Abstract:
A highly efficient and simple power conversion circuit ( 300 ) having zero voltage switching (ZVS) includes a novel switch timing technique, such that the need for an leakage inductor connected in series with the primary circuit of the converter and rectifier diodes is eliminated. A switch timing circuit ( 351 ) located in an output side circuit ( 350 ) enables the use of the natural stored magnetic energy in the output side circuit ( 350 ) to drive the critical switching transitions to accomplish soft switching for all of the switches ( 314-317 ) in a full bridge forward converter ( 300 ) for all transitions. This power conversion circuit ( 300 ) includes a full bridge circuit ( 310 ) having plurality of switching devices ( 314-317 ) that intermittently couple the primary winding ( 327 ) to the input of the power converter ( 300 ). A transformer ( 326 ) couples to receive power from the full bridge circuit ( 310 ) into its primary winding ( 327 ). The output side circuit ( 310 ) includes the switching circuit ( 351 ) coupled to provide stored magnetic energy to drive the switching transitions of the switching devices ( 314-317 ) in the full bridge circuit ( 310 ). The switching circuit ( 351 ) closes at a predetermined time delay after the switching transitions in the full bridge circuit ( 310 ) to accomplish zero voltage switching for the plurality of switches in the full bridge circuit ( 310 ). A converter controller provides control signals to the full bridge and the switching circuits ( 310, 351 ).

Description:
This application claims the benefit of Provisional application Ser. No. 60/201,821, filed May 4, 2000. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the application Ser. No. 09/312,091, filed May 15, 1999, for “Dual Opposed Interleaved Coupled Inductor Soft Switching Converters,” which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power conversion circuitry and, more particularly, to an apparatus having an efficient power conversion to accomplish zero voltage switching using a novel switch timing technique. 
     BACKGROUND OF THE INVENTION 
     A known full bridge forward DC-to-DC converter having zero voltage switching typically includes a DC-to-AC converter circuit and an AC-to-DC converter circuit linked together by a high frequency AC link, where isolation is provided on the intermediate AC link. This type of converter is a common circuit topology used to transform electric energy from a source at a given potential to a destination load at a different potential. It typically includes four switches, typically power metal-oxide semiconductor field-effect transistors (MOSFETs), operated in alternating pairs, an input/output isolation and step-up/step-down transformer, an output rectifier, and an output filter. A feedback regulator or controller is included to control the switches. 
     The main advantages of this converter topology include: constant frequency operation, which allows optimum design of magnetic filter components, pulse width modulation (PWM) control, minimum voltage and current (VA) stresses, and good control range and controllability. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunications and computer systems wherein high voltages are converted down to lower voltages needed to operate the systems. Power converters generally suffer from problems such as switching losses, switching noise and common-mode power transformer noise. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment. 
     When switching devices turn on and off, there is a power loss associated with this action. The power loss relates to the current through the switch and the voltage across the switch during the switching transition. The greatest loss is associated with the turn on of the switch. Zero voltage switching, however, provides a means for eliminating switching losses particularly in higher line voltages. The resulting converter will be more efficient by dissipating less heat. Zero voltage switching is achieved by adding a controlled dead time at the turn on of each stage. 
     A full bridge converter of this type operates generally as follows. The switches are arranged in two diagonal pairs that are alternately turned on for a fraction of a switching period to apply opposite polarities of the input DC voltage across the primary of the transformer. The operation of the switches produce a zero voltage across the transformer by turning off only one switch of the pair. A switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate at zero voltage through the two switches. The two switches clamp the voltage across the transformer at zero, thereby eliminating the ringing behavior suffered by the conventional bridge when the switches are off. Thus the switches operate to convert the input DC voltage into an AC voltage required to properly operate the transformer. 
     Different schemes have been developed to reduce the additional switching losses caused by high frequency switching of conventional converters. For example, semiconductor switching losses can be reduced by using reactive snubber elements. In FIG. 1. a first snubber circuit is implemented in zero voltage switching converter  40 . As illustrated, a snubber capacitor  64  may be connected in parallel with a converter semiconductor switch  56 , having an anti-parallel connected diode  60 . This snubber element  64  tends to limit the rate of rise of voltage experienced by the switching device  56 . Thus, snubber element  64  provides an easy method to divert the energy that would be dissipated in the switching device  56  during switching. However, the energy stored in the snubber element  64  needs to be dissipated during a subsequent part of the switching cycle. Each converter semiconductor switch  57 - 59  are connected in parallel with a snubber capacitor  65 - 67  and in anti-parallel with a diode  61 - 63 , respectively. 
