Self-adaptive soft switching control method for cyclic wave conversion type high-frequency chain inverter

Abstract

The invention provides a full-range soft switching implementation method of a cyclic wave conversion type high-frequency chain inverter, and belongs to the technical field of power electronics. On the basis of adopting secondary side modulation, the M1, M2, M3 and M4 are enabled to realize full-range zero-voltage turn-on by controlling self-adaptive dead time and device switching time; the M5, the M6, the M7 and the M8 realize full-range zero-voltage turn-on and zero-current turn-off; the M9 and the M11 (M10 and M12) realize full-range zero-voltage turn-on and zero-current turn-off; and the M10 and the M12 (M9 and M11) realize full-range zero-current turn-off. Compared with other soft switching control, the full-range soft switching implementation method of the cycle conversion type high-frequency chain inverter has the advantages that switching loss is effectively reduced, conduction loss of a semiconductor device is further reduced, and accordingly energy conversion efficiency is remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a soft switching method of a cycloconversion type high frequency link inverter. Background Art

[0002] As the requirements for volume and power density of power electronic converters are increasing, high-frequency link inverters are widely used in renewable energy grid connection, electric vehicle power systems, and distributed energy storage. Among them, the frequency conversion type high-frequency link inverter significantly reduces the volume, weight, and complexity of the system by removing the intermediate DC energy storage link, while improving the energy transmission efficiency. In addition, its single-stage power conversion structure simplifies the system design and improves the reliability and stability of the system.

[0003] However, the coordinated operation of up to 12 power switching devices in this topology leads to serious switching loss and conduction loss problems. Therefore, soft switching technology has become a key measure to reduce switching loss and improve efficiency. Existing soft switching schemes mainly rely on the traditional setting of fixed dead time and switching time to achieve zero voltage switching and zero current switching. However, in actual working conditions, due to load fluctuations and changes in conditions, the fixed time control method is difficult to achieve soft switching in the full range of working conditions and will generate large conduction losses during the redundant time. Therefore, a soft switching method that can achieve a full load range is urgently needed. Summary of the invention

[0004] Aiming at the large switching loss and conduction loss problem of the frequency conversion type high frequency link inverter, the present invention provides an adaptive soft switch control method. Based on the secondary side modulation, the method realizes full range zero voltage switching of M1, M2, M3 and M4 through adaptive dead time and switch time control; M5, M6, M7 and M8 realize full range zero voltage switching and zero current switching; M9 and M10 realize full range zero voltage switching and zero current switching; M11, M12, M13 and M14 realize full range zero voltage switching and zero current switching; M12, M14, M15, M16, M17 and M18 realize full range zero voltage switching and zero current switching; M13, M14, M15, M16, M17 and M18 realize full range zero voltage switching and zero current switching; ... 11 (M 10 、M 12 ) Achieve full range zero voltage turn-on and zero current turn-off;

[0005] M 10 、M 12 (M9, M 11 ) to achieve full range zero current shutdown. The present invention not only effectively reduces switching losses by optimizing soft switch control, but also further reduces conduction losses of semiconductor devices, thereby significantly improving energy conversion efficiency.

[0006] In order to achieve the above functions, the technical solution adopted by the present invention is: an adaptive soft switching control method for a frequency conversion type high frequency link inverter, the frequency conversion type high frequency link inverter includes a primary side full bridge circuit, an isolation transformer, a secondary side frequency conversion circuit, a filter circuit, a drive module, an isolation IC, a sampling unit and a controller, the output end of the primary side full bridge circuit is connected to the primary side of the isolation transformer, the secondary side of the isolation transformer is connected to the input end of the secondary side frequency conversion circuit, the output end of the secondary side frequency conversion circuit is connected to the input end of the filter circuit, the output end of the filter circuit is connected to the load, the sampling unit collects the voltage and current at the output end of the frequency conversion type high frequency link inverter, and connects the collected voltage and current signals to the controller through the isolation IC, and the controller is connected to the primary side full bridge circuit and the secondary side frequency conversion circuit through the isolation IC and the drive module; wherein the primary side full bridge circuit includes power switch tubes M1, M2, M3 and M4; the secondary side frequency conversion circuit includes power switch tubes M5, M6, M7, M8, M9, M10, M11, M12, M13, M14, M15, M16, M17, M18, M19, M20, M21, M22, M23, M24, M25, M26, M27, M28, M29, M30, M31, M32, M33, M34, M35, M36, M37, M38, M39, M40, M41, M42, M43, M44, M45, M46, M47, M48, M49, M50, M51, M52, M53, M54, M55 10 、M 11 and M 12 ; The filter circuit includes a filter inductor L f And filter capacitor C f ; The controller includes an ePWM dead zone control unit, an ePWM switching time control unit, a digital ePWM unit and a voltage loop control unit; the sampling unit includes an output voltage sampling unit and an output current sampling unit.

[0007] The specific steps of the adaptive soft switching method are:

[0008] Step 1: According to the voltage v between the midpoints of the two bridge arms of the primary full-bridge circuit AB The polarity of the isolation transformer primary leakage inductance current i p The direction and the voltage v between the midpoints of the two bridge arms of the secondary side cycloconversion circuit ab The polarity combination of the switch tube in the primary side full-bridge circuit is divided into nine modes, the mode conversion boundary conditions are defined, and the circuit parameters i under each mode are established by Laplace transform. p , Isolation transformer secondary leakage current i s 、v AB and v ab Time domain analytical model of

[0009] Step 2: Establish the commutation time model of the frequency conversion high-frequency link inverter and optimize M9, M 10 、M 11 、M 12 ZCS shutdown condition;

[0010] Step 3: Based on the time domain analytical model of step 1, predict the optimal dead time t1 for M1, M2, M3 and M4 to achieve ZVS opening, and use polynomial fitting and exponential fitting to establish the inverter output current I o The curve about t1 is used to generate the primary side adaptive dead time curve;

