阅读说明：本技术 一种软开关逆变器 (Soft switching inverter ) 是由 胡平 杨迎化 张林森 刘杰峰 刘凯 潘逊 钱超 于 2020-06-22 设计创作，主要内容包括：本发明提供一种软开关逆变器,其包括谐振电路和三相全桥逆变器电路,谐振电路包括辅助开关管T<Sub>1</Sub>、二极管D<Sub>1</Sub>、辅助开关管T<Sub>2</Sub>、二极管D<Sub>2</Sub>、谐振电感L<Sub>r</Sub>以及谐振电容C<Sub>r</Sub>,三相全桥逆变器电路为三相PWM逆变器,三相PWM逆变器的负载为交流电机,三相PWM逆变器的负载电感L<Sub>0</Sub>大于谐振电感L<Sub>r</Sub>,在一个开关周期中,负载电流I<Sub>L</Sub>在一个谐振周期中不变,负载电流I<Sub>L</Sub>数值取决于各相电流的瞬时值及三相PWM逆变器的六个开关器件的开关状态,直流母线电压为电压源U<Sub>s</Sub>。本发明将软开关技术引入到PWM逆变器中,使它既能保持原有的特点,又能实现软开关工作。(The invention provides a soft switching inverter, which comprises a resonant circuit and a three-phase full-bridge inverter circuit, wherein the resonant circuit comprises an auxiliary switching tube T 1 Diode D 1 Auxiliary switch tube T 2 Diode D 2 Resonant inductor L r And a resonant capacitor C r The three-phase full-bridge inverter circuit is a three-phase PWM inverter, the load of the three-phase PWM inverter is an alternating current motor, and the load inductance L of the three-phase PWM inverter 0 Greater than resonant inductance L r In a switching cycle, the load current I L Constant in one resonance period, load current I L The value depends on the instantaneous value of each phase current and the switching state of six switching devices of the three-phase PWM inverter, and the DC bus voltage is a voltage source U s . The invention introduces the soft switching technology into the PWM inverter, so that the soft switching technology can not only keep the original characteristics, but also realize the soft switching work.)

1. A soft-switched inverter, characterized by: it comprises a resonant circuit and a three-phase capacitorA bridge inverter circuit, wherein the resonance circuit and the three-phase full-bridge inverter circuit are sequentially connected, and the resonance circuit comprises a switch T₁Diode D₁Switch T₂Diode D₂Resonant inductor L_rAnd a resonant capacitor C_rSwitch T₁Resonant inductor L_rAnd a switch T₂Connected in series in turn, diode D₁And switch T₁Parallel connected, diode D₂And switch T₂The two are connected in parallel,

the three-phase full-bridge inverter circuit is a three-phase PWM inverter, the load of the three-phase PWM inverter is an alternating current motor, and the load inductance L of the three-phase PWM inverter₀Greater than resonant inductance L_rIn a switching cycle, the load current I_LConstant in one resonance period, load current I_LThe value depends on the instantaneous value of each phase current and the switching state of six switching devices of the three-phase PWM inverter, and the DC bus voltage is a voltage source U_s，

Voltage source U of the resonant circuit_sBoth ends of the filter capacitor C are connected in parallel_dFilter capacitor C_dPositive pole of (2) connecting switch T₁First terminal of (1), switch T₁And diode D₁Antiparallel, switch T₁Second ends of the two terminals are respectively connected with a resonance inductor L_rFirst terminal, resonant capacitor C_rA first terminal, a positive input terminal of a three-phase full-bridge inverter, and a resonant inductor L_rSecond terminal of (2) is connected with a switch T₂A first terminal, a switch T₂And diode D₂Anti-parallel, filter capacitor C_dNegative electrodes of the switches are respectively connected with the switches T₂Second terminal of (1), resonant capacitor C_rAnd a negative input terminal of the three-phase full-bridge inverter;

two input ends of the three-phase full-bridge inverter are respectively connected with a resonant capacitor C_rThe three-phase full-bridge inverter is of a three-phase full-bridge structure and comprises 6 switching devices, each switch is respectively connected with a diode and a capacitor in parallel, the three-phase full-bridge inverter outputs three-phase alternating current, and the three-phase alternating current is connected with the alternating current motor;

switch T₁Switch T₂And 6The switching device is controlled to be switched on or switched off by the controller, and the controller simultaneously detects the three-phase alternating current output by the three-phase full-bridge inverter, so that each switch is controlled.

