Abstract:
A resonant, bi-directional, DC to DC voltage converter with loss-less (soft) switching having regulated output and capable of converting power between two, high-potential and low-potential DC voltage sources. The converter&#39;s semiconductor and magnetic components provide both, output regulation and soft switching in both (step-down and step-up) directions of power conversion which reduces total component count, cost and volume and enhances power conversion efficiency.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a bi-directional and isolated DC to DC power converters featuring soft, loss-less switching operation and output voltage controllability in both directions of power transfer. In addition, the present invention maintains the soft-switching operation and output voltage controllability within the entire load operating range (i.e. from no-load to full-load). 
     BACKGROUND OF THE INVENTION 
     Today&#39;s DC to DC switch mode power converters are typically required to provide insulation between the primary and secondary sides and to have high power density, high efficiency and low cost. In addition, many applications including uninterruptable power supplies (UPS), power supplies utilizing renewable energy sources (e.g. solar, wind, fuel cells), as well as aerospace power supplies require bi-directional (step-up and step-down) power conversion with isolated and regulated output. Examples of isolated and pulse width modulation (PWM) regulated bi-directional DC to DC converters are described in U.S. Pat. No. 5,140,509, U.S. Pat. No. 5,255,174, U.S. Pat. No. 7,433,207, U.S. Pat. No. 6,370,050 and U.S. Pat. No. 6,205,035. The pulse width modulation techniques control techniques employed in these converters typically feature so called “hard-switching” which can lead to significant switching losses and adversely impact the ability to achieve high power densities and high power conversion efficiencies. 
     Zero-voltage switching (ZVS) and zero-current switching (ZCS) are well established switching techniques for reducing switching losses which in turn allows for higher switching frequencies, reduced size of magnetic components, increased power density and reduced cost. U.S. Pat. No. 5,539,630, U.S. Pat. No. 6,370,050 and U.S. Pat. No. 6,330,170 describe bi-directional converters that feature ZVS but only in one of the directions of power conversion. 
     There is a need for an improved bi-directional DC to DC converter having a wide range of output voltage controllability in both directions of power transfer. 
     There is a need for an improved bi-directional DC to DC converter having a wide range of output voltage controllability in both directions of power transfer, the bi-directional DC to DC converter further providing galvanic isolation between the power source and the load. 
     There is a need for an improved bi-directional DC to DC converter having a wide range of output voltage controllability in both directions of power transfer, the bi-directional DC to DC converter employing the same components for power conversion in both directions of power transfer to reduce costs. 
     There is a need for series type, frequency controlled, bi-directional DC to DC resonant converter having a wide range output voltage controllability in both directions of power transfer, the resonant converter providing for loss-less switching operation in both directions of power transfer; loss-less switching operation within the whole range of load conditions (i.e. from no-load to full-load) and loss-less switching operation for all semiconductor devices in the circuitry. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention relates to improved bi-directional DC to DC converters. In particular, the present invention provides for an improved series type, frequency controlled, bi-directional DC to DC resonant converter that not only allows for a full control of the output voltage in both direction of power transfer, but when properly dimensioned, can provide ZVS for the input section devices (i.e. the ones connected to the power source) and ZCS for the output section devices (i.e. the ones connected to the load) in both directions of power transfer and for all load conditions. The combination of ZVS and ZCS for all devices enhances the power conversion efficiency and the use of the same components for bi-directional power conversion is a major contributor of achieving very high power density. The loss-less switching provided by embodiments of the present invention allows for further increase in the power density by operating at higher switching frequencies. It is well known that the increase of the switching frequency reduces the size of all magnetic and filter components. This is a distinctive advantage of the present invention compared with conventional PWM-controlled, bi-directional converters that feature hard-switching in at least one of the directions of power conversion. 
     Various embodiments of the present invention can employ input, or primary section devices, that are connected in full-bridge, half-bridge, or push-pull switcher (“chopper”) configurations that chop the power source voltage (i.e. with the switching frequency), which is then applied to the resonant network circuit of the present invention, while the output, or secondary section devices, are connected in a full-bridge, half-bridge, or push-pull configurations and are controlled in a synchronous rectification manner. To reverse the direction of power transfer the control functions of the primary section devices and the secondary section devices are swapped (i.e. the devices that have performed synchronous rectification perform the “chopping” function while the former chopper devices perform the synchronous rectification function). The resonant circuit of various embodiments of the present invention is arranged in such a way that when its input/output terminals are swapped, which is the default function of the bi-directional converter, both loss-less switching (i.e. ZVS and ZCS operation) and the output voltage controllability of the circuitry is maintained. 
     In a first aspect of the invention, there is provided a bi-directional DC to DC converter that includes a first resonant tank circuit employed during power transfer along a first direction through the bi-directional DC to DC converter and a second resonant tank circuit employed during power transfer along a second direction through the bi-directional DC to DC converter. The second direction opposes the first direction. 
     In a feature of this aspect of the invention, the first resonant tank circuit shares at least two common resonant components with the second resonant tank circuit and the first resonant tank circuit further includes a first resonant component that is different from a second resonant component of the second resonant tank circuit. 
     In another feature of this aspect, the at least two common resonant components include a capacitor connected in series with an inductor. The first resonant component includes a first inductor connected in series with the at least two common resonant components and the second resonant component includes a second inductor connected in series with the at least two common resonant components. 
     In yet another feature of this aspect, the at least two common resonant components are connected in series with a first load circuit and the first inductor is connected in parallel with the first load circuit during the power transfer along the first direction. The at least two common resonant components are connected in series with a second load circuit and the second inductor is connected in parallel with the second load circuit during the power transfer along the second direction. 
     In another feature of this aspect, the bi-directional DC to DC converter includes a transformer, the transformer including a primary side connected in series with the at least two common resonant components and connected in parallel with the first inductor. The at least two common resonant components are connected in series with a full-bridge switcher circuit and the second inductor is connected in parallel with the full-bridge switcher circuit. The transformer includes a secondary side connected to one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. 
     In yet another feature of this aspect, the bi-directional DC to DC converter includes a transformer, the transformer including a primary side connected in series with the at least two common resonant components and connected in parallel with the first inductor. The at least two common resonant components are connected in series with a half-bridge switcher circuit and the second inductor is connected in parallel with the half-bridge switcher circuit. The transformer includes a secondary side connected to one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. 
     In yet another feature of this aspect, the bi-directional DC to DC converter includes a transformer, the transformer including a primary side connected in series with the at least two common resonant components and connected in parallel with the first inductor. The at least two common resonant components are connected in series with a push-pull switcher circuit and the second inductor is connected in parallel with the push-pull switcher circuit. The transformer includes a secondary side connected to one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. 
     In yet another feature of this aspect, the first resonant tank circuit and the second resonant tank circuit include the same resonant configuration. 
     In a second aspect of the present invention, there is provided a bi-directional DC to DC converter that includes an electronic circuit adapted to provide a first resonant tank circuit during a first power transfer mode through the bi-directional DC to DC converter and a second resonant tank circuit during a second power transfer mode through the bi-directional DC to DC converter. The electronic circuit includes a first terminal set, a second terminal set, at least one capacitor and a plurality of inductors. The plurality of inductors include a first inductor positioned between at least two terminals in the first terminal set and a second inductor positioned between at least two terminals in the second terminal set. 
     In a feature of this aspect of the invention, a first load circuit is connected to the first terminal set during the first power transfer mode and a second load circuit is connected to the second terminal set during the second power transfer mode. The at least one capacitor and at least one inductor of the plurality of inductors is connected in series with the first load circuit and the first inductor is connected in parallel with the first load circuit during the first power transfer mode. The at least one capacitor and the at least one inductor of the plurality of inductors is connected in series with the second load circuit and the second inductor is connected in parallel with the second load circuit during the second power transfer mode. 
     In another feature of this aspect, the bi-directional DC to DC includes a transformer, the transformer including a primary side connected to the first terminal set, wherein the at least one capacitor and at least one inductor of the plurality of inductors is connected in series with the transformer. The bi-directional DC to DC converter includes a full-bridge switcher circuit connected to the second terminal set, and the at least one capacitor and the at least one inductor of the plurality of inductors are connected in series with the full-bridge switcher circuit. The transformer includes a secondary side connected to one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. 
     In yet another feature of this aspect, the bi-directional DC to DC includes a transformer, the transformer including a primary side connected to the first terminal set, wherein the at least one capacitor and at least one inductor of the plurality of inductors is connected in series with the transformer. The bi-directional DC to DC converter includes a half-bridge switcher circuit connected to the second terminal set, and the at least one capacitor and the at least one inductor of the plurality of inductors are connected in series with the half-bridge switcher circuit. The transformer includes a secondary side connected to one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. 
     In yet another feature of this aspect, the bi-directional DC to DC includes a transformer, the transformer including a primary side connected to the first terminal set, wherein the at least one capacitor and at least one inductor of the plurality of inductors is connected in series with the transformer. The bi-directional DC to DC converter includes a push-pull switcher circuit connected to the second terminal set, and the at least one capacitor and the at least one inductor of the plurality of inductors are connected in series with the push-pull switcher circuit. The transformer includes a secondary side connected to one of a full-bridge rectifier circuit, a full-bridge synchronous rectifier circuit, a half-bridge rectifier circuit, a half bridge synchronous rectifier circuit, a push-pull rectifier circuit and a push-pull synchronous rectifier circuit. 
     In another feature of this aspect, the bi-directional DC to DC converter includes a transformer and the first inductor is implemented as a magnetizing inductor of the transformer and the second inductor is implemented as an external inductor. 
     In a third aspect of the invention, there is provided bi-directional DC to DC converter that includes a switcher circuit adapted for generating a square-wave voltage waveform; a transformer that includes a primary side and secondary side; a first resonant tank circuit connected between the switcher circuit and the transformer, the first resonant tank circuit including a first inductor connected in parallel with the primary side of the transformer; a second resonant tank circuit connected between the switcher circuit and the transformer, the second resonant tank circuit including a second inductor connected in parallel with the switcher circuit; and a synchronous rectifier circuit connected to the secondary side of the transformer. 
     In a feature of this aspect, each of the first resonant tank circuit and the second resonant tank circuit include at least one capacitor and at least one inductor that are connected in series with the switcher circuit and the primary side of the transformer. 
     In another feature of this aspect, each of the first resonant tank circuit and the second resonant tank circuit share at least one capacitor and at least one inductor that are connected in series with the switcher circuit and the primary side of the transformer. 
     In another feature of this aspect, the switcher circuit includes one of a full-bridge switcher circuit, a half-bridge switcher circuit and a push-pull switcher circuit. The synchronous rectifier circuit includes one of a full-bridge synchronous rectifier circuit, a half-bridge synchronous rectifier circuit and a push-pull synchronous rectifier circuit. The synchronous rectifier circuit also can include one of a dissipative snubber and a non-dissipative snubber. 
