Abstract:
The invention relates to a soft switching topological circuit. A zero voltage turn-on is realized when the main switch is turned on, by utilizing the resonance of the resonant inductor and the resonant capacitor after the auxiliary switch is turned on. Moreover, during the turn-off of the main switch, the resonant inductor withstands a voltage drop, which causes the energy-feed device corresponding to the auxiliary switch to feed no energy out when the auxiliary switch is turned on, thereby realizing zero current turn-on of the auxiliary switch, and increasing the circuit running efficiency.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a soft switching topological circuit, more particularly to a soft switching topological circuit in a boost or buck converter circuit and a bridge circuit. 
     BACKGROUND OF THE INVENTION 
     The prior art working procedures of a ZVT-BOOST circuit are shown as in FIGS. 2A-2F. FIG. 2A shows the waveform of the gate driving signal voltage V gs2  of the auxiliary MOSFET  106 ; FIG. 2B shows the waveform of the gate driving signal voltage V gs1  of a main MOSFET  103 ; FIG. 2C shows the waveform of the current I lr  in resonant inductor  105 ; FIG. 2D shows the waveform of current I Dmain  of main ultrafast recovery diode  107 ; FIG. 2E shows the waveform of the current I Daux  of auxiliary ultrafast recovery diode  108 ; FIG. 2F shows the waveform of the voltage V dsmain  between the source and the drain of main MOSFET  103 . It can be seen from the above drawings: 
     When t=to, auxiliary MOSFET  106  is turned on, since the current flowing through an inductor, can not change abruptly, so, when auxiliary MOSFET  106  is turned on, the current in resonant inductor  105  increases gradually from an initial value, therefore the current flowing through main ultrafast recovery diode  107  decreases, and gradually to zero, thereby a soft turn-off of main ultrafast recovery diode  107  is realized by means of resonant inductor  105 ; 
     It can be seen from FIG. 2D, at the moment t=t 1 , the forward current of main ultrafast recovery diode  107  reduces to zero smoothly, thereby realizing a soft turn-off of main ultrafast recovery diode  107 ; 
     After the soft turn-off of main ultrafast recovery diode  107 , resonant inductor  105  resonates with resonant capacitor  104 , as shown in FIG. 2F, at the moment t=t 2 , when the voltage on resonant capacitor  104  resonates to zero, i.e., the voltage V dmain  between the drain and source of main MOSFET  103  is also zero, the parasitic diode of main MOSFET starts turn-on and freewheel. 
     During the freewheeling period of the parasitic diode of main MOSFET  103 , at the moment t=t 3 , main MOSFET is turned on while auxiliary MOSFET  106  is turned off, thus, realizing a zero voltage turn on of main MOSFET  103 , at this moment, the stored energy in resonant inductor  105  is fed into output filter capacitor  109  through auxiliary ultrafast recovery diode  108 , since the voltage between the drain and source of auxiliary MOSFET  106  is limited by the voltage Vo on output filter capacitor  109  through auxiliary ultrafast recovery diode  108 , thereby also realizing a voltage clamping of auxiliary MOSFET  106  when it is turned off; 
     As shown in FIG. 2E, at the moment t=t 4 , the stored energy in resonant inductor  105  is completely released, i.e. the current flowing through auxiliary ultrafast recovery diode  108  is reduced smoothly to zero, and a soft turn-off of auxiliary ultrafast recovery diode  108  is realized, 
     At the moment t=t 5 , main MOSFET  103  is turned off, resonant capacitor connected in parallel to main MOSFET  103  accomplishes a zero voltage turn-off of main MOSFET  103  as shown in FIG. 2F; along with the rise of the voltage V dsmain  between the drain and source of main MOSFET  103 , voltage V dsaux  between the drain and source of auxiliary MOSFET  106  will also rise due to the resonance of resonant inductor  105  and the output parasitic capacitor of auxiliary MOSFET  106 , and the current flowing through resonant inductor  105  also rises resonantly, as shown in FIG. 2C; 
     At the moment t=t 6 , when the voltage V dsaux  between the drain and source of auxiliary MOSFET  106  equals the voltage on output filter capacitor  109 , i.e. equals to the voltage Vo on load resistor  110 , the current in resonant inductor  105  will flow to the output filter capacitor  109  through auxiliary ultrafast recovery diode  108 , while at this moment main ultrafast recovery diode  107  is turned on, thus the voltage drop withstood on resonant inductor  105  is zero, it can be seen according to V=Lr·di/dt=0, the current flowing through resonant inductor  105  remains unchanged until auxiliary MOSFET  106  is turned on, therefore, at the moment t=t 7 , when auxiliary MOSFET  106  is turned on again periodically, it is a non-zero current turn-on. 