2. The soft-switched inverter of claim 1, wherein: the soft switching inverter comprises six working modes in one switching period, wherein the six working modes are a working mode I, a working mode II, a working mode III, a working mode IV, a working mode V and a working mode VI respectively.

3. The soft-switched inverter of claim 2, wherein: the first working mode is as follows: at a time [ t ]₀-t₁]Carrying out a first working mode, wherein t is t as the initial time of the first working mode₀Has a 1_i＝I_L，i_T1＝I_L，i_T2＝0，i_DF＝0，i_Lr＝0，u_Cr＝U_SSwitch T₁On, switch T₂Obtaining a conducting signal, and starting the working mode when the current of the resonant inductor is t ═ t₁The switch T is turned off when the threshold current set by the controller is reached₁Ending the first working mode; in the first operating mode, the energy stored in the resonant inductor must be able to compensate for the energy loss caused by resonance, so as to ensure that the dc side voltage value can be raised again to the power supply voltage value U through resonance after being reduced to zero_SSwitch T₂And is switched on in a zero current state.

4. The soft-switched inverter of claim 2, wherein: the second working mode is specifically as follows: at a time [ t ]₁-t₂]Performing a second operation mode, and before the second operation mode is started, switching to the second operation mode by a switch T₁Has a current of I_L+I_LrThrough a switch tube T₂Has a current of I_LrWhen switching on or off T₁At t ═ t₁After the moment is turned off, the second working mode begins, and at the moment, the following moments are: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lr，i_DF＝0，i_Lr＝I_Lr，U_Cr＝U_iAnd, from L_r、T₂、C_rThe resonant circuit is set to resonate at t ═ t₂Time of day, capacitance C_rThe voltage at the two ends is reduced to zero, the second working mode is finished, and the switch T is switched on₁And is turned off in a state of zero voltage.

5. The soft-switched inverter of claim 2, wherein: the third working mode is as follows: at a time [ t ]₂-t₃]Performing a third working mode, when the voltage on the direct current side is t ═ t₂The time drops to zero, and the reverse recovery diode D of the inverter_FConduction, the beginning of the third working mode, and at the beginning time of the third working mode, the following steps are carried out: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lrmax，i_DF＝I_Lrmax+I_L，i_Lr＝I_Lrmax，U_CrWhen the switch T is equal to 0₂And when the power is turned off, the third working mode is finished.

6. The soft-switched inverter of claim 2, wherein: the fourth working mode is specifically as follows: at a time [ t ]₃-t₄]Operating in the fourth mode, wherein t is t₃Time of day, switch T₂Turn-off, diode D_FTurn off while diode D₂Conduction, the working mode four begins, and at the moment: i.e. i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_Lrmax，U_Cr0 and is represented by L_r、D₂、C_rThe resulting resonant circuit begins to resonate.

7. The soft-switched inverter of claim 1, wherein: the fifth working mode is specifically as follows: at a time [ t ]₄-t₅]Carrying out a fifth working mode, wherein t is t₄Time of day, diode D₁Conducting, starting the working mode five, and the moment: i.e. i_i＝I_L-I_lr，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_lr，U_Cr＝U_SDiode D₁A current of i_Lr-I_LWhen i is_Lr＝I_LDiode D₁And ending the fifth working mode.

8. The soft-switched inverter of claim 1, wherein: wherein, the sixth working mode specifically is: at a time [ t ]₅-t₆]Carrying out a sixth working mode, wherein t is the initial time t of the sixth working mode₅When there is i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_L，U_Cr＝U_iAt this time, the diode D₁Cut-off, switch T₁Obtaining a turn-on signal, starting the working mode six, and when the current of the resonant inductor is t ═ t₆The time drops to zero and the sixth operating mode ends, in which the energy stored in the inductor is supplied to the load and the diode D₂At the end of the six working modes, the DC power supply is cut off in a zero current state, and at the end of the six working modes, the DC power supply passes through a switch T₁The load is powered up, and by this point one resonant period ends, waiting for the next resonant period to begin.

