Bridge type constant current source clamping circuit, alternating current SSPC and control method

Abstract

The invention discloses a bridge type constant current source clamping circuit, an alternating current SSPC and a control method. The clamping circuit comprises a full-bridge circuit, a constant current source circuit and a one-way power switch. Two alternating-current input ends of the full-bridge circuit are respectively connected with a power input end and a power ground of the alternating-current solid-state power controller, and the constant-current source circuit and the one-way power switch are connected in series between two direct-current output ends of the full-bridge circuit; the control input end of the one-way power switch is connected with a control instruction. The bridge type constant current source clamping circuit not only provides a path for leakage current, but also greatly reduces equivalent resistance of a load side, and realizes constant current discharge of a capacitive load while reducing voltage of the load side.

Description

Technical Field

[0001] The invention relates to a bridge type constant current source clamping circuit, an AC SSPC and a control method, and belongs to the fields of power electronics technology and electrical engineering technology. Background Art

[0002] The Solid State Power Controller (SSPC) uses power semiconductor devices as switches and integrates the switch control function of a relay and the protection function of a circuit breaker. It has the advantages of fast response, no arc, no mechanical contacts, low on-state power consumption, strong load adaptability, and high reliability. It is the key core of advanced solid-state power distribution systems.

[0003] In aviation solid-state power distribution systems, AC SSPC has microampere leakage current in the off state. When connected to a capacitive load, the leakage current will cause the voltage on the load side to continue to accumulate. When unloaded, a "virtual voltage" close to the power supply voltage will even appear on the load side. As the system voltage level increases, it will endanger the normal operation of the system and personnel safety. Therefore, it is of great significance to study the leakage current problem of AC SSPC and its solution.

[0004] The conventional solution is to connect a bleeder resistor in parallel on the load side. R When the AC SSPC is turned off, the discharge resistor is connected to the load side. This method reduces the equivalent resistance on the load side and consumes the energy of the capacitor on the discharge resistor. There is a current spike during the initial discharge. R The instantaneous power consumption is the largest, and then the discharge current decays exponentially. In terms of clamping time, the natural discharge of the capacitor depends on the power resistor, and the discharge current decays exponentially. The discharge time is more than 100ms, which is a long time. In terms of safety, due to the instability of the discharge current, there is a current spike during the initial discharge, and the safety of the discharge process is relatively low. In terms of volume and weight, since the energy is consumed in the discharge resistor, the volume of the resistor is relatively large, resulting in a large total volume and weight of the power device.

[0005] In order to further shorten the capacitor discharge time, reduce the volume and weight of power devices, and improve the safety of AC SSPC after shutdown, it is necessary to conduct in-depth research on the clamping circuit of AC SSPC. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a bridge constant current source clamping circuit, AC SSPC and control method to address the defects and shortcomings in the aforementioned background, which can not only clamp the load voltage to a lower safety range, but also provide an energy release channel for the load capacitor, and make the energy release time controllable.

[0007] In order to solve the above technical problems, an embodiment of the present application first provides a bridge constant current source clamping circuit, which is applied to an AC solid-state power controller, and is characterized in that it includes a full-bridge circuit, a constant current source circuit and a unidirectional power switch; the two AC input ends of the full-bridge circuit are respectively connected to the power input end and the power ground of the AC solid-state power controller, and the constant current source circuit and the unidirectional power switch are connected in series between the two DC output ends of the full-bridge circuit; the control input end of the unidirectional power switch is connected to a control instruction.

[0008] The embodiment of the present application also provides a main power circuit, a clamping circuit and a clamping circuit control logic circuit, characterized in that the clamping circuit includes a full-bridge circuit, a constant current source circuit and a unidirectional power switch; the two AC input ends of the full-bridge circuit are respectively connected to the power input end and the power ground of the AC solid-state power controller, and the constant current source circuit and the unidirectional power switch are connected in series between the two DC output ends of the full-bridge circuit; the control input end of the unidirectional power switch is connected to a control instruction output by the clamping circuit control logic circuit.

[0009] Preferably, the main power circuit includes a first power tube and a second power tube connected in anti-series, the drain of the first power tube is connected to an AC power supply as a power input end of an AC solid-state power controller, the drain of the second power tube is connected to a clamping circuit and a load as a power output end of the AC solid-state power controller, the gate of the first power tube and the source of the second power tube are connected to the same analog ground, and the gate of the first power tube and the gate of the second power tube are connected to different switching instructions.