[0011] Step 4: Based on the time domain analytical model in step 1, calculate the ZVS turn-on delay time t of M5, M6, M7, and M8 on and ZCS turn-off delay time t off , fitting generates I o -t on / t off Adaptive control curve of

[0012] Step 5: Based on the commutation time model in step 2, calculate M9 and M 10 、M 11 、M 12 The ZCS delay turn-off time t off1 , fitting generates I o -t off1 Adaptive ZCS shutdown control curve;

[0013] Step 6: Input the output results of Step 3, Step 4 and Step 5 into the controller, sample the inverter output current in real time, and dynamically calculate the time parameters t1 and t on ,t off and t off1 , and configure it to the ePWM dead zone control unit and the ePWM switch time control unit to dynamically adjust the dead zone time of M1, M2, M3 and M4, the switch time of M5, M6, M7, M8, and the switch time of M9, M 10 、M 11 、M 12 The turn-off time is shortened to achieve full-range adaptive soft switching control.

[0014] The present invention provides an adaptive soft switching control method for a cycloconversion type high frequency link inverter, wherein the nine modes in step 1 are:

[0015] Mode I: v AB ≠±V in 、i p >0,v ab >0;

[0016] Mode II: v AB =-V in 、i p >0,v ab >0;

[0017] Mode III: v AB ≠±Vin 、i p <0,v ab >0;

[0018] Mode IV: v AB =V in 、i p <0,v ab >0;

[0019] Mode V: v AB = ±V in 、i p >0,v ab =0;

[0020] Mode VI: v AB ≠±V in 、i p >0,v ab <0;

[0021] Mode VII: v AB =-V in 、i p >0,v ab <0;

[0022] Mode VIII: v AB ≠±V in 、i p <0,v ab <0;

[0023] Mode IX: v AB =V in 、i p <0,v ab <0;

[0024] Its V in is the DC power supply at the input side;

[0025] The present invention provides an adaptive soft switching control method for a cycloconversion type high frequency link inverter, wherein the conversion boundary conditions of the nine modes in step 1 are:

[0026] The nine modes can be divided into modes I-IVv ab >0, mode Vv ab = 0 and mode VI-IXv ab <0, within the dead time, the circuit first enters the state v ab >0 in mode I, and perform mode conversion according to the following rules:

[0027] S1. After mode I ends, if v AB =-V in , the circuit enters mode II, at this time i p Start to decline when i p= 0, the circuit further enters mode III; if mode I ends i p = 0 and v AB ≠-V in , the circuit directly enters mode III, this process usually occurs under light load conditions;

[0028] S2. After mode III ends, if v AB =V in , the circuit enters mode IV, at this time i p Start to rise, when i p = 0, the circuit returns to mode I; if mode III ends, i p = 0 and v AB ≠V in , the circuit returns directly to mode Ⅰ;

[0029] S3. The circuit will continue to repeat S1 and S2 until v ab drops to zero and enters the next state; if state v ab >0 when i ends p >0, the circuit enters state v ab =0, at this time i p Start to decline when i p = 0, the circuit enters state v ab <0; if state v ab >0 when i ends p ≤0, the circuit enters state v ab <0;

[0030] S4. When the circuit enters state v ab <0, the conversion law between modes is similar to the state v ab >0 is the same.

[0031] The present invention provides an adaptive soft switch control method for a cycloconversion type high frequency link inverter, wherein the step 1 is to control the primary leakage inductance current i of the isolation transformer under each mode. p , Isolation transformer secondary leakage current i s , the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit v AB The voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit is v ab The time domain analytical model is as follows:

[0032] Mode I:

[0033]

[0034] Where n is the turns ratio of the isolation transformer, C p is the parasitic capacitance of the power switch tube of the primary full-bridge circuit, C s is the parasitic capacitance of the power switch tube of the secondary side cycloconversion circuit, Io is the inverter output current, L k_eq To calculate the equivalent leakage inductance of the secondary side of the isolation transformer, L m is the magnetizing inductance of the isolation transformer, T s is the switching period of the cycloconversion high-frequency link inverter, V in is the input side DC power supply, i m is the peak value of the excitation current, i p1 is the primary leakage inductance current of the isolation transformer at the initial moment of mode I, i p1 =nI o +i m , ω1 is the first series resonant angular frequency,

[0035]

[0036] Mode II and Mode VII:

[0037]

[0038] where i p2 is the primary leakage inductance current of the isolation transformer at the initial moment of mode II or mode VII, v ab2 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode II or mode VII, A1 is the coefficient to be determined, under mode II Mode VII ω2 is the second series resonant angular frequency,

[0039] Mode III, Mode VI and Mode VIII:

[0040]

[0041] where i p3 is the primary leakage inductance current of the isolation transformer at the initial moment under mode III, mode VI or mode VIII, v AB3 is the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit at the initial moment in mode III, mode VI or mode VIII, v ab3 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment under mode III, mode VI or mode VIII, A2 and A3 are unknown coefficients, A2 = nv AB3 -v ab3 ,

[0042] Mode IV and Mode IX:

[0043]

[0044] where i p4 is the primary leakage inductance current of the isolation transformer at the initial moment of mode IV or mode IX, vab4 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode IV or mode IX;

[0045] Under mode V, the time domain analytical model is:

[0046]

[0047] where i p5 is the primary leakage inductance current of the isolation transformer at the initial moment of mode V.

[0048] The present invention provides an adaptive soft switching control method for a cyclotron type high frequency link inverter. The commutation time model of the cyclotron type high frequency link inverter is established in step 2 as follows:

[0049] Based on the steady-state operating point of the cycloconversion high-frequency link inverter, the state of each component during the commutation time is obtained;

[0050] According to the state of each component during the commutation time, the commutation time equivalent circuit of the cyclotron type high-frequency link inverter is established by Laplace transform. The leakage inductance current i of the secondary side of the isolation transformer is obtained by solving the equivalent circuit. s The time domain expression of is the commutation time model of the cycloconversion high-frequency link inverter:

[0051]

[0052] Where V in is the input DC power supply, L k_eq is the equivalent leakage inductance converted to the secondary side of the isolation transformer; n is the turns ratio of the isolation transformer.

