Abstract:
A bidirectional converter circuit includes a voltage source which provides an input voltage, an energy storage set connected to the voltage source and receives the input voltage, a switch set connected to the energy storage set, wherein the switch set includes a first switch and a second switch; an operating switch set connected to the switch set, wherein the operating switch set includes a first operating switch, a second operating switch, a third operating switch and a fourth operating switch. The bidirectional converter further includes a blocking capacitor set and a (input/output) capacitor set. Wherein, the blocking capacitor set is connected to the switch set and the operating switch set. The first operating switch and the second operating switch are driven complementarily with the first switch, and the third operating switch and the fourth operating switch are driven complementarily with the second switch.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Inventions 
         [0002]    The present invention relates to a non-isolated bidirectional DC/DC converter with high conversion ratio and low switch voltage stress characteristic, in particularly, to a novel transformer-less two-phase interleaved bidirectional DC/DC converter with high efficiency. 
         [0003]    2. Description of Related Art 
         [0004]    Recently bidirectional dc-dc converters (BDC) have received a lot of attention due to the increasing need to systems with the capability of bidirectional energy transfer between two dc buses. Apart from traditional application in dc motor drives, new applications of BDC include energy storage in renewable energy systems, fuel cell energy systems, hybrid electric vehicles (REV), uninterruptible power supplies (UPS), PV hybrid power systems and battery chargers. 
         [0005]    Various BDCs can be divided into the non-isolated BDCs and isolated BDCs. Non-isolated BDCs (NBDC)are simpler than isolated BDCs (IBDC) and can achieve better efficiency. 
         [0006]    For non-isolated applications, the non-isolated bidirectional DC-DC converters, which include the conventional boost/buck (step-up/step-down) types, multi-level type, three-level type, sepic/zeta type, switched-capacitor type and coupled-inductor type, are presented. The multi-level type is a magnetic-less converter, but more switches are used in this converter. If higher step-up and step-down voltage conversion ratios are required, much more switches are needed. This control circuit becomes more complicated. In the three-level type, the voltage stress across the switches on the three-level type is only half of the conventional type. However, the step-up and step-down voltage conversion ratios are low. Since the sepic/zeta type is combined of two power stages, the conversion efficiency will be decreased. The switched capacitor and coupled-inductor types can provide high step-up and step-down voltage gains. However, their circuit configurations are complicated. The interleaved structure is another effective solution to increase the power level, which can minimize the current ripple, can reduce the passive component size, can improve the transient response, and can realize the thermal distribution. For example, a two-phase conventional interleaved boost/buck converter is presented. However, the step-up and step-down voltage conversion ratios also are low. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention presents a novel interleaved bidirectional DC-DC converter with low switch voltage stress characteristic for the low-voltage distributed energy resource applications. In boost mode, the module is combined with interleaved two-phase boost converter for providing a much higher step-up voltage gain without adopting an extreme large duty ratio. In buck mode, the module is combined with interleaved two-phase buck converter in order to get a high step-down conversion ratio without adopting an extreme short duty ratio. Based on the concepts of the voltage division and the voltage summation of the capacitor voltage, the energy can be stored in the blocking capacitor set of the bidirectional converter circuit for increasing the voltage conversion ratio and for reducing the voltage stresses of the switches. As a result, the invention converter topology possesses the low switch voltage stress characteristic. This will allow one to choose lower voltage rating MOSFETs to reduce both switching and conduction losses, and the overall efficiency is consequently improved. In addition, due to the charge balance of the blocking capacitor, the converter features automatic uniform current sharing characteristic of the interleaved phases without adding extra circuitry or complex control methods. 