[0010] Preferably, the clamp circuit control logic circuit includes an NOR gate, an OR gate and an RS trigger, the two input ends of the NOR gate are respectively connected to the switch instructions of the first power tube and the second power tube, the four input ends of the OR gate are respectively connected to the switch instructions of the first power tube and the second power tube, and the positive and negative logic signals of the load current direction; the output end of the NOR gate is connected to the RS trigger The output of the OR gate is connected to the RS flip-flop end, the RS flip-flop The terminal outputs the control instruction.

[0011] The embodiment of the present application also provides a control method of the above-mentioned AC solid-state power controller, which is characterized by comprising: When the AC solid-state power controller is turned on: in the positive half cycle of the AC power supply voltage, the second power tube is turned on first, and the first power tube is turned on after the voltage passes through zero, and the unidirectional power switch is turned off; in the negative half cycle of the AC power supply voltage, the first power tube is turned on first, and the second power tube is turned on after the voltage passes through zero, and the unidirectional power switch is turned off; When the AC solid-state power controller is turned off: in the positive half cycle of the load current, the second power tube is turned off first, and the first power tube is turned off after the current passes through zero, and the unidirectional power switch is turned on; in the negative half cycle of the load current, the first power tube is turned off first, and the second power tube is turned off after the current passes through zero, and the unidirectional power switch is turned on.

[0012] The present invention has the following beneficial effects: 1. The clamping circuit of the present invention can reduce the "virtual voltage" on the load side caused by the leakage current when the AC solid-state power controller is unloaded to within a safe range; 2. The clamping circuit of the present invention provides an energy release path for the capacitive load, preventing the existence of leakage current from causing continuous accumulation of voltage on the load side. The energy discharge current of the capacitive load is stable, without current spikes, and the discharge current and discharge time are controllable; 3. The present invention is simple to control and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is an AC SSPC circuit diagram based on a bridge constant current source clamp circuit; Figure 2 It is a depletion type constant current source circuit diagram; Figure 3 It is the control logic circuit diagram of the clamp circuit; Figure 4 This is the overall control timing diagram of the AC SSPC based on the bridge constant current source clamp circuit; Figure 5 It is the working timing diagram of the bridge constant current source clamp circuit; Figure 6 It is the working circuit diagram when the load is unloaded and the SSPC is in the steady-state off condition; Figure 7 is the operating circuit diagram of the capacitive load and SSPC in steady-state off condition. DETAILED DESCRIPTION

[0014] Some key technologies involved in the present invention are described in detail below in conjunction with the drawings to support the claims; in order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the drawings and embodiments.

[0015] Embodiment 1 This embodiment provides a bridge type constant current source clamping circuit, and provides an AC solid-state power controller based on the bridge type constant current source clamping circuit.

[0016] The bridge constant current source clamp circuit of this embodiment is applied to an AC solid-state power controller. Figure 1 As shown, the bridge constant current source clamp circuit of this embodiment includes a full bridge circuit, a constant current sourceI CCS and unidirectional power switch S aux The AC side of the full-bridge circuit is connected in parallel with the load, and the DC side is connected with the constant current source. I CCS , unidirectional power switch S aux Specifically, the two AC input terminals of the full-bridge circuit are respectively connected to the power output terminals of the AC solid-state power controller. Powerout and power ground PGND , a constant current source is connected in series between the two DC output terminals of the full-bridge circuit I CCS and unidirectional power switch S aux .

[0017] The full bridge circuit of this embodiment is composed of the first to fourth rectifier diodes (D 1 ~D 4 ) is composed of the first rectifier diode D 1 and the second rectifier diode D 2 The first bridge arm is formed in series, and the third rectifier diode D 3 and the fourth rectifier diode D 4 The two bridge arms are connected in series to form the second bridge arm, and then the two bridge arms are connected in parallel to form a complete bridge structure. 1 The cathode and the second rectifier diode D 2 The anode connection end is used as the positive pole of the AC input end of the full-bridge circuit and is connected to the power output end of the AC solid-state power controller. Powerout The third rectifier diode D 3 Anode and fourth rectifier diode D 4 The cathode connection end is used as the negative terminal of the AC input of the full-bridge circuit and is connected to the power ground of the AC solid-state power controller. PGND The first rectifier diode D 1 Anode and fourth rectifier diode D 4 The anode connection end is used as the negative DC output terminal of the full-bridge circuit and is connected to the analog ground AGND. 2 The third rectifier diode D 3 The cathode and the second rectifier diode D 2 The cathode connection end is used as the positive DC output terminal of the full bridge circuit + V DD .

