Abstract:
The present invention provides a bi-directional DC/DC converter and a control method thereof. The bi-directional DC/DC converter comprises a push-pull circuit, a chargeable and dischargeable device, a transformer, a half bridge circuit, a rectifying and filtering circuit and a first switch. The primary side of the transformer is connected to an output of the push-pull circuit. The output of the rectifying and filtering circuit is connected across a bridge arm of the half bridge circuit. The first switch is configured to connect the secondary side of the transformer to an input of the rectifying and filtering circuit, or to connect a part of windings of the secondary side of the transformer to an output of the half bridge circuit while disconnecting the part of windings of the secondary side of the transformer from two terminals of the input of the rectifying and filtering circuit. The bi-directional DC/DC converter of the present invention has fewer components and is low in cost.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to power electronics, and particularly, to a bi-directional DC/DC converter and a control method thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    A bi-directional DC/DC converter is a DC converter which can adjust bi-directional power transfer as required. It has been widely applied in DC uninterruptible power systems, aerospace power systems, battery energy storage, electric vehicles, hybrid energy vehicles, fuel cells, DC power amplifiers etc. 
         [0003]    A conventional DC/DC converter converts DC of a rechargeable battery to AC, and then converts AC to a required DC through a rectifying and filtering circuit, so as to achieve discharge process of the rechargeable battery. During the process of charging the rechargeable battery, a separate charging circuit is required. Therefore, the DC/DC converter has more components and is complex in structure. 
         [0004]    Therefore, it is expected to use a bi-directional DC/DC converter with fewer components and lower cost to achieve the charge and discharge processes of the rechargeable battery, namely to achieve bi-directional power transfer. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the above-mentioned prior art, the present invention provides a bi-directional DC/DC converter, which comprises: 
         [0006]    a push-pull circuit or a full bridge circuit; 
         [0007]    a chargeable and dischargeable device for providing DC to the push-pull circuit or the full bridge circuit; 
         [0008]    a transformer, the primary side of which is connected to an output of the push-pull circuit or the full bridge circuit; 
         [0009]    a half bridge circuit; 
         [0010]    a rectifying circuit or a rectifying and filtering circuit, an output of which is connected to across a bridge arm of the half bridge circuit; 
         [0011]    a first switch, which is configured to connect the secondary side of the transformer to an input of the rectifying circuit or the rectifying and filtering circuit, or connect a part of windings of the secondary side of the transformer to an output of the half bridge circuit while 
         [0012]    disconnecting the part of windings of the secondary side of the transformer from two terminals of the input of the rectifying circuit or the rectifying and filtering circuit. 
         [0013]    Preferably, two switching tubes of the push-pull circuit are connected with anti-parallel diodes. 
         [0014]    Preferably, four switching tubes of the full bridge circuit are connected with anti-parallel diodes. 
         [0015]    Preferably, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a center tap of the primary side of the transformer. 
         [0016]    Preferably, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a terminal of the input of the full bridge circuit. 
         [0017]    The present invention provides a control method for the above-mentioned bi-directional DC/DC converter. The control method comprises: controlling the first switch to connect the secondary side of the transformer to the input of the rectifying circuit or the rectifying and filtering circuit; controlling any one or two switching tubes of the half bridge circuit to switch off; and controlling the push-pull circuit or the full bridge circuit to operate in a pulse width modulation (PWM) mode so as to discharge the chargeable and dischargeable device and charge two capacitors of the half bridge circuit. In another embodiment of the present invention, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a center tap of the primary side of the transformer, and the control method of the present invention further comprises: controlling the second switch to switch on. In another embodiment of the present invention, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a terminal of the input of the full bridge circuit, and the control method of the present invention further comprises: controlling the second switch to switch on. 