     In yet another feature of this aspect, the first resonant tank circuit and the second resonant tank circuit include the same resonant configuration. 
     In other aspects, the invention provides combinations and subsets of the aspects described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and applications of the invention are illustrated by the attached non-limiting drawings. The attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
         FIG. 1  shows a schematic circuit of an example embodiment of the present invention employing a full-bridge primary section and a push-pull secondary section; 
         FIG. 2  shows a schematic circuit of an example embodiment of the present invention employing a full-bridge primary section and a full-bridge secondary section; 
         FIG. 3  shows a schematic circuit of an example embodiment of the present invention employing a half-bridge primary section and a push-pull secondary section; 
         FIG. 4  shows a schematic circuit of an example embodiment of the present invention employing a push-pull primary section and a push-pull secondary section; 
         FIG. 5  shows a schematic circuit of an example embodiment of the present invention employing a half-bridge primary section and a half-bridge secondary section; 
         FIG. 6  shows a schematic circuit of an example embodiment of the present invention employing a full-bridge primary section and a push-pull secondary section in which the push-pull switching devices are equipped with individual non-dissipative or dissipative snubbers; 
         FIG. 7  shows a schematic circuit of an example embodiment of the present invention employing a full-bridge primary section and a push-pull secondary section in which the push-pull switching devices are equipped with a common non-dissipative or dissipative snubber connected to the switching devices through two common cathode diodes; 
         FIG. 8  shows an equivalent circuit of the example embodiment of  FIG. 1  during power transfer from the primary section to the secondary section; 
         FIG. 9  shows an equivalent circuit of the example embodiment of  FIG. 1  power conversion from the secondary section to the primary section; 
         FIGS. 10  ( a - c ) show respectively the current thorough the additional inductor Lnew A , the currents through the synchronous rectifiers and the currents through the switching devices of the circuit of  FIG. 1 ; 
         FIGS. 11  ( a - b ) show combined voltage and current waveforms plots respectively of a synchronous rectifier and a switcher employed by an LLC converter; and 
         FIG. 12  shows a typical surface plot of the DC voltage gain of a LLC converter with variables that include the normalised switching frequency and the Q-factor, and with a constant ratio between the magnetising inductance and the resonant inductance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. 
     A schematic of an example embodiment of the bi-directional converter  100 A embodying the principles of the present invention is shown in  FIG. 1 . In the case of power transfer from the left hand side to the right hand side of the circuitry in  FIG. 1 , a full-bridge switcher circuit  220 A containing controlled switching devices  4 ,  5 ,  6 , and  7  that include embedded, or external, anti-parallel diodes  8 ,  9 ,  10 , and  11  is connected to a DC voltage source  1 . Switching devices such as switching devices  4 ,  5 ,  6 , and  7  employed by the present invention can include MOSFETs, IGBTs, GTOs, BJTs by way of non-limiting example. A filter capacitor  2  is connected in parallel to the DC voltage source  1  to filter out switching ripple. Switching devices  4 ,  5 ,  6  and  7  are turned on and off with approximately 50% duty cycle width and their switching frequency is controlled so that full-bridge switcher circuit  220 A produces square-wave voltage waveform with 50% duty cycle and variable frequency on terminals  3  and  12 . An inductor  13  is connected across the terminals  3  and  12 . A series network that includes inductor  14 , capacitor  15  and magnetising inductor  16  is connected to terminals  3  and  12 . Magnetising inductor  16  is connected in parallel with the primary winding  17  located on the primary side  18 A of transformer  18 . It is noted that transformers such as transformer  18  are schematically represented by dashed lines in associated ones of the Figures. To reduce the number of magnetic components magnetising inductor  16  is usually embedded in the magnetic structure of transformer  18 . The value of the magnetising inductance can be controlled in such case by introducing an air gap in the magnetic core and adjusting its length. The secondary windings  19  and  20  located on the secondary side  18 B of transformer  18  include equal number of winding turns and are connected in centre-tap configuration in which the centre-tap terminal is connected to the positive terminal of a load impedance  26 , while the ends of the secondary windings are connected to a push-pull rectifier circuit  230 A that includes controlled switching devices  21  and  22  that include embedded, or external, anti-parallel diodes  23  and  24 . Switching devices  21  and  22  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulse-width to rectify the square-wave voltage produced by the secondary windings  19  and  20 . A common point of switching devices  21  and  22 , and anti-parallel diodes  23  and  24  is connected to the negative terminal of the load impedance  26 . A filter capacitor  25  is connected in parallel to load impedance  26  in this load circuit. In this example embodiment, push-pull rectifier circuit  230 A is employed as push-pull synchronous rectifier circuit. 
     In case of power transfer from the right hand side to left hand side of the circuitry in  FIG. 1 , the power source and the load exchange their places, i.e. load impedance  26  becomes a DC voltage source, while DC voltage source  1  becomes a load. In addition, the switching devices  21  and  22  become a push-pull controlled switcher with controlled switching frequency and approximately 50% duty cycle width that produces square-wave voltage with variable frequency across the terminals  27  and  28  of the primary side  18 A of transformer  18 . Furthermore, the full-bridge switcher circuit  220 A becomes a synchronously controlled rectifier circuit with approximately 50% duty cycle control pulse-width that rectifies the square-wave voltage on terminals  3  and  12  produced by the switching devices  21  and  22 . 
     A multi-resonant converter, known also in the power electronics field as a “LLC converter” is a series type, frequency controlled, resonant converter typically having three resonant components: a resonant capacitor, a resonant inductor and a magnetizing inductor. The resonant components of the LLC converter can be selected (in relation to the operating frequency) in such a way that the converter will provide zero voltage switching (ZVS) for the switching devices connected to the power source (i.e. the primary section of the converter) and zero current switching (ZCS) for the switching devices connected to load (i.e. the secondary section of the converter). In addition, the resonant component selection can be done in a way that the ZVS and ZCS can be maintained when operating from no-load to full-load conditions. An LLC converter design procedure for meeting the above features is outlined in a paper by R. Petkov entitled “Analysis and Optimisation of a Multi-Resonant Converter Employed in a Telecom Rectifier”, 21st International Telecommunication Energy Conference Intelec &#39;99, Copenhagen, Denmark, June 1999, poster 41, as well as by Diambo Fu et al. in a paper entitled “1 MHz High Efficiency LLC Resonant Converters with Synchronous Rectifier” 38-th Annual Power Electronics Specialists Conference PESC &#39;07, Orlando, Fla., USA, June 2007, pp. 2404-2410. The optimal selection of the resonant components typically results in magnetising inductance value being much larger than the resonant inductance value. It is noted that conventional multi-resonant converters or LLC converters typically work only in a single direction of the power transfer. The present inventors believe that by replacing the output diodes of the classical LLC converter design with synchronously controllable switches, like Mosfets for example, a circuit configuration that, when appropriately controlled, can provide bi-directional power transfer. In this circuit configuration however, the output voltage in one of the direction cannot be controlled. 
     Referring back to  FIG. 1 , the network of inductors  13 ,  14 ,  16  and capacitor  15  are employed in resonant network circuit  240 A. It should be noted that inductors  13 ,  14 ,  16  and capacitor  15  are resonant components and that various ones of these resonant components will combine to form a same resonant configuration when exited from the terminals  3  and  12  and loaded across the terminals  27  and  28 , as well as when exited from the terminals  27  and  28  and loaded across the terminals  3  and  12  (i.e. such change in the excitation and loading terminals of the resonant network circuit  240 A happens when switching devices  4 ,  5 ,  6  and  7  and switching devices  21  and  22  change their control functions from a switcher to a synchronous rectifier and vice versa). This interchanging of the functions of the switcher circuit (i.e. also referred to as a chopper circuit) and the synchronous rectifier circuit when the source and the load exchange places is schematically illustrated in  FIG. 8  and  FIG. 9  which are simplified versions of  FIG. 1  during power conversion in both directions of power transfer through bi-directional converter  100 A. In  FIG. 8  the power transfer is from the left hand side to the right hand side (i.e. along a first direction  222 ) through bi-directional converter  100 A, while in  FIG. 9  the power transfer is from the right hand side to left hand side (i.e. along an opposing second direction  224 ) through bi-directional converter  100 A. In  FIG. 8 , power is transferred through bi-directional converter  100 A during a first power transfer mode along a direction that is different than the direction that power is transferred through bi-directional converter  100 A during a second power transfer mode as shown in  FIG. 9 . In  FIG. 8  the switcher block contains the switching devices  4 ,  5 ,  6  and  7  (not shown) and anti-parallel diodes  8 ,  9 ,  10  and  11  (also not shown) from  FIG. 1 , while the synchronous rectifier block contains the switching devices  21  and  22  (not shown) and anti-parallel diodes  23  and  24  (also not shown) from  FIG. 1 . In  FIG. 9  the switcher block contains the switching devices  21  and  22  (not shown) and anti-parallel diodes  23  and  24  (also not shown) from  FIG. 1 , while the synchronous rectifier block contains the switching devices  4 ,  5 ,  6  and  7  (not shown) and anti-parallel diodes  8 ,  9 ,  10  and  11  (also not shown) from  FIG. 1 . 
     As shown in each of  FIGS. 8 and 9  the electronic circuit includes a first terminal set  29  that includes terminals  27  and  28  and a second terminal set  39  that includes terminals  3  and  12 . The electronic circuit further includes a capacitor Cr and a plurality of inductors including Lnew A , Lr, and Lm. In this example embodiment, a first inductor (i.e. Lm) is positioned between various terminals in the first terminal set  29  (i.e. terminals  28  and  27  in this illustrated embodiment) and a second inductor (i.e. Lnew A ) is positioned between various terminals in the second terminal set  39  (i.e. terminals  3  and  12  in this illustrated embodiment). In this example embodiment, a first load circuit is connected to the first terminal set  29  during the first power transfer mode and a second load circuit is connected to the second terminal set  39  during the second power transfer mode. In this example embodiment, the capacitor Cr and the inductor Lr are connected in series with the first load circuit and the first inductor (i.e. Lm) is connected in parallel with the first load circuit during the first power transfer mode. In this example embodiment, the capacitor Cr and the inductor Lr are connected in series with the second load circuit and the second inductor (i.e. Lnew A ) is connected in parallel with the second load circuit during the second power transfer mode. In this example embodiment, capacitor Cr and inductor Lr are connected in series with the primary side  18 A of transformer  18  while inductor Lm is connected in parallel with the primary side  18 A. 
     In this example embodiment, the resonant network between the switcher blocks and their associated synchronous rectifier blocks is of the same type for both directions of power transfer and yet it is equal to the resonant network of a conventional LLC converter. For example, in  FIG. 8  the resonant components involved in the power transfer mechanism and determining the DC voltage gain characteristic (i.e. the ratio between the output voltage and the input voltage) are Lr (i.e. inductor  14 ), Lm (i.e. magnetizing inductor  16 ) and Cr (i.e. capacitor  15 ) with the load section connected across Lm. The inductor Lnew A  (i.e. inductor  13 ) in  FIG. 8  is connected directly across the output terminals of the switcher block and therefore does not take part in power transfer mechanism, (i.e. Lnew A  does not affect the DC voltage gain (Vout/Vinp) characteristics of the resonant circuit. Accordingly, a first resonant tank circuit  300  (i.e. shown in dot-dash lines) that includes inductor  14 , magnetizing inductor  16  and capacitor  15  is provided by the electronic circuit. Similarly, in  FIG. 9  the resonant components involved in the power transfer mechanism are Lr (i.e. inductor  14 ), Lnew A  (i.e. inductor  13 ) and Cr (i.e. capacitor  15 ) with the load section connected across Lnew A . Inductor Lm in  FIG. 9  being directly connected to the switcher block (i.e. through transformer  18 ) will not affect the DC voltage gain characteristics of the resonant circuit either. Accordingly, a second resonant tank circuit  310  (i.e. shown in dot-dash lines) that includes inductor  14 , inductor  13  and capacitor  15  is provided by the electronic circuit. This very desirable equality of the resonant configurations in both directions of the power transfer is due to inductor Lnew A  which is a resonant component that is not found in conventional LLC resonant converters. In this example embodiment, Lnew A  is implemented as an external component. In this example embodiment, first resonant tank circuit  300  has the same resonant configuration as second resonant tank circuit  310 . That is, the combination of inductors Lr, Lm and capacitor Cr employed by the first resonant tank circuit  300  have the same resonant configuration as the combination of inductors Lr, Lnew A  and capacitor Cr employed by the second resonant tank circuit  310 . 
     The parameters governing resonant characteristics of an LLC converter can be represented by the following relationships: 
               fo   =     1     LrCr         ;         
where fo is a resonant frequency
 