[0018] like Figure 2 As shown, the constant current source of this embodiment I CCS By depletion mode N-MOSFET S 1 and negative feedback resistor R Composition, negative feedback resistor R The unidirectional power switch S of this embodiment is an adjustable resistor.aux It is an enhancement mode N-MOSFET. Depletion mode N-MOSFETS 1 The drain is connected to +V DD , source and negative feedback resistor R One end of the negative feedback resistor is connected R The other end is connected to the depletion mode N-MOSFET S 1 The gate and unidirectional power switch S aux The source of the unidirectional power switch S aux The gate is used as the control input of the clamp circuit to connect the control command CMD aux , unidirectional power switch S aux The drain of the 2 .

[0019] like Figure 1 The AC SSPC based on the bridge constant current source clamp circuit of this embodiment is composed of an AC power supply V s The AC SSPC power circuit consists of three parts: the main power circuit, the above-mentioned bridge constant current source clamping circuit and the load. The AC power supply is a 115V AC source, and the load is composed of a resistor and a capacitor in parallel. Figure 3 The clamp circuit shown controls the logic circuit.

[0020] The main power circuit of the AC SSPC consists of the first power tube S upp , the second power tube S low Two identical N-channel enhancement-type Si MOSFETs are connected in anti-series. upp The drain of the AC solid-state power controller is connected to the AC power supply as the power input terminal, and the second power tube S low The drain of the AC solid-state power controller is connected to the clamping circuit and the load as the power output terminal. upp The gate of the second power tube S low The source is connected to the same analog ground AGND 1 , the first power tube S upp The gate of the first switch command CMD is connected upp , the second power tube S low The gate of the second switch command CMD is connected low .

[0021] The constant current source of this embodiment is as follows: when the depletion type N-MOSFET S 1 When working in the saturation region, the drain current I D for:

[0022] in, μ 0 is the electron mobility, C ox is the gate oxide capacitance per unit area, W is the channel width, L is the effective channel length; V GS and V TH They represent the gate-source voltage and threshold voltage of the MOS tube respectively.

[0023] Depletion-mode N-MOSFET S 1 Gate-source voltage zero bias ( V GS =0V), the drain current I DSS for:

[0024] Adding adjustable resistor R After that, the depletion-mode N-MOSFET S 1 The drain current I D It can be expressed as:

[0025] Combining the above two formulas, we get:

[0026] Therefore, the depletion-mode N-MOSFET S 1 Drain Current I D The drain current when the gate-source voltage is zero biased I DSS , threshold voltage V TH and adjustable resistor R After determining the specific depletion-mode MOSFET device, the drain current I D The adjustable resistor R The only thing that is certain is adjustable resistance. R The calculation expression for the value is:

[0027] At the same time, the adjustable resistance between the gate and source of the depletion-mode MOSFET R It has negative feedback function, which can effectively suppress the V DD Therefore, the depletion-type N-MOSFET S 1The current source formed by the adjustable resistor R has the advantages of wide allowable input voltage range and high output current accuracy.

[0028] like Figure 3 The clamp circuit control logic circuit of this embodiment includes an OR gate 1 , an OR gate 2 RS flip-flop composed of two NAND gates, and NOR gate 1 The two input terminals are connected to the first switch command CMD upp , the second switch command CMD low , OR gate 2 The four input terminals are connected to the first switch command CMD upp , the second switch command CMD low , and load current direction positive and negative logic signals I + and I -. I +、 I - are logic signals indicating the direction of load current, high level indicates valid, I + indicates the current direction is from the power input terminal of the AC SSPC Powerin Flow to power output Powerout , I - indicates the opposite direction of current. 1 The output of the RS flip-flop is connected to Terminal, OR gate 2 The output of the RS flip-flop is connected to The RS flip-flop Output clamp circuit control command CMD aux .