         [0018]    The present invention further provides a control method for the above-mentioned bi-directional DC/DC converter. The control method comprises: controlling the first switch to connect a part of windings of the secondary side of the transformer to the output of the half bridge circuit; controlling switching tubes of the push-pull circuit or the full bridge circuit to switch off, and controlling the half bridge circuit to operate in a PWM mode so as to discharge two capacitors of the half bridge circuit alternately and charge the chargeable and dischargeable device. In another embodiment of the present invention, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a center tap of the primary side of the transformer, and the control method of the present invention further comprises: controlling the second switch to switch off. In another embodiment of the present invention, the bi-directional DC/DC converter further comprises a first inductor and a second switch, wherein the first inductor and the second switch are connected in parallel and then connected in series to the chargeable and dischargeable device, and are connected in parallel between the positive of the chargeable and dischargeable device and a terminal of the input of the full bridge circuit, and the control method of the present invention further comprises: controlling the second switch to switch off. 
         [0019]    The bi-directional DC/DC converter of the present invention has fewer components and is low in cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Below, embodiments of the present invention are further described with reference to the drawings, wherein: 
           [0021]      FIG. 1  is a circuit diagram of a bi-directional DC/DC converter of a first embodiment of the present invention. 
           [0022]      FIG. 2  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 1  in the back-up mode. 
           [0023]      FIG. 3  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 1  in the charge mode. 
           [0024]      FIG. 4  is a circuit diagram of a bi-directional DC/DC converter of a second embodiment of the present invention. 
           [0025]      FIG. 5  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 4  in the back-up mode. 
           [0026]      FIG. 6  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 4  in the charge mode. 
           [0027]      FIG. 7  is a circuit diagram of a bi-directional DC/DC converter of a third embodiment of the present invention. 
       
    
    
     REFERENCE SYMBOLS 
       [0028]    B rechargeable battery 
         [0029]    Tr 1  transformer 
         [0030]    Tr 2  transformer 
         [0031]    Q 1 ˜Q 4 , Q 7 ˜Q 10  metal oxide semiconductor field effect transistor 
         [0032]    D 1 ˜D 10  diode 
         [0033]    S 1 , S 2  switch 
         [0034]    C 1 , C 2  capacitor 
         [0035]    L 1 ˜L 3  inductor 
         [0036]      1  push-pull circuit 
         [0037]      2  full bridge rectifying circuit 
         [0038]      3  half bridge circuit 
         [0039]      4  center tap 
         [0040]      5  node 
         [0041]      6  terminal 
         [0042]      7  terminal 
         [0043]      8  terminal 
         [0044]      9  terminal 
         [0045]      10  node 
         [0046]      11  node 
         [0047]      12  node 
         [0048]      13  node 
         [0049]      14  terminal 
         [0050]      15  rectifying and filtering circuit 
         [0051]      16  full bridge circuit 
         [0052]      17  terminal 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail by using the specific embodiments below, with reference to the drawings. It should be understood that the specific embodiments described herein are only used for explaining the present invention, and are not intended to limit the present invention. 
         [0054]      FIG. 1  is a circuit diagram of a bi-directional DC/DC converter of a first embodiment of the present invention. As shown in  FIG. 1 , it comprises a push-pull circuit  1 , a full bridge rectifying circuit  2 , a half bridge circuit  3 , a transformer Tr 1  and a switch S 1 . The push-pull circuit  1  comprises metal oxide semiconductor field effect transistors Q 1  and Q 2 , wherein metal oxide semiconductor field effect transistors Q 1  and Q 2  have a parasitic diode D 1  and a parasitic diode D 2  respectively. The drain of the metal oxide semiconductor field effect transistor Q 1  and the drain of the metal oxide semiconductor field effect transistor Q 2  act as the output of the push-pull circuit  1  and is connected to the primary side of the transformer Tr 1 . The source of the metal oxide semiconductor field effect transistor Q 1  and the source of the metal oxide semiconductor field effect transistor Q 2  are connected to form a node  5 . The center tap  4  of the primary side of the transformer Tr 1  and the node  5  act as the input of the push-pull circuit  1 . A rechargeable battery B which can provide DC is connected to the input of the push-pull circuit  1 . The full bridge rectifying circuit  2  comprises diodes D 3 ˜D 6 , wherein the anode of the diode D 3  and the cathode of the diode D 4  are connected to form a node  10 , the anode of the diode D 5  and the cathode of the diode D 6  are connected to form