input voltage VIN during operation. Third, when the system stops working, the presence of the body diode of the PMOS switch M2 prevents the low-impedance loop from the input to the output from being completely disconnected. If a fault occurs (e.g., an output short circuit), the current will become uncontrollable, threatening system safety. Summary of the Invention

[0004] In view of this, the present invention proposes a control method and a switching circuit for a switching circuit. By setting the connection method of the body diode of the synchronous switching transistor and controlling the turn-on voltage of the synchronous switching transistor, the problems existing in the switching circuits of the prior art can be solved.

[0005] According to the control method of the switching circuit of the present invention, the switching circuit includes an inductor, a power switch, and a synchronous switch. The inductor is connected between the input voltage of the switching circuit and the common connection point of the power switch and the synchronous switch. The shutdown process of the switching circuit includes the following steps:

[0006] The control terminal of the power switching transistor is grounded;

[0007] When the power is off, the output voltage is higher than the input voltage, and the control terminal of the synchronous switch is grounded. When the inductor current drops to zero, if the output voltage is greater than the input voltage, the control terminal of the synchronous switch is connected to the output voltage, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the common connection point to the output terminal. Alternatively, when the inductor current drops to zero, if the output voltage is less than the input voltage, the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the output terminal to the common connection point.

[0008] When the power is off, the output voltage is lower than the input voltage, and the control terminal of the synchronous switch is connected to the input voltage.

[0009] Furthermore, when the gate of the control synchronous switch is connected to a DC input voltage, the synchronous switch operates in saturation until the inductor current drops to zero, at which point the synchronous switch is turned off.

[0010] A switching circuit includes an inductor, a power switch, and a synchronous switch. The inductor is connected between the input voltage of the switching circuit and the common connection point of the power switch and the synchronous switch. The synchronous switch has a body diode comprising a first diode and a second diode, with the cathodes of the first and second diodes connected together. The anode of the first diode is connected to the output terminal, and the anode of the second diode is connected to the common connection point of the power switch and the synchronous switch. The power-off process of the switching circuit includes:

[0011] The control terminal of the power switching transistor is grounded;

[0012] When the power is off, the output voltage is higher than the input voltage, and the control terminal of the synchronous switch is grounded. When the inductor current drops to zero, if the output voltage is greater than the input voltage, the control terminal of the synchronous switch is connected to the output voltage, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the common connection point to the output terminal. Alternatively, when the inductor current drops to zero, if the output voltage is less than the input voltage, the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the output terminal to the common connection point.

[0013] When the power is off, the output voltage is lower than the input voltage, and the control terminal of the synchronous switch is connected to the input voltage.

[0014] Furthermore, when the gate of the control synchronous switch is connected to a DC input voltage, the synchronous switch operates in saturation until the inductor current drops to zero, at which point the synchronous switch is turned off.

[0015] Furthermore, the switching circuit further includes a first switching transistor, which is connected in parallel across the first diode.

[0016] Furthermore, the switching circuit further includes a second switching transistor, which is connected in parallel across the two ends of the second diode.

[0017] Furthermore, the switching circuit further includes a third switching transistor and a fourth switching transistor, wherein the third switching transistor is connected in parallel across the first diode and the fourth switching transistor is connected in parallel across the second diode.

[0018] In summary, according to the control method and switching circuit of the present invention, during the shutdown process of the switching circuit, the control terminals of the power switching transistor and the synchronous switching transistor are controlled, and the orientation of the body diode of the synchronous switching transistor is controlled in conjunction with the relationship between the input voltage and the output voltage. This ensures that the inductor current always has a freewheeling path during the shutdown process and prevents voltage spikes. Furthermore, after the main circuit stops working, its input and output terminals are completely blocked, providing timely and effective protection even in the event of a circuit fault, resulting in high reliability. Attached Figure Description

[0019] Figure 1 The diagram shown is a schematic of a synchronous rectification switching circuit in the prior art.

[0020] Figure 2a The diagram shows a first implementation of a switching circuit for reducing inrush current in the prior art;

[0021] Figure 2b The diagram shows a second implementation of a switching circuit for reducing inrush current in the prior art.

