Patent Publication Number: US-2023136912-A1

Title: Dc-dc converter with bridge circuit for voltage-free switching, and associated method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the U.S. National Phase of PCT/EP2020/051222, filed on 20 Jan. 2020, which claims priority to German Patent Application No. 10 2019 101 748.3, filed on 24 Jan. 2019, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     The invention relates to the technical area of bridge circuits. In particular, the present invention relates to a bridge circuit, a DC-DC converter having the bridge circuit, a method for operating a bridge circuit, and a program element. 
     Related Art 
     For effective operation of bridge circuits, the goal is to switch them as much as possible when no voltage is applied. This type of switching is known as ZVS (Zero Voltage Switching) switching. In order to keep the circuits simple, the challenge in the case of ZVS is to set the switching times optimally and not to use complex additional circuits, such as measuring circuits that give feedback to the controller for the switches. 
     Particularly when a bridge circuit is operated in a high-voltage direct current network, as is used, for example, in the high-voltage circuit of an electric car, high losses can occur due to the high voltages used if the switches are not switched at the correct moment. 
     SUMMARY 
     It can be considered to be an object of the present invention to enable effective ZVS switching. 
     Accordingly, a bridge circuit, a DC/DC converter having the bridge circuit, a method for operating a bridge circuit, and a program element are specified. 
     The subject matter of the invention is specified by the features of the independent claims. Example embodiments and further aspects of the invention are specified by the dependent claims and the following description. 
     According to one aspect of the invention, a bridge circuit is specified. The bridge circuit has a first and a second high-side switch, a first and a second low-side switch, a transformer having a primary coil and a secondary coil, a coil, and a current injection device. In this bridge circuit, the first high-side switch and the first low-side switch are connected at a first bridge terminal in a series circuit to form a first bridge branch. In addition, the second high-side switch and the second low-side switch are connected at a second bridge terminal in a series circuit to form a second bridge branch. 
     The first and second bridge branch are each connected at a first and a second input terminal in a parallel circuit, wherein the secondary coil has a first and a second output terminal. The primary coil and the coil or the inductance are connected in a series circuit to connect the first bridge terminal to the second bridge terminal. The current injection device is configured to inject a predetermined current into the coil at a predetermined point in time. 
     According to a further aspect of the present invention, a DC/DC converter (direct current/direct current converter) having the bridge circuit according to the invention is described. 
     According to yet another aspect of the present invention, a method for operating a bridge circuit is specified, wherein the method includes operating the switches of the bridge circuit in such a way that a predetermined current is injected by the current injection device into the coil at a predetermined point in time. 
     According to another aspect of the present invention, a program element is described, having a program code which, when it is executed by a processor, is configured to execute the method for operating a bridge circuit. 
     According to yet another aspect of the present invention, a computer-readable storage medium is provided, on which a program code is stored which, when it is executed by a processor, executes the method for operating a bridge circuit. 
     A floppy disk, a hard disk, a USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), or an EPROM (Erasable Programmable Read Only Memory) may be used as a computer-readable storage medium. An ASIC (application-specific integrated circuit) or an FPGA (field-programmable gate array) as well as an SSD (solid-state drive) technology or a flash-based storage medium can also be used as a storage medium. A web server or a cloud can also be used as a storage medium. A communication network, for example, the Internet, which may permit the downloading of a program code, may also be considered to be a computer-readable storage medium. A radio-based network technology and/or a wired network technology can be used. 
     The use of a current injection device can ensure that energy present is withdrawn from a switch of the bridge circuit to switch the switch in a state that is as deenergized as possible. In particular, the current injection device can ensure that the switch is assisted during switching upon the withdrawal of the energy. This injected current can also enable a live node to be discharged rapidly and thus promote ZVS switching. For example, the output capacitance or parasitic capacitance of the first high-side switch is discharged and the output capacitance or parasitic capacitance of the first low-side switch is charged and the first bridge terminal moves from an upper potential to a lower potential or ground potential, whereby ZVS switching can then be achieved for the first low-side switch. 
     According to another aspect of the present invention, the current injection device is a further coil which, in combination with the coil, forms a second transformer or an additional transformer. 