               Q   =         Lr   ⁢     /     ⁢   Cr       R       ;         
where Q is a Q-factor
 
               fnorm   =     fsw   fo       ;         
where fnorm is a normalised switching frequency
 
                 n   prsec     =     Lm   Lr       ;         
where n prsec  is an inductances ratio during primary-secondary power transfer
 
                 n   secpr     =     Lnew   Lr       ;         
where n secpr  is an inductances ratio during secondary-primary power transfer
 
     The surface plot shown in  FIG. 12  shows the DC voltage gain (Vout/Vinp) of a conventional LLC resonant converter, as well as its loss-less (ZVS/ZCS) operating area as a function of the above parameters. The broken line in  FIG. 12  identifies the boundary of the ZVS/ZCS operating area of the circuit (i.e. all combinations of the 3D plot parameters lying on the surface in front of the broken line will provide loss-less switching operation with waveforms as shown in  FIGS. 11   a  and  11   b .  FIG. 11   a  and  FIG. 11   b  show combined current and voltage waveform plots respectively of the synchronous rectifier devices (i.e. “IsynhRect” and “VsynhRect”) and the switcher devices (“Iswitch” and “Vswitch”) employed in a LLC converter. These waveforms emphasize that the devices of the LLC converter operate in loss-less switching conditions (i.e. the switcher devices operate in ZVS, while the synchronous rectifier devices operate in ZCS). 
     There are a couple of important conclusions that can be derived from the surface plot in  FIG. 12 :
         The DC voltage gain varies below and above the unity value, i.e. the LLC converter can perform both, step-up and step-down voltage conversion. This feature is very desirable for all bi-directional power converters, especially when a battery is connected to their terminals.   The step-up and step-down voltage conversion of the appropriately dimensioned LLC converter is accompanied with loss-less switching that is very desirable in achieving high power conversion efficiency and high power density.       