     The converter circuit has been disclosed in China Patent Application CN 95190525.2. The circuit diagrams and the working procedures are shown in FIGS. 1 and 2. When the circuit is at the moment t= 6 , and the voltage V dsaux  between the drain and source terminal of an auxiliary MOSFET  106  equals the voltage on an output filter capacitor  109 , i.e. the voltage Vo on load resistor  110 , the current of resonant inductor  105  flows to output filter capacitor  109  through auxiliary ultrafast recovery diode  108 , but at this time, main ultrafast recovery diode  107  is turned on, therefore, the voltage drop of resonant inductor  105  is zero, it can be seen from V=Lr·di/dt=0 that before auxiliary MOSFET  106  is turned on, the current flowing through the resonant inductor  105  remains unchanged so, therefore at the moment t=t 7 , when auxiliary MOSFET  106  is turned on again periodically, it is a nonzero current turn-on. 
     Due to the above reason, the turn-on of auxiliary MOSFET  106  at the moment t=to is a non-zero current turn-on, thereby resulting in a fact that the turn-off of auxiliary ultrafast recovery diode  108  at t=to is a hard turn-off, so the turn-on loss of auxiliary MOSFET  106  and the turn-off loss of the corresponding auxiliary ultrafast recovery diode  108  are relatively large. 
     SUMMARY OF THE INVENTION 
     The invention gives out an improved ZVT power converter circuit, through which the drawbacks of the above-mentioned invention can be overcome, thus realizing a zero-current turn-on for the auxiliary MOSFET and a soft turn-off for auxiliary ultrafast recovery diode. 
     A basic principle on the invention is to utilize the resonance of a resonant inductor and a resonant capacitor after the auxiliary switch is turned on to realize a zero-voltage turn-on for the main switch. What is more important is that the energy feed device has no energy feed-out when the auxiliary switch is turned on, thereby achieving a zero current turn-on for the auxiliary switch, and the circuit running efficiency is raised. 
     The invention includes the following circuit, which comprising: a main switch, an auxiliary switch, a freewheel diode in parallel with the main switch, a resonant capacitor, a current source, a resonant inductor, a main diode, an energy-feed device and a voltage source. In which the resonant capacitor is connected to the main switch in parallel, the main and auxiliary switches are turned on and off periodically, at the same time when the auxiliary switch is turned off, the main switch is turned on simultaneously, but the auxiliary switch is not turned on until the main switch is turned off for a period of time. In the boost converter circuit, said current source and said auxiliary switch form a loop, wherein the cathode of said main diode is connected to the positive electrode of said voltage source to form a serial branch, which is connected in parallel to said main switch; in the buck converter circuit, said voltage source, main switch and main diode form a loop, wherein the negative electrode of said voltage source is connected to the anode of the main diode, the current source is connected in parallel to the serial branch formed by the main diode and said resonant inductor. In these two converters, the resonant inductor is inserted between the current source and the connecting point of the main diode and said main switch, said auxiliary switch is connected in parallel to the serial branch formed by said resonant inductor and said main switch, said energy-feed device feeds out the residual energy of the resonant inductor when said auxiliary switch is turned off, and meanwhile feeds out the energy of the current source. 
     The basic principle on the invention is to utilize the resonance of a resonant inductor and a resonant capacitor after the auxiliary switch is turned on to realize a zero-voltage turn-on for the main switch. What is more important is that the energy feed device has no energy feed-out when the auxiliary switch is turned on, thereby achieving a zero current turn-on for the auxiliary switch, and the circuit running efficiency is raised. 
     The task of the invention is solved through the following circuit, which comprising: a main switch, an auxiliary switch, a freewheel diode in parallel with the main switch, a resonant capacitor, a current source, a resonant inductor, a main diode, an energy-feed device and a voltage source. In which the resonant capacitor is connected to the main switch in parallel, the main and auxiliary switches are turned on and off periodically, at the same time when the auxiliary switch is turned off, the main switch is turned on simultaneously, but the auxiliary switch is not turned on until the main switch is turned off for a period of time. In the boost converter circuit, said current source and said auxiliary switch form a loop, wherein the cathode of said main diode is connected to the positive electrode of said voltage source to form a serial branch, which is connected in parallel to said main switch; in the buck converter circuit, said voltage source, main switch and main diode form a loop, wherein the negative electrode of said voltage source is connected to the anode of the main diode, the current source is connected in parallel to the serial branch formed by the main diode and said resonant inductor. In these two converters, the resonant inductor is inserted between the current source and the connecting point of the main diode and said main switch, said auxiliary switch is connected in parallel to the serial branch formed by said resonant inductor and said main switch, said energy-feed device feeds out the residual energy of the resonant inductor when said auxiliary switch is turned off, and meanwhile feeds out the energy of the current source. 