     Converters that allow lossless resetting of the reactive snubber energy are referred to as “soft-switching” converters. Soft-switching converters may be broadly categorized as zero voltage switching. Various zero voltage switching schemes and converter topologies have been proposed in an attempt to achieve increased performance over conventional hard-switching converters. Many are disclosed in U.S. Pat. No. 5,781,419 which is incorporated herein. 
     An exemplary known soft-switching converter circuit topology is the full-bridge PWM converter shown at  40  in FIG.  1 . This converter topology  40  achieves PWM control with resonant switching of the converter semiconductor switches. The basic DC-to-DC converter circuit topology  40  includes an input side circuit  42  and an output side circuit  44  with the input circuit  42  and output circuit  44  linked by a transformer  46 . The transformer  46  includes a primary winding  48 , a secondary winding  50 , and is characterized by a leakage inductance  52 . The primary  48  of the transformer  46  is connected to a DC input voltage source  54  by a bridge of converter switches that forms the input circuit  42 . Four semi-conductor switching devices  56 - 59 , e.g., transistors, form the input side circuit  42  converter bridge. Each switching device  56 - 59  includes an anti-parallel connected diode  60 - 63  and parallel connected capacitor  64 - 67 . The output side circuit  44  connects the secondary winding  50  of the transformer  46  to a load, shown here as a resistance load  68 , by a diode bridge including four diodes  70 - 73 . An output side filter inductor  74  is connected in series between the diode bridge and the load  68 . An output side capacitor  76  is connected in parallel with the load  68 . In operation, a PWM controller is used to switch the input side circuit switching devices  56 - 59  in a sequence to generate an AC signal from the DC voltage source  54  across the primary winding  48  of the transformer  46 . The resulting AC signal appearing on the secondary winding  50  of the transformer  46  is rectified by the diodes  70 - 73  of the output side circuit  44  to provide a DC output voltage to the load  68 . The output side inductor  74  and capacitor  76  filter high frequency and transient voltages from the output voltage applied to the load  68 . The magnitude of the DC output voltage applied to the load  68  is determined by the magnitude of the DC source voltage, the duty cycle of the PWM controller, and the turns ratio of the transformer  46 . 
     In the DC-to-DC converter topology  40 , the leakage  52  and magnetizing inductance&#39;s of the transformer  46  are effectively utilized to achieve zero voltage switching of the switching devices  56 - 59 . The operation of the full-bridge PWM converter  40 , to achieve zero voltage switching, is as follows. With input circuit switching devices  56  and  59  initially turned on and conducting, the voltage applied across the primary winding  48  of the transformer  46  will be the voltage level of the voltage source  54 , V in . A corresponding voltage will appear on the secondary winding  50  of the transformer  46 , causing an output current to flow through diodes  70  and  73 . When switching device  59  in the input side circuit  42  is turned off, the input voltage  54  is disconnected from the primary winding  48 . With the input voltage Vin no longer applied to the primary winding  48  of the transformer  46 , the current in the output side circuit  44  will free wheel through all of the output side diodes  70 - 73 . A current thus continues to flow through the output side filter inductor  74  to provide power to the load  68 . This stored energy in the output side circuit  44  is reflected back through the transformer  46  to the input side circuit  42 . The continued flow of current through the primary winding  48  charges the output capacitance  67  of the input side switching device  59 , and discharges the output capacitance  66  of input side switching device  58 . This causes the anti-parallel connected diode  62  of switching device  58  to conduct. Thus, at this point, switching device  58  can be turned on under zero voltage switching conditions. Since the energy available for achieving zero voltage switching for the leading leg switching devices  58  and  59  is the energy stored in the output filter inductor  74 , zero voltage switching can be achieved even at light loads. 