a node  11 , the nodes  10  and  11  act as the input of the full bridge rectifying circuit  2 , and the cathode of the diode D 5  and the anode of the diode D 6  act as the output of the full bridge rectifying circuit  2 . The half bridge circuit  3  comprises metal oxide semiconductor field effect transistors Q 3  and Q 4 , capacitors C 1  and C 2 . The capacitors C 1  and C 2  are connected to form a node  13 . The source of the metal oxide semiconductor field effect transistor Q 3  and the drain of the metal oxide semiconductor field effect transistors Q 4  are connected to form a node  12 , wherein the metal oxide semiconductor field effect transistors Q 3  and Q 4  act as a bridge arm of the half bridge circuit  3 , and the nodes  12  and  13  act as the output of the half bridge circuit  3 . The output of the full bridge rectifying circuit  2  is connected across the bridge arm of the half bridge circuit  3 , i.e. the output of the full bridge rectifying circuit  2  is connected to the drain of the metal oxide semiconductor field effect transistor Q 3  and the source of the metal oxide semiconductor field effect transistor Q 4 . The secondary side of the transformer Tr 1  has three windings, wherein the first winding has terminals  6  and  14 , the second winding has terminals  7  and  8 , and the third winding has terminals  8  and  9 . The terminal  8  is a common terminal of the second winding and the third winding. The terminal  14  is connected to the node  10 , and the terminal  8  is connected to the node  13 , wherein the terminals  14  and  9  act as the two terminals of the secondary side of the transformer Tr 1 . The switch S 1  is a double pole double throw switch, which comprises a suspension terminal  17 . The switch S 1  is controlled to connect the terminal  6  to the terminal  7  and connect the terminal  9  to the node  11  in the first state, so that the two terminals of the secondary side of the transformer Tr 1  are connected to the input of the full bridge rectifying circuit  2 . The switch S 1  is controlled to connect the terminal  7  to the node  12  and disconnect the terminal  9  from the node  11  (i.e. connect the terminal  9  to the terminal  17 ), so that the output of the half bridge circuit  3  is connected to a winding between the terminal  7  and the terminal  8  of the transformer Tr 1 , and the two terminals of the input of the full bridge rectifying circuit  2  (i.e. the nodes  10  and  11 ) are disconnected from the winding between the terminal  7  and the terminal  8  of the transformer Tr 1 . In other embodiments of the present invention, the node  13  can be connected to the grounding. 
         [0055]    The operating principle of the bi-directional DC/DC converter shown in  FIG. 1  will be described below with reference to  FIGS. 2 and 3 . 
         [0056]      FIG. 2  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 1  in the back-up mode. In the back-up mode, i.e. in the discharging process of the rechargeable battery B, the switch S 1  is controlled to connect two terminals of the secondary side of the transformer Tr 1  (the terminals  9  and  14 ) to the input of the full bridge rectifying circuit  2  (the nodes  10  and  11 ), the metal oxide semiconductor field effect transistors Q 3  and/or Q 4  are controlled to switch off, the equivalent circuit diagram is shown in  FIG. 2 . In the present embodiment, the push-pull circuit  1  can be controlled by using the control method in the prior art, so as to discharge the rechargeable battery B and charge the capacitors C 1  and C 2 . For example, a pulse width modulation (PWM) signal is provided to the gates of the metal oxide semiconductor field effect transistors Q 1  and Q 2 , and in the first time period, the metal oxide semiconductor field effect transistor Q 1  is controlled to switch on and the metal oxide semiconductor field effect transistor Q 2  is controlled to switch off, thus the rechargeable battery B discharges through the metal oxide semiconductor field effect transistor Q 1  at the primary side of the transformer Tr 1 , and the secondary side of the transformer Tr 1  charges the capacitors C 1  and C 2  simultaneously through the full bridge rectifying circuit  2 . In the second time period, the metal oxide semiconductor field effect transistor Q 1  is controlled to switch off and the metal oxide semiconductor field effect transistor Q 2  is controlled to switch on, thus the rechargeable battery B discharges through the metal oxide semiconductor field effect transistor Q 2  at the primary side of the transformer Tr 1 , and the secondary side of the transformer Tr 1  charges the capacitors C 1  and C 2  simultaneously through the full bridge rectifying circuit  2 . Then the control methods in the first time period and the second time period are repeated alternately, so as to achieve the transfer of the power from the rechargeable battery B to the capacitors C 1  and C 2 . In the present embodiment, the duty cycle of the PWM signal applied to the metal oxide semiconductor field effect transistors Q 1  and Q 2  may be constant or variable. In other embodiments of the present invention, the voltages across the capacitor C 1  and the capacitor C 2  may be positive voltage and negative voltage respectively when the node  13  is grounded. 