[0053] The present invention provides an adaptive soft switching control method for a cycloconversion type high frequency link inverter, wherein step 3 is specifically as follows:

[0054] The optimal dead time t1 refers to the drain-source voltage V ds From V in To be specific, after M1 and M4 are turned off, if v AB =-V in , then V of M2 and M3 ds The minimum time required to drop to zero is t1; after M2 and M3 are turned off, if v AB =+V in , then V of M1 and M4 ds The minimum time required to drop to zero is t1;

[0055] By analyzing the output current I of different inverters o The circuit below satisfies v AB = ±V inThe modal conversion process of the conditions is analyzed, and the sum of the duration of each mode is calculated according to the time domain analytical model, so as to determine the optimal dead time t1 and fit it to generate a piecewise adaptive curve.

[0056] The present invention provides an adaptive soft switch control method for a cycloconverter type high frequency link inverter, wherein the ZVS opening delay time t on and ZCS turn-off delay time t off The method to determine is:

[0057] ZVS opening delay time t of M5, M6, M7, and M8 on Defined as the time from when the primary switch is turned off to when v ab =0, specifically: in the positive half cycle, M6 and M8 remain on, M1 and M4 are turned off, when v ab = 0, V of M7 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M5 ds drops to zero to realize ZVS opening; in the negative half cycle, M5 and M7 remain turned on, and after M1 and M4 are turned off, when v ab = 0, V of M6 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M8 ds Drop to zero to achieve ZVS opening;

[0058] ZCS shutdown delay time t of M5, M6, M7, and M8 off Defined as the time from when the primary switch is turned off to when i s =0, specifically: in the positive half cycle, after M1 and M4 are turned off, when i s = 0, the drain current i of M5 D drops to zero to realize ZCS shutdown; after M2 and M3 are turned off, when i s = 0, i of M7 D drops to zero to realize ZCS shutdown; after M1 and M4 are turned off in the negative half cycle, when i s = 0, i of M8 D drops to zero to realize ZCS shutdown; after M2 and M3 are turned off, when i s = 0, i of M6 D Drop to zero to achieve ZCS shutdown;

[0059] By analyzing the output current I of different inverters o The circuit below satisfies v ab =0 and i s = 0 condition, and calculate the sum of the duration of each mode according to the time domain analytical model to determine ton / t off .

[0060] The present invention provides an adaptive soft switch control method for a cycloconverter type high frequency link inverter, wherein the ZCS delay shutdown time t off1 The method to determine is:

[0061] M9, M 10 、M 11 、M 12 ZCS delay shutdown t off1 Defined as M9 and M 10 or M 11 and M 12 Open to i s =i o The time interval is as follows: in the positive half cycle, M9, M 10 After activation, when i s =I o When M 11 The drain current i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When the M9 i D Drop to zero to achieve ZCS shutdown; in the negative half cycle, M9, M 10 After activation, when i s =I o When M 12 i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When M 10 i D Drop to zero to achieve ZCS shutdown;

[0062] According to the commutation time model, the ZCS delayed shutdown time t off1 for:

[0063]

[0064] The present invention provides an adaptive soft switch control method for a cycloconversion type high frequency link inverter, wherein step 6 is specifically as follows:

[0065] S1. Adaptive optimal dead time curves of M1, M2, M3 and M4, adaptive soft switching curves of M5, M6, M7, M8 and M9, M 10 、M 11 、M 12 The ZCS off time toff1 The calculation formula is input into the controller;

[0066] S2. Obtain the inverter output current I through the sampling unit o and transmitted to the controller according to I o Calculate the optimal dead time t1, the ZVS opening delay time t of M5, M6, M7, and M8 in real time on and ZCS turn-off delay time t off , M9, M 10 、M 11 and M 12 ZCS turn-off delay time t off1 ;

[0067] S3. Configure t1 to the ePWM dead zone control unit to adjust the dead zone time between the complementary drive signals of the primary-side full-bridge switches M1, M2, M3 and M4 so that after the primary-side full-bridge switches are turned off, their drain-source voltage V ds It drops to zero within t1 time to achieve zero voltage turn-on in the full load range; on and t off Configured in the ePWM switch time control unit, so that the turn-on time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t on , ensure that v ab =0 to complete the zero voltage turn-on, and the turn-off time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t off , ensure i s = 0 to complete the zero current shutdown; off1 Configured to the ePWM switch time control unit, so that M9, M 10 、M 11 and M 12 The turn-off time lags behind M9 and M 10 or M 11 and M 12 Opening time t off1 , ensure i s =I o Zero current shutdown is achieved at the moment of

[0068] Compared with the prior art, the present invention has the following advantages:

[0069] (1) The present invention provides an adaptive soft switching control method for a frequency conversion type high frequency link inverter, which calculates the optimal dead time for M1, M2, M3 and M4 to achieve zero voltage switching under different load conditions according to a dead time model, and realizes dynamic control through an adaptive dead time curve. This not only ensures that the primary side of the converter achieves full range zero voltage switching, but also determines the optimal dead time, thereby avoiding problems such as performance degradation, loss increase and EMI aggravation caused by unreasonable dead time setting.

[0070] (2) The present invention provides an adaptive soft switching control method for a frequency-conversion high-frequency link inverter, which can calculate the specific conditions for M5, M6, M7, and M8 to achieve ZVS opening and ZCS closing, and optimize their switching time, which can effectively reduce switching losses and the loss of the freewheeling diode, thereby improving the overall efficiency of the converter.

[0071] (3) The present invention provides an adaptive soft switching control method for a cycloconverter type high frequency link inverter, which calculates M9 and M9 under different load conditions based on a commutation time model. 10 、M 11 and M 12 The ZCS turn-off condition is achieved and adaptive turn-off control is implemented, further reducing switching losses.