Technical Field

The invention relates to a power electronic technology, in particular to an inverter based on a soft switching technology.

Background

In the traditional PWM three-phase inverter, the output pulses of the inverter in one period are increased due to high frequency, and the equivalent voltage waveform is closer to a sine wave, so that the output harmonic wave is reduced, and the speed regulation performance is improved. However, the increase of the switching frequency also brings a series of new problems, which mainly appear in the following aspects.

(1) Switching losses and device damage problems

The power switch tube in the main circuit of the conventional PWM inverter is Hard Switching (Hard Switching), i.e. it is turned on when the voltage is not zero, or turned off when the current is not zero, and is in the forced Switching process. The power switching device is not an ideal device, and the turn-on and turn-off thereof are not instantaneous and require a certain time. During this time, the voltage (or the passing current) across the switch tube decreases, and the current (or the voltage across the switch tube) passing through the switch tube increases simultaneously, so that the waveforms of the voltage and the current overlap, thereby generating a Switching Loss (Switching Loss), including a turn-on Loss and a turn-off Loss. Fig. 1 shows voltage and current waveforms when the power switching device is turned on and off, and the hatched area in the figure is an area where switching loss occurs.

The switching loss of a power switch is proportional to its carrier frequency, i.e., as the carrier frequency increases, the switching loss will increase linearly in proportion. The switching time of the high power transistor BJT is about tens of ns to several us, and the switching time of the turn-off devices GTO and IGBT is about tens of ns. The switching frequency of the frequency converter typically formed by these devices is 2-3 KHz. After the high frequency, the switching frequency of the frequency converter is increased to dozens of KHz, resulting in increased switching loss. Theoretical analysis shows that the switching loss of the device in one period is 30% -40% of the total average loss, and increases with the increase of the switching frequency. Due to the limitation of junction temperature, the increase of working frequency is limited, and the current and voltage capacity of the device can not operate under rated conditions.

(2) Diode reverse recovery problem

Another source of switching losses is the reverse recovery current of the anti-parallel diode on the power switch, which will cause significant turn-on losses under hard switching conditions. When the diode is switched from on to off, reverse recovery time exists, the diode is still in an on state in the period, and if a switching device connected with the diode in series is immediately switched on, a direct-current power supply is easily subjected to instantaneous short circuit to generate large impact current, so that the power consumption of the switching device and the diode is rapidly increased if the direct-current power supply is light, and the switching device and the diode are damaged if the direct-current power supply is heavy. The higher the frequency, the greater the inrush current, which is detrimental to the safe operation of the device. It can be seen that the power loss increases rapidly with increasing operating frequency.

(3) Inductive turn-off problem

Inductive elements (parasitic inductance or entity inductance such as lead inductance, transformer leakage inductance and the like) are inevitably present in the circuit, and when the switching device is turned off, because di/dt passing through the inductive elements is large, a high peak voltage is induced to be applied to two ends of the switching device, and the higher the switching frequency is, the faster the turn-off is, the higher the induced voltage is. The voltage is applied to two ends of the switching device, which easily causes device breakdown.

(4) Problem of capacitive opening

When the switching device is turned on at a very high voltage, the energy stored in the junction capacitor of the switching device is totally dissipated in the switching device, and the higher the frequency, the larger the turn-on current spike, thereby causing the device to be damaged by overheating.

(5) Safe working area

Voltage and current spikes generated at the turn-on and turn-off moments will possibly cause the operation track of the switching device to exceed the Safe Operating Area (SOA), affecting the reliable operation of the power switch. In the PWM control technique, the on and off traces of the switching device experience a large switching stress interval, i.e., the switch is subjected to a high switching voltage and a large switching current at the same time in some time, which is likely to damage the switching device.