         [0008]    The present invention provides a bidirectional DC-DC converter, comprising: a voltage source for providing an input voltage; an energy storage set connected to the voltage source and receiving the input voltage; a switch set including a first switch and a second switch, wherein the first switch and the second switch are respectively connected to the energy storage set; an operating switch set connected to the switch set, wherein the operating switch set includes a first operating switch, a second operating switch, a third operating switch and a fourth operating switch; a blocking capacitor set respectively connected to the switch set and the operating switch set; and an output capacitor set receiving energy from the energy storage set and the input voltage and providing a power to a load; wherein, the first operating switch and the second operating switch are driven complementarily with the first switch, and the third operating switch and the fourth operating switch are driven complementarily with the second switch. 
         [0009]    The present invention utilizes voltage adding and voltage dividing concept of the capacitor to increase the conversion ratio for boost or buck, and further reduce the switch across voltage. Therefore, the circuit can use the elements with lower switch cross voltage in order to reduce the switching loss and conduction loss to increase the conversion efficiency of the converter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of an interleaved bidirectional DC-DC converter circuit showing embodiment of the invention; 
           [0011]      FIG. 2(   a ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 1 and mode 3 under the step-up mode of the invention; 
           [0012]      FIG. 2(   b ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 2 under the step-up mode of the invention; 
           [0013]      FIG. 2(   c ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 4 under the step-up mode of the invention; 
           [0014]      FIG. 3  key waveforms of the converter operating at CCM which include gating signals of the active switches, voltage stress of switches and inductors current in different operating modes under the step-up mode of the interleaved bidirectional DC-DC converter; 
           [0015]      FIG. 4(   a ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 1 under the step-down mode of the invention; 
           [0016]      FIG. 4(   b ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 2 and 4 under the step-down mode of the invention; 
           [0017]      FIG. 4(   c ) is an equivalent circuit of the interleaved bidirectional DC-DC converter showing the operating mode 3 under the step-down mode of the invention; and 
           [0018]      FIG. 5  key waveforms of the converter operating at CCM which include gating signals of the active switches, voltage stress of switches and inductors current in different operating modes under the step-down mode of the interleaved bidirectional DC-DC converter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    The following content combines with the drawings and the embodiment for describing the present invention in detail. 
         [0020]    With reference to  FIG. 1 , the DC-DC converter  10  is comprised of a switch set  12  which have a first switch S 1  and a second switch S 2 , an operating switch set  14  which have four operating switches, a first operating switch S 1a , a second operating switch S 1b , a third operating switch S 2a , and a fourth operating switch S 2b , two blocking capacitors C A  and C B , two inductors L 1  and L 2  and two capacitors C 1  and C 2 . Wherein, one end of the inductors L 1  and L 2  is connected to a first voltage source  16 , and the other end of the inductors L 1  and L 2  is connected to the first switch S 1  and the second switch S 2  respectively. Two capacitors C 1  and C 2  are connected in series and the other end of the capacitors C 1  and C 2  is connected to second voltage source  18  in parallel. In order to simplify the circuit analysis of the invention converter, some assumptions are made as follows. All components are ideal components and the capacitors are sufficiently large, such that the voltages across them can consider as constant approximately. 
       A. Step-Up Mode 
       [0021]    Some key waveforms of the converter under step-up mode are shown in  FIG. 3  and the corresponding equivalent circuits are shown in  FIG. 2(   a )˜ FIG. 2(   c ). 
         [0022]    In one embodiment, that operation of active switches S 1a  and S 1b  (S 2a  and S 2b ) are complementary to S 1 (S 2 ) and the phase shift between two phases is 180°. In the step-up mode, the first voltage source  16  is as an input voltage, the second voltage source  18  at the output side is replaced by a load  20 . The capacitors C 1  and C 2  at the output side are as the output capacitors. The load  20  is connected to the capacitors C 1  and C 2 . Prior to mode 1, the switches S 1a  and S 1b  are turned off. During dead time the inductor current i L1  would be forced to flow through the body diodes of switch S 1a  and switch S 1b  respectively. Also the inductor current i L2  flows through the switch S 2 . 