     According to a further aspect of the present invention, coupling between the further coil and the coil is low. In other words, the coupling between the further coil and the coil is less than the coupling between the primary coil and the secondary coil of the transformer. For example, a coupling of the further coil and the coil has a lower magnetic coupling factor than the coupling between the primary coil and the secondary coil of the transformer. 
     This low coupling factor of the ZVS transformer may allow a current to be injected into the coil but not to load the circuit with a high voltage. In other words, the low coupling factor of the ZVS transformer can enable providing a high leakage inductance of the ZVS transformer, which allows magnetic energy to be stored, but which essentially only has a minor influence on the output capability of an inverter. If the ZVS transformer, which includes the coil and the current injection device, had a high or good coupling factor in contrast to the low coupling factor, the remaining inductance would not be sufficient to combine or withdraw the energy which is required for ZVS conditions. The high leakage inductance or the leakage inductance of the ZVS transformer is used to generate or inject a current that is required to achieve ZVS (Zero Voltage Switching). 
     The magnetic coupling factor between the further coil and the coil may have a value of approximately 0.9 with a maximum possible value of 1. Coupling factors of typical power transformers may be in the range of 0.995. The main transformer may also have a magnetic coupling factor of approximately 0.995, which is thus significantly greater than the magnetic coupling factor of the ZVS additional transformer, which is approximately 0.9. 
     According to another aspect of the present invention, the bridge circuit furthermore has a high-side capacitor and a low-side capacitor. The high-side capacitor and the low-side capacitor are connected in series at a third bridge terminal to form a third bridge branch, wherein the third bridge branch is connected to the first and second input terminal and wherein the further coil connects the third bridge terminal to at least one of the first bridge terminal and the second bridge terminal. The two capacitors, the high-side capacitor and the low-side capacitor, keep the ZVS transformer at a medium voltage potential. The magnetic core of the ZVS transformer is thus balanced and the first high-side switch and the first low-side switch can be controlled using a symmetrical switching pattern. 
     According to yet another aspect of the present invention, the bridge circuit has a synchronous rectifier. The synchronous rectifier is connected to the first and second output terminals. 
     In contrast to diodes, the synchronous rectifier can be actively controlled. The control can be designed in such a way that the synchronous rectifier is short-circuited for a predeterminable duration during a freewheeling phase of the bridge circuit or the phase-shifted full bridge. By short-circuiting the synchronous rectifier during the freewheeling phase of the bridge circuit, the current in the ZVS additional transformer can be increased, in particular, an additional current can be injected into a coil of the ZVS additional transformer T ZVS . This additional current can be used to enable ZVS switching and/or ZCS switching of the high-side switch and/or low-side switch associated with the respective switching phase in that this switch carries out a transition from one switching state to the other, essentially without a voltage being applied across this switch. 
     According to yet another aspect of the present invention, the bridge circuit has a control unit which is connected to each of the first and second high switches and low switches. The control unit is configured to operate the switches in such a way that the predetermined current is injected into the coil by the current injection device at the predetermined point in time. Secondary-side switches may also be used for the injection, for example, switches of a secondary-side rectifier and/or of the synchronous rectifier. This switching of the switches may take place during a freewheeling phase. 
     The control unit may, for example, be configured in such a way that it operates secondary-side switches so that the predetermined current is injected into the coil. The secondary-side switch or switches may be switches of a secondary-side rectifier and/or of a synchronous rectifier. The secondary switch and/or the plurality of secondary switches may be implemented by MOSFET components. The level of the injected current may be indirectly determinable by the time duration for which one, the two, and/or the plurality of secondary switches are switched simultaneously and thus the one, the two, and/or the plurality of secondary coils are short-circuited. This short-circuiting of the secondary coil and/or of the plurality of secondary coils may take place during a freewheeling phase of one of the high-side switches and/or the low-side switches. 
     According to yet another aspect of the present invention, the control unit is furthermore configured to operate the high-side switch and/or switches and/or the low-side switch and/or switches in such a way that the predetermined current is injected into the coil when the current through the coil is below a predeterminable threshold value at the predetermined point in time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further example embodiments of the present invention are described hereinafter with reference to the figures. 
         FIG.  1    shows a block diagram of a DC-DC converter having a bridge circuit according to one example embodiment of the present invention. 