[0022] Figure 2c The figure shows a third implementation of a switching circuit for reducing inrush current in the prior art;

[0023] Figure 3a The diagram shows a first implementation of an input / output blocking switching circuit in the prior art;

[0024] Figure 3b The diagram shows a second implementation of an input / output blocking switching circuit in the prior art;

[0025] Figure 4a The diagram shows a first implementation of the switching circuit according to the present invention;

[0026] Figure 4b The diagram shows a second implementation of the switching circuit according to the present invention;

[0027] Figure 4c The diagram shows a third implementation of the switching circuit according to the present invention;

[0028] Figure 4dThe diagram shows a fourth implementation of the switching circuit according to the present invention;

[0029] Figure 5a The diagram shown is a first type of circuit control diagram during the operation of the switching circuit according to the present invention;

[0030] Figure 5b The diagram shown is a second type of circuit control diagram during the operation of the switching circuit according to the present invention;

[0031] Figure 6 The diagram shown is a schematic diagram of the power-off working principle of the switching circuit according to the present invention.

[0032] Figure 7a The diagram shows a flowchart of a control method for the startup process of a switching circuit according to the present invention.

[0033] Figure 7b The diagram shows a flowchart of a control method for the shutdown process of a switching circuit according to the present invention. Detailed Implementation

[0034] Some preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention is not limited thereto.

[0035] As shown in the background section, traditional boost circuits suffer from inrush current. To reduce the damage caused by inrush current to circuit components, the existing technology offers the following solutions:

[0036] 1) Connect bypass diode D1 between the input terminal VIN and the output terminal VOUT of the boost circuit, such as... Figure 2a As shown, when the input voltage VIN is applied, most of the inrush current can be charged to the output capacitor C2 through the bypass diode D1, thereby reducing damage to the components in the main circuit.

[0037] 2) Add a current-limiting resistor during system startup, such as... Figure 2b As shown, the current-limiting resistor R1 is connected in series between the input terminal VIN and the inductor L, and the time-delay switch M... DELAY It is connected in parallel with the current-limiting resistor R1. At the moment the input voltage VIN is powered on, the time-delay switch M... DELAY When in the off state, the current-limiting resistor R1 is connected in series in the main power circuit, and the input voltage VIN charges the output capacitor C2 through the current-limiting resistor R1. After a certain delay, the delay switch M is controlled. DELAY When the circuit is turned on, the current-limiting resistor R1 is short-circuited and no longer functions, thereby reducing damage to components in the main circuit.

[0038] 3) Add a current limiting control loop, such as Figure 2cAs shown, during the system startup phase, the current and voltage detection unit detects the voltage drop and current of the synchronous rectifier transistor M2, and controls the gate voltage of the power switch transistor M1 through a loop to control the current of M1. When the voltage drop of the power switch transistor M1 is large, the output capacitor C2 is charged with a small constant current; as the output voltage rises, the voltage drop of the power switch transistor M1 drops to a certain threshold, and then switches to a larger output current until the output voltage VOUT exceeds the input voltage VIN, at which point it switches to the normal boost mode.

[0039] However, all three methods mentioned above have drawbacks: adding a bypass diode between the input and output of the boost circuit does not reduce the startup inrush current; it only diverts most of the inrush current, thus requiring a diode with a relatively large rated current, which increases system cost. Adding a current-limiting resistor and a time-delay switch also increases system cost, and after the startup process ends, the time-delay switch remains connected in series in the main power circuit, inevitably leading to system efficiency loss. Adding a current-limiting control circuit can solve the startup inrush problem, but it requires an additional feedback control loop, complicating the system, and the output can only handle a relatively light load during startup.