     The energy-feed device of the invention can be a diode, i.e. an auxiliary diode, and the auxiliary diode is connected in parallel to a serial branch formed by the resonant inductor and the main diode. 
     The above resonant capacitor can be a parasitic capacitor of said main switching device, said freewheel diode may be an inverse-parallel diode or a parasitic diode of the main switching device. 
     The circuit of the invention ensures that the auxiliary diode is definitely cut off before the auxiliary switch is turned on, thus ensuring a zero current turn-on of the auxiliary switch, and also avoiding the hard turn-off of the auxiliary diode, and raising the circuit efficiency. However, the auxiliary switch is still a hard turn-off. 
     To solve the hard turn-off problems of auxiliary switch, a lossless snubber diode and a lossless snubber capacitor can be added in the above-mentioned circuit. Wherein the lossless snubber diode is connected in series to said auxiliary diode, and the serial branch thus formed is connected in parallel to a serial branch formed by said resonant inductor and main diode, and said lossless snubber capacitor is a cross the connecting point of said lossless snubber diode and said auxiliary diode and the connecting point of said resonant inductor and the main diode. 
     The circuit realizes a zero current turn-on and zero voltage turn-off of the auxiliary switch, and further increases the efficiency of the circuit. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described in detail via embodiments in conjunction with drawings as follows, in these drawings identical or similar portions are represented by identical symbols. 
     FIG. 1 is a topology of a prior art ZVT-BOOST circuit; 
     FIGS. 2A-2F show the working procedures of a prior art ZVT-BOOST circuit; 
     FIGS. 3A-3D show the topological structure view of the circuit of the present invention: 
     FIGS. 4A-4F show the working procedures of an application in a BOOST circuit of the invention; 
     FIG. 5 is a schematic diagram of an application in a BOOST circuit of the present invention; 
     FIG. 6 is a schematic diagram of an application in a BOOST circuit of the present further improved invention; 
     FIG. 7 is a schematic diagram of an application in a BUCK circuit of the present invention; 
     FIG. 8 is a schematic diagram of an application in a BUCK circuit of the present further improved invention; 
     FIG. 9 is a schematic diagram of application in a bridge circuit of the present invention; 
     FIG. 10 is a schematic diagram of an application in a bridge circuit of the present further improved invention; 
     FIG. 11 is an application embodiment in a 2 kW Power Factor Correcting (PFC) circuit of the present invention; 
     FIG. 12 is an application embodiment in a 2 kW Power Factor Correcting (PFC) circuit of the present further improved invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A circuit topological structure of the invention is shown in FIG. 3, where FIGS. 3A-3C are several connecting modes of the invention, which are applied in various circuits. 
     FIG. 3A is a topological structure applied in a BOOST circuit, current source  305 , resonant inductor  105 , main diode  107  and a first voltage source  308  are connected in order to form a main circuit, and other parts of the circuit are connected as the description for a scheme of the invention. The topological circuit includes current source  305 , main switch  302 , main diode  107 , auxiliary switch  301 , auxiliary diode  108 , resonant inductor  105 , resonant capacitor  304 , freewheel diode  303 , and first voltage source  308 , wherein current source  305 , resonant inductor  105 , main diode  107 , first voltage source  308  are connected in order to form a serial loop; and the positive electrode of current source  305  is connected to one end of resonant inductor  105 , and the other end of the inductor is connected to the anode of main diode  107 . The cathode of main diode  107  is connected to the positive electrode of first voltage source  308 . Auxiliary switch  301  is connected in parallel to current source  305 . The anode of auxiliary diode  108  is connected to the positive electrode of current source  305 , and the cathode of auxiliary diode  108  is connected to the positive electrode of first voltage source  308 . Main switch  302  is connected across the anode of main diode  107  and the negative electrode of first voltage source  308 . Resonant capacitor  304  and freewheel diode  303  are in parallel with a main switch  302 , wherein the cathode of the freewheel diode is connected to the anode of main diode  107 . 