     Accordingly, the resonant configuration of a conventional LLC converter provides all desirable characteristics of the bi-directional converter, but only in one of the directions of power conversion. To maintain these desirable characteristics in the other direction of the power conversion one has to maintain the same resonant configuration in that direction of power conversion also. Referring back to  FIG. 8  and  FIG. 9  that represented simplified versions of the  FIG. 1  circuit during both directions of power transfer, the addition of the inductor Lnew A  advantageously provides the needed equality in both of the resonant configurations. 
     It follows from the equality of the resonant configurations in both directions of power transfer in the circuit of the example embodiment shown in  FIG. 1 , that the resonant characteristics, and specifically the DC voltage gain versus the normalised switching frequency and the Q-factor, will have similar shape to the surface plot of  FIG. 12  (i.e. all desirable features of the LLC converter represented in  FIG. 12  will be valid during power conversion in both directions in the circuit of  FIG. 1 ). The inductor Lnew A  in the example embodiment of the invention represented in  FIG. 1  equalises the resonant configurations in both directions of power transfer resulting in step-down/step-up voltage conversion accompanied with loss-less, ZVS/ZCS operation in both directions of power conversion. The exact values of resonant characteristics in both directions of power transfer are governed by the inductance ratios Lnew A /Lr and Lm/Lr (in addition to normalised switching frequency and the Q-factor). In an idealised case in which the turns ratio of transformer  18  is unity, Lnew A  is equal to Lm and the input/output terminals of the circuit are equally loaded (during the bi-directional transfer), then the bi-directional converter  100 A will exhibit exactly the same DC-voltage gain and ZVS/ZSC characteristics in both directions of power transfer. It is noted that in some example embodiments of the invention, various ones of the corresponding resonant components employed to establish equal resonant configurations in both directions of power transfer have different values. In some example embodiments of the invention, a value of a resonant component employed in a first resonant tank circuit is different from a value of a corresponding resonant component employed by a second resonant tank circuit that has the same resonant configuration as the first resonant tank circuit. In other example embodiments of the invention, resonant circuits having different resonant configurations are employed in each direction of power transfer 
       FIG. 10   a  shows the waveform of the currents (i.e. ILnew A , ILm) through Lnew A  and Lm inductors of  FIG. 8  and  FIG. 9 .  FIG. 10   b  shows the waveforms (i.e. IsynhRect 1  and IsynhRect 2 ) of the current through the synchronous rectifier devices of  FIG. 8  and  FIG. 9 .  FIG. 10   b  shows that at the switching instances t 1  and t 2  during which one of the synchronous rectifier devices turns off and the opposite synchronous rectifier device turns on the currents through these devices is equal to zero, which results in zero switching loss (i.e. zero current switching (ZCS)).  FIG. 10   c  shows the waveforms of the currents (i.e. Iswitch 1  and Iswitch 2 ) through the switcher devices of  FIG. 8  and  FIG. 9  and one can see that just after the switching instances t 1  and t 2  these currents are negative, i.e. they flow not through device channel but through the anti-parallel diodes of the switcher devices. It follows that the voltage drop across the switcher devices at these instances is very small and equal to the voltage drop across the junction of a forward biased diode (i.e. typically less than 1V). A very small switching loss (i.e. zero voltage switching (ZVS) results. 
     Referring back to  FIGS. 8 and 9 , it is to be noted that costs are advantageously reduced in this example embodiment since the first resonant tank circuit  300  shares at least two common resonant components (i.e. Lr connected in series with Cr in this illustrated embodiment) with the second resonant tank circuit  310 . In this illustrated embodiment, each of the first and second resonant tank circuits  300  and  310  include only a single different component. Specifically, in this illustrated embodiment, the first resonant tank circuit  300  include a first resonant component (i.e. Lm) that is different than a second resonant component (i.e. Lnew A ) employed by the second resonant tank circuit  310 . In various example embodiments of the present invention, a plurality of resonant tank circuits is employed, each of the resonant tank circuits including at least one capacitor and at least one inductor that are connected in series. 
     A bi-directional converter  100 B as per another example embodiment of present invention is shown in  FIG. 2 . This circuit configuration is suitable for bi-directional power conversion of DC voltages with higher amplitudes. In the case of power transfer from the left hand side to the right hand side of the circuitry shown in  FIG. 2 , a full-bridge switcher circuit  220 B that includes controlled switching devices  34 ,  35 ,  36  and  37  which include embedded, or external, anti-parallel diodes  52 ,  53 ,  54  and  55  is connected to a DC voltage source  31 . A filter capacitor  32  is connected in parallel to the DC voltage source  31  to filter switching frequency ripple. Switching devices  34 ,  35 ,  36  and  37  are turned on and off with approximately 50% duty cycle width and their switching frequency is controlled, so that full-bridge switcher circuit  220 B produces square-wave voltage with 50% duty cycle and variable frequency at terminals  33  and  38 . An inductor  39  is connected across terminals  33  and  38 . A series network that includes inductor  40 , capacitor  41  and magnetizing inductor  42  is connected to terminals  33  and  38 . Magnetising inductor  42  is connected in parallel with the primary winding  43  located on the primary side  45 A of transformer  45 . In this example embodiment, magnetizing inductor  42  is an embedded magnetising inductor of transformer  45 . The secondary winding  44  located on the secondary side  45 B of transformer  45  is connected to terminals  60  and  61  of a full-bridge rectifier circuit  230 B that includes controllable switching devices  46 ,  47 ,  48  and  49  that include embedded or external anti-parallel diodes  56 ,  57 ,  58  and  59 . In this example embodiment, switching devices  46 ,  47 ,  48  and  49  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulses width, so they rectify the square-wave voltage across terminals  60  and  61  produced by the secondary winding  44 . A common cathode point of anti-parallel diodes  56  and  57  is connected to the positive terminal of the load impedance  51 , while a common anode point of diodes  58  and  59  is connected to the negative terminal of the load impedance  51 . A filter capacitor  50  is connected across the load impedance  51  to filter out switching ripple. In this example embodiment, full-bridge rectifier circuit  230 B is employed as a full-bridge synchronous rectifier circuit. 
     In case of power transfer from the right hand side to left hand side of the circuitry shown in  FIG. 2 , the power source and the load impedance swap their places (i.e. load impedance  51  becomes a DC voltage source, while DC voltage source  31  becomes a load impedance). In addition, the switching devices  46 ,  47 ,  48  and  49  become a full-bridge switcher circuit with controlled switching frequency and approximately 50% duty cycle width that produces square-wave voltage with variable frequency across terminals  60  and  61  of the secondary winding  44  of transformer  45 . In addition, the switching devices  34 ,  35 ,  36  and  37  become a full-bridge synchronously controlled rectifier circuit with approximately 50% duty cycle control pulses width that rectify the square wave voltage on terminals  33  and  38  produced by the full-bridge switcher circuit. Similarly to the example embodiment of  FIG. 1 , a resonant network circuit  240 B that includes inductors  39 ,  40 ,  42  and capacitor  41  forms the same resonant configuration when excited from terminals  33  and  38  and loaded across terminals  62  and  63 , as well as when excited from terminals  62  and  63  and loaded across terminals  33  and  38 . This change in the excitation and loading terminals of the resonant network circuit  204 B can happen when switching devices  34 ,  35 ,  36  and  37  and switching devices  46 ,  47 ,  48  and  49  change their control functions from a switcher to a synchronous rectifier and vice versa. It is noted that the loading/excitation across terminals  62  and  63  is firmly linked to loading/excitation across terminals  60  and  61  by the turns ratio of the primary winding  43  and secondary winding  44  of transformer  45 . In this example embodiment, inductor  39  is referred to Lnew B  which allows bi-directional converter  100 B to have the same resonant configurations in both directions of power transfer. 