[0029] The following table is the function table of the clamp circuit control logic circuit:

[0030] Figure 4 This is the overall control timing diagram of the AC SSPC based on the bridge constant current source clamp circuit. The control timing diagram corresponds to the power supply voltage V s In the negative half cycle, the AC SSPC is turned on, and the load current I load In the positive half cycle, the AC SSPC is turned off, t 0 ~ t 2 Corresponding to the opening process of AC SSPC, t 3 ~ t 5 Corresponding to the shutdown process of AC SSPC,t 5 Then the bridge constant current source clamp circuit works.

[0031] t 0 :The main switch command CMD of AC SSPC becomes high, AC SSPC is turned on, due to the power supply voltage V s In the negative half cycle, the first power tube S of the main power circuit is turned on first. upp However, due to the second power tube S of the main power circuit low It has not been opened yet, and the main power circuit is still in the reverse cut-off state; t 1 : Supply voltage V s When entering the positive half cycle, the second power tube S of the main power circuit low The body diode and the first power tube S of the main power circuit that has been turned on upp The channel forms a forward path to achieve the change of load current from zero; t 2 : Turn on the second power tube S of the main power circuit low , the AC SSPC enters the steady-state conduction state; t 3 :The total switch command CMD of AC SSPC becomes low, AC SSPC is turned off, and the load current I load In the positive half cycle, first turn off the second power tube S of the main power circuit low , the second power tube S of the main power circuit low Current switches from the MOSFET channel to the body diode, and the load current continues; t 4 : Load current I load When entering the negative half cycle, the AC SSPC is in the negative cut-off state, and the current naturally crosses zero and shuts off; t 5 : Turn off the first power tube S of the main power circuit upp , the AC SSPC enters a stable off state with bidirectional cutoff, the load voltage is negative, and the control instruction CMD of the clamping circuit aux Go high, unidirectional power switch S aux Open, the loop current flows from the power ground PGND Through the third rectifier diode D 3 , constant current source I CSS , the first rectifier diode D1 Flow to the power output Powerout , and the load voltage gradually decreases.

[0032] The following table summarizes the control logic of the AC SSPC zero-crossing switch.

[0033]

[0034] The AC solid-state power controller based on the bridge constant current source clamping circuit of the present invention, when the AC solid-state power controller is turned on, the unidirectional power switch S aux When the AC solid-state power controller is turned off, the unidirectional power switch S aux Open, the clamp circuit is turned on, the clamp circuit is connected in series with the load, and the load capacitance ( C load ) The energy stored in the load resistor is dissipated in R load ). This circuit can provide a path for the leakage current of the AC solid-state power controller and realize constant current discharge of the capacitive load. Since the equivalent resistance of the clamping circuit is much smaller than the equivalent resistance of the AC solid-state power controller when it is turned off, the voltage division value on the load side caused by the leakage current after turning off the AC solid-state power controller can be controlled within 1V, and the discharge time is reduced to within 10ms, which effectively improves the safety of the AC solid-state power controller.

[0035] Figure 5 This is the working timing diagram of the bridge constant current source clamp circuit. When the unidirectional power switch S aux When turned on, if the power output terminal of the AC solid-state power controller Powerout The voltage is higher than the power ground PGND The voltage of the clamp circuit is shown in (a); on the contrary, if the unidirectional power switch S aux When turned on, the power output terminal of the AC solid-state power controller Powerout The voltage is lower than the power ground PGND The voltage of the clamp circuit is (b).

[0036] t 0 : The main switch command CMD of AC SSPC is set low, and AC SSPC is turned off.

[0037] t 1 :Control instruction CMD of clamp circuit aux Set high level, unidirectional power switch S aux If the AC solid-state power controller is turned on, the power output Powerout The voltage is higher than the power ground PGND The voltage of the second rectifier diode D2 , the fourth rectifier diode D 4 Working, the first rectifier diode D 1 , the third rectifier diode D 3 Reverse cutoff, current I CSS +Through the second rectifier diode D 2 , the fourth rectifier diode D 4 , constant current source I CSS Clamping load side voltage V C ; If the power output terminal of the AC solid-state power controller Powerout The voltage is lower than the power ground PGND The voltage of the first rectifier diode D 1 , the third rectifier diode D 3 Working, the second rectifier diode D 2 , the fourth rectifier diode D 4 Reverse cutoff, current I CSS - through the first rectifier diode D 1 , the third rectifier diode D 3 , constant current source I CSS Clamping load side voltage V C .