     After input side switching device  58  is turned on, input side switching device  56  may be turned off, at a later point in time. Current still flows through the primary winding  48  of the transformer  46  due to energy stored in the leakage inductance  52  of the transformer  46 . When switching device  56  is turned off, this current charges the output capacitance  64  of switching device  56  and discharges the output capacitance  65  of switching device  57 , causing the anti-parallel connected diode  61  of switching device  57  to conduct. Thus, switching device  57  may now be turned on under zero voltage switching conditions, to once again apply the input voltage −V in  to the primary winding  48  of the transformer  46 . The switching sequence is then repeated for the turn-off of input side switching devices  58  and  57 , and the turn-on of input side switching devices  56  and  59 . In order to ensure zero voltage turn-on of the switching devices  56  and  57  in the lagging leg of the input side circuit bridge  42 , enough energy needs to be stored in the leakage inductance  52  of the transformer  46  to provide for charging and discharging of the switching device capacitors  64 - 67  throughout the switching sequence. Since the energy in the leakage inductance  52  is a function of the current to the load  68 , zero voltage switching will be lost below a certain load level for the lagging leg switching devices  56  and  57 . One way to extend the load range of the DC-to-DC converter  40  is thus by properly sizing the leakage inductance  52  of the transformer  46 . In addition the effect of leakage inductance  52  on the circuit is to create a ringing condition with the parasitic capacitance associated with the secondary switches  70 - 73 . The ringing is undamped and results in increased electromagnetic interference. Damping the ringing with a snubber circuit including rectifier diodes only results in additional energy losses. 
     Conventional soft switching full bridge converters do not enable the primary switch currents to decrease during the reset time of the output choke. In addition, these converters rely upon maintaining the stored energy in primary circuit magnetic elements for driving the switching transitions. The stored energy mechanisms used to drive the switching transitions are typically either relatively small chokes added specifically for the purpose or increased leakage inductance or reduced magnetizing inductance of the main transformer. In either case the amount of energy stored is small. 
     Thus, there is a need for a simple and efficient power conversion circuit having zero voltage switching. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the conventional full bridge converter having zero voltage switching, the present invention is directed to a highly efficient and simple power conversion circuit that accomplishes zero voltage switching (ZVS) using a novel switch timing technique. This circuit eliminates the need for an leakage inductor connected in series with the primary circuit of the converter and rectifier diodes, ultimately reducing size, weight and cost of the converter. A power conversion circuit having features of the present invention includes a full bridge circuit having a plurality of switching devices that intermittently couple the primary winding to the input of the power converter. A transformer couples to the full bridge circuit to receive power from the full bridge circuit into its primary winding. An output side circuit, connected to the secondary winding of the transformer, includes a switching circuit coupled to provide stored magnetic energy to drive the switching transitions of the switching devices in the full bridge circuit. The switching circuit closes at a predetermined time delay after the switching transitions in the full bridge circuit to accomplish zero voltage switching for the plurality of switches in the full bridge circuit. A converter controller provides control signals to the full bridge and the switching circuits. 
     This power conversion circuit provides a solution using a unique timing mechanism. The switch timing circuit enables the use of the natural stored magnetic energy in the output side circuit of the converter to drive the critical switching transitions to accomplish soft switching for all of the switches in a full bridge forward converter for all transitions. 
     The previously described version of the present invention has many advantages, including a simple and more reliable energy mechanism for driving the switching transitions and lower switch conduction losses by comparison to conventional full bridge forward converters. Since the energy storage elements used to store the energy to drive the transition are large in the present invention, the stored energy available to drive the transitions is larger than that known in the art. As a result of the large amount of stored energy available to drive the switching transitions, the range over which soft switching can be realized is larger. In addition, the circuit for driving the switching transitions does not require the reversal of the magnetizing current which leads to higher conduction and core losses in known converters. Moreover, the embodiments of the present invention reduce the conduction losses by comparison to other known soft switching full bridge forward converters, since the primary switch currents decrease with the output choke current during the reset time of the output choke. Furthermore, the stored energy used to drive the primary switching transitions is the energy stored in the large output choke(s), which is the natural stored magnetic energy component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
     FIG. 1 is a known power conversion circuit; 
     FIG. 2 is a first embodiment of a power converter in accordance with the present invention; 
     FIGS. 3 a-f  are timing diagrams demonstrating switch operation for each switching device of the power converter of FIG. 3 in accordance with the present invention; 
     FIG. 4 is a second embodiment of a power converter in accordance with the present invention; 
     FIG. 5 is a third embodiment of a power converter in accordance with the present invention; and 
     FIG. 6 is a fourth embodiment of a power converter in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in FIG. 2, the full bridge power converter  300  in accordance with the present invention eliminates the need for a leakage inductor  52 , output side filter inductor  74 , and rectifying diodes  70 - 73  as shown in FIG.  1 . This converter circuit topology  300  includes an input side circuit  310  and an output side circuit  350  linked together by a transformer  326 . The transformer  326  includes a primary winding  327  and a secondary winding  328 . The primary  327  of the transformer  326  is connected to a DC input voltage source  312  by a bridge of converter switches  314 - 317  that forms the input circuit  310 . Four semiconductor switching devices  314 - 317 , e.g., transistors, form the input side circuit  310  converter bridge. This converter is particularly useful when the following types of devices are used for the switching devices  314 - 317 : Insulated Gate Bipolar Transistors (IGBT), Bipolar Junction Transistors (BJT), Metal Oxide Semiconductor Controlled Thyristors (MCT), and Field Effect Transistors (FET). Each switching device  314 - 317  includes an anti-parallel connected diode  318 - 321  and parallel connected capacitor  322 - 325 . 