         [0023]    At t 0 , when into operating mode 1, switch S 1  is turned on. The current that had been flowing through the body diodes of the S 1a  and S 1b  now flows switch S 1 . Since both switches S 1  and S 2  are conducting, switches S 1a , S 1b , S 2a , and S 2b  are all off. The corresponding equivalent circuit is shown in  FIG. 2(   a ). From  FIG. 2(   a ) it is seen that both i L1  and i L2  are increasing to store energy in L 1  and L 2  respectively. The voltages across switches S 1a  and S 2  clamped to capacitor voltage V CA  and V CB  respectively and the voltages across the switches S 1b  and S 2b  are clamped to V C2  minus V CB  and V C1  minus V CA  respectively. Also, the load  20  is supplied from capacitors C 1  and C 2 . 
         [0024]    At t 1 , when into operating mode 2, switch S 2  is turned off. After a short dead time, S 2a  and S 2b  are turned on while their body diodes are conducting. In other words, S 2a  and S 2b  are turned on with zero voltage switching (ZVS). The corresponding equivalent circuit is shown in  FIG. 2(   b ). It is seen from  FIG. 2(   b ) that part of stored energy in inductor L 2  as well as the stored energy of C A  is now released to output capacitor C 1  and the load  20 . Meanwhile, part of stored energy in inductor L 2  is stored in C B . In this mode, capacitor voltage V C1  is equal to V CB  plus V CA . During this mode, i L1  increases continuously and i L2  decreases linearly. 
         [0025]    At t 2 , when into operating mode 3, S 2a  and S 2b  are turned off. After a short dead time, S 2  is turned on. The current that had been flowing through body diodes of S 2a  and S 2b  flows into switch S 2 . The corresponding equivalent circuit turns out to be the same as Mode 1. 
         [0026]    At t 3 , when into operating mode 4, S 1  is turned off. After a short dead time, S 1a  and S 1b  are turned on while their body diodes are conducting. Similarly, S 1a  and S 1b  are turned on with ZVS. The corresponding equivalent circuit is shown in  FIG. 2(   c ). It is seen from  FIG. 2(   c ) that part of stored energy in inductor L 1  as well as the stored energy of C B  is now released to output capacitor C 2  and the load  20 . Meanwhile, part of stored energy in inductor L 1  is stored in C A . In this mode the output capacitor voltage V C2  is equal to V CB  plus V CA . During this mode, i L2  still increases continuously and i L1  decreases linearly. 
       B. Step-Down Mode 
       [0027]    Some key waveforms of the converter under step-down mode are shown in  FIG. 5  and the corresponding equivalent circuits are shown in  FIG. 4(   a )- FIG. 4(   c ). 
         [0028]    In one embodiment, that operation of active switches S 1a  and S 1b  (S 2a  and S 2b ) are complementary to S 1 (S 2 ) and the phase shift between two phases is 180°. In the step-down mode, when the interleaved bidirectional DC-DC converter  10  is operated as a step-down converter, the second voltage source  18  is as an input voltage, the first voltage source  16  at the input side is replaced by a load  22  and an output capacitor Co is connected in parallel. Prior to Mode 1, S 2  is off. During dead time inductor current i L2  would be forced to flow through the body diode of switch S 2  and inductor current i L1  still flows through the switch S 1 . 
         [0029]    At t 0 , when into operating mode 1, S 2a  and S 2b  are turned on. Current i L2  that had been flowing through the body diode of S 2  flows into S 1  and S 2a . The corresponding equivalent circuit is shown in  FIG. 4(   a ). From  FIG. 4(   a ) one can see that during this mode current i L1  freewheels through S 1  and L 1  is releasing energy to the output capacitor C O  and the load  22 . However, current i L2  provides two separate current paths through C A  and C B . The first path starts from C 1 , through S 2b , C A , L 2 , C O  and R, S 1  and then back to C 1  again. Hence, the stored energy of C 1  is discharged to C A , L 2 , and output capacitor C O  and the load  22 . The second path starts from C B , through L 2 , C O  and R, S 2a  and then back to C B  again. In other words, the stored energy of C B  is discharged to L 2  and output capacitor C O  and the load  22 . Therefore, during this mode, i L2  is increasing and i L1  is decreasing as can be seen from  FIG. 5 . Also, from  FIG. 4(   a ), one can see that, V C1  is equal to V CA  plus V CB  due to conduction of S 2a , S 2b  and S 1 . Since V C1 =V H /2 (V H  is voltage source  18 ), and V CA =V CB =V C1 /2=V H /4, one can observe from  FIG. 4(   a ) that the voltage stress of S 2  is equal to V CH =V H /4 and the voltage stresses of S 1a  and S 1b  are clamped to V C1 =V H /2 and V C2 =V H /2 respectively. 