[0072] (4) The present invention provides an adaptive soft switching control method for a cycloconversion type high frequency link inverter, according to the voltage v between the midpoints of the two bridge arms of the primary side full bridge circuit. AB The polarity of the isolation transformer primary leakage inductance current i p The direction and the voltage v between the midpoints of the two bridge arms of the secondary side cycloconversion circuit ab The dead time of the primary-side switch is divided into nine modes based on the polarity combination, and the mode conversion boundary conditions are determined. An accurate dead time model is established by using Laplace transform to ensure that the output results of the model are highly accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0074] Figure 1 The invention discloses a topological connection relationship of an adaptive soft switch control method for a cycloconversion type high frequency link inverter.

[0075] Figure 2 The secondary side modulation method of the embodiment corresponds to the switching timing of the semiconductor device. Figure 3 1 is a conversion relationship diagram of nine modes within the dead time of the switch tube of the primary side full-bridge circuit of the embodiment.

[0076] Figure 4 1 is an equivalent circuit diagram corresponding to nine modes within the dead time of the embodiment.

[0077] Figure 5 Schematic diagram of primary-side adaptive dead time control in an embodiment.

[0078] Figure 6 (a) is a schematic diagram of the adaptive ZVS turn-on delay time control of M5, M6, M7, and M8 on the secondary side of the embodiment.

[0079] Figure 6 (b) is a schematic diagram of the adaptive ZCS turn-off delay time control of M5, M6, M7, and M8 on the secondary side of the embodiment.

[0080] Figure 7 Equivalent circuit diagram during the commutation time of the embodiment.

[0081] Figure 8 FIG. 4 is a block diagram of an adaptive soft switch control embodiment.

[0082] Fig. 9 This is the zero voltage turn-on simulation waveform of Example M1.

[0083] Fig.10 (a) is the zero voltage turn-on simulation waveform of embodiment M5 during the positive half cycle of the power frequency.

[0084] Fig.10 (b) is the zero voltage turn-on simulation waveform of embodiment M7 during the positive half cycle of the power frequency.

[0085] Fig.10 (c) is the zero current shutdown simulation waveform of Example M5 during the positive half cycle of the power frequency.

[0086] Fig.10 (d) is the zero current shutdown simulation waveform of Example M7 during the positive half cycle of the power frequency.

[0087] Fig.11 (a) is the positive half cycle of the power frequency, the zero voltage turn-on simulation waveform of Example M9.

[0088] Fig.11 (b) is the positive half cycle of the power frequency, embodiment M 11 Zero voltage turn-on simulation waveform.

[0089] Fig.11 (c) is the zero current shutdown simulation waveform of Example M9 during the positive half cycle of the power frequency.

[0090] Fig.11 (d) is the positive half cycle of the power frequency, embodiment M 11 Zero current shutdown simulation waveform.

[0091] Fig.12(a) is the positive half cycle of the power frequency, embodiment M 10 Zero current shutdown simulation waveform.

[0092] Fig.12 (b) is the positive half cycle of the power frequency, embodiment M 12 Zero current shutdown simulation waveform. DETAILED DESCRIPTION

[0093] The present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0094] Embodiment 1: This embodiment involves an adaptive soft switching control method for a frequency conversion type high frequency link inverter, and a method for establishing a time domain analytical model of the dead time of the power switch tube of the primary side full bridge circuit of the frequency conversion type high frequency link inverter as follows:

[0095] like Figure 2 As shown, the modulation wave of the PWM control signal is a sine wave with a frequency of 50Hz; the carrier wave is a descending triangle wave with a frequency twice the expected switching frequency f of the frequency conversion type high-frequency link inverter. s . The PWM control signal level is determined by the sinusoidal modulation wave and the triangular carrier. When the amplitude of the triangular carrier is lower than the amplitude of the sinusoidal modulation wave, the PWM control signal is high, and when the amplitude of the triangular carrier is higher than the amplitude of the sinusoidal modulation wave, the PWM control signal is low. The positive sinusoidal half-cycle of the modulation wave is called the positive half-cycle of the power frequency, and the negative sinusoidal half-cycle of the modulation wave is called the negative half-cycle of the power frequency. The dead time is defined as the time periods t0-t0” and t2-t2” in the positive half-cycle of the power frequency and the time periods t5-t5” and t7-t7” in the negative half-cycle of the power frequency. The switching time is defined as the time periods t1-t1' and t3-t3' in the positive half-cycle of the power frequency and the time periods t6-t6' and t8-t8' in the negative half-cycle of the power frequency. The positive and negative half-cycles of the frequency conversion type high-frequency link inverter are symmetrical. Taking the positive half-cycle of the input AC voltage as an example, the dead time model establishment process of the primary side switch is explained. According to the voltage v between the midpoints of the two bridge arms of the primary side full-bridge circuit AB The polarity of the isolation transformer primary leakage inductance current i p The direction and the voltage v between the midpoints of the two bridge arms of the secondary side cycloconversion circuit ab The circuit state within the dead time is divided into nine typical modes according to the polarity combination of , as shown in Table 1.

[0096] Table 1

[0097]

[0098]

[0099] Where Vin is the DC power supply at the input side;

[0100] like Figure 3 As shown, the nine modes can be divided into modes I-IVv ab >0, mode Vv ab = 0 and mode VI-IXv ab <0, within the dead time, the circuit first enters the state v ab >0 in mode I, and perform mode conversion according to the following rules:

[0101] S1. After mode I ends, if v AB =-V in , the circuit enters mode II, at this time i p Start to decline when i p = 0, the circuit further enters mode III; if mode I ends i p = 0 and v AB ≠-V in , the circuit directly enters mode III, this process usually occurs under light load conditions;

[0102] S2. After mode III ends, if v AB =V in , the circuit enters mode IV, at this time i p Start to rise, when i p = 0, the circuit returns to mode I; if mode III ends, i p = 0 and v AB ≠V in , the circuit returns directly to mode Ⅰ;

[0103] S3. The circuit will continue to repeat S1 and S2 until v ab drops to zero and enters the next state; if state v ab >0 when i ends p >0, the circuit enters state v ab =0, at this time i p Start to decline when i p = 0, the circuit enters state v ab <0; if state v ab >0 when i ends p ≤0, the circuit enters state v ab <0;

[0104] S4. When the circuit enters state v ab <0, the conversion law between modes is similar to the state v ab >0 is the same.