         [0057]      FIG. 3  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 1  in the charge mode. In the charge mode, i.e. in the charge process of the rechargeable battery B, the switch S 1  is controlled to connect the output of the half bridge circuit  3  to the winding between the terminal  7  and the terminal  8  of the transformer Tr 1 , i.e. the node  12  and the node  13  are connected to the terminal  7  and the terminal  8  respectively while disconnecting the terminal  9  from the node  11 , the metal oxide semiconductor field effect transistors Q 1  and Q 2  are controlled to switch off, the equivalent circuit diagram is shown in  FIG. 3 . In the present embodiment, the half bridge circuit  3  can be controlled by using the control method in the prior art, so as to discharge the capacitors C 1  and C 2  and charge the rechargeable battery B. For example, a PWM signal is provided to the gates of the metal oxide semiconductor field effect transistors Q 3  and Q 4 , and in the first time period, the metal oxide semiconductor field effect transistor Q 3  is controlled to switch on and the metal oxide semiconductor field effect transistor Q 4  is controlled to switch off, thus the capacitor C 1  discharges, and the current generated in the primary side of the transformer Tr 1  flows to the positive of the rechargeable battery B through the diode D 2 , so as to charge the rechargeable battery B. In the second time period, the metal oxide semiconductor field effect transistor Q 3  is controlled to switch off and the metal oxide semiconductor field effect transistor Q 4  is controlled to switch on, thus the capacitor C 2  discharges, and the current generated in the primary side of the transformer Tr 1  flows to the positive of the rechargeable battery B through the diode D 1 , so as to charge the rechargeable battery B. Then the control method in the first time period and the second time period are repeated alternately, so as to continually charge the rechargeable battery B by alternately discharging the capacitor C 1  and the capacitor C 2 . In the present embodiment, the duty cycle of the PWM signal applied to the metal oxide semiconductor field effect transistors Q 3  and Q 4  may be constant or variable. 
         [0058]      FIG. 4  is a circuit diagram of a bi-directional DC/DC converter of a second embodiment of the present invention, which is similar to that shown in  FIG. 1 . The difference is that the bi-directional DC/DC converter of  FIG. 4  further comprises an inductor L 1 , a switch S 2 , an inductor L 2  and an inductor L 3 , wherein the inductor L 1  and the switch S 2  are connected in parallel and then connected in series to the rechargeable battery B, and are connected in parallel between the positive of the rechargeable battery B and the center tap  4 , the inductor L 2  is connected between the diode D 5  and the drain of the metal oxide semiconductor field effect transistor Q 3 , the inductor L 3  is connected between the anode of the diode D 6  and the source of the metal oxide semiconductor field effect transistor Q 4 , and the inductors L 2 , L 3  and the full bridge rectifying circuit  2  constitute the rectifying and filtering circuit  15 . Thus from the circuit shown in  FIG. 4 , it can be understood that the output of the rectifying and filtering circuit  15  is connected across the capacitors C 1  and C 2  of the half bridge circuit  3 . 