         FIG.  2    shows a block diagram of a DC-DC converter having a bridge circuit and two main transformers according to one example embodiment of the present invention. 
         FIG.  3    shows a block diagram of a DC-DC converter having a bridge circuit and a main transformer having center tap according to one example embodiment of the present invention. 
         FIG.  4   a    shows diagrams of various signal profiles of a PSFB without use of the additional transformer according to one example embodiment of the present invention. 
         FIG.  4   a    shows diagrams of various signal profiles of a PSFB with use of the additional transformer according to one example embodiment of the present invention. 
         FIG.  5   a    shows a detail from diagram  4   a  according to one example embodiment of the present invention. 
         FIG.  5   b    shows a detail from diagram  4   b  according to one example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrations in the figures are schematic and not to scale. In the following description of  FIG.  1    to  FIG.  5   b   , the same reference numbers are used for the same or corresponding elements. 
     In this text, the terms “capacitor” and “capacitance” as well as “coil” or “choke” and “inductance” may be used synonymously and should not be interpreted restrictively unless otherwise specified. 
     The term “high-side” may refer to a connection to a live potential. The term “low-side” may refer to a connection to a reference potential. 
       FIG.  1    shows a block diagram of a DC-DC converter  100  having a bridge circuit  101  according to one example embodiment of the present invention. Using this switching arrangement, good switching conditions for the switching devices or switches A, B, C, D of a phase-shifted full bridge (PSFB)  101  or a bridge circuit  101  having phase-shifted switching behavior can be achieved. These good switching conditions may be achieved if a substantially voltage-free switching of the respective active switches A, B, C, D can be produced. The voltage-free switching, ZVS (Zero Voltage Switching) or zero voltage switching enables high switching losses to be avoided, which can arise in particular when switching high voltages due to parasitic elements in switches A, B, C, D, since energy may be stored in these parasitic elements against which it is necessary to work when switching the switches A, B, C, D. Alternatively or additionally to ZVS switching, currentless switching can also be achieved (ZCS, Zero Current Switching). 
     A bridge circuit  101  can be used, for example, in a DC-DC converter  100 , to convert an input voltage V in  into an output voltage V out . V in  and V out  are DC voltages (DC, direct current). On the way from the input to the output, the input DC voltage V in  is converted by the bridge circuit  101  into an AC voltage (AC, alternating current) and converted again into the output DC voltage by rectification. Particularly in applications that are used in an OBC unit (On Board Charging Unit) of an electric or hybrid vehicle, it can be necessary to convert very high voltages V in  (HV) into typical on-board voltages V out  of approximately 12V, which can be used to operate a radio, for example. The voltages V in  are provided, for example, by the direct current intermediate circuit of the electric vehicle. Alternatively, the voltage V in  can also come from the on-board component of a charging device. 
     The DC-DC converter  100  can be used instead of a generator (alternator) of a vehicle to provide the on-board voltage 12V. In one example, the 12V on-board voltage is not generated directly by mechanical work, but rather by the DC-DC converter  100  converting the high voltage (HV) of an HV battery (DC voltage, DC) into the 12V on-board voltage of an EV (electrical vehicle) or PHEV (plug-in hybrid electric vehicle). The HV is present in a load circuit or intermediate circuit of a power supply system of a vehicle. The energy withdrawn from the HV circuit is used to charge a 12V on-board supply battery to which the 12V consumers are connected. If the 12V battery were not continuously recharged from the HV circuit via the DC-DC converter, the connected consumers would discharge the 12V battery, similar to how the alternator would fail when using a mechanical energy supply. 
     The OBC unit (not shown in  FIG.  1   ) that supplies V in  is used to charge the HV battery of the intermediate circuit. The voltage of the HV battery can be V in =400V or 800V. The OBC unit draws its energy itself, for example, from an AC power supply (also not shown in  FIG.  1   ), the so-called mains, for example, via an alternating current or three-phase connection. Therefore, the voltages V in  of the HV DC voltages (DC) can be in the range of 400V-800V or in a range less than 800V. The bridge circuit  101  is configured so that it can deal with voltages of an appropriate dimension and variation range. 