[0105] like Figure 4 As shown, the primary leakage inductance current i of the isolation transformer in each mode is p, Isolation transformer secondary leakage current i s , the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit v AB The voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit is v ab The time domain analytical model is as follows:

[0106] Mode I:

[0107]

[0108] Where n is the turns ratio of the isolation transformer, C p is the parasitic capacitance of the power switch tube of the primary full-bridge circuit, C s is the parasitic capacitance of the power switch tube of the secondary side cycloconversion circuit, I o is the inverter output current, L k_eq To calculate the equivalent leakage inductance of the secondary side of the isolation transformer, L m is the magnetizing inductance of the isolation transformer, T s is the switching period of the cycloconversion high-frequency link inverter, V in is the input side DC power supply, i m is the peak value of the excitation current, i p1 is the primary leakage inductance current of the isolation transformer at the initial moment of mode I, i p1 =nI o +i m , ω1 is the first series resonant angular frequency,

[0109]

[0110] Mode II and Mode VII:

[0111]

[0112] where i p2 is the primary leakage inductance current of the isolation transformer at the initial moment of mode II or mode VII, v ab2 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode II or mode VII, A1 is the coefficient to be determined, under mode II Mode VII ω2 is the second series resonant angular frequency,

[0113] Mode III, Mode VI and Mode VIII:

[0114]

[0115] where i p3 is the primary leakage inductance current of the isolation transformer at the initial moment under mode III, mode VI or mode VIII, v AB3is the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit at the initial moment in mode III, mode VI or mode VIII, v ab3 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment under mode III, mode VI or mode VIII, A2 and A3 are unknown coefficients, A2 = nv AB3 -v ab3 ,

[0116] Mode IV and Mode IX:

[0117]

[0118] where i p4 is the primary leakage inductance current of the isolation transformer at the initial moment of mode IV or mode IX, v ab4 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode IV or mode IX;

[0119] Mode V:

[0120]

[0121] where i p5 is the primary leakage inductance current of the isolation transformer at the initial moment of mode V.

[0122] Embodiment 2: This embodiment involves an adaptive soft switching control method for a cyclotron type high frequency link inverter, and a method for establishing a commutation time model for the cyclotron type high frequency link inverter is specifically as follows:

[0123] like Figure 7 As shown, based on the steady-state operating point of the cycloconversion high-frequency link inverter, the state of each component during the commutation time is obtained;

[0124] According to the state of each component during the commutation time, the commutation time equivalent circuit of the cyclotron type high-frequency link inverter is established by Laplace transform. The leakage inductance current i of the secondary side of the isolation transformer is obtained by solving the equivalent circuit. s The time domain expression of is the commutation time model of the cycloconversion high-frequency link inverter:

[0125]

[0126] Where V in is the input DC power supply, L k_eq is the equivalent leakage inductance converted to the secondary side of the isolation transformer; n is the turns ratio of the isolation transformer.

[0127] Embodiment 3: The method for calculating the optimal dead time t1 and its adaptive dead time curve of the primary side switch tubes M1-M4 for realizing ZVS opening is specifically as follows:

[0128] The optimal dead time t1 refers to the drain-source voltage V ds From V in To achieve the minimum time of ZVS opening, after M1 and M4 are turned off, if v AB =-V in , then V of M2 and M3 ds The minimum time required to drop to zero is t1; after M2 and M3 are turned off, if v AB =+V in , then V of M1 and M4 ds The minimum time required to drop to zero is t1;

[0129] like Figure 5 As shown, by analyzing the output current I of different inverters o The circuit below satisfies v AB = ±V in The modal conversion process of the conditions is analyzed, and the sum of the duration of each mode is calculated according to the time domain analytical model, so as to determine the optimal dead time t1 and fit it to generate a piecewise adaptive curve.

[0130] Embodiment 4: The method for calculating the adaptive soft switching control curves of M5, M6, M7, and M8 of a frequency-converting high-frequency link inverter according to the present embodiment is as follows:

[0131] ZVS opening delay time t of M5, M6, M7, and M8 on Defined as the time from when the primary switch is turned off to when v ab =0, specifically: in the positive half cycle, M6 and M8 remain on, M1 and M4 are turned off, when v ab = 0, V of M7 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M5 ds In the negative half cycle, M5 and M7 remain on, and after M1 and M4 are turned off, when v ab = 0, V of M6 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M8 ds Drop to zero to achieve ZVS.

[0132] ZCS shutdown delay time t of M5, M6, M7, and M8off Defined as the time from when the primary switch is turned off to when i s =0, specifically: in the positive half cycle, after M1 and M4 are turned off, when i s = 0, the drain current i of M5 D drops to zero to realize ZCS shutdown; after M2 and M3 are turned off, when i s = 0, i of M7 D After M1 and M4 are turned off in the negative half cycle, when i s = 0, i of M8 D drops to zero to realize ZCS shutdown; after M2 and M3 are turned off, when i s = 0, i of M6 D Drop to zero to achieve ZCS shutdown.

[0133] By analyzing the output current I of different inverters o The circuit below satisfies v ab =0 and i s = 0 condition, and calculate the sum of the duration of each mode according to the time domain analytical model to determine t on / t off .