(6) Electromagnetic interference

When the frequency converter works, Electromagnetic Interference (EMI) is easily generated to other equipment. The electromagnetic interference problem is particularly prominent for an electrodynamic propulsion system, the frequency converter of the electrodynamic propulsion system is easy to generate electromagnetic interference to other equipment when working, along with the increasing of the frequency, the electromagnetic interference is more and more serious, and relates to strong conduction type electromagnetic interference caused by excessive dv/dt and di/dt in the switching process of a high-power device in a main circuit of the device, and strong electromagnetic field (especially near field) radiation is also caused, and the electromagnetic interference can not only cause the false operation of a control circuit of the electromagnetic interference device, but also cause the electromagnetic interference to other equipment such as an internal guidance device and the like, and the normal and reliable work of the electromagnetic interference is influenced.

Disclosure of Invention

Based on the defects of the prior art, the invention provides a soft switching inverter, which adds a soft switching technology into a PWM inverter, so that the soft switching inverter can maintain the original characteristics and realize the soft switching operation.

Specifically, the invention provides a soft switching inverter, which comprises a resonance circuit and a three-phase full-bridge inverter circuit, wherein the resonance circuit and the three-phase full-bridge inverter circuit are sequentially connected, and the resonance circuit comprises a switch T₁Diode D₁Switch T₂Diode D₂Resonant inductor L_rAnd a resonant capacitor C_rSwitch T₁Resonant inductor L_rAnd a switch T₂Connected in series in turn, diode D₁And switch T₁Parallel connected, diode D₂And switch T₂The two are connected in parallel,

the three-phase full-bridge inverter circuit is a three-phase PWM inverter, the load of the three-phase PWM inverter is an alternating current motor, and the load inductance L of the three-phase PWM inverter₀Greater than resonant inductance L_rIn a switching cycle, the load current I_LConstant in one resonance period, load current I_LThe value depends on the instantaneous value of each phase current and the switching state of six switching devices of the three-phase PWM inverter, and the DC bus voltage is a voltage source U_s，

Voltage source U of the resonant circuit_sTwo of (2)The ends are connected with a filter capacitor C in parallel_dFilter capacitor C_dPositive pole of (2) connecting switch T₁First terminal of (1), switch T₁And diode D₁Antiparallel, switch T₁Second ends of the two terminals are respectively connected with a resonance inductor L_rFirst terminal, resonant capacitor C_rA first terminal, a positive input terminal of a three-phase full-bridge inverter, and a resonant inductor L_rSecond terminal of (2) is connected with a switch T₂A first terminal, a switch T₂And diode D₂Anti-parallel, filter capacitor C_dNegative electrodes of the switches are respectively connected with the switches T₂Second terminal of (1), resonant capacitor C_rAnd the negative input of the three-phase full-bridge inverter.

Two input ends of the three-phase full-bridge inverter are respectively connected with a resonant capacitor C_rThe three-phase full-bridge inverter is of a three-phase full-bridge structure and comprises 6 switches S₁～S₆Each switch is respectively connected with a diode and a capacitor in parallel, the three-phase full-bridge inverter outputs three-phase alternating current, and the three-phase alternating current is connected with the alternating current motor;

switch T₁Switch T₂And 6 switches S₁～S₆The controller controls the on or off of the three-phase full-bridge inverter, and the controller simultaneously detects the three-phase alternating current output by the three-phase full-bridge inverter, so that each switch is controlled.

Preferably, the soft switching inverter includes six operating modes in one switching cycle, and the six operating modes are an operating mode one, an operating mode two, an operating mode three, an operating mode four, an operating mode five, and an operating mode six, respectively.

Preferably, the first operation mode is specifically: at a time [ t ]₀-t₁]Carrying out a first working mode, wherein t is t as the initial time of the first working mode₀Has a 1_i＝I_L，i_T1＝I_L，i_T2＝0，i_DF＝0，i_Lr＝0，u_Cr＝U_SSwitch T₁On, switch T₂Obtaining a conducting signal, and starting the working mode when the current of the resonant inductor is t ═ t₁Time-of-day controllerConstant threshold current, turn off switch T₁Ending the first working mode; in the first operating mode, the energy stored in the resonant inductor must be able to compensate for the energy loss caused by resonance, so as to ensure that the dc side voltage value can be raised again to the power supply voltage value U through resonance after being reduced to zero_SSwitch T₂And is switched on in a zero current state.