     The output side circuit  350  connects the secondary winding  328  of the transformer  326  to a load, shown here as a resistance load  362 , by two switch/inductor pairs  352 ,  354 ,  356 ,  358 . The output side circuit  350  includes a switching circuit  351  coupled to provide stored magnetic energy to drive the switching transitions of the switching devices in the full bridge circuit, wherein the switching circuit closes at a predetermined time delay after the switching transitions in the full bridge circuit to accomplish zero voltage switching for the plurality of switches in the full bridge circuit. An output side capacitor  360  is connected in parallel with the load  362  to provide filtering of the voltage applied to load  362 . In operation, a controller  364  is used to switch the input side circuit switching devices  314 - 317  in a sequence to generate an AC signal from the DC voltage source  312  across the primary winding  327  of the transformer  326 . The controller is also used to switch the output side circuit switching devices  352  and  356 . The resulting AC signal appearing on the secondary winding  328  of the transformer is driven by the energy stored in inductors  354  and  358  and by the timing of switches  352  and  356  of the output side circuit  350  to provide a DC output voltage to the load  362 . The output side capacitor  360  filters high frequency and transient voltages from the output voltage applied to the load  362 . The magnitude of the DC output voltage applied to the load  362  is determined by the magnitude of the DC source voltage, the duty cycle of the controller, and the turns ratio of the transformer  326 . 
     In the DC-to-DC converter topology  300 , the energy stored in inductors  354  and  358  effectively achieve zero voltage switching of the switching devices  314 - 317 . The operation of the full-bridge converter  300 , to achieve zero voltage switching, is as follows. With input circuit switching devices  314  and  317  initially turned on and conducting, the voltage applied across the primary winding  327  of the transformer  326  will be the voltage level of the voltage source  312 , V in . In order to achieve zero voltage switching, it is necessary to close switching device  356  at a predetermined time delay after the switching transition of one of the switching devices  314 - 317  in the input side circuit  310 . In particular, when switching devices  315  and  317  are on the voltages applied to both the primary winding  327  and the secondary winding  328  are zero. While switches  315  and  317  are on switch  352  is on simultaneously. When switch  315  is turned off, switch  356  remains off while energy stored in inductor  358  drives the transition of the two primary switches  315  and  314 , subsequent to which switch  314  is turned on at zero voltage. During this primary switching transition inductor  358  will force the voltage at the undotted terminal of secondary winding  328  to become negative with respect to the voltage at the dotted terminal of winding  328 . After the completion of the transition in which switch  315  is turned off and switch  314  is turned on, switch  356  is turned on at a time determined by the predetermined time delay and switch  352  is turned off. Thus, while switches  314  and  317  are initially transitioning closed, switch  356  remains off and the energy stored in inductor  358  is available to drive the turn on transition of switches  314  and  317 . During these transitions and the entire operating cycle, current continues to flow through both inductors  354  and  358  to output capacitor  360  and load  362 . 
     The predetermined time delay may be on the order of one hundred nano-seconds for a midrange power converter of 100-500 W. It depends on parameters of the converter  300 . If the converter  300  is a high power circuit on the order of 500 W or above, the time delay must be longer due to larger parasitic capacitors (not shown) in the windings of transformer  326  and switching devices  314 - 317 , which translates into a more lengthy switching transition. If the converter  300  is a low power circuit on the order of 50-100 W, the time delay would need to be shorter due to a shorter switching transition. 