     FIG. 3B is a topological structure applied in BUCK circuit. Voltage source  309 , main switch  302 , resonant inductor  105 , and current source  310  are connected in order to form a main loop, other portions of the circuit are connected as the description for the scheme of the invention. The topological circuit consists of voltage source  309 , main switch  302 , main diode  107 , auxiliary switch  301 , auxiliary diode  108 , resonant inductor  105 , resonant capacitor  304 , freewheel diode  303 , and current source  310 , Wherein main diode  107 , resonant inductor  105  and current source  310  are connected in order to form a serial loop, and the cathode of main diode  107  is connected to one end of resonant inductor  105 , and the other end of inductor  105  is connected to the negative electrode of current source  310 . Auxiliary diode  108  is in parallel with current source  310 , wherein the cathode of auxiliary diode  108  is connected to the negative electrode of current source  310 . A serial branch formed by freewheel diode  303  and voltage source  309  is in parallel with main diode  107 , wherein the anode of freewheel diode  303  is connected to the cathode of main diode  107 , and the cathode of freewheel diode  303  is connected to the positive electrode of the voltage source  309 . Both main switch  302  and resonant capacitor  304  are in parallel with freewheel diode  303 . Auxiliary switch  301  is connected across the positive electrode of voltage source  309  and the cathode of auxiliary diode  108 . 
     FIG. 3B can be further improved. To be illustrated, a lossless snubber diode ( 307 ) is included between the auxiliary diode ( 108 ) and the current source ( 310 ), wherein the cathode of the lossless snubber diode ( 307 ) is connected to the anode of the auxiliary diode ( 108 ), and the anode of the lossless snubber diode ( 307 ) is connected to the positive electrode of the current source ( 310 ); a lossless snubber capacitor ( 306 ) is included, one end of the capacitor is connected to the cathode of the main diode ( 107 ), the other end of the capacitor is connected to the connecting point between the auxiliary diode ( 108 ) and the lossless snubber diode ( 307 ). 
     FIG. 3C is a topological structure of an arm of a bridge circuit when it is applied in the bridge circuit, one arm can be regarded as a combination of a BOOST circuit and a BUCK circuit, wherein, a voltage source  311 , the first of main switches  302 , the first of resonant inductors  105 , current source  312 , the first of resonant capacitors  304 , the first of freewheel diodes  303 , the first of auxiliary switches  301 , the first of auxiliary diodes  108 , and the first of main diodes  107  form a BUCK circuit; current source  312 , the second of resonant inductors  105 ′, the second of main diodes  107 ′, voltage source  311 , the second of main switches  302 ′, the second of resonant capacitors  304 ′, the second of freewheel diodes  303 ′, the second of the auxiliary switches  301 ′, and the second of auxiliary diodes  108 ′ form a BOOST circuit, both circuits share a current source  312  and a voltage source  311  to form an arm in the bridge circuit. 
     The topological circuit consists of voltage source  311 , the first of auxiliary switches  301 , the second of the auxiliary switches  301 ′, the first of auxiliary diodes  108 , the second of the auxiliary diodes  108 ′, the first of main switches  302 , the second of main switches  302 ′, the first of resonant inductors  105 , the second of the resonant inductors  105 ′, the first of resonant capacitors  304 , the second of resonant capacitors  304 ′, the first of freewheel diodes  303 , the second of the freewheel diodes  303 ′, the first of main diodes  107 , the second of main diodes  107 ′ and current source  312 , wherein the serial branch formed the first of auxiliary diodes  108  and the second of auxiliary diodes  108 ′ is in parallel with a voltage source  311 , and the anode of the first of auxiliary diodes  108  is connected to the negative electrode of a voltage source  311 , the cathode of the second of auxiliary diodes  108 ′ is connected to the positive electrode of voltage source  311 . The first of auxiliary switches  301  is in parallel with the second of the auxiliary diodes  108 ′, and the second of auxiliary switches  301 ′ is connected in parallel to the first of auxiliary diodes  108 . The serial branch formed by the first of freewheel diodes  303  and the first of resonant inductors  105  is in parallel with the second of auxiliary diodes  108 ′, wherein the cathode of the first of freewheel diodes  303  is connected to the cathode of the second of auxiliary diodes  108 ′. The serial branch formed by the second of freewheel diodes  303 ′ and the second of resonant inductors  105 ′ is in parallel with the first of auxiliary diodes  108 , wherein the anode of the second of freewheel diodes  303 ′ is connected to the anode of the first of auxiliary diodes  108 , both the first of main switches  302  and the first of resonant capacitors  304  are connected in parallel to the first of freewheel diodes  303 . Both the second of main switch  302 ′ and the second of resonant capacitors  304 ′ are connected in parallel to the second of freewheel diodes  303 ′. The anode of the first of main diodes  107  is connected to the negative electrode of voltage source  311 , and the cathode of the first of main diodes  107  is connected to the anode of the first of freewheel diodes  303 . The anode of the second of main diodes  107 ′ is connected to the cathode of the second of freewheel diodes  303 ′, and the cathode of the second of main diodes  107 ′ is connected to the positive electrode of voltage source  311 . One end of current source  312  is connected to the cathode of the first of auxiliary diodes  108 , and the other end of current source  312  is connected to the other arm in the bridge circuit. 