     The voltage V in  is supplied to the bridge circuit  101  via a first input terminal  102  and via a second input terminal  103 . The first input terminal  102  may be referred to as the high-side terminal  102  and the second input terminal  103  may be referred to as the low-side terminal  103 . These input terminals  102 ,  103  form a parallel circuit of the first  107  and second  108  bridge branches. The first bridge branch  107  is formed from a series circuit of the first high-side switch A and the first low-side switch B. The second bridge branch  108  is formed from the series circuit of the second high switch C and the second low-side switch D. The first high-side switch A has the control terminal  104   a , the first low-side switch B has the control terminal  104   b , the second high-side switch C has the control terminal  104   c , and the second low-side switch D has the control terminal  104   d . The control terminals  104   a,    104   b,    104   c,    104   d  are connected to a control unit (not shown in  FIG.  1   ), which ensures the phase-shifted control of the switches A, B, C, D. The control is carried out by the control unit in such a way that the first high-side switch A and the second low-side switch D are switched essentially simultaneously. And so that the second high-side switch C and the second low-side switch B are switched simultaneously. It can also be provided that a pause or dead time is provided between the switching of the switches associated with one another, during which no switch is switched and during which all switches are open. During the switching process, a duty cycled of 50% is essentially provided, so that the switch combinations A, D and B, C are active for essentially the same length of time. 
     The switch pairs A, D and B and C, which are switched essentially simultaneously, are arranged diagonally to the coil T 1   A  and/or the coil T 3   B , so that the paired switching of the switch pairs A, D and/or B, C ensures a reversed current flow through the coil T 3   B  in each case. The control pattern for the phase-shifted control of the switches A, B, C, D essentially corresponds to a conventional control pattern or control scheme used for a phase-shifted switching full bridge (PSFB). 
       FIG.  4   a    shows diagrams of various signal curves, in particular voltage curves and current curves as a function of a switching behavior of a bridge circuit  101  and/or a synchronous rectifier SR 1 , SR 2  without using the additional transformer according to one example embodiment of the invention. 
       FIG.  4   a    shows a diagram  400   a  of an expanded signal curve of a PSFB with lagging A/B without using the additional transformer according to an example embodiment of the present invention. Circuit diagram  400   a  shows a selection of signal curves for an operation of a phase-shifted full bridge converter circuit without ZVS transformer T ZVS . In the switching phase  406   a  or transition phase  406   a , as shown at point  405   a , the current I T1A  through the primary coil decreases, since the leading branch C, D is switched in this phase. During the freewheeling phase II, which follows the phase  406   a , the current I T1A  decreases further, since, due to the simultaneous connection of switches B, D, a circuit having switch B, switch D, and the primary coil T 1   A  is formed. The current decreases due to the current flow circulating in this freewheeling circuit. The circuit formed in the freewheeling phase II behaves like an RL circuit, which is formed from the line resistances and the primary coil T 1A . The line resistances result in losses caused by the current that flows to dissipate the stored magnetic energy. Due to the losses occurring during the freewheeling phase II, the magnetic energy stored in the inductance T 1   A  during the switching or transition phase  404   a  of the lagging branch  107  (lagging leg transition) A, B is lower than in the transition phase  406   a  of the leading branch  108  (leading leg transition) C, D. As a result, there is not enough magnetic energy available to completely discharge the parasitic capacitances of switches A, B of the lagging branch, for example, the parasitic capacitances of a MOSFET switch A, B. 
       FIG.  4   a    shows diagrams of various signal curves, in particular voltage curves and current curves as a function of a switching behavior of a bridge circuit  101  and/or a synchronous rectifier SR 1 , SR 2  without using the additional transformer T ZVS  according to one example embodiment of the invention. In particular, during the end phase II B  of the freewheeling phase II, in which the low-side switches B  104   b  and D  104   d  and synchronous rectifiers SR 1  and SR 2  are switched simultaneously and form the low-side freewheeling circuit  104   b,    104   d , T 3   B , and T 1   A , the current I T1A  rises further after the switching phase  406   b  because the T 3   B  winding of the T ZVS  transformer is short-circuited, while at the same time essentially half the input voltage is applied to the T 3   A  winding of T ZVS . The increase of the primary current I T1A  lasts until the switching time  404   b  of the lagging branch (lagging leg) A/B, in which the low-side switch B  104   b  is switched off and the high-side switch A  104   a  is switched on. In other words, the increase in the primary current I T1A  allows the available magnetic energy to increase during the transition  404   b  of the lagging branch (lagging leg) A/B. Thus, the transition phase  404   b  of switching the lagging branch  107  A/B can take place as a soft transition and soft ZVS switching on of the switch A  104   a  can be carried out. 
     The circuit diagram  400   b  shows, as drain-source voltage Vds A , the curve of the voltage across the high-side switch A, i.e., the curve of the voltage between terminal  102  and bridge point  105  in the first bridge branch  107  for the case that the additional transformer T ZVS  is used according to  FIG.  1   . Circuit diagram  400   a  shows, as drain-source voltage Vds A , the corresponding voltage curve for the case that no additional transformer is used and thus only the primary coil T 1   A  is solely responsible for the switching of the high switch A. 