         [0030]    At t 1 , when into operating mode 2, S 2a and S 2b  are turned off. After a short dead time, S 2  is turned on while its body diode is conducting. In other words, S 2  is turned on with zero voltage switching (ZVS). The corresponding equivalent circuit is shown in  FIG. 4(   b ). From  FIG. 4(   b ), one can see that i L1  and i L2  are freewheeling through S 1  and S 2  respectively. Both V L1  and V L2  are equal to −V CO , and hence, i L1  and i L2  decrease linearly. L 1  and L 2  are releasing energy to output capacitor C O  and the load  22 . During this mode, the voltage across S 2b , namely V S2b , is equal to the difference of V C1  and V CA  and V S2a  is clamped at V CB . Similarly, the voltage across S 1b , namely V S1b , is equal to the difference of V C2  and V CB  and V S1a  is clamped at V CA . 
         [0031]    At t 2 , when into operating mode 3, S 1  is turned off and inductor current i L1  flows through the body diode of switch S 1 . After a short dead time, S 1a  and S 1b  are turned on. The current that had been flowing through the body diode of S 1  flows into S 2 . The corresponding equivalent circuit is shown in  FIG. 4(   c ). From  FIG. 4(   c ) one can see that during this mode current i L2  freewheels through S 2  and L 2  is releasing energy to output load. However, current i L1  provides two separate current paths through C A  and C B . The first path starts from C 2 , through L 1 , C O  and R, S 2 , C B , S 1b , and then back to C 2  again. Hence, the stored energy of C 2  is discharged to C B , L 1  and output capacitor C O  and the load  22 . The second path starts from C A , through S 1a , L 1 , C O  and R, S 2 , and then back to C A  again. In other words, the stored energy of C A  is discharged to L 1  and output capacitor C O  and the load  22 . Therefore, during this mode, i L1  is increasing and i L2  is decreasing as can be seen from  FIG. 5 . Also, from  FIG. 4(   c ), one can see that, V C2  is equal to V CA  plus V CB  due to conduction of S 1a  and S 1b . Since V C2 =V H /2, and V CA =V CH =V C2 /2=V H /4, one can observe from  FIG. 4(   c ) that the voltage stress of S 1  is equal to V CA =V H /4 and the voltage stresses of S 2b  and S 2a  are clamped to V C1 =V H /2 and V CB =V H /4 respectively. 
         [0032]    At t 3 , when into operating mode 4, S 1a  and S 1b  are turned off. After a short dead time, S 1  is turned on while its body diode is conducting. Similarly, S 1  is turned on with zero voltage switching (ZVS). The corresponding equivalent circuit turns out to be the same as  FIG. 4(   b ) and its operation is the same as that of mode 2. 
         [0033]    In summary, in one embodiment, in the step-up mode, the high step-up voltage conversion ratio is 4*V L /(1−D) times under the duty cycle (0.5&lt;D&lt;1). In the step-down mode, the high step-down conversion ratio is D*V H /4 times under the duty cycle (0&lt;D&lt;0.5). According to the voltage adding and voltage dividing principle of the capacitor, the main purpose of the new capacitive switching circuit of the DC/DC converter is not only storing the energy in the blocking capacitor to increase the conversion ratio but also reducing the voltage stress of the active switches. As a result, the proposed converter topology possesses the low switch voltage stress characteristic. This will allow one to choose lower voltage rating MOSFETs to reduce both switching and conduction losses, and the overall efficiency is consequently improved. In addition, due to the charge balance of the blocking capacitor, the converter features both automatic uniform current sharing characteristic of the interleaved phases and without adding extra circuitry or complex control methods. 
         [0034]    The present invention mainly is comprised of the internal capacitive switching circuit which equally distributes the charge energy on the interleaved input/output inductor circuits so as to achieve active current sharing on the inductor circuits so that it can reduce conduction losses and increase the conversion efficiency of the converter. 
         [0035]    For demonstrating the performance of the invention converter, the invention converter is compared with conventional boost DC-DC converter, as shown in Table 1, wherein, D is the duty cycle. 
         [0036]    Table. 1 summarizes the voltage conversion ratio and normalized voltage stress of active switches for reference. It shows a comparison table for the interleaved bidirectional DC-DC converter under step-up mode according to an embodiment of the present invention and the conventional boost DC-DC converter. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of the steady state characteristics for four converter. 
               