[0040] To address the challenge of achieving complete output-input isolation in traditional boost circuits, existing technologies typically employ the following measures:

[0041] 1) Connect a switching transistor M in series at the input terminal VIN or the output terminal VOUT. SCP ,like Figure 3a Or the switching transistor M shown in Figure 3b SCP As shown, when it is necessary to control the boost circuit to stop working, the switching transistor M is switched off. SCP Shutdown, due to M SCP The body diode is in the opposite direction to the body diode of the synchronous switching transistor in the boost circuit, thus enabling complete shutdown of the circuit input and output;

[0042] 2) Connect a resettable fuse F1 in series at the circuit input or output. When a system fault occurs, the increased current flowing through the resettable fuse generates heat, causing the fuse to trip and providing protection. Figure 3a Or as shown in F1 of 3b;

[0043] However, the above solutions also have shortcomings: the system cost is high because additional switching transistors or resettable fuses are required; or the system efficiency is low because the switching transistors or resettable fuses need to be connected in series in the main power circuit during normal circuit operation; in addition, since resettable fuses protect the circuit through the principle of thermal effect, the protection speed is relatively slow and it is not easy to achieve precise control, which reduces the reliability of the system.

[0044] Therefore, based on the above research, the inventors proposed a switching circuit, such as... Figure 4a , Figure 4b , Figure 4c and Figure 4d The diagram shown is a circuit diagram of a first embodiment of a switching circuit according to the present invention. Figure 4a , Figure 4b , Figure 4c and Figure 4d As shown, the switching circuit includes an inductor L1, a power switch M1 (taking an NMOS switch as an example), and a synchronous switch M2 (taking a PMOS switch as an example). The inductor L1 is connected to the positive terminal of the DC input voltage VIN of the switching circuit and the common connection point SW of the power switch M1 and the synchronous switch M2. The other end of the power switch M1 is connected to the negative terminal of the input voltage VIN, and the other end of the synchronous switch is connected to the positive terminal of the output voltage VOUT of the switching circuit. The negative terminal of the output voltage VOUT is grounded. The input capacitor CIN is connected in parallel across the DC input voltage, and the output capacitor COUT is connected in parallel across the output voltage.

[0045] like Figure 4a As shown, the body diode of the synchronous switch includes a first diode D1 and a second diode D2. The cathodes of the first diode D1 and the second diode D2 are connected. The anode of the first diode D1 is connected to the output terminal, and the anode of the second diode D2 is connected to the common connection point SW of the power switch and the synchronous switch. The connection point of the body diode of the synchronous switch is denoted as the B terminal. In this embodiment, the B terminal of the substrate of the synchronous switch is left floating.

[0046] according to Figure 4a In the circuit structure, when the switching circuit is started, the output voltage VOUT is zero, and the output voltage VOUT is less than the input voltage VIN. At this time, the base of the body diode of the synchronous switch can be left floating. The control terminal (gate) of the synchronous switch M2 is connected to the DC input voltage VIN, and the control terminal (gate) of the power switch M1 receives the PWM control signal, such as... Figure 5aThe diagram shows the gate connection of the power switch and synchronous switch. The gate-source voltage of power switch M1 is greater than its turn-on voltage. When power switch M1 is on, the voltage at the common connection point SW is pulled down to zero. At this time, the output voltage VOUT is obviously less than the DC input voltage VIN. The gate of synchronous switch M2 has the highest potential relative to its source and drain. Therefore, synchronous switch M2 is turned off, the current in inductor L1 rises, and power switch M1 is turned on for a certain period of time or when the inductor current rises to a set reference, at which point M1 is turned off. When power switch M1 is turned off, since the inductor current flows from the input terminal to the common connection point SW, the voltage at the common connection point SW rises. When the voltage at the common connection point SW rises to a level that causes the gate-source voltage VOUT of synchronous switch M2 to drop to zero... GS Below its turn-on voltage V TH When, i.e., V GS <V TH V TH This is the turn-on voltage of the synchronous switch M2. When the synchronous switch M2 is turned on, it's easy to understand why, since the synchronous switch M2 is a PMOS switch, the voltage V... GS and voltage V TH Both are negative voltages. Then the output voltage VOUT starts to rise. In order to keep the synchronous switch M2 on, the voltage at the point of common coupling SW should be at least higher than the DC input voltage VIN by the turn-on voltage V. TH The magnitude of the voltage is given. At this time, since the output voltage VOUT is still less than the input voltage VIN, it can be known that the synchronous switch M2 is working in saturation. The voltage drop of the synchronous switch M2 is approximately VIN + VTH + VDSAT - VOUT, where VDSAT is the saturation voltage of the synchronous switch M2. The inductor current decreases, and when the inductor current decreases to the set reference value, the control power switch M1 is turned on, and then the above process is repeated.