     The inductors  354  and  358 , equal in size and inductance, share the average load current equally. During the first interval, current in inductor  354  increases, current in inductor  358  decreases and current in the primary winding  327  of transformer  326  increases. The energy in inductor  354  will drive the dotted side of the transformer  326  to decrease in voltage. Since the energy between the primary and secondary windings,  327  and  328 , are coupled, the decrease in voltage at the dotted side of the secondary winding  328  will decrease the voltage at the dotted side of the primary winding  327 . The current in the primary winding  327  of transformer  326  has two components, one component is a relatively small magnetizing current and the other component is equal to the current in the inductor  354  multiplied by the secondary to primary turns ratio of the power transformer  326 . Thus, the current that is proportional to the current through inductor  354  dominates. 
     When switching device  317  in the input side circuit  310  is turned off, the input voltage  312  is disconnected from the primary winding  327 . With the input voltage V in  no longer applied to the primary winding  327  of the transformer  326 , the current in the output side circuit  350  will free wheel through the switching device  356  and inductor  358 . Current thus continues to flow through both inductors  354  and  358  to the output capacitor  360  and to the load  362 . The current of inductor  354  is reflected back through the transformer  326  to the input-side circuit  310 . The continued flow of current through the primary winding  327  charges the output capacitance  321  connected to switching device  317 , and discharges the output capacitance  324  connected to switching device  316 . This causes the anti-parallel connected diode  320  of switching device  316  to conduct. Thus, at this point, switching device  316  can be turned on under zero voltage switching conditions. Since the energy available for achieving zero voltage switching for the leading leg switching devices  316  and  317  is the energy stored in the inductor  354 , zero voltage switching can be achieved even at light loads. 
     After switching device  316  is turned on, switching device  314  may be turned off, at a later point in time. Current still flows through the primary winding  327  of the transformer  326  due to energy stored in the inductor  354 . When switching device  314  is turned off, this current charges the output capacitance  322  of switching device  314  and discharges the output capacitance  323  of switching device  315 , causing the anti-parallel connected diode  319  of switching device  315  to conduct. Thus, switching device  315  may now be turned on under zero voltage switching conditions, to once again apply the input voltage −V in  to the primary winding  327  of the transformer  326 . Switching device  352  is closed after a predetermined time delay after the closing of switching device  315  closes. The switching sequence is then repeated for the turn-off of input side switching devices  316  and  315 , and the turn-on of input side switching devices  314  and  317 . 
     In order to ensure zero voltage turn-on of the switching devices  314  and  315  in the lagging leg of the input side circuit bridge  310 , enough energy needs to be stored in the inductors  354  and  358  of the output circuit  350  to provide for charging and discharging of the switching device capacitors  322 - 325  throughout the switching sequence. Since the energy in the inductors  354  and  358  is a function of the current to the load  362 , zero voltage switching will be lost below a certain load level for the lagging leg switching devices  314  and  315 . One way to extend the load range of the DC-to-DC converter  300  is thus by properly sizing inductors  354  and  358 . 
     FIG. 3 illustrates a graphical representation of a plurality of switching transitions as described for selected elements of the full bridge converter  300  shown in FIG. 2 in a conventional mode of operation. In particular, FIGS. 3 a ,  3   b ,  3   c ,  3   d ,  3   e , and  3   f  represent control voltages applied to the switching elements  314 ,  315 ,  316 ,  317 ,  352 , and  356 , respectively. The switches  314 - 317  in the full bridge converter  300  are divided into two alternately conducting diagonal pairs. During a first interval (time t 0  to t 2 ), for a first duty cycle, the first and fourth switches,  314  and  317 , conduct to apply an input voltage V in  across the primary winding  327  of the transformer  326 . Switch  356  conducts after a predetermined time delay (time t 1 ) to apply the voltage V in ′ transferred from the primary circuit  310  to inductor  354  and load  362 . During a second interval (time t 2  to t 3 ), the first and third switches,  314  and  316 , conduct. During this second interval the voltages applied to the transformer windings,  327  and  328 , are zero and switch  356  continues to conduct. During a third interval (time t 3  to t 5 ), the third and second switches,  316  and  315 , conduct to apply an input voltage V in  across the primary winding  327  of the transformer  326 . Switch  352  conducts at a delayed time (time t 4 ) to apply the voltage V in ′ transferred from the primary circuit  310  to inductor  358  and load  362 . During a fourth interval (time t 5  to t 6 ), the second and fourth switches,  315  and  317 , conduct. During this fourth interval, the voltages applied to the windings of the transformer  328  and  327 , are zero and switch  352  continues to conduct. Accordingly, the current of inductor  358  is reflected back through transformer  326  to the input side circuit  310 . A similar pattern of switching transitions exist during time t 7  through time t 13 . 