     A bi-directional converter  100 C as per another example embodiment of present invention is shown in  FIG. 3 . This circuit configuration is suitable for bi-directional power conversion of DC voltages with medium to low amplitudes. In case of power transfer from the left hand side to the right hand side of the circuitry in  FIG. 3 , a half-bridge switcher circuit  220 C that includes controlled switching devices  74  and  75  which include embedded, or external, anti-parallel diodes  72  and  73  is connected to DC voltage source  65 . A filter capacitor  66  is connected in parallel to the DC voltage source  65  to filter switching frequency ripple. Two resonant capacitors  67  and  69  are connected in series with a common point located at terminal  68  and their free ends are connected to the positive and the negative terminals of the DC voltage source  65 . Switching devices  74  and  75  are turned on and off with approximately 50% duty cycle width and their switching frequency is controlled, so that half-bridge switcher circuit  220 C produces square-wave voltage with 50% duty cycle and variable frequency on terminals  68  and  71 . An inductor  70  is connected across terminals  68  and  71 . A series network that includes inductor  76  and magnetizing inductor  77  which is an embedded magnetising inductor of a transformer  80  in this example embodiment is connected to terminals  68  and  71 . Magnetizing inductor  77  is also connected in parallel with the primary winding  90  located on the primary side  80 A of transformer  80 . The secondary windings  78  and  79  located on the secondary side  80 B of transformer  80  have equal number of turns and are connected in centre-tap configuration in which the centre-tap terminal is connected to the positive terminal of a load impedance  86 , while the free ends of the secondary windings are connected to controlled switching devices  81  and  82  that include embedded, or external, anti-parallel diodes  83  and  84 . The switching devices  81  and  82  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulses width, so they rectify the square-wave voltage produced by the secondary windings  78  and  79 . The common point of switching devices  81  and  82  is connected to the negative terminal of the load impedance  86 . A filter capacitor  85  is connected in parallel to the load impedance  86 . In this example embodiment switching devices  81  and  82  are arranged in a push-pull rectifier circuit  230 C. In this example embodiment, push-pull rectifier circuit  230 C is employed as a push-pull synchronous rectifier circuit. 
     In the case of power transfer from the right hand side to left hand side of the circuitry in  FIG. 3 , the power source and the load swap their places, (i.e. load impedance  86  becomes a DC voltage source, while DC voltage source  65  becomes a load). In addition, switching devices  81  and  82  become a push-pull controlled switcher with controlled switching frequency and approximately 50% duty cycle width that produces square-wave voltage with variable frequency across terminals  87  and  88  located at the primary winding  90  of transformer  80 . Furthermore, the switching devices  74  and  75  become a synchronously controlled rectifier with approximately 50% duty cycle control pulses width that rectifies the square wave voltage on terminals  68  and  71  produced by the push-pull switcher created by switching devices  81  and  82 . It can be noted that the resonant network circuit  240 C that includes inductors  70 ,  76 ,  77  and resonant capacitors  67  and  69  will form the same resonant configuration when excited from terminals  68  and  71  and loaded across terminals  87  and  88 , as well as when exited from terminals  87 ,  88  and loaded across terminals  68  and  71 . This change in the excitation and loading terminals of the resonant network of capacitors  67  and  69 , and inductors  70 ,  76  and  77  happens when switching devices  74 ,  75 ,  81  and  82  change their control functions from a switcher to a synchronous rectifier and vice versa. It is noted that loading/excitation across terminals  87  and  88  is firmly linked to loading/excitation across terminals  91  and  92  by the turns ratio of the primary winding  90  and secondary windings  78  and  79  of transformer  80 . In this example embodiment, inductor  70  is referred to Lnew C  which allows bi-directional converter  100 C to have the resonant configurations in both directions of power transfer. 
     A bi-directional converter  100 D as per another example embodiment of present invention is shown in  FIG. 4 . This circuit configuration is suitable for bi-directional power conversion of DC voltages with lower amplitudes. In case of power transfer from the left hand side to the right hand side of the circuitry in  FIG. 4 , a push-pull switcher circuit  220 D that includes controlled switching devices  103  and  104  which include embedded, or external, anti-parallel diodes  105  and  106  is connected to a DC voltage source  101 . A filter capacitor  102  is connected in parallel to the DC voltage source  101  to filter switching frequency ripple. Switching devices  103  and  104  are turned on and off with approximately 50% duty cycle width and their switching frequency is controlled to produce square-wave voltage with 50% duty cycle and variable frequency on terminals  110  and  111 . An inductor  107  having two, connected in series and magnetically coupled sections with equal number of turns 108 and 109 is connected across terminals  110  and  111 . A series network that includes inductor  112 , capacitor  113  and magnetising inductor  114  is also connected to terminals  110  and  111 . In this example embodiment, magnetising inductor  114  is an embedded inductor of transformer  124 . Magnetising inductor  114  is also connected in parallel with the primary winding 115  located on the primary side  124 A of transformer  124 . The secondary windings  116  and  117  located on the secondary side  124 B of transformer  124  have equal number of turns and are connected in centre-tap configuration in which the centre-tap terminal is connected to the positive terminal of a load impedance  123 , while the free ends of the secondary windings  116  and  117  are connected to controlled switching devices  118  and  119  that include embedded, or external, anti-parallel diodes  120  and  121 . The switching devices  118  and  119  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulses width, so they rectify the square-wave voltage produced by the secondary windings  116  and  117 . A common point of switching devices  118  and  119  is connected to the negative terminal of the load impedance  123 . A filter capacitor  122  is connected in parallel to the load impedance  123 . In this example embodiment switching devices  118  and  119  are arranged in a push-pull rectifier circuit  230 D. In this example embodiment, push-pull rectifier circuit  230 D is employed as push-pull synchronous rectifier circuit. 
     In case of power transfer from the right hand side to left hand side of the circuitry in  FIG. 4 , the power source and the load swap their places (i.e. load impedance  123  becomes a DC voltage source, while DC voltage source  101  becomes a load). In this case the switching devices  118  and  119  become a push-pull controlled switcher with controlled switching frequency and approximately 50% duty cycle width that produces square-wave voltage with variable frequency across terminals  125  and  126  located at the primary winding  115  of transformer  124 . Furthermore, the switching devices  103  and  104  become a synchronously controlled rectifier with approximately 50% duty cycle control pulses width that rectifies the square wave voltage on terminals  110  and  111  produced by the push-pull switcher created by switching devices  118  and  119 . It can be noted that the series resonant network circuit  240 D that include inductors  107 ,  112  and  114  and the resonant capacitor  113  will form the same resonant configuration when excited from terminals  110  and  111  and loaded across terminals  125  and  126 , as well as when excited from terminals  125  and  126  and loaded across terminals  110  and  111 . This change in the excitation and loading terminals of the resonant network circuit  240 D can happen when switching devices  103 ,  104 ,  118 , and  119  change their control functions from a switcher to a synchronous rectifier and vice versa. It is noted that loading/excitation across terminals  125  and  126  is firmly linked to loading/excitation across terminals  127  and  128  by the turns ratio of the primary winding  115  and secondary windings  116  and  117  of transformer  124 . In this example embodiment, inductor  107  is referred to Lnew D  which allows bi-directional converter  100 D to have the resonant configurations in both directions of power transfer. 
     A bi-directional converter  100 E as per another example embodiment of present invention is shown in  FIG. 5 . This circuit configuration is suitable for bi-directional power conversion of DC voltages with medium amplitudes. In case of power transfer from the left hand side to the right hand side of the circuitry in  FIG. 5 , a half-bridge switcher circuit  220 E that includes controlled switching devices  198  and  200  which include embedded, or external, anti-parallel diodes  197  and  195  is connected to a DC voltage source  190 . A filter capacitor  191  is connected in parallel to the DC voltage source  190  to filter switching frequency ripple. Two resonant capacitors  192  and  194  are connected in series with a common point at terminal  193  and their free ends are connected to the positive and negative terminals of the DC voltage source  190 . The switching devices  198  and  200  are turned on and off with approximately a 50% duty cycle width and their switching frequency is controlled, so that half-bridge switcher circuit  220 E produces square-wave voltage with 50% duty cycle and variable frequency on terminals  193  and  199 . An inductor  196  is connected across terminals  193  and  199 . A series network that includes inductor  201  and magnetizing inductor  206  is also connected to terminals  193  and  199 . In this example embodiment, magnetizing inductor  206  is an embedded magnetising inductor of a transformer  203 . Magnetizing inductor  206  is connected in parallel with the primary winding  205  located on the primary side  203 A of transformer  203 . The secondary winding  204  located on the secondary side  203 B of transformer  203  is connected to terminals  215  and  216  which are input terminals of a half-bridge rectifier circuit  230 E that includes controllable switching devices  209  and  210  which include embedded or external anti-parallel diodes  208  and  211 . The switching devices  209  and  210  are connected in series with a common point located at terminal  216 , while their free ends are connected to the positive and negative terminals of the load impedance  214 . Two filter capacitors  212  and  213  are also connected in series with a common point at terminal  215 , while their free ends are connected to the positive and negative terminals of the load impedance  214 . The switching devices  209  and  210  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulses width, so they rectify the square-wave voltage across terminals  215  and  216  produced by the secondary winding  204 . In this example embodiment, half-bridge rectifier circuit  230 E is employed as a half-bridge synchronous rectifier circuit. 