Preferably, the second operation mode specifically includes: at a time [ t ]₁-t₂]Performing a second operation mode, and before the second operation mode is started, switching to the second operation mode by a switch T₁Has a current of I_L+I_LrThrough a switch tube T₂Has a current of I_LrWhen switching on or off T₁At t ═ t₁After the moment is turned off, the second working mode begins, and at the moment, the following moments are: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lr，i_DF＝0，i_Lr＝I_Lr，U_Cr＝U_iAnd, from L_r、T₂、C_rThe resonant circuit is set to resonate at t ═ t₂Time of day, capacitance C_rThe voltage at the two ends is reduced to zero, the second working mode is finished, and the switch T is switched on₁And is turned off in a state of zero voltage.

Preferably, the third operating mode is specifically: at a time [ t ]₂-t₃]Performing a third working mode, when the voltage on the direct current side is t ═ t₂The time drops to zero, and the reverse recovery diode D of the inverter_FConduction, the beginning of the third working mode, and at the beginning time of the third working mode, the following steps are carried out: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lrmax，i_DF＝I_Lrmax+I_L，i_Lr＝I_Lrmax，U_CrWhen the switch T is equal to 0₂And when the power is turned off, the third working mode is finished.

Preferably, the fourth operation mode is specifically: at a time [ t ]₃-t₄]Operating in the fourth mode, wherein t is t₃Time of day, switch T₂Turn-off, diode D_FTurn off while diode D₂Conduction, the working mode four begins, and at the moment: i.e. i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_Lrmax，U_Cr0 and is represented by L_r、D₂、C_rThe resulting resonant circuit begins to resonate.

Preferably, the fifth operating mode is specifically: at a time [ t ]₄-t₅]Carrying out a fifth working mode, wherein t is t₄Time of day, diode D₁Conducting, starting the working mode five, and the moment: i.e. i_i＝I_L-I_lr，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_lr，U_Cr＝U_SDiode D₁A current of i_Lr-I_LWhen i is_Lr＝I_LDiode D₁And ending the fifth working mode.

Preferably, the sixth operating mode is specifically: at a time [ t ]₅-t₆]Carrying out a sixth working mode, wherein t is the initial time t of the sixth working mode₅When there is i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_L，U_Cr＝U_iAt this time, the diode D₁Cut-off, switch T₁Obtaining a turn-on signal, starting the working mode six, and when the current of the resonant inductor is t ═ t₆The time drops to zero and the sixth operating mode ends, in which the energy stored in the inductor is supplied to the load and the diode D₂At the end of the six working modes, the DC power supply is cut off in a zero current state, and at the end of the six working modes, the DC power supply passes through a switch T₁The load is powered up, and by this point one resonant period ends, waiting for the next resonant period to begin.

Compared with the prior art, the invention has the following beneficial effects:

(1) the invention provides a soft switching inverter, which can add a soft switching technology into a PWM inverter, so that the soft switching inverter can keep the original characteristics, can realize soft switching work and provides a new inverter under the condition of not increasing the cost.

(2) The soft switching inverter only adds a resonance link at the input end of the traditional three-phase full-bridge inverter, and the resonance link only adds a few components, which are a switch T1, a diode D1, a switch T2, a diode D2, a resonance inductor Lr and a resonance capacitor Cr, and the turning on and off of the switch T1 and the switch T2 are also soft switching processes, so that the increased electric energy loss is very small.