[0134] Embodiment 5: An adaptive soft switching control method for a frequency conversion type high frequency link inverter according to this embodiment, calculating M9, M 10 、M 11 、M 12 The ZCS delay turn-off time t off1 The specific method is:

[0135] M9, M 10 、M 11 、M 12 ZCS delay shutdown t off1 Defined as M9, M 10 (M 11 、M 12 )Open to i s =i o The time interval is as follows: in the positive half cycle, M9, M 10 After activation, when i s =I o When M 11 The drain current i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When the M9 i D When the negative half cycle is 10 After activation, when is =I o When M 12 i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When M 10 i D Drop to zero to achieve ZCS shutdown.

[0136] According to the commutation time model, the ZCS delayed shutdown time t off1 for:

[0137]

[0138] Embodiment 6: This embodiment involves an adaptive soft switching control method of a frequency conversion type high frequency link inverter, Figure 8 The method of full range adaptive soft switching control shown is specifically:

[0139] S1. Adaptive optimal dead time curves of M1, M2, M3 and M4, adaptive soft switching curves of M5, M6, M7, M8 and M9, M 10 、M 11 、M 12 The ZCS off time t off1 The calculation formula is input into the controller;

[0140] S2. Obtain the inverter output current I through the sampling unit o and transmitted to the controller according to I o Calculate the optimal dead time t1, the ZVS opening delay time t of M5, M6, M7, and M8 in real time on and ZCS turn-off delay time t off , M9, M 10 、M 11 and M 12 ZCS turn-off delay time t off1 ;

[0141] S3. Configure t1 to the ePWM dead zone control unit to adjust the dead zone time between the complementary drive signals of the primary-side full-bridge switches M1, M2, M3 and M4 so that after the primary-side full-bridge switches are turned off, their drain-source voltage V ds It drops to zero within t1 time to achieve zero voltage turn-on in the full load range; on and t off Configured in the ePWM switch time control unit, so that the turn-on time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t on , ensure that vab =0 to complete the zero voltage turn-on, and the turn-off time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t off , ensure i s = 0 to complete the zero current shutdown; off1 Configured to the ePWM switch time control unit, so that M9, M 10 、M 11 and M 12 The turn-off time lags behind M9 and M 10 or M 11 and M 12 Opening time t off1 , ensure i s =I o Zero current shutdown is achieved at the moment of

[0142] Embodiment 7: This embodiment is based on Figure 1 The circuit is constructed by using the cycloconversion type high frequency link inverter topology and the adaptive soft switching method proposed in the present invention. The circuit parameters are input voltage V in =48V, transformer leakage inductance L k =880nH, transformer excitation inductance L m =100μH, transformer turns ratio N p :N s =1:8, filter inductor L f =2mH, switching frequency f s =40kHz, filter capacitor C f =0.68μF, load R = 120Ω, output voltage V o =220V.

[0143] Fig. 9 Figure 2 is the drain-source voltage waveform of the MOSFET M1 on the primary side of the converter. The figure shows that the primary side switch has a zero voltage turn-on characteristic.

[0144] Fig.10 (a)(b) are the drain-source voltage waveforms of MOSFET M5 and M7 on the secondary side of the converter, and (c)(d) are the drain-source current waveforms. The figure shows that M5 and M7 have zero voltage turn-on and zero current turn-off characteristics in the positive half cycle of the power frequency.

[0145] Fig.11 (a) (b) are the MOSFETs M9 and M9 on the secondary side of the converter. 11 The drain-source voltage waveform (c) and (d) are the drain-source current waveforms. The figure shows that M9 and M 11 It has zero voltage turn-on and zero current turn-off characteristics in the positive half cycle of the power frequency.

[0146] Fig.12is the MOSFET M on the secondary side of the converter 10 and M 12 The drain-source current waveform of M 10 and M 12 It has zero current shutdown characteristics in the positive half cycle of the power frequency.

[0147] Similarly, in the negative half cycle of the power frequency, M6 and M8 have zero voltage turn-on and zero current turn-off characteristics, and M0 and M 12 With zero voltage turn-on and zero current turn-off characteristics; M9, M 11 It has zero current shutdown characteristic.

[0148] Comparative Example

[0149] The comparative ratio is also based on Figure 1 The circuit parameters and control parameters of the cycloconversion high-frequency link inverter topology shown in the figure are the same as those in the embodiment, and the traditional fixed dead time and switching time method is adopted. When using this modulation method, the power switch tubes M1, M2, M3 and M4 cannot achieve full range soft switching, M5 and M7 only have zero voltage turn-on characteristics in the positive half cycle of the power frequency, and M9 and M 11 In the positive half cycle of the power frequency, it only has the zero voltage turn-on characteristic; M6 and M8 only have the zero voltage turn-on characteristic in the negative half cycle of the power frequency. 10 、M 12 It only has zero voltage turn-on characteristics in the negative half cycle of the power frequency.