     In case of power transfer from the right hand side to left hand side of the circuitry in  FIG. 5 , the power source and the load impedance swap their places (i.e. load impedance  214  becomes a DC voltage source, while DC voltage source  190  becomes a load). In addition, the switching devices  209  and  210  become a half-bridge switcher circuit with controlled switching frequency and approximately 50% duty cycle width that produces square-wave voltage with variable frequency across terminals  215  while the switching devices  198  and  200  become a half-bridge synchronously controlled rectifier with approximately 50% duty cycle control pulses width that rectify the square wave voltage on terminals  193  and  199  produced by the half-bridge switcher circuit. It is noted that a resonant network circuit  240 E that includes inductors  196 ,  201 ,  206  and capacitors  192 ,  194  forms the same resonant configuration when excited from terminals  193  and  199  and loaded across terminals  202  and  207 , as well as when excited from terminals  202  and  207  and loaded across terminals  193  and  199 . This change in the excitation and loading terminals of the resonant network circuit  240 E can happen when switching devices  198 ,  200 ,  209  and  210  change their control functions from a switcher to a synchronous rectifier and vice versa. It is noted that the loading/excitation across terminals  202  and  207  is firmly linked to loading/excitation across terminals  215  and  216  by the turns ratio of the primary winding  205  and secondary winding  204  of transformer  203 . In this example embodiment, inductor  196  is referred to Lnew E  which causes bi-directional converter  100 E to have the resonant configurations in both directions of power transfer. 
     A bi-directional converter  100 F as per another example embodiment of present invention is shown in  FIG. 6 . The  FIG. 6  schematic and principle of operation is similar to that described for the example embodiment of  FIG. 1 . In the case of power transfer from the left hand side to the right hand side of the circuitry in  FIG. 6 , a full-bridge switcher circuit  220 F containing controlled switching devices  134 ,  135 ,  136  and  137  that include embedded, or external, anti-parallel diodes  138 ,  139 ,  140  and  141  is connected to a DC voltage source  131 . A filter capacitor  132  is connected in parallel to the DC voltage source  131  to filter out the switching ripple. Switching devices  134 ,  135 ,  136  and  137  are turned on and off with approximately 50% duty cycle width and their switching frequency is controlled, so that full-bridge switcher circuit  220 F produces square-wave voltage waveform with 50% duty cycle and variable frequency on terminals  133  and  143 . An inductor  142  is connected across the terminals  133  and  143 . A series network that includes inductor  144 , capacitor  145  and magnetising inductor  146  is connected to terminals  133  and  143 . Magnetising inductor  146  is connected in parallel with the primary winding  147  located on the primary side  150 A of transformer  150 . In this example embodiment, magnetising inductor  146  is embedded in the magnetic structure of transformer  150 . The secondary windings  148  and  149  located on the secondary side  150 B of transformer  150  include equal number of winding turns and are connected in centre-tap configuration in which the centre-tap terminal is connected to the positive terminal of a load impedance  158 , while the ends of the secondary windings  148  and  149  are connected to a push-pull rectifier circuit  230 F that includes controlled switching devices  151  and  152  which include embedded, or external, anti-parallel diodes  155  and  156 . The switching devices  151  and  152  are controlled in a synchronous rectification manner with approximately 50% duty cycle control pulses width, so they rectify the square-wave voltage produced by the secondary windings. A common point of switching devices  151  and  152 , and anti-parallel diodes  155  and  156  is connected to the negative terminal of the load impedance  158 . A filter capacitor  157  is connected in parallel to load impedance  158 . In this example embodiment, push-pull rectifier circuit  230 F is employed as a push-pull synchronous rectifier circuit. 
     One difference between the example embodiment of  FIG. 1  and this illustrated embodiment includes the presence of two dissipative or non-dissipative snubber networks  153  and  154  connected in parallel to the switching devices switches  151  and  152 . In various applications switching devices  151  and  152  are equipped with parallel snubbers (or voltage clamps) that clamp voltage spikes across switching devices  151  and  152  that can be generated by leakage inductances of transformer  150 . Although the secondary windings  148  and  149  located on the secondary side  150 B of the centre-tap transformer  150  are typically designed to have very strong magnetic coupling, a small leakage inductance is usually present in secondary windings  148  and  149 . This inductance can cause voltage spikes across the switching devices  151  and  152  as they undergo a “turn-off” cycle. The amplitude of these voltage spikes is typically a function of the leakage inductance value, as well as the rate of change of the turn-off current and sometimes it can be dangerously high for the safe operation of switching devices  151  and  152 . The schematics of these dissipative, or non-dissipative, snubbers are of a large variety, and can vary from those illustrated in  FIG. 6 . 
     Similarly to the example embodiment of  FIG. 1 , a resonant network circuit  240 F that includes inductors  142 ,  144 ,  146  and capacitor  145  forms the same resonant configuration when excited from terminals  133  and  143  and loaded across terminals  159  and  160 , as well as when excited from terminals  159  and  160  and loaded across terminals  133  and  143 . This change in the excitation and loading terminals of the resonant network circuit  240 F can happen when switching devices  134 ,  135 ,  140  and  141  and switching devices  151  and  152  change their control functions from a switcher to a synchronous rectifier and vice versa. In this example embodiment, inductor  142  is referred to Lnew F  which allows bi-directional converter  100 F to have the resonant configurations in both directions of power transfer. 
     A bi-directional converter  100 G as per another example embodiment of present invention is shown in  FIG. 7 . Its schematic and principle of operation is similar to the example embodiment of  FIG. 6  and similar components are accordingly identified using identical part numbers. One difference between the two embodiments is that the two dissipative, or non-dissipative snubbers (or voltage clamps)  153  and  157  employed by the push-pull rectifier circuit  230 F of  FIG. 6  are replaced in  FIG. 7  with a single dissipative or non-dissipative snubber (or voltage clamp)  185  employed by push-pull rectifier circuit  230 G. Snubber  185  is connected in parallel to the switching devices  151  and  152  via diodes  184  and  186 . Diodes  184  and  186  have a common cathode point connected to the top terminal of the snubber  185 , while the anodes of diodes  184  and  186  are connected to the top terminals of switching devices  151  and  152 , respectively. The bottom terminals of switching devices  151  and  152  are connected to the bottom terminal of the snubber  185  and to the negative terminal of the load  158 . In this example embodiment, power is transferred bi-directionally between full-bridge switcher circuit  220 G and push-pull rectifier circuit  230 G via resonant network circuit  240 G in a manner similar to that described in other example embodiments. Inductor  142  which is referred to as Lnew G  allows bi-directional converter  100 G to have resonant configurations in both directions of power transfer. 
     Various embodiments of the invention have now been described in detail. Without limitation, the various embodiments of the invention described can be combined to provide other example embodiments. The scope of the invention is to be construed in accordance with the substance defined by the following claims. As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications to the above-described best mode are possible without departing from the spirit or scope thereof. For example, certain modifications, permutations, additions and sub-combinations of the features described herein will be apparent to those skilled in the art. It is intended that the following appended claims and the claims hereafter introduced should be interpreted broadly so as to encompass all such modifications, permutations, additions and sub-combinations as are consistent with the language of the claims, broadly construed. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 
                 DC voltage source 
               