[0047] Subsequently, when the output voltage VOUT rises to be equal to or greater than the input voltage, the base (B) of the body diode of the synchronous switch M2 is left floating. The control terminals of both the power switch M1 and the synchronous switch M2 receive the PWM control signal. The gate connection methods of the power switch and the synchronous switch are as follows: Figure 5b As shown, the power switch M1 and the synchronous switch M2 are switched on and off alternately, and the circuit enters the normal boost operation mode.

[0048] As can be seen from the above-described startup operating mode, the switching circuit of the present invention can cause the output voltage VOUT to rise from zero during startup, and the current during startup is controlled, so it will not cause impact on the circuit and will not damage the components in the circuit. Based on the above circuit structure, the embodiment of the present invention adopts PWM control, which does not require an additional feedback loop, making it simple and reliable to implement, and enabling the output terminal to start with full load.

[0049] refer to Figure 4b This is a circuit diagram according to a second embodiment of a switching circuit of the present invention. Figure 4a In this embodiment, a first switching transistor S1 is added, which is connected in parallel across the first diode D1. Similarly, during circuit startup, when the output voltage VOUT is less than the input voltage VIN, the first switching transistor S1 is turned off; when the output voltage VOUT is greater than the input voltage VIN, the first switching transistor S1 remains off. Figure 4b The connection method of the control terminals of the power switch and synchronous switch in the embodiment shown is the same as Figure 4a The processes are all the same; the working process of this embodiment is the same as that of the previous one during startup. Figure 4a The same applies, so I won't elaborate further. Relative to... Figure 4a In contrast, connecting the first switching transistor in parallel across the first diode D1 can improve system efficiency and reduce the likelihood of latch-up.

[0050] refer to Figure 4c This is a circuit diagram according to a third embodiment of a switching circuit of the present invention. This embodiment is... Figure 4a In this embodiment, a second switch S2 is added, which is connected in parallel across the second diode D2. Similarly, during circuit startup, when the output voltage VOUT is less than the input voltage VIN, the second switch S2 is closed; when the output voltage VOUT is greater than the input voltage VIN, the second switch S2 is opened. Figure 4c The connection method of the control terminals of the power switch and synchronous switch in the embodiment shown is the same as Figure 4a The processes are all the same; the working process of this embodiment is the same as that of the previous one during startup. Figure 4a The same applies, so I won't go into details here.

[0051] Figure 4d This is a circuit diagram according to a fourth embodiment of a switching circuit of the present invention. Figure 4a In this embodiment, a third switch S3 and a fourth switch S4 are added. The third switch S3 is connected in parallel across the first diode D1, and the fourth switch S4 is connected in parallel across the second diode D2. Similarly, during circuit startup, when the output voltage VOUT is less than the input voltage VIN, the third switch S3 is turned off and the fourth switch S4 is turned on; when the output voltage VOUT is greater than the input voltage VIN, the third switch S3 is turned on and the fourth switch S4 is turned off. Figure 4d The connection method of the control terminals of the power switch and synchronous switch in the embodiment shown is the same as Figure 4aThe processes are all the same; the working process of this embodiment is the same as that of the previous one during startup. Figure 4a The same applies, so I won't go into details here.

[0052] Figure 5a The diagram shows the connection method of the control terminals of power switch M1 and synchronous switch M2 when the output voltage VOUT is less than the input voltage VIN, as well as the control method of the body diode of synchronous switch M2. The dashed line indicates that when the output voltage VOUT < the input voltage VIN, the switch connected in parallel with the body diode of synchronous switch M2 is optional. Figure 5b The diagram shows the connection method of the control terminals of power switch M1 and synchronous switch M2 when the output voltage VOUT is greater than the input voltage VIN, as well as the control method of the body diode of synchronous switch M2. The dashed line indicates that the switch transistor connected in parallel with the body diode of synchronous switch M2 is optional when the output voltage VOUT ≥ the input voltage VIN. According to... Figure 5a and 5b As can be seen from the embodiments, when the output voltage is less than the input voltage, the direction of the body diode of the synchronous switch is from the output terminal to the common connection point of the power switch and the synchronous switch; when the output voltage is equal to or greater than the input voltage, the direction of the body diode of the synchronous switch is from the common connection point of the power switch and the synchronous switch to the output terminal. Through the direction control of the body diode of the synchronous switch in this embodiment, the output voltage VOUT can be made to rise from zero during startup of the switching circuit, and the current during startup is controlled, preventing impact on the circuit and damage to the components.