         [0059]      FIG. 5  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 4  in the back-up mode, which is similar to that shown in  FIG. 2 . The difference is that the full bridge rectifying circuit  2  in the  FIG. 2  is replaced with a rectifying and filtering circuit  15  and the switch S 2  is controlled to switch on all the time. In the present embodiment, the push-pull circuit  1  can be controlled by using the control method in the prior art, so as to discharge the rechargeable battery B and charge the capacitors C 1  and C 2 . For example, it can be realized through the following control method: a PWM signal is provided to the gates of the metal oxide semiconductor field effect transistors Q 1  and Q 2 , and in the first time period, the metal oxide semiconductor field effect transistor Q 1  is controlled to switch on and the metal oxide semiconductor field effect transistor Q 2  is controlled to switch off, thus the rechargeable battery B discharges through the metal oxide semiconductor field effect transistor Q 1  at the primary side of the transformer Tr 1 , and the secondary side of the transformer Tr 1  charges the capacitors C 1  and C 2  simultaneously through the rectifying and filtering circuit  15 . In the second time period, the metal oxide semiconductor field effect transistors Q 1  and Q 2  are controlled to switch off, thus electric energy stored in the inductors L 2  and L 3  charges the capacitors C 1  and C 2  at the secondary side of the transformer Tr 1 . In the third time period, the metal oxide semiconductor field effect transistor Q 1  is controlled to switch off and the metal oxide semiconductor field effect transistor Q 2  is controlled to switch on, thus the rechargeable battery B discharges through the metal oxide semiconductor field effect transistor Q 2  at the primary side of the transformer Tr 1 , and the secondary side of the transformer Tr 1  charges the capacitors C 1  and C 2  simultaneously through the rectifying and filtering circuit  15 . In the fourth time period, the metal oxide semiconductor field effect transistors Q 1  and Q 2  are controlled to switch off, thus electric energy stored in the inductors L 2  and L 3  charges the capacitors C 1  and C 2  at the secondary side of the transformer Tr 1 . Then the control methods from the first time period to the fourth time period are repeated sequentially, so as to achieve the transfer of the power from the rechargeable battery B to the capacitors C 1  and C 2 . 
         [0060]      FIG. 6  is an equivalent circuit diagram of the bi-directional DC/DC converter shown in  FIG. 4  in the charge mode, which is similar to that shown in  FIG. 3 . The difference is that the switch S 2  of  FIG. 6  is controlled to switch off, so that the inductor L 1  is connected between the center tap  4  and the positive of the rechargeable battery B. In the present embodiment, the half bridge circuit  3  can be controlled by using the control method in the prior art, so as to discharge the capacitors C 1  and C 2  and charge the rechargeable battery B. For example, a PWM signal is provided to the gates of the metal oxide semiconductor field effect transistors Q 3  and Q 4 , and in the first time period, the metal oxide semiconductor field effect transistor Q 3  is controlled to switch on and the metal oxide semiconductor field effect transistor Q 4  is controlled to switch off, thus the capacitor C 1  discharges, and the primary side of the transformer Tr 1  charges the rechargeable battery B through the diode D 2  and the inductor L 1  In the second time period, the metal oxide semiconductor field effect transistors Q 3  and Q 4  are controlled to switch off, thus electric energy stored in the inductor L 1  charges the rechargeable battery B. In the third time period, the metal oxide semiconductor field effect transistor Q 3  is controlled to switch off and the metal oxide semiconductor field effect transistor Q 4  is controlled to switch on, thus the capacitor C 2  discharges and the primary side of the transformer Tr 1  charges the rechargeable battery B through the diode D 1  and the inductor L 1 . In the fourth time period, the metal oxide semiconductor field effect transistors Q 3  and Q 4  are controlled to switch off, thus electric energy stored in the inductor L 1  charges the rechargeable battery B. Then the control methods from the first time period to the fourth time period are repeated sequentially, so as to achieve the transfer of the power from the rechargeable battery B to the capacitors C 1  and C 2 . 
         [0061]    In the present embodiment, the rectifying and filtering circuit  15  can effectively filter ripple current in the back-up mode (i.e. during the discharge process of the rechargeable battery B), so as to reduce impulse and damage to the capacitors C 1  and C 2 . In addition, the inductor L 1  can effectively filter ripple current in the charge mode, so as to reduce damage to the rechargeable battery B. 