[0038] Figure 6 The working circuit diagram is shown in Figure 1. The load is unloaded and the SSPC is in a steady-state off condition. Since the main power topology of the single-phase AC SSPC adopts two identical N-channel enhancement MOSFETs connected in anti-series, according to the circuit equivalent principle, the resistance can be established. R SSPC With capacitor C SSPC The equivalent two-terminal network model of the parallel SSPC, where the equivalent resistance of the SSPC in the steady-state conduction state is R SSPC It is in the milliohm level and in the megohm level in the steady-state off state.

[0039] If the clamp circuit is not added, in the SSPC control circuit, due to the presence of the load voltage sampling and conditioning circuit, that is, the differential amplifier circuit composed of the load and the operational amplifier is connected in parallel, and the differential input impedance of the operational amplifier Z diff Generally, it is 100MΩ, so the no-load is approximately equivalent to a resistive load of 100MΩ. Assuming that the equivalent resistance is 10MΩ when the SSPC main power circuit is turned off, when the load is unloaded and the SSPC is in a steady-state off condition, the voltage on the load side is:

[0040] After adding the clamp circuit, the equivalent resistance of the clamp circuit is much smaller than the equivalent resistance of the SSPC shutdown. R SSPC Therefore, the partial voltage generated by the leakage current on the load side is much lower than the power supply voltage. The specific no-load clamping voltage V C As follows: V C =2 V F + V DS_aux +( V DS_CCS + Ri leak ) (7)

[0041] In the above formula V F is the forward conduction voltage drop of a single diode in the rectifier bridge, V DS_aux is the drain-source conduction voltage drop of the enhancement MOSFET, V DS_CCS is the drain-source conduction voltage drop of the depletion-mode MOSFET, R is the negative feedback resistor in the constant current source circuit, i leak is the branch leakage current. After adding the clamping circuit, the load side voltage is much lower than the load side voltage without the clamping circuit, which greatly improves the safety of the system.

[0042] Figure 7 The working circuit diagram of the capacitive load and SSPC in steady-state off condition is shown below. (a) is the circuit without clamping circuit. For solid-state switches such as MOSFET or IGBT, these devices still have very small leakage current (in the order of μA) even in the off state. When SSPC is turned off, this leakage current will slowly charge the capacitive load:

[0043] Although the off-leakage current of SSPC is small, the continuous charging of the capacitive load will cause the voltage to rise linearly over time. This voltage may cause harm to personnel or equipment and needs to be suppressed through circuit design or protection measures.

[0044] Figure 7 (b) is the circuit after adding the clamping circuit. Ignoring the current of the capacitive load, the voltage on the load side is: V C =2 V F + V DS_aux +(V DS_CCS + Ri leak ) (9)

[0045] in V F is the forward conduction voltage drop of a single diode in the rectifier bridge, V DS_aux is the drain-source conduction voltage drop of the enhancement MOSFET, V DS_CCS is the drain-source conduction voltage drop of the depletion-mode MOSFET, R is the negative feedback resistor in the constant current source circuit, i leak is the branch leakage current.

[0046] When the power MOSFET is turned on, its drain-source voltage is usually maintained at the millivolt level; in the off state, the leakage current is only in the microampere level. By introducing a clamping circuit and setting a negative feedback resistor of tens of ohms, the load-side voltage can be effectively limited to a few volts. Compared with the case without adding a clamping circuit, after adding a clamping circuit, the load-side voltage can remain basically stable, avoiding the continuous accumulation of "virtual voltage" caused by leakage current. This design not only ensures system reliability, but also eliminates potential safety hazards, effectively protecting the safety of personnel and equipment.

[0047] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A bridge constant current source clamp circuit, applied to an AC solid-state power controller, characterized in that: It includes a full-bridge circuit, a constant current source circuit and a unidirectional power switch; the two AC input ends of the full-bridge circuit are respectively connected to the power input end and the power ground of the AC solid-state power controller, and the constant current source circuit and the unidirectional power switch are connected in series between the two DC output ends of the full-bridge circuit; the control input end of the unidirectional power switch is connected to a control instruction.