[0053] The above describes the operation of the switching circuit according to the present invention during startup. During normal operation, if it is necessary to control the output voltage VOUT to be higher than the input voltage VIN, the circuit operates in the conventional boost mode. Figure 5b As shown, if it is necessary to control the output voltage VOUT to be lower than the input voltage VIN, then the circuit operating mode is the same as... Figure 5a Similarly, the switching circuit of the present invention can operate in a state where the output voltage is higher than the input voltage, or in a state where the output voltage is lower than the input voltage.

[0054] Finally, when the switching circuit needs to enter the stop working state, the control terminal (gate) of the power switch M1 is first grounded. Then, when the output voltage VOUT is higher than the input voltage VIN at the power-off moment, the gate of the synchronous switch M2 is grounded first, and the inductor current freewheels through the synchronous switch M2, causing the inductor current to decrease. When the inductor current drops to zero, if the output voltage VOUT > the DC input voltage VIN, the gate of the synchronous switch M2 is connected to the output voltage VOUT. The connection method of the base diode (B) of the synchronous switch M2 is the same as... Figure 5b Similarly; when the inductor current drops to zero, if the output voltage VOUT < DC input voltage VIN, then the gate of synchronous switch M2 is connected to the input voltage VIN, and the base of the body diode of synchronous switch M2 is connected to... Figure 5a resemblance.

[0055] If the output voltage VOUT of the switching circuit is lower than the input voltage VIN at the time of power-off, the gate of the synchronous switch M2 is connected to the DC input voltage VIN, the synchronous switch M2 operates in saturation, and the synchronous switch M2 is turned off after the inductor current drops to zero.

[0056] The shutdown working principle of the switching circuit is as follows: Figure 6 As shown, the dashed line indicates that the parallel connection of the body diode of the synchronous switch M2 is optional in the two cases of output voltage VOUT < input voltage VIN and output voltage VOUT ≥ input voltage VIN. MAX indicates that the larger value between the output voltage VOUT and the DC input voltage VIN is selected as the gate signal of the synchronous switch M2 after the inductor current drops to zero. From the above description of the shutdown control circuit process, it can be seen that if the inductor current direction at the shutdown moment is from the input terminal to the common connection point SW, the inductor current will freewheel through the synchronous switch M2 to the output voltage VOUT. When the inductor current drops to zero, the synchronous switch M2 is completely turned off, and the main circuit stops working. If the inductor current direction at the shutdown moment is from the common connection point SW to the input terminal, the inductor current will freewheel through the body diode of the power switch M1, causing the inductor current to drop to zero, and the main circuit stops working. Therefore, the control method and switching circuit of the present invention ensure that the inductor current always has a freewheeling path during the system shutdown process, and no voltage spikes are generated. After the main circuit stops working, its input and output terminals are completely blocked. Even if a circuit fault occurs, such as an output short circuit or overload, the system can be protected in a timely and effective manner because the circuit input and output are completely blocked, resulting in high reliability.

[0057] refer to Figure 7a The control method for the startup process of the switching circuit according to the present invention includes the following steps: during the startup phase of the switching circuit, when the output voltage is less than the input voltage, the body diode of the synchronous switch is left floating or the direction of the body diode of the synchronous switch is controlled to point from the output terminal to the common connection point of the power switch and the synchronous switch. The control terminal of the power switch receives a PWM control signal, and the control terminal of the synchronous switch receives the input voltage signal.

[0058] When the output voltage is equal to or greater than the input voltage, the body diode of the synchronous switch is left floating or the direction of the body diode of the synchronous switch is controlled to point from the common connection point of the power switch and the synchronous switch to the output terminal. The control terminals of the power switch and the synchronous switch both receive the PWM control signal.