         [0062]      FIG. 7  is a circuit diagram of a bi-directional DC/DC converter of a third embodiment of the present invention, which is similar to that shown in  FIG. 4 . The difference is that the transformer Tr 1  in the  FIG. 4  is replaced with a transformer Tr 2  and the push-pull circuit  1  in the  FIG. 4  is replaced with a full bridge circuit  16 , wherein the full bridge circuit  16  comprises a metal oxide semiconductor field effect transistor Q 7  having an anti-parallel diode D 7 , a metal oxide semiconductor field effect transistor Q 8  having an anti-parallel diode D 8 , a metal oxide semiconductor field effect transistor Q 9  having an anti-parallel diode D 9 , and a metal oxide semiconductor field effect transistor Q 10  having an anti-parallel diode D 10 . The metal oxide semiconductor field effect transistors Q 7  and Q 8  constitute a first bridge arm. The metal oxide semiconductor field effect transistors Q 9  and Q 10  constitute a second bridge arm. Intermediate nodes of the first bridge arm and the second bridge arm act as the output of the full bridge circuit  16 . The secondary side of the transformer Tr 2  is the same as that of the transformer Tr 1 , which is not discussed here. The two terminals of the primary side of the transformer Tr 2  are connected to the output of the full bridge circuit  16 . Two terminals of the first bridge arm or the second bridge arm act as the input of the full bridge circuit  16 . The inductor L 1  and the switch S 2  are connected in parallel and then connected in series to the rechargeable battery B, and are connected in parallel between the positive of the rechargeable battery B and the drain of the metal oxide semiconductor field effect transistor Q 9 . In this circuit diagram, in the back-up mode, the switch S 2  is controlled to switch on, the switch S 1  is controlled to connect the secondary side of the transformer Tr 2  to the input of the rectifying and filtering circuit  15 , the metal oxide semiconductor field effect transistors Q 3  or/and Q 4  are controlled to switch off, and the full bridge circuit  16  is controlled to work in PWM mode in the prior art. Therefore, an alternating magnetic field is generated in the transformer Tr 2 , and the induced current charges the capacitors C 1  and C 2  through the rectifying and filtering circuit  15 , so as to achieve the transfer of the power from the rechargeable battery B to the capacitors C 1  and C 2 . In the charge mode, the switch S 2  is controlled to switch off, the switch S 1  is controlled to connect the output of the half bridge circuit  3  to the winding between the terminals  7  and  8  of the secondary side of the transformer Tr 2 , the metal oxide semiconductor field effect transistors Q 7 ˜Q 10  are controlled to switch off, and the half bridge circuit  3  is controlled by the same control method as that of the above second embodiment, so as to achieve discharging of the capacitors C 1  and C 2  and charging of the rechargeable battery B. 
         [0063]    In the above embodiment, a control device for controlling the operating state of the metal oxide semiconductor field effect transistors Q 1 ˜Q 4  and the metal oxide semiconductor field effect transistors Q 7 ˜Q 10 . Those skilled in the art will appreciate that any control device providing the above control signals can be employed. In addition, the metal oxide semiconductor field effect transistor in above embodiment can be replaced with an insulated gate bipolar transistor connecting anti-parallel diode. 
         [0064]    In the above embodiments of the present invention, there is no limit to the direction of the coil windings of the transformers Tr 1  and Tr 2 . In the above embodiments of the present invention, the node  13  can be grounded. In the other embodiments of the present invention, the push-pull circuit  1  of the first embodiment can be replaced with the full bridge circuit  16 . In the other embodiments of the present invention, the primary side of the transformer Tr 1  can be composed of two coil windings having the same turns connected in series, and the number of windings of the secondary side of the transformers Tr 1  and Tr 2  is not limited to three in the above embodiments, so long as the following conditions are met: when the switch is in the first state, two terminals of the secondary side of the transformer are connected to the input of the rectifying circuit  2  or rectifying and filtering circuit  15 ; and when the switch is in the second state, a part of windings of the secondary side of the transformer is connected to the output of the half bridge circuit  3 , and is disconnected from the two terminals of the input of the rectifying circuit  2  or rectifying and filtering circuit  15 . 
         [0065]    Although the present invention has been described with reference to preferred embodiments, the present invention is not limited to the embodiments described herein. And the features and operations of the invention as described are susceptible to various modifications and alterations, without departing from the scope of the invention.