     As shown in FIG. 4, another embodiment  500  in accordance with the present invention accomplishes secondary switching through the use of a series combination of rectifier diodes,  556  and  562 , and saturable core inductors,  554  and  560 . In the case of low output voltages used for digital logic, the saturable inductors need only be the size of a small bead with a single winding turn. The saturable inductors,  554  and  560 , provide a brief delay in the switch “turn-on” timing of diodes,  556  and  562 , due to the volt second product of the saturable inductor which must be overcome before it saturates. The size of the saturable inductor and the number of turns on the saturable inductor can be selected to provide an appropriate time delay. The circuit of FIG. 4 provides a converter  500  which achieves soft switching for every switch,  514 - 517 , for every transition and eliminates first order switching losses. By comparison to other full bridge forward converters that offer soft switching this converter offers advantages in efficiency, cost, and complexity. The efficiency advantage is a result of decreasing circulating currents during the second and fourth switch states,  515  and  517 , in which known converters maintain peak primary current flowing in the two active primary switches during these states. In contrast, the converter illustrated in FIG. 4 provides primary currents that ramp down quite rapidly. Another advantage of the present embodiment is that the transitions are driven from a large source of energy, namely, the energy stored in an output energy storage inductor,  552  and  558 , whereas in conventional converters the magnetizing energy of a small inductor or the leakage or magnetizing inductance of the transformer are relied upon to provide energy to drive the switching transitions. 
     As shown in FIG. 5, another embodiment  600  in accordance with the present invention requires a single output choke  668 , two additional secondary switches,  628  and  662 , and two additional saturable core inductors,  652  and  660 , in order to accomplish equivalent operation as that of converter  500  shown in FIG.  4 . 
     The necessary switch delay can also be provided by a single saturable reactor inductor  754  in series with the secondary winding  728 , as shown in FIG. 6 with additional saturable inductors,  758  and  764 , placed in series with the reset diodes,  756  and  762 , respectively. In comparison to FIG. 5, this configuration eliminates the need for four saturable inductors. Yet, saturable inductor  754  swings from one saturation extreme to an opposite saturation extreme each cycle creating maximum core losses in the bead. Saturable inductors,  758  and  764 , however, operate over a fraction of their magnetic induction/field (BH) loops such that core losses are not considerable. 
     The advantages of the present invention include and are not limited to a simple and more reliable energy mechanism for driving the switching transitions and lower switch conduction losses by comparison to conventional full bridge forward converters. Since the energy storage elements used to store the energy to drive the transition are large, the stored energy available to drive the transitions is larger than that known in the art. Thus, due to the increased stored energy available to drive the switching transitions, the range over which soft switching can be realized is larger. In addition, the switching circuit in accordance with the present invention for driving the switching transitions does not require the reversal of the magnetizing current which leads to higher conduction and core losses in known converters. Moreover, the embodiments of the present invention reduce the conduction losses by comparison to other known soft switching full bridge forward converters, since the primary switch currents decrease with the output choke current during the reset time of the output choke. Furthermore, the stored energy used to drive the primary switching transitions is the energy stored in the large output choke(s), which is a natural stored magnetic energy component. 
     The aforementioned embodiments in accordance with the present invention have been described using phase shift modulation of the primary switches, however, it is clear that similar benefits and operation are accomplished by using pulse width modulation. In some cases pulse width modulation will have advantages over phase shift modulation. These differences and advantages and disadvantages are well understood by those skilled in the art of power conversion. Moreover, although only one output is shown in the figures it is clear that this technique can be extended to converters with multiple outputs by providing additional secondary windings, rectifiers, saturable inductors, output capacitors and loads. Furthermore, the embodiments disclosed may include synchronous rectifiers coupled to appropriate gate drive control signals. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All the features disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.