       FIG.  5   a    shows a detail from diagram  4   a  according to one example embodiment of the present invention. This shows the switching phase  404   a  of the switch A of the lagging branch  107 , in particular the control voltage of the switch A, for example, the gate voltage if the switch A is implemented as a MOSFET. The presence of a Miller plateau  408   a  in the voltage curve A indicates that the switch A cannot be discharged before the switching process is carried out, as can also be seen from the point  407   a  of the curve of the switching signal Vds A , so that only hard switching takes place. 
       FIG.  5   b    shows a detail from diagram  4   b  according to one example embodiment of the present invention.  FIG.  5    shows the ZVS switching process of switch A  104   a . Assuming that the switch A is implemented as a MOSFET, there is no Miller plateau at point  408   b  of the curve of the gate voltage of the switch A after the switching process in switching phase  404   b . The voltage across the switch A  104   a , for example, the drain-source voltage Vds A , has already dropped to 0V when it is switched, as is illustrated at point  407   b . This enables complete ZVS switching and a soft transition. 
     The voltage curve of the control voltage of the input switches A, B when controlled by PWM (pulse-width modulation) is shown in  FIG.  4   b    when the additional transformer T ZVS  is used. 
     In  FIGS.  4   a ,  4   b ,  5   a ,  5   b   , the signal A corresponds to the gate voltage Vg at the switch A  104   a , the signal B to the gate voltage Vg at the switch B  104   b , the signal C to the gate voltage Vg at the Switch C  104   c , and the signal D to the gate voltage Vg at switch D  104   d . According to  FIG.  1   , switches A  104   a , B  104   b , C  104   c , D  104   d  are designed as normally blocking MOSFETs. This means that the application of a voltage to the respective gate or a high pulse in diagram  400   a,    400   b  corresponds to a closed switch through which current can flow. The use of self-conducting MOSFETS is also possible with an inverse sign. 
     The signal SR 1  corresponds to the gate voltage at the switch SR 1 . The signal SR 2  corresponds to the gate voltage at the switch SR 2 . 
     The signal I T1A  corresponds to the time curve of the primary current through the coil T 1   A , in particular through the primary coil T 1   A . 
     The signal Vds A  corresponds to the time curve of the drain-source voltage in the switch A. 
     To simplify and to illustrate the influence of T ZVS , in particular the increase in current of I T1A  caused thereby, the comparisons in  FIGS.  4   a ,  5   a ,  4   b ,  5   b    show the same control pattern of switches A, B, C, D, SR 1 , SR 2 , although a different control pattern would possibly be used in a PSFB without T ZVS  according to  FIG.  4   a   , since, for example, the switches SR 1 , SR 2  would not be short-circuited simultaneously during the freewheeling phase II to achieve ZVS switching. 
     A bridge branch  107 ,  108  can be referred to as a leg  107 ,  108 . In the PSFB switching method considered below, the second bridge branch  108  or leg CD  108 , which has switches C and D, is controlled as a leading branch (leading leg)  108 . 
     The first bridge branch  107  or leg AB  107 , which has the switches A and B, is controlled as a lagging branch (lagging leg)  107 . The reverse control is also possible. In the PSFB switching method, leg CD  108  is phase shifted with respect to leg AB in order to control and/or regulate the output voltage V out  by way of the phase offset. 
     In a PSFB switching method or control method, there are essentially four main phases or four main events. In the following, the differences compared to a standard PSFB, which arise due to the use of the ZVS transformer T ZVS , are discussed. 
     Diagram  400   a  shows the control pattern for switches A, B, C, D for the case that no ZVS transformer T ZVS , that is to say no coil T 3   A  and no coil T 3   B , is used. The time curve of diagram  400   a  is essentially divided into four phases I, II, III, and IV. 
     In contrast, the diagram  400   b  shows the control pattern for the switches A, B, C, D for the case that the ZVS transformer T ZVS  is used, that is to say, that the coil T 3   A  and the coil T 3   B  are used. 
     The control patterns for the switches A, B, C, D essentially match both for diagram  400   a  and also for diagram  400   b . Likewise, the division into phases I, II, III and IV. 
     The control patterns of the synchronous rectifiers SR 1 , SR 2  in the diagrams  400   a  and  400   b  also correspond. 
     The individual phases I, II, III, IV are described below. 
     1. In phase I, the switches B  104   b  and C  104   c  are switched on or activated (“B &amp; C on”). This phase I is called the energizing phase. During this phase, energy and/or power is transmitted from the supply source V in , for example, the HV battery, which is connected to the nodes  102 ,  103 , to the load (not shown in  FIG.  1   ) at the terminals  110   a,    110   b , V out . The current flow thus takes place in phase I essentially via node  102 , switch C, primary coil T 1   A  and, if the coil T 3   B  is present, via T 3   B , via switch B to node  103 . 
     1.a) In the following, phase I is considered for the case that no transformer T ZVS  is provided, thus for the case that neither coil T 3   A  nor coil T 3   B  is present. The associated signal curves are shown in  FIGS.  4   a   ,  5   a.  For the case that no transformer T ZVS  is provided, after the switching of the switch B, due to the switching of the lagging branch  107 , which initiates phase I, the current I T1A  increases in the main transformer T 1   A  with a slope according to the formula: 
     