[0059] refer to Figure 7b A control method for the shutdown process of a switching circuit according to the present invention includes the step of: when the switching circuit stops working,

[0060] The control terminal of the power switch transistor is grounded.

[0061] If the output voltage is greater than the input voltage at this time, the control terminal of the synchronous switch receives the output voltage signal, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled to point from the common connection point of the power switch and the synchronous switch to the output terminal.

[0062] If the output voltage is less than the input voltage, the control terminal of the synchronous switch receives the input voltage signal, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled to point from the output terminal to the common connection point of the power switch and the synchronous switch.

[0063] The control method and switching circuit of the switching circuit according to the preferred embodiment of the present invention have been described in detail above. However, the circuit and beneficial effects of this patent should not be considered as being limited to what has been described above. The disclosed embodiments and drawings can better understand the present invention. Therefore, the disclosed embodiments and the description and drawings above are for better understanding of the present invention. The protection of the present invention is not limited to the scope of this disclosure. All substitutions and modifications to the embodiments of the present invention by those skilled in the art are within the protection scope of the present invention.

Claims

1. A control method for a switching circuit, the switching circuit comprising an inductor, a power switch, and a synchronous switch, the inductor being connected between the input voltage of the switching circuit and the common connection point of the power switch and the synchronous switch, characterized in that, The power-off process of the switching circuit includes the following steps: The control terminal of the power switching transistor is grounded; When the power is off, the output voltage is higher than the input voltage, and the control terminal of the synchronous switch is grounded. When the inductor current drops to zero, if the output voltage is greater than the input voltage, the control terminal of the synchronous switch is connected to the output voltage, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the common connection point to the output terminal. Alternatively, when the inductor current drops to zero, if the output voltage is less than the input voltage, the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the output terminal to the common connection point. When the power is off, the output voltage is lower than the input voltage, and the control terminal of the synchronous switch is connected to the input voltage.

2. The control method as described in claim 1, characterized in that, When the gate of the control synchronous switch is connected to a DC input voltage, the synchronous switch operates in saturation until the inductor current drops to zero, at which point the synchronous switch is turned off.

3. A switching circuit, comprising an inductor, a power switch, and a synchronous switch, wherein the inductor is connected between the input voltage of the switching circuit and the common connection point of the power switch and the synchronous switch, characterized in that, The body diode of the synchronous switch includes a first diode and a second diode, the cathodes of the first diode and the second diode are connected, the anode of the first diode is connected to the output terminal, and the anode of the second diode is connected to the common connection point of the power switch and the synchronous switch. The power-off process of the switching circuit includes: The control terminal of the power switching transistor is grounded; When the power is off, the output voltage is higher than the input voltage, and the control terminal of the synchronous switch is grounded. When the inductor current drops to zero, if the output voltage is greater than the input voltage, the control terminal of the synchronous switch is connected to the output voltage, and the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the common connection point to the output terminal. Alternatively, when the inductor current drops to zero, if the output voltage is less than the input voltage, the body diode of the synchronous switch is either left floating or the direction of the body diode of the synchronous switch is controlled from the output terminal to the common connection point. When the power is off, the output voltage is lower than the input voltage, and the control terminal of the synchronous switch is connected to the input voltage.

4. The switching circuit as described in claim 3, characterized in that, When the gate of the control synchronous switch is connected to a DC input voltage, the synchronous switch operates in saturation until the inductor current drops to zero, at which point the synchronous switch is turned off.

5. The switching circuit as described in claim 3, characterized in that, The switching circuit further includes a first switching transistor, which is connected in parallel across the first diode.

6. The switching circuit as described in claim 3, characterized in that, The switching circuit further includes a second switching transistor, which is connected in parallel across the two ends of the second diode.

7. The switching circuit as described in claim 3, characterized in that, The switching circuit further includes a third switching transistor and a fourth switching transistor, wherein the third switching transistor is connected in parallel across the two ends of the first diode, and the fourth switching transistor is connected in parallel across the two ends of the second diode.

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