     FIG. 3D is a topological structure of the further improved invention applied in a BOOST circuit, in the scheme illustrated in FIG. 3A, two components, i.e. lossless snubber capacitor  306  and lossless snubber diode  307  are added in the topological structure, wherein, a serial branch formed lossless snubber diode  307  and auxiliary diode  108  is in parallel with a serial branch formed by resonant inductor  105  and main diode  107 ; lossless snubber capacitor  306  is connected across the two connecting points of the above two branches. 
     The topological circuit includes current source  305 , auxiliary switch  301 , auxiliary diode  108 , lossless snubber capacitor  306 , lossless snubber diode  307 , resonant inductor  105 , resonant capacitor  304 , main switch  302 , main diode  107 , freewheel diode  303 , and first voltage source  308 , wherein a serial loop is formed in series by current source  305 , resonant inductor  105 , main diode  107  and firsts voltage source  308 , and the positive electrode of a current source  305  is connected to one end of resonant inductor  105 , the other end of resonant inductor  105  is connected to the anode of main diode  107 , and the cathode of main diode  107  is connected to the positive electrode of first voltage source  308 . Auxiliary switch  301  is in parallel with current source  305 . The anode of freewheel diode  303  is connected to the negative electrode of first voltage source  308 , and the cathode of freewheel diode  303  is connected to the anode of main diode  107 . Both main switch  302  and resonant capacitor  304  are in parallel with freewheel diode  303 . The anode of auxiliary diode  108  is connected to the positive electrode of current source  305 , the cathode of auxiliary diode  108  is connected to the anode of lossless snubber diode  307 , the cathode of lossless snubber diode  307  is connected to the cathode of main diode  107 , and lossless snubber capacitor  306  is connected across the anode of lossless snubber diode  307  and the anode of main diode  107 . 
     Similar to the topological structure of a circuit shown in FIG. 3D, the application of the invention in a BUCK circuit can also be further improved, wherein another lossless snubber diode  307  is also included between auxiliary diode  108  and current source  310 , and the cathode of lossless snubber diode  307  is connected to the anode of auxiliary diode  108 , and the anode of lossless snubber diode  307  is connected to the positive electrode of current source  310 ; and another lossless snubber capacitor  306  is also included, one end of the capacitor is connected to the cathode of main diode  107 , the other end of the capacitor is connected to the connecting point of auxiliary diode  108  and lossless snubber diode  307 . 
     FIG. 5 is a schematic diagram of an application of the invention in a BOOST circuit. Its core is a topological structure shown in FIG.  3 A. Its circuit comprises voltage source  501 , energy-storage inductor  502 , main switch  302 , freewheel diode  303 , resonant capacitor  304 , resonant inductor  105 , auxiliary switch  301 , main diode  107 , auxiliary diode  108 , output filter capacitor  503 , and load resistor  504 , wherein the current source in FIG. 3A is replaced by voltage source  501  and energy-storage inductor  502 , and output filter capacitor  503  together with a load resistor  504  serve as an output circuit of the circuit. 