       
         
           
             
               
                 
                   
                     di 
                     dt 
                   
                   = 
                   
                     
                       
                         V 
                         in 
                       
                       - 
                       
                         V 
                         0 
                         ′ 
                       
                     
                     
                       L 
                       1 
                       ′ 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, i denotes the current I T1A  through the primary coil T 1   A , Vo′ denotes the voltage at the primary coil T 1   A , which is reflected by the voltage V out  at the terminals  110   a ,  110   b  at the load (not shown) via the transformer T 1  on the primary side of the transformer T 1   A . Vo′ corresponds to n*V out , wherein n is the number of turns of the transformer T 1 . L 1 ′ denotes the inductance of the coil T 1   A . The bar at V o ′ and L 1 ′ indicates that they are values which have been reflected in the primary coil T 1   A . 
     During phase I, the current I T1A  flows from node  102 , via switch C, via the primary coil T 1   A , via switch B into node  103 . 
     Only during the further energizing phase III “A&amp;D on” described below is the reflected output inductance L 1 ′ assumed to be very much greater than the leakage inductance of the transformer T 1 . The leakage inductance that results during the transmission from T 1   A  to T 1   B  of the transformer T 1  is not shown in  FIG.  1   , since it is a fictitious quantity that does not correspond to any physical component. However, it can be set via the type of the transformer. 
     1.b) If in addition, as shown in  FIGS.  4   b ,  5   b   , the ZVS transformer T ZVS  is provided in series with T 1 , the ZVS transformer T ZVS  with its coils T 3   A  and T 3   B  increases the voltage across the primary coil T 1   A  of the main transformer T 1  during phase I. Thus, the slope of the current changes in relation to formula (1) during the energizing phase I, if the ZVS transformer T ZVS  is provided, to: 
     
       
         
           
             
               
                 
                   
                     di 
                     dt 
                   
                   = 
                   
                     
                       
                         
                           V 
                           in 
                         
                         ( 
                         
                           1 
                           + 
                           
                             1 
                             
                               2 
                               ⁢ 
                               
                                 n 
                                 ZVS 
                               
                             
                           
                         
                         ) 
                       
                       - 
                       
                         V 
                         0 
                         ′ 
                       
                     
                     
                       L 
                       1 
                       ′ 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Here, i again denotes the primary current I T1A , n ZVS  the number of turns of the ZVS transformer T ZVS , and 2n ZVS  twice the number of turns, wherein it is assumed that the coils T 3   A  and T 3   B  have the same number of turns n. In the energizing phase I “B&amp;C on”, only the change in the current I T1A  through the primary coil over time is affected by the provision of the ZVS transformer T ZVS  and therefore essentially no changes result in relation to the control method of a PSFB circuit in which the ZVS transformer T ZVS  is not is provided. The greater the number of turns n ZVS  of the ZVS transformer T ZVS , the less noticeable the ZVS transformer T ZVS  is. 
     2. The transition phase  406   a,    406   b  between phase I and phase II arises due to the essentially simultaneous switching of switches C, D of the leading branch  108 . During the transition phase  406   a,    406   b , the switch C  104   c  is turned off, via which the current I T1A  has been supplied in phase I, and the switch D  104   d  is turned on (turn-off C/turn-on D). This transition phase  406   a,    406   b  is referred to as the transition of the leading bridge branch  108  (leading leg transition). 
     2.a) In the case of  FIGS.  4   a ,  5   a    that no transformer T ZVS  is provided, in order to achieve a soft transition (ZVS) and avoid hard switching, the parasitic capacitance of switches C and D is essentially completely discharged or charged during transition phase  406   a . The energy used for the discharging and/or charging is absorbed or emitted by the leakage inductance (not shown in  FIG.  1   ) of the primary winding T 1   A  in the form of magnetic energy. The magnetic energy of the leakage inductance of the primary winding T 1   A  is determined as follows: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       E 
                       L 
                     