Drawings

FIG. 1 is a graph of voltage and current waveforms for a power switching device of the present invention turning on and off under hard switching conditions;

FIG. 2 is a schematic diagram of a soft-switched inverter configuration of the present invention;

FIG. 3 is an equivalent circuit diagram of the soft-switched inverter of the present invention;

FIG. 4a is an equivalent circuit for soft-switched inverter mode one of operation of the present invention;

FIG. 4b is an equivalent circuit of the second mode of operation of the soft-switched inverter of the present invention;

FIG. 4c is an equivalent circuit for mode three of the soft-switched inverter of the present invention;

FIG. 4d is an equivalent circuit for mode four of the soft-switched inverter of the present invention;

FIG. 4e is an equivalent circuit for soft-switched inverter mode five of operation of the present invention;

FIG. 4f is an equivalent circuit for soft-switched inverter operating mode six of the present invention; and

fig. 5 is a graph of various parameters of the soft-switched inverter of the present invention in different operating modes.

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The soft switching inverter is composed of a resonant link and a three-phase full bridge inverter circuit, as shown in fig. 1 and 2. The resonance link is the core of the invention and is composed of an auxiliary switch tube T₁Diode D₁Auxiliary switch tube T₂Diode D₂Resonant inductor L_rAnd a resonant capacitor C_rAnd (4) forming. To simplify the analysis, the following assumptions are made: used in electric circuitsThe components are all ideal.

The load of the three-phase PWM inverter is an alternating current motor, and a load inductor L₀Much larger than the resonance inductance L_rThus, in one switching cycle, the load current I_LThe value of which is approximately constant during a resonance period and depends on the instantaneous value of the respective phase current and the switching state of the 6 switching devices of the inverter bridge. The ripple of the DC bus voltage is negligible, so it can be equivalent to an ideal voltage source U_s. The soft-switched inverter architecture can be equivalent to the circuit shown in fig. 3. D_F、I_LAnd S_invAn equivalent circuit of the inverter and the alternating current motor is formed.

Voltage source U of resonant circuit_sBoth ends of the filter capacitor C are connected in parallel_dFilter capacitor C_dPositive pole of (2) connecting switch T₁First terminal of (1), switch T₁And diode D₁Antiparallel, switch T₁Second ends of the two terminals are respectively connected with a resonance inductor L_rFirst terminal, resonant capacitor C_rA first terminal, a positive input terminal of a three-phase full-bridge inverter, and a resonant inductor L_rSecond terminal of (2) is connected with a switch T₂A first terminal, a switch T₂And diode D₂Anti-parallel, filter capacitor C_dNegative electrodes of the switches are respectively connected with the switches T₂Second terminal of (1), resonant capacitor C_rAnd a negative input terminal of the three-phase full-bridge inverter;

two input ends of the three-phase full-bridge inverter are respectively connected with a resonant capacitor C_rThe three-phase full-bridge inverter is of a three-phase full-bridge structure and comprises 6 switches S₁～S₆Each switch is respectively connected with a diode and a capacitor in parallel, the three-phase full-bridge inverter outputs three-phase alternating current, and the three-phase alternating current is connected with the alternating current motor;

switch T₁Switch T₂And 6 switches S₁～S₆The controller controls the on or off of the three-phase full-bridge inverter, and the controller simultaneously detects the three-phase alternating current output by the three-phase full-bridge inverter, so that each switch is controlled.

In the practical application of the present patent, the hall sensor can be used to detect the current, and if the detected current is large, the ac engine speed is large, and if the speed needs to be reduced, the frequency of each switch needs to be reduced, and conversely, if the speed of the motor needs to be increased, the frequency of each switch needs to be increased.

The soft switching inverter comprises six operation modes in total, and during actual work, the operation modes of the soft switching inverter are as follows:

the soft switching inverter can be divided into 6 modes in one switching period, the equivalent circuit of each mode is shown in fig. 4, and the parameter curves in different operation modes are shown in fig. 5.

Preferably, as shown in fig. 4a, the first operation mode is specifically: at a time [ t ]₀-t₁]At the initial time t equal to t of the first operation mode₀Has a 1_i＝I_L，i_T1＝I_L，i_T2＝0，i_DF＝0，i_Lr＝0，u_Cr＝U_SSwitch T₁On, switch T₂Obtaining a conducting signal, and starting the working mode when the current of the left resonant inductor is t ═ t₁The switch T is turned off when the threshold current is reached₁Ending the first working mode; in this mode, the energy stored in the inductor must be sufficient to compensate for the energy loss due to resonance, so as to ensure that the dc-side voltage value can be raised again to the supply voltage value U through resonance after dropping to zero_SSwitch T₂And is switched on in a zero current state.