2. The bridge type constant current source clamping circuit as claimed in claim 1, characterized in that: The constant current source circuit includes a depletion-type N-MOSFET and an adjustable resistor, the drain of the depletion-type N-MOSFET is connected to the positive output end of the two DC output ends of the full-bridge circuit, the source of the depletion-type N-MOSFET is connected to one end of the adjustable resistor, and the other end of the adjustable resistor is connected to the gate of the depletion-type N-MOSFET and the unidirectional power switch.

3. The bridge type constant current source clamping circuit as claimed in claim 1, characterized in that: The unidirectional power switch is an enhancement type N-MOSFET, the gate of the enhancement type N-MOSFET is connected to the control instruction, the source of the enhancement type N-MOSFET is connected to the constant current source circuit, and the drain of the enhancement type N-MOSFET is connected to the negative output terminal of the two DC output terminals of the full-bridge circuit.

4. An AC solid-state power controller, comprising a main power circuit, a clamping circuit and a clamping circuit control logic circuit, characterized in that: The clamping circuit includes a full-bridge circuit, a constant current source circuit and a unidirectional power switch; the two AC input ends of the full-bridge circuit are respectively connected to the power input end and the power ground of the AC solid-state power controller, and the constant current source circuit and the unidirectional power switch are connected in series between the two DC output ends of the full-bridge circuit; the control input end of the unidirectional power switch is connected to a control instruction output by the clamping circuit control logic circuit.

5. The AC solid-state power controller according to claim 4, characterized in that: The constant current source circuit includes a depletion-type N-MOSFET and an adjustable resistor, the drain of the depletion-type N-MOSFET is connected to the positive output end of the two DC output ends of the full-bridge circuit, the source of the depletion-type N-MOSFET is connected to one end of the adjustable resistor, and the other end of the adjustable resistor is connected to the gate of the depletion-type N-MOSFET and the unidirectional power switch.

6. The AC solid-state power controller according to claim 4, characterized in that: The unidirectional power switch is an enhancement type N-MOSFET, the gate of the enhancement type N-MOSFET is connected to the control instruction, the source of the enhancement type N-MOSFET is connected to the constant current source circuit, and the drain of the enhancement type N-MOSFET is connected to the negative output terminal of the two DC output terminals of the full-bridge circuit.

7. The AC solid-state power controller according to any one of claims 4 to 6, characterized in that: The main power circuit includes a first power tube and a second power tube connected in anti-series, the drain of the first power tube being connected to an AC power supply as a power input end of an AC solid-state power controller, the drain of the second power tube being connected to a clamping circuit and a load as a power output end of the AC solid-state power controller, the gate of the first power tube and the source of the second power tube being connected to the same analog ground, and the gate of the first power tube and the gate of the second power tube being connected to different switching instructions.

8. The AC solid-state power controller according to claim 7, characterized in that: The clamp circuit control logic circuit includes an NOR gate, an OR gate and an RS trigger. The two input ends of the NOR gate are respectively connected to the switch instructions of the first power tube and the second power tube. The four input ends of the OR gate are respectively connected to the switch instructions of the first power tube and the second power tube, as well as the positive and negative logic signals of the load current direction. The output end of the NOR gate is connected to the RS trigger. The output of the OR gate is connected to the RS flip-flop end, the RS flip-flop The terminal outputs the control instruction.

9. The control method of the AC solid-state power controller according to claim 7, characterized in that: include: When the AC solid-state power controller is turned on: in the positive half cycle of the AC power supply voltage, the second power tube is turned on first, and the first power tube is turned on after the voltage passes through zero, and the unidirectional power switch is turned off; in the negative half cycle of the AC power supply voltage, the first power tube is turned on first, and the second power tube is turned on after the voltage passes through zero, and the unidirectional power switch is turned off; When the AC solid-state power controller is turned off: in the positive half cycle of the load current, the second power tube is turned off first, and the first power tube is turned off after the current passes through zero, and the unidirectional power switch is turned on; in the negative half cycle of the load current, the first power tube is turned off first, and the second power tube is turned off after the current passes through zero, and the unidirectional power switch is turned on.

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