               
                   
                  2 
                 filter capacitor 
               
               
                   
                  3 
                 terminal 
               
               
                   
                  4 
                 switching device 
               
               
                   
                  5 
                 switching device 
               
               
                   
                  6 
                 switching device 
               
               
                   
                  7 
                 switching device 
               
               
                   
                  8 
                 anti-parallel diode 
               
               
                   
                  9 
                 anti-parallel diode 
               
               
                   
                  10 
                 anti-parallel diode 
               
               
                   
                  11  
                 anti-parallel diode 
               
               
                   
                  12  
                 terminal 
               
               
                   
                  13  
                 inductor 
               
               
                   
                  14  
                 inductor 
               
               
                   
                  15  
                 capacitor 
               
               
                   
                  16  
                 magnetizing inductor 
               
               
                   
                  17  
                 primary winding 
               
               
                   
                  18  
                 transformer 
               
               
                   
                  18A 
                 primary side 
               
               
                   
                  18B 
                 secondary side 
               
               
                   
                  19  
                 secondary winding 
               
               
                   
                  20  
                 secondary winding 
               
               
                   
                  21  
                 switching device 
               
               
                   
                  22  
                 switching device 
               
               
                   
                  23  
                 anti-parallel diode 
               
               
                   
                  24  
                 anti-parallel diode 
               
               
                   
                  25  
                 filter capacitor 
               
               
                   
                  26  
                 load impedance 
               
               
                   
                  27  
                 terminal 
               
               
                   
                  28  
                 terminal 
               
               
                   
                  29  
                 first terminal set 
               
               
                   
                  31  
                 DC voltage source 
               
               
                   
                  32  
                 filter capacitor 
               
               
                   
                  33  
                 terminal 
               
               
                   
                  34  
                 switching device 
               
               
                   
                  35  
                 switching device 
               
               
                   
                  36  
                 switching device 
               
               
                   
                  37  
                 switching device 
               
               
                   
                  38  
                 terminal 
               
               
                   
                  39  
                 second terminal set 
               
               
                   
                  40  
                 inductor 
               
               
                   
                  41  
                 capacitor 
               
               
                   
                  42  
                 magnetizing inductor 
               
               
                   
                  43  
                 primary winding 
               
               
                   
                  44  
                 secondary winding 
               
               
                   
                  45  
                 transformer 
               
               
                   
                  45A 
                 primary side 
               
               
                   
                  45B 
                 secondary side 
               
               
                   
                  46  
                 switching device 
               
               
                   
                  47  
                 switching device 
               
               
                   
                  48  
                 switching device 
               
               
                   
                  49  
                 switching device 
               
               
                   
                  50  
                 filter capacitor 
               
               
                   
                  51  
                 load impedance 
               
               
                   
                  52  
                 anti-parallel diode 
               
               
                   
                  53  
                 anti-parallel diode 
               
               
                   
                  54  
                 anti-parallel diode 
               
               
                   
                  55  
                 anti-parallel diode 
               
               
                   
                  56  
                 anti-parallel diode 
               
               
                   
                  57  
                 anti-parallel diode 
               
               
                   
                  58  
                 anti-parallel diode 
               
               
                   
                  59  
                 anti-parallel diode 
               
               
                   
                  60  
                 terminal 
               
               
                   
                  61  
                 terminal 
               
               
                   
                  62  
                 terminal 
               
               
                   
                  63  
                 terminal 
               
               
                   
                  65  
                 DC voltage source 
               
               
                   
                  66  
                 filter capacitor 
               
               
                   
                  67  
                 capacitor 
               
               
                   
                  68  
                 terminal 
               
               
                   