[0150] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. An adaptive soft switching control method for a frequency conversion type high frequency link inverter, the frequency conversion type high frequency link inverter comprises a primary side full bridge circuit, an isolation transformer, a secondary side frequency conversion circuit, a filter circuit, a drive module, an isolation IC, a sampling unit and a controller, the output end of the primary side full bridge circuit is connected to the primary side of the isolation transformer, the secondary side of the isolation transformer is connected to the input end of the secondary side frequency conversion circuit, the output end of the secondary side frequency conversion circuit is connected to the input end of the filter circuit, and the output end of the filter circuit is connected to a load; the sampling unit collects the voltage and current at the output end of the frequency conversion type high frequency link inverter, and connects the collected voltage and current signals to the controller through the isolation IC, and the controller is connected to the primary side full bridge circuit and the secondary side frequency conversion circuit through the isolation IC and the drive module; wherein, The primary side full bridge circuit includes power switch tubes M1, M2, M3 and M4; the secondary side cycloconversion circuit includes power switch tubes M5, M6, M7, M8, M9, M 10 、M 11 and M 12 ; The filter circuit includes a filter inductor L f And filter capacitor C f ; The controller includes an ePWM dead zone control unit, an ePWM switching time control unit, a digital ePWM unit and a voltage loop control unit; the sampling unit includes an output voltage sampling unit and an output current sampling unit; it is characterized in that the adaptive soft switching control method of the frequency-converting high-frequency link inverter is based on the secondary side modulation mode, realizes the full range zero voltage switching of the primary side through adaptive dead zone time, and realizes the secondary soft switching control by dynamically adjusting the switching time, and the specific steps are: Step 1: According to the voltage v between the midpoints of the two bridge arms of the primary full-bridge circuit AB The polarity of the isolation transformer primary leakage inductance current i p The direction and the voltage v between the midpoints of the two bridge arms of the secondary side cycloconversion circuit ab The polarity combination of the primary side full-bridge circuit is used to divide the dead time of the switch tube into nine modes, define the mode conversion boundary conditions, and use Laplace transform to establish the primary side leakage inductance current i of the isolation transformer under each mode. p , Isolation transformer secondary leakage current i s , the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit v AB The voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit is v ab The time domain analytical model of Step 2: Establish the commutation time model of the frequency conversion high-frequency link inverter and optimize M9, M 10 、M 11 、M 12 ZCS shutdown condition; Step 3: Based on the time domain analytical model of step 1, predict the optimal dead time t1 for M1, M2, M3 and M4 to achieve ZVS opening, and use polynomial fitting and exponential fitting to establish the output current I of the cycloconverter high-frequency link inverter in sections o The curve about t1 is used to generate the primary side adaptive dead time curve; Step 4: Based on the time domain analytical model in step 1, calculate the ZVS turn-on delay time t of M5, M6, M7, and M8 on and ZCS turn-off delay time t off , fitting generates I o -t on / t off Adaptive control curve of Step 5: Based on the commutation time model in step 2, calculate M9 and M 10 、M 11 、M 12 The ZCS delay turn-off time t off1 , fitting generates I o -t off1 Adaptive ZCS shutdown control curve; Step 6: Input the output results of Step 3, Step 4 and Step 5 into the controller, sample the output current of the frequency conversion type high frequency link inverter in real time, and dynamically calculate the time parameters t1 and t on ,t off and t off1 , and configure it to the ePWM dead zone control unit and the ePWM switch time control unit to dynamically adjust the dead zone time of M1, M2, M3 and M4, the switch time of M5, M6, M7, M8, and the switch time of M9, M 10 、M 11 、M 12 The turn-off time is shortened to achieve full-range adaptive soft switching control.

2. The adaptive soft switching control method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: The nine modes in step 1 are: Mode I: v AB ≠±V in 、i p >0,v ab >0; Mode II: v AB =-V in 、i p >0,v ab >0; Mode III: v AB ≠±V in 、i p <0,v ab >0; Mode IV: v AB =V in 、i p <0,v ab >0; Mode V: v AB = ±V in 、i p >0,v ab =0; Mode VI: v AB ≠±V in 、i p >0,v ab <0; Mode VII: v AB =-V in 、i p >0,v ab <0; Mode VIII: v AB ≠±V in 、i p <0,v ab <0; Mode IX: v AB =V in 、i p <0,v ab <0; Where V in It is the DC power supply at the input side.

3. The adaptive soft switching control method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: The modal conversion boundary condition in step 1 is: The nine modes can be divided into modes I-IVv ab >0, mode Vv ab = 0 and mode VI-IXv ab <0, within the dead time, the circuit first enters the state v ab >0 in mode I, and perform mode conversion according to the following rules: S1. After mode I ends, if v AB =-V in , the circuit enters mode II, at this time i p Start to decline when i p = 0, the circuit further enters mode III; if mode I ends i p = 0 and v AB ≠-V in , the circuit directly enters mode III, this process usually occurs under light load conditions; S2. After mode III ends, if v AB =V in , the circuit enters mode IV, at this time i p Start to rise, when i p = 0, the circuit returns to mode I; if mode III ends, i p = 0 and v AB ≠V in , the circuit returns directly to mode Ⅰ; S3. The circuit will continue to repeat S1 and S2 until v ab drops to zero and enters the next state; if state v ab >0 when i ends p >0, the circuit enters state v ab =0, at this time i p Start to decline when i p = 0, the circuit enters state v ab <0; if state v ab >0 when i ends p ≤0, the circuit enters state v ab <0; S4. When the circuit enters state v ab <0, the conversion law between modes is similar to the state v ab >0 is the same.

4. The adaptive soft switching control method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: In step 1, the primary leakage inductance current i of the isolation transformer under each mode is p , Isolation transformer secondary leakage current i s , the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit v AB The voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit is v ab The time domain analytical model is as follows: Mode I: Where n is the turns ratio of the isolation transformer, C p is the parasitic capacitance of the power switch tube of the primary full-bridge circuit, C s is the parasitic capacitance of the power switch tube of the secondary side cycloconversion circuit, I o is the inverter output current, L k_eq To calculate the equivalent leakage inductance of the secondary side of the isolation transformer, L m is the magnetizing inductance of the isolation transformer, T s is the switching period of the cycloconversion high-frequency link inverter, V in is the input side DC power supply, i m is the peak value of the excitation current, i p1 is the primary leakage inductance current of the isolation transformer at the initial moment of mode I, i p1 =nI o +i m , ω1 is the first series resonant angular frequency, Mode II and Mode VII: where i p2 is the primary leakage inductance current of the isolation transformer at the initial moment of mode II or mode VII, v ab2 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode II or mode VII, A1 is the coefficient to be determined, and under mode II Mode VII ω2 is the second series resonant angular frequency, Mode III, Mode VI and Mode VIII: where i p3 is the primary leakage inductance current of the isolation transformer at the initial moment under mode III, mode VI or mode VIII, v AB3 is the voltage between the midpoints of the two bridge arms of the primary full-bridge circuit at the initial moment in mode III, mode VI or mode VIII, v ab3 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment under mode III, mode VI or mode VIII, A2 and A3 are unknown coefficients, A2 = nv AB3 -v ab3 , Mode IV and Mode IX: where i p4 is the primary leakage inductance current of the isolation transformer at the initial moment of mode IV or mode IX, v ab4 is the voltage between the midpoints of the two bridge arms of the secondary side cycloconversion circuit at the initial moment of mode IV or mode IX; Mode V: where i p5 is the primary leakage inductance current of the isolation transformer at the initial moment of mode V.