Preferably, as shown in fig. 4b, the second operation mode specifically includes: time [ t ]₁-t₂]Before the start of the second operating mode, by means of a switch T₁Has a current of I_L+I_LrThrough a switch tube T₂Has a current of I_LrWhen switching on or off T₁At t ═ t₁After the moment is turned off, the second working mode begins, and at the moment, the following moments are: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lr，i_DF＝0，i_Lr＝I_Lr，U_Cr＝U_iAt t ═ t₁At a time from L_r、T₂、C_rThe resonant circuit is set to resonate at t ═ t₂Time of day, capacitance C_rThe voltages at the two ends are reduced to zero, the second working mode is ended, meanwhile, the current of the resonant inductor and the stored energy thereof reach the maximum value, and the capacitor C_rVoltage across the terminals from U_SSlowly drops to zero, switch T₁And is turned off in a state of zero voltage.

Preferably, as shown in fig. 4c, the third operating mode specifically includes: at a time [ t ]₂-t₃]When the DC side voltage is t ═ t₂The time drops to zero, and the reverse recovery diode D of the inverter_FConduction, the beginning of the third working mode, and at the beginning time of the third working mode, the following steps are carried out: i.e. i_i＝0，i_T1＝0，i_T2＝I_Lrmax，i_DF＝I_Lrmax+I_L，i_Lr＝I_Lrmax，U_CrWhen the switch T is equal to 0₂When turned off, the present mode ends. In this mode, the capacitance C_rThe voltage across can be kept at zero and the duration can be chosen arbitrarily as required. Therefore, all switching tubes of the inverter can easily realize zero-voltage switching in this stage.

Preferably, as shown in fig. 4d, the fourth operation mode is specifically: at a time [ t ]₃-t₄]At t ═ t₃Time, T-T switching tube₂Turn-off, diode D_FTurn off while diode D₂Conduction, the working mode four begins, and at the moment: i.e. i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_Lrmax，U_Cr0, by L_r、D₂、C_rThe resulting resonant circuit begins to resonate. At t ═ t₄At all times, the DC side voltage (i.e. the capacitance C)_rVoltage across) to an input dc voltage U_SDiode D₁And (5) conducting, and ending the working mode. The DC side voltage slowly rises from zero to U due to the resonance_SSwitch tube T₂And is turned off in a state of zero voltage.

Preferably, among others, as shown in fig. 4e, the operation modeThe fifth concrete step is: at a time [ t ]₄-t₅]At t ═ t₄Time of day, diode D₁Conducting, starting the working mode five, and the moment: i.e. i_i＝I_L-I_lr，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_lr，U_Cr＝U_SDiode D₁Flowing current (i)_Lr-I_L) When i is_Lr＝I_LDiode D₁And ending the fifth working mode. In this mode, a portion of the energy stored in the inductor is fed back to the dc power source and another portion is supplied to the load. At the end time t of this mode, t is equal to t₅Switch T₁And is turned on in a zero voltage state.

Preferably, as shown in fig. 4f, the sixth operating mode is specifically: at a time [ t ]₅-t₆]At the initial time t ═ t of this mode₅When there is i_i＝0，i_T1＝0，i_T2＝0，i_DF＝0，i_Lr＝I_L，U_Cr＝U_iDiode D₁Cut-off, switch T₁Obtaining a turn-on signal, starting the working mode six, and when the current of the resonant inductor is t ═ t₆The time drops to zero and the sixth operating mode ends. In this mode, the energy stored in the inductor is supplied to the load, and the diode D₂And is turned off in a zero current state at the end of this mode. At the end of this mode, the DC power supply passes through switch T₁To power the load. At this point, one resonance period ends, waiting for the next resonance period to begin.

The above-mentioned embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.