             
          
           
               
                   
                   
                   
                   
                 An embodi- 
               
               
                   
                   
                 High 
                 Ultra high 
                 ment of the 
               
               
                 Gain/voltage 
                 Voltage 
                 boost ratio 
                 boost ratio 
                 present 
               
               
                 stress 
                 doubler 
                 converter 
                 converter 
                 invention 
               
               
                   
               
               
                 Conversion 
                 2/(1 − D) 
                 (3 − D)/(1 − D) 
                 (3 + D)/(1 − D) 
                 4/(1 − D) 
               
               
                 ratio 
               
               
                 The voltage 
                 1/2 
                 1/(3 − D) 
                 2/(3 + D) 
                 1/4 
               
               
                 stress on 
               
               
                 the switch 
               
               
                 of the low 
               
               
                 voltage side 
               
               
                 The voltage 
                 1 
                 2/(3 − D) 
                 2/(3 + D) 
                 1/2 
               
               
                 stress on 
               
               
                 the switch 
               
               
                 of the high 
               
               
                 voltage side 
               
               
                   
               
             
          
         
       
     
         [0037]    For demonstrating the performance of the invention converter, the invention converter is also compared with conventional buck DC-DC converter, as shown in Table 2, wherein, D is the duty cycle. 
         [0038]    Table. 2 summarizes the voltage conversion ratio and normalized voltage stress of active switches for reference. It shows a comparison table for the interleaved bidirectional DC-DC converter under step-down mode according to an embodiment of the present invention and the conventional buck DC-DC converter. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Comparison of the steady state characteristics for three converter. 
               
             
          
           
               
                   
                 Traditional 
                 Interleaved 
                   
               
               
                   
                 interleaved 
                 buck converter 
                 An embodiment 
               
               
                 Gain/Voltage 
                 buck 
                 with expanded 
                 of the present 
               
               
                 stress 
                 converter 
                 duty cycle 
                 invention 
               
               
                   
               
               
                 Conversion ratio 
                 D 
                 D/2 
                 D/4 
               
             
          
           
               
                 The voltage stress 
                 1 
                 1/2 
                 S 1a   
                 1/2 
               
               
                 on the switch S a  of 
                   
                   
                 S 2a   
                 1/4 
               
               
                 the high voltage 
               
               
                 side 
               
               
                 The voltage stress 
                 1 
                 1 
                 S 1b   
                 1/2 
               
               
                 on the switch S b  of 
                   
                   
                 S 2b   
               
               
                 the high voltage 
               
               
                 side 
               
             
          
           
               
                 The voltage stress 
                 1 
                 1/2 
                 1/4 
               
               
                 on the switch of the 
               
               
                 low voltage side 
               
               
                   
               
             
          
         
       
     
         [0039]    The present invention discloses a simple, practical and effective bidirectional DC-DC converter. The converter is comprised of six switches, two capacitors, and two inductors to form a bidirectional boost-buck converter circuit, which can effectively increase the performance, the ratio for boost or buck, the life time, and decreases the requirement for the sustain voltage of the components and system costs. 
         [0040]    The above embodiments of the present invention are not used to limit the claims of this invention. Any use of the content in the specification or in the drawings of the present invention which produces equivalent structures or equivalent processes, or directly or indirectly used in other related technical fields is still covered by the claims in the present invention.