                   
                   = 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       L 
                       lk 
                     
                     ⁢ 
                     
                       I 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Herein ΔE L  denotes the change in energy in the primary-side leakage inductance L lk  of transformer T 1  and I denotes the current I T1A  through the primary coil T 1   A . However, this formula generally relates to achieving a ZVS condition by the switches A and B and is not limited to phase II. If there is sufficient current in the main transformer T 1 A, C and D are switched and ZVS can also be achieved for these two switches. To achieve a soft transition when switching A and B (ZVS), the following condition is to be met: 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                         E 
                         L 
                       
                     
                     &gt; 
                     
                       Δ 
                       ⁢ 
                       
                         E 
                         c 
                       
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             4 
                             3 
                           
                           ⁢ 
                           
                             C 
                             mos 
                           
                         
                         + 
                         
                           
                             1 
                             2 
                           
                           ⁢ 
                           
                             C 
                             tr 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       V 
                       in 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The change in the inductive energy stored in the primary-side leakage inductance of T 1  is to be greater than the change in the capacitive energy ΔE C  stored in the parasitic capacitances C mos  of the switches A and B. In other words, the primary-side leakage inductance is to be dimensioned so that this condition is met. Formula (4) expresses that the energy stored in the leakage inductance of the coil T 1   A  has to be greater than the energy which is stored in the parasitic capacitances C mos  of the switches C and D and the energy which is stored in the parasitic capacitance C tr  of the transformer T 1 . Usually, the switching process “turn-off C/turn-on D” of the leading branch  108  is a soft transition during the transition phase  406   a , since the current I TA1  is at its maximum value and the energy of the leakage inductance is sufficiently large to completely charge or discharge the capacitances C mos  and C tr . 
     During the transition phase  406   a , which follows the end of the energizing phase I, the two switches C, D of the leading branch  108  (leading leg)  108  are switched essentially simultaneously. There is only a short dead time between the switching off of C and the switching on of D. The closed switch C of the leading branch  108  is opened during the transition phase  406   a  and the open switch D of the leading branch  108  is closed “turn-off C/turn-on D”. Since the switches A, B of the lagging branch  107  are not yet actuated, switch A remains open and switch B remains closed. 
     Due to this asymmetrical and chronologically differing switching of the switches C, D of the leading branch  108  and the switches A, B of the lagging branch  107 , during the transition phase  406   a , the state of phase II results, in which the high switches A, C are open and the low switches B, D are closed simultaneously. Due to this switching behavior, a loop or freewheeling loop is formed in the lower region in the vicinity of the low-side node  103 . The primary coil T 1   A  drives the current flowing during phase I via the primary coil T 1   A  and via switch B, via node  103 , and via switch D. Since switches A, C are open and/or are opened during transition phase  406   a  and since the current still flowing through T 1   A  is sufficiently large to discharge parasitic charges from the bridge point  106  and thus from the switches C, D, both switch C and switch D can essentially be switched under ZVS conditions in the transition phase  406   a.    
     2.b) If the ZVS transformer is provided, the diagrams  400   b  result, as shown in  FIGS.  4   b   ,  5   b.  During the transition phase  406   b , these diagrams essentially do not differ from the transition process  406   a  described in section 2.a), in which no ZVS transformer is provided. The control method is also essentially the same. However, in the case of the coil T 3   B  connected in series in addition to the primary coil T 1   A , the primary coil T 1   A , because of the drop of the current, continues to drive the current I T1A  flowing during phase via the primary coil T 1   A  and via the coil T 3   B , and via the switch B, the node  103 , and the switch D. The freewheeling loop thus has the primary coil T 1   A , the coil T 3   B , the switch B, the low-side node  103 , and the switch D. 
     3. Phase II following the transition phase  406   a,    406   b  is referred to as the freewheeling phase II. During this freewheeling phase II, the low-side switches B  104   b  and D  104   d  are switched on (B &amp; D on), i.e., closed and the high-side switches A, C are open. 
     3.a) In the case of  FIGS.  4   a ,  5   a   , that no transformer T ZVS  is present, during this phase II “B &amp; D on”, the two low-side switches B and D are switched on or closed and the two terminals  105 ,  106  of the primary coil T 1   A  of the main transformer T 1  are connected to the input terminal  103 . Both input terminals  105 ,  106  of the transformer T 1  are thus at the same potential and there is no voltage applied across the transformer T 1 . However, the primary coil T 1   A  continues to drive the current I T1A . However, this current I T1A  through the primary winding T 1   A  decreases exponentially according to an RL circuit—a circuit having coil and resistor: 
     