5. The adaptive soft switching control method of the cycloconversion type high frequency link inverter according to claim 1, characterized in that: The commutation time model of the cycloconversion type high frequency link inverter established in step 2 is specifically as follows: Based on the steady-state operating point of the cyclotron type high-frequency link inverter, the state of each component during the commutation time is obtained; according to the state of each component during the commutation time, the commutation time equivalent circuit of the cyclotron type high-frequency link inverter is established by Laplace transform, and the leakage inductance current i on the secondary side of the isolation transformer is obtained by solving the equivalent circuit. s The time domain expression of is the commutation time model of the cycloconversion high-frequency link inverter: Where V in is the input DC power supply, L k_eq is the equivalent leakage inductance converted to the secondary side of the isolation transformer; n is the turns ratio of the isolation transformer.

6. The adaptive soft switching control method of a cycloconverter type high frequency link inverter according to claim 1, characterized in that: The step 3 is specifically as follows: The optimal dead time t1 refers to the drain-source voltage V ds From V in To achieve the minimum time of ZVS opening, after M1 and M4 are turned off, if v AB =-V in , then V of M2 and M3 ds The minimum time required to drop to zero is t1; after M2 and M3 are turned off, if v AB =+V in , then V of M1 and M4 ds The minimum time required to drop to zero is t1; By analyzing the output current I of different inverters o The circuit below satisfies v AB = ±V in The modal conversion process of the conditions is analyzed, and the sum of the duration of each mode is calculated according to the time domain analytical model, so as to determine the optimal dead time t1 and fit it to generate a piecewise adaptive curve.

7. The adaptive soft switching control method of a cycloconverter type high frequency link inverter according to claim 1, characterized in that: In step 4, the ZVS opening delay time t on and ZCS turn-off delay time t off The method for determining is: ZVS opening delay time t of M5, M6, M7, and M8 on Defined as the time from when the primary switch is turned off to when v ab =0, specifically: in the positive half cycle, M6 and M8 remain on, M1 and M4 are turned off, when v ab = 0, V of M7 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M5 ds drops to zero to realize ZVS opening; in the negative half cycle, M5 and M7 remain turned on, and after M1 and M4 are turned off, when v ab = 0, V of M6 ds drops to zero to realize ZVS opening; after M2 and M3 are turned off, when v ab = 0, V of M8 ds Drop to zero to achieve ZVS opening; ZCS shutdown delay time t of M5, M6, M7, and M8 off Defined as the time from when the primary switch is turned off to when i s =0, specifically: in the positive half cycle, after M1 and M4 are turned off, when i s = 0, the drain current i of M5 D Drop to zero to achieve ZCS shutdown; After M2 and M3 are turned off, when i s = 0, i of M7 D drops to zero to realize ZCS shutdown; after M1 and M4 are turned off in the negative half cycle, when i s = 0, i of M8 D Drop to zero to achieve ZCS shutdown; After M2 and M3 are turned off, when i s = 0, i of M6 D Drop to zero to achieve ZCS shutdown; By analyzing the output current I of different inverters o The circuit below satisfies v ab =0 and i s = 0 condition, and calculate the sum of the duration of each mode according to the time domain analytical model to determine t on / t off .

8. The adaptive soft switching control method of a cycloconverter type high frequency link inverter according to claim 1, characterized in that: In step 5, the ZCS delay shutdown time t off1 The method for determining is: M9, M 10 、M 11 、M 12 ZCS delay shutdown t off1 Defined as M9 and M 10 or M 11 and M 12 Open to i s =i o The time interval is as follows: in the positive half cycle, M9, M 10 After activation, when i s =I o When M 11 The drain current i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When the M9 i D Drop to zero to achieve ZCS shutdown; in the negative half cycle, M9, M 10 After activation, when i s =I o When M 12 i D Drop to zero to achieve ZCS shutdown; M 11 、M 12 After activation, when i s =I o When M 10 i D Drop to zero to achieve ZCS shutdown; According to the commutation time model, the ZCS delayed shutdown time t off1 for:

9. The adaptive soft switching control method of a cycloconverter high frequency link inverter according to claim 1, characterized in that: The step 6 is specifically as follows: S1. Adaptive optimal dead time curves of M1, M2, M3 and M4, adaptive soft switching curves of M5, M6, M7, M8 and M9, M 10 、M 11 、M 12 The ZCS off time t off1 The calculation formula is input into the controller; S2. Obtain the inverter output current I through the sampling unit o and transmitted to the controller according to I o Calculate the optimal dead time t1, the ZVS opening delay time t of M5, M6, M7, and M8 in real time on and ZCS turn-off delay time t off , M9, M 10 、M 11 and M 12 ZCS turn-off delay time t off1 ; S3. Configure t1 to the ePWM dead zone control unit to adjust the dead zone time between the complementary drive signals of the primary-side full-bridge switches M1, M2, M3 and M4 so that after the primary-side full-bridge switches are turned off, their drain-source voltage V ds It drops to zero within t1 time to achieve zero voltage turn-on in the full load range; on and t off Configured in the ePWM switch time control unit, so that the turn-on time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t on , to ensure zero voltage switching, the turn-off time of M5, M6, M7, and M8 lags behind the turn-off time of the primary side full-bridge switch tube t off , to ensure zero current shutdown; off1 Configured to the ePWM switch time control unit, so that M9, M 10 、M 11 and M 12 The turn-off time lags behind M9 and M 10 or M 11 and M 12 Opening time t off1 , ensuring zero current shutdown.

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