                  69  
                 capacitor 
               
               
                   
                  70  
                 inductor 
               
               
                   
                  71  
                 terminal 
               
               
                   
                  72  
                 anti-parallel diode 
               
               
                   
                  73  
                 anti-parallel diode 
               
               
                   
                  74  
                 switching device 
               
               
                   
                  75  
                 switching device 
               
               
                   
                  76  
                 inductor 
               
               
                   
                  77  
                 magnetizing inductor 
               
               
                   
                  78  
                 secondary winding 
               
               
                   
                  79  
                 secondary winding 
               
               
                   
                  80  
                 transformer 
               
               
                   
                  80A 
                 primary side 
               
               
                   
                  80B 
                 secondary side 
               
               
                   
                  81  
                 switching device 
               
               
                   
                  82  
                 switching device 
               
               
                   
                  83  
                 anti-parallel diode 
               
               
                   
                  84  
                 anti-parallel diode 
               
               
                   
                  85  
                 filter capacitor 
               
               
                   
                  86  
                 load impedance 
               
               
                   
                  87  
                 terminal 
               
               
                   
                  88 
                 terminal 
               
               
                   
                  90 
                 primary winding 
               
               
                   
                  91 
                 terminal 
               
               
                   
                  92 
                 terminal 
               
               
                   
                 100A 
                 bi-directional converter 
               
               
                   
                 100B 
                 bi-directional converter 
               
               
                   
                 100C 
                 bi-directional converter 
               
               
                   
                 100D 
                 bi-directional converter 
               
               
                   
                 100E 
                 bi-directional converter 
               
               
                   
                 100F 
                 bi-directional converter 
               
               
                   
                 100G 
                 bi-directional converter 
               
               
                   
                 101  
                 DC voltage source 
               
               
                   
                 103  
                 switching device 
               
               
                   
                 104  
                 switching device 
               
               
                   
                 105  
                 anti-parallel diode 
               
               
                   
                 106  
                 anti-parallel diode 
               
               
                   
                 107  
                 inductor 
               
               
                   
                 108  
                 turns 
               
               
                   
                 109  
                 turns 
               
               
                   
                 110  
                 terminal 
               
               
                   
                 111  
                 terminal 
               
               
                   
                 112  
                 inductor 
               
               
                   
                 113  
                 capacitor 
               
               
                   
                 114  
                 magnetizing inductor 
               
               
                   
                 115  
                 primary winding 
               
               
                   
                 116  
                 secondary winding 
               
               
                   
                 117  
                 secondary winding 
               
               
                   
                 118  
                 switching device 
               
               
                   
                 119  
                 switching device 
               
               
                   
                 120  
                 anti-parallel diode 
               
               
                   
                 121  
                 anti-parallel diode 
               
               
                   
                 122  
                 filter capacitor 
               
               
                   
                 123  
                 load impedance 
               
               
                   
                 124  
                 transformer 
               
               
                   
                 124A 
                 primary side 
               
               
                   
                 124B 
                 secondary side 
               
               
                   
                 131  
                 DC voltage source 
               
               
                   
                 132  
                 filter capacitor 
               
               
                   
                 133  
                 terminal 
               
               
                   
                 134  
                 switching device 
               
               
                   
                 135  
                 switching device 
               
               
                   
                 136  
                 switching device 
               
               
                   
                 137  
                 switching device 
               
               
                   
                 138  
                 anti-parallel diode 
               
               
                   
                 139  
                 anti-parallel diode 
               
               
                   
                 140  
                 anti-parallel diode 
               
               
                   
                 141  
                 anti-parallel diode 
               
               
                   
                 142  
                 inductor 
               
               
                   
                 143  
                 terminal 
               
               
                   
                 144  
                 inductor 
               
               
                   
                 145  
                 capacitor 
               
               
                   
                 146  
                 magnetizing inductor 
               
               
                   
                 147  
                 primary winding 
               
               
                   
                 148  
                 secondary winding 
               
               
                   
                 149  
                 secondary winding 
               
               
                   
                 150  
                 transformer 
               
               
                   
                 150A 
                 primary side 
               
               
                   
                 150B 
                 secondary side 
               
               
                   
                 151  
                 switching device 
               
               
                   
                 152  
                 switching device 
               
               
                   
                 153  
                 snubber network 
               
               
                   
                 154  
                 snubber network 
               
               
                   
                 157  
                 filter capacitor 
               
               
                   
                 158  
                 load impedance 
               
               
                   
                 159  
                 terminal 
               
               
                   
                 160  
                 terminal 
               
               
                   
                 171  
                 inductor 
               
               
                   
                 184  
                 diode 
               
               
                   
                 185  
                 snubber 
               
               
                   
                 186  
                 diode 
               
               
                   
                 190  
                 DC voltage source 
               
               
                   
                 191  
                 filter capacitor 
               
               
                   
                 192  
                 resonant capacitor 
               
               
                   
                 193  
                 terminal 
               
               
                   
                 194  
                 resonant capacitor 
               
               
                   
                 195  
                 anti-parallel diode 
               
               
                   
                 197  
                 anti-parallel diode 
               
               
                   
                 198  
                 switching device 
               
               
                   
                 199  
                 terminal 
               
               
                   
                 200  
                 switching device 
               
               
                   
                 201  
                 inductor 
               
               
                   
                 203  
                 transformer 
               
               
                   
                 203A 
                 primary side 
               
               
                   
                 203B 
                 secondary side 
               
               
                   
                 204  
                 secondary winding 
               
               
                   
                 205  
                 primary winding 
               
               
                   
                 206  
                 magnetizing inductor 
               
               
                   
                 207  
                 terminal 
               
               
                   
                 208  
                 anti-parallel diode 
               
               
                   
                 209  
                 switching device 
               
               
                   
                 210  
                 switching device 
               
               
                   
                 211  
                 anti-parallel diode 
               
               
                   
                 212  
                 filter capacitor 
               
               
                   
                 213  
                 filter capacitor 
               
               
                   
                 214  
                 load impedance 
               
               
                   
                 215  
                 terminal 
               
               
                   
                 216  
                 terminal 
               
               
                   
                 220A 
                 full-bridge switcher circuit 
               
               
                   
                 220B 
                 full-bridge switcher circuit 
               
               
                   
                 220C 
                 half-bridge switcher circuit 
               
               
                   
                 220D 
                 push-pull switcher circuit 
               
               
                   
                 220E 
                 half-bridge switcher circuit 
               
               
                   
                 220F 
                 full-bridge switcher circuit 
               
               
                   
                 220G 
                 full-bridge switcher circuit 
               
               
                   
                 222  
                 first direction 
               
               
                   
                 224  
                 second direction 
               
               
                   
                 230A 
                 push-pull rectifier circuit 
               
               
                   
                 230B 
                 full-bridge rectifier circuit 
               
               
                   
                 230C 
                 push-pull rectifier circuit 
               
               
                   
                 230D 
                 push-pull rectifier circuit 
               
               
                   
                 230E 
                 half-bridge rectifier circuit 
               
               
                   
                 230F 
                 push-pull rectifier circuit 
               
               
                   
                 230G 
                 push-pull rectifier circuit 
               
               
                   
                 240A 
                 resonant network circuit 
               
               
                   
                 240B 
                 resonant network circuit 
               
               
                   
                 240C 
                 resonant network circuit 
               
               
                   
                 240D 
                 resonant network circuit 
               
               
                   
                 240E 
                 resonant network circuit 
               
               
                   
                 240F 
                 resonant network circuit 
               
               
                   
                 240G 
                 resonant network circuit 
               
               
                   
                 300  
                 first resonant tank circuit 
               
               
                   
                 310  
                 second resonant tank circuit