       
         
           
             
               
                 
                   
                     I 
                     ⁡ 
                     ( 
                     t 
                     ) 
                   
                   = 
                   
                     
                       I 
                       p 
                     
                     ⁢ 
                     
                       e 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               
                                 r 
                                 
                                   ds 
                                   , 
                                   on 
                                 
                               
                             
                             
                               L 
                               lk 
                             
                           
                           ) 
                         
                         ⁢ 
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Here, I p  is the peak current that flows during the transition phase  406   a  “transition of the leading branch (leading leg transition)” described in section 2. following the energizing phase I. The resistance value r ds,on  is the on resistance of the switch B or D, for example, of the MOSFET B or D. 
     The end of the freewheeling phase II determines the further transition phase  404   a , which is characterized in that the switches A, B of the lagging branch  107  are switched in this further transition phase  404   a . The high-side switch A is switched on or closed and the low-side switch B is switched off or opened. If the leakage inductance is too small and/or insufficient current I T1A  flows through the primary winding  T1A  of the main transformer T 1 , ZVS switching cannot be achieved for this switching process of the lagging branch  107  in the transition phase  404   a . This is because if only the leakage inductance of T 1  were increased, this would influence the output capability. Therefore, increasing the leakage inductance is substantially avoided. The current cannot readily be increased either. However, the use of the ZVS transformer and the simultaneous activation of the switches SR 1  and SR 2  during the freewheeling phase were capable of increasing the current. 
     3.b) If, as provided in  FIGS.  4   b ,  5   b   , according to  FIG.  1    the ZVS (Zero Voltage Switching) transformer T 3A , T 3B  is additionally provided as a series circuit to the transformer T 1  between the bridge points  105 ,  106 , there are differences in the control from a case in which the ZVS transformer T ZVS  is not provided. This is because even if the leakage inductance is too small and/or if sufficient current I T1A  would not flow through the primary winding T 1   A  of the main transformer T 1 , the current I T1A  through the primary winding T 1   A  of the main transformer T 1  can be increased if the ZVS transformer takes advantage of the fact that during the freewheeling phase II before the transition phase  404   b , the synchronous rectifiers SR 1 , SR 2  are activated or closed at the same time. Before the switches A, B of the lagging branch  107  are switched, the synchronous rectifiers SR 1 , SR 2  are activated or closed at the same time, whereby the output  110   a,    110   b  and in particular the secondary coil T 1 B are short-circuited. This simultaneous activation of SR 1 , SR 2  generates an additional current pulse in the primary coil T 1   A , which increases the current I T1A  and thus also has the effect of increasing the current through the ZVS transformer. 
     Therefore, no change needs to be made to the switching behavior of the synchronous rectifiers SR 1 , SR 2  in relation to  FIG.  4     a.  The actuation of the switches SR 1 , SR 2  according to  FIG.  4   a    could be omitted for the control of a PSFB without ZVS transformer and is only shown in  FIG.  4   a    for a better comparison. It also shows that the current does not increase without a ZVS transformer despite switching SR 1 , SR 2 . 
     If, however, the ZVS transformer is present, the ZVS transformer can be used to simultaneously activate the synchronous rectifiers SR 1 , SR 2  during the freewheeling phase II, II B  even during the transition phase  404   b , to provide ZVS conditions for switching the switches A, B during the transition phase  404   b.    
     In other words, in order to take advantage of the presence of the ZVS transformer T ZVS , even during phase II “B &amp; D on”, during which the low-side switches B and D switch the bridge points  105 ,  106  on the primary side to the same potential, the secondary-side coil T 1   B  of the transformer T 1  is short-circuited by the two switches SR 1  and SR 2  of the output rectifier circuit  112  or the