     FIG. 4 shows the working procedures of the circuit shown in FIG.  5 . FIG. 4A shows the waveform of gate driving signal voltage V 1  of auxiliary switch  106 ; FIG. 4B shows the waveform of gate driving signal voltage V 2  of main switch  302 ; FIG. 4C shows the waveform of current I lr  in resonant inductor  105 ; FIG. 4D shows the waveform of current I Daux  of auxiliary diode  108 ; FIG. 4B shows the waveform of current I Dmain  of main diode  107 ; FIG. 4F shows the waveform of the voltage V Qmain  main switch  302 . It can be seen from the above drawings: 
     At the moment t=t 0 , auxiliary switch is turned on, since the currents of energy storage inductor  502  and resonant inductor  105  can not change abruptly, therefore at the instant of turning on the current of auxiliary switch  301  is zero. Thus, the circuit realized zero current turn-on for auxiliary switch  301  by means of resonant inductor  105 ; 
     Starting from the moment t 0 , the current of resonant inductor  105 , i.e. the current on main diode  107 , decreases gradually, and at the moment t=t 1  reduces to zero, thereby realizing a soft turn-off of main diode  107  by utilizing resonant inductor  105 ; 
     After the soft turn-off of main diode  107 , resonant inductor  105  will resonate with resonant capacitor  304  as shown in FIG. 4F, at the moment t=t 2 , the voltage on resonant capacitor  304  will resonate to zero, i.e. the voltage V Qmain  of main switch  302  is also zero, and afterwards freewheel diode  303  starts turn-on; 
     During the freewheeling period of freewheel diode  303 , at the moment t=t 3 , main switch  302  is turned on, at the same time auxiliary switch  301  is turned off, so that zero voltage turn-on of main switch  302  is realized. At this time, the current in energy storage inductor  502  plus the resonant current in a resonant inductor  105  flows to output filter capacitor  503  through auxiliary diode  108 , since the voltage on both ends of auxiliary switch  301  is limited by the voltage on output filter capacitor  503  through auxiliary diode  108 , thereby a voltage clamping is realized when auxiliary switch  301  is turned off; 
     At the moment t=t 4 , when the current in a resonant inductor  105  inverts and gradually increases to the current value of energy-storage inductor  502 , the current of auxiliary diode  108  reduces gradually to zero, thereby a soft turn-off of auxiliary diode  108  is realized; 
     At the moment t=t 5 , main switch  302  is turned off, and a zero voltage turn-off of main switch  302  is realized by resonant capacitor  304  in parallel with main switch  302 ; 
     At the moment t=t 6 , the voltage of resonant capacitor  304  rises to a voltage as that of output filter capacitor  503 , the turn-on of main diode  107  limits the voltage overshoot of the main switch via a voltage clamping circuit formed by main diode  107  and output filter capacitor  503 , as shown in FIG. 4F; 
     At the moment t=t 7 , auxiliary switch  301  is turned on again, the above procedures are periodically repeated. 
     It can be seen from the above, the invention has solved both the problem of non-zero current turn-on of auxiliary switch  301 , and at the same time the problem of hard turn-off of auxiliary diode  108 . 
     The difference between soft switching circuit shown in FIG.  6  and the circuit shown in FIG. 5 lies in that a lossless snubber diode  307  and a lossless snubber capacitor  306  are added, i.e. the circuit is an application of the topological structure in FIG.  3 D. Its topological circuit comprises a second voltage source  601 , energy-storage inductor  602 , main switch  302 , freewheel diode  303 , resonant capacitor  304 , resonant inductor  105 , auxiliary switch  301 , main diode  107 , auxiliary diode  108 , lossless snubber diode  307 , lossless snubber capacitor  306 , output filter capacitor  603  and load resistor  604 , wherein the current source ( 305 ) in FIG. 3D is replaced by a serial circuit formed via the second voltage source ( 601 ) in series with the energy-storage inductor ( 602 ), and the first voltage source ( 308 ) in FIG. 3D is replaced by a parallel circuit formed via the output filter capacitor ( 603 ) in parallel with the load resistor ( 604 ). 
     The working procedures of the circuit shown in FIG.  6  and that of the circuit shown in FIG. 5 differ in: 
     1. At the moment t=t 3  when auxiliary switch  301  is turned off, the current in energy-storage inductor  602  plus the resonant current in resonant inductor  105  flows to lossless snubber capacitor  306  through auxiliary diode  108 , thereby, a zero voltage turn-off of auxiliary switch  301  is realized; 
     2. At the moment t=t 5  when main switch  302  is turned off, the energy stored in lossless snubber capacitor  306  feeds to output filter capacitor  603  via a lossless snubber diode  307 . 
     The soft switching topological circuit shown in FIG. 7 is an application of the invention in a BUCK circuit. Its core is the topological structure shown in FIG.  3 B. Its circuit comprises a voltage source  701 , an energy-storage inductor  702 , an auxiliary switch  301 , a main switch  302 , a freewheel diode  303 , a resonant capacitor  304 , a resonant inductor  105 , a main diode  107 , an auxiliary diode  108 , an output filter capacitor  703 , and a load resistor  704 . The positive electrode of the voltage source  701  is connected to the cathode of the freewheel diode  303 , the negative electrode of the voltage source  701  is connected to the anode of the main diode  107 ; the current source  310  in FIG. 3B is a branch constituted by the electrolytic capacitor  703  in series with the energy-storage inductor  702 , one end of the branch is connected to the negative electrode of the voltage source  701  and is connected to the anode of the main diode  107 , the other end of the branch is connected to the connecting point of the resonant inductor  105  and the auxiliary switch  301 . The load resistor  704  can be added in parallel with the electrolytic capacitor  703 . Its idea of realizing ZVT is the same as the ZVT-BOOST circuit shown in FIG. 5, and the specific working procedures are as follows; 
     When auxiliary switch  301  is turned on, a soft turn-off of main diode  107  and a zero current turn-on of auxiliary switch  301  are realized by means of resonant inductor  105 ; 
     After the soft turn-off of main diode  107 , resonant capacitor  304  resonates with resonant inductor  105 , when the voltage drop on resonant capacitor  304  is zero, freewheel diode  303  starts turn on. During the turn-on period of freewheel diode  303 , main switch  302  is turned on, thereby realizing zero voltage turn-on of main switch  302 ; 
     At the same time as main switch  302  is turned on an auxiliary switch  301  is turned off, at this moment, auxiliary diode  108  is turned on to provide freewheel for energy-storage inductor  702 , and resonant inductor  105 ; 
     After a main switch  302  is turned on, the current in a resonant inductor  105  rises gradually, so the soft turn-off of auxiliary diode  108  is realized; 
     When main switch  302  is turned off, resonant capacitor  304  in parallel with main switch  302  realizes a zero voltage turn-off of the main switch; 
     When the voltage of resonant capacitor  304  rises to the same level as for the voltage source  701 , main diode  107  is turned on. 
     At a certain time afterwards, auxiliary switch is turned on again, and repeats periodically the above procedures. 
     Application of the further improved invention in a BUCK circuit is seen in FIG.  8 . The circuit is based on the circuit shown in FIG. 7 with an addition of lossless snubber diode  307  and lossless snubber capacitor  306 . The circuit comprises a voltage source  801 , an auxiliary switch  301 , a main switch  302 , a freewheel diode  303 , a resonant capacitor  304 , a resonant inductor  105 , a main diode  107 , an auxiliary diode  108 , a lossless snubber diode  307 , a lossless snubber capacitor  306 , an energy-storage inductor  802 , an output filter capacitor  803  and a load resistor  804 . 
     The difference of the working procedure of the circuit shown in FIG. 8 from that in FIG. 7 lies in: 
     1. when auxiliary switch  301  is turned off, a freewheel is providing to energy-storage inductor  802  via main switch  302 , lossless snubber capacitor  306 , and auxiliary diode  108 , meanwhile lossless snubber capacitor  306  and auxiliary diode  108  also provide freewheel to resonant inductor  105 , and zero voltage turn-off of auxiliary switch  301  is realized via charging lossless snubber capacitor  306 ; 
     2. when main switch  302  is off, energy stored in lossless snubber capacitor  306  feeds energy to resonant inductor  105  and energy-storage inductor  802  via lossless snubber diode  307 . 
     Application of the invention in a bridge circuit is shown in FIG.  9 . The circuit shown in FIG. 9 is a schematic diagram of an arm in a bridge circuit, its core is the topological structure shown in FIG.  3 C. The current source  312  in FIG. 3C is replaced by an inductor  902 . The circuit comprises a voltage source  901 , the first auxiliary switches  301 , the second of auxiliary switches  301 ′, the first of main switches  302 , the second of main switches  302 ′, the first of freewheel diodes  303 , the second freewheel diodes  303 ′, the first resonant capacitors  304 , the second of resonant capacitors  304 ′, the first of resonant inductors  105 , the second of resonant inductors  105 ′, the first of main diodes  107 , the second of main diodes  107 ′, the first of auxiliary diodes  108 , the second of auxiliary diodes  108 ′, the inductor  902 , wherein voltage source  901 , the first of main switches  302 , the first of resonant inductors  105 , the inductor  902 , the first of resonant capacitors  304 , the first of freewheel diodes  303 , the first of auxiliary switches  301  and the first of auxiliary diodes  108  form a ZVT-BUCK circuit, inductor  902 , the second of resonant inductors  105 ′, the second of main diodes  107 ′, voltage source  901 , the second of main switches  302 ′, the second of resonant capacitors  304 ′, the second of freewheel diodes  303 ′, the second of auxiliary switches  301 ′ and the second of auxiliary diodes  108 ′ form a ZVT-BOOST circuit, two circuits share an inductor  902  and a voltage source  901 , forming an arm in the bridge circuit, the other end(C) of inductor  902  is connected to other arms of the bridge. 
     The circuit shown in FIG. 10 is a schematic diagram of an application of the further improved invention in an arm of the bridge circuit. The circuit is based on FIG. 9 with an addition of The first of lossless snubber capacitors  306 , the second of lossless snubber capacitors  306 ′, the first of lossless snubber diodes  307  and the second of lossless snubber diodes  307 ′, wherein the first of lossless snubber capacitors  306  and the first of lossless snubber diodes  307  are added to the ZVT-BUCK circuit shown in FIG. 9 to form an improved ZVT-BUCK circuit; the second of lossless snubber capacitors  306 ′ and the second of lossless snubber diodes  307 ′ are added to the ZVT-BOOST circuit shown in FIG. 9 to form an improved ZVT-BOOST circuit. The first of the lossless snubber diodes  307  is added between the first of the auxiliary diodes  108  and the voltage source  901 , the anode of the first of the lossless snubber diodes  307  is connected to the negative electrode of the voltage source  901 , its cathode is connected to the anode of the first of the auxiliary diodes  108 . The first of the lossless snubber diodes  306  is also included, its one end is connected to the connecting point of the first of the auxiliary diodes  108  and the first of the lossless snubber diodes  307 , its other end is connected to the connecting point of the first of the resonant inductors  105  and the first of the main switches  302 . The second of the lossless snubber diodes  307 ′ is added between the second of the auxiliary diodes  108 ′ and the voltage source  901 , the cathode of the second of the lossless snubber diodes  307 ′ is connected to the positive electrode of the voltage source  901 , its anode is connected to the cathode of the second of the auxiliary diodes  108 ′. The second of the lossless snubber capacitors  306 ′ is included, its one end is connected to the connecting point of the second of the auxiliary diodes  108 ′ and the second of the lossless snubber diodes  307 ′ , its other end is connected to the connecting point of the second of the resonant inductors  105 ′ and the second of the main switches  302 ′. 
     A circuit of the invention applied in 2 kW Power Factor Correction (PFC) is shown in FIG.  11 . It is a ZVT-BOOST circuit, its input is 220 V single-phase AC voltage, after being filtered by a filter network  1101 , and after being rectified by rectifying bridge  1002 , it is sent to the main circuit as a voltage source; the inductance value of energy-storage inductor  1103  in the main loop is set at 300 μH, the value of resonant inductor  105  is set at 20 μH, main diode  107  comprises DSEI 30-06 A (600 V, 37 A), auxiliary diode  108  comprises DSEI 12-06 A (600 V,  14 A), main switch  302  comprises two MOSFETs with model number of IXFH 32N50 (500 V, 32 A) in parallel, while auxiliary switch  301  comprise a MOSFET with a model number of IXFH 20N60 (600 V, 20 A), the driving control circuit of main switch  302  and auxiliary switch  301  comprises a special ZVT-PFC control chip with model number of UC 3855BN, the resonant capacitor comprise a 4n7 non-inductive capacitor. The output filter capacitor comprises three electrolytic capacitors of 330 μF/450 V in parallel. This circuit can provide a direct current with an output voltage of 450 V, and a power of 2 kW achieving a satisfactory result with efficiency as high as 97.3%. 
     An application of the further improved invention in a 2 KW PFC circuit is shown as FIG.  12 . The values set for other components and devices are basically the same as in FIG. 11, while the value of the additional lossless snubber diode is set at DSEI 12-60 A (600 V, 14 A), and the lossless snubber capacitor is set at a 6n6 non-inductive capacitance. 
     The efficiency of application of the further improved invention in a 2 KW PFC circuit reached as high as 97.5%. 
     Although the main technical features and advantages of the invention have been described in detail with the above preferred embodiments, obviously the protection scope of the invention is not limited to the above embodiments, but include a variety of obviously alternative schemes in accordance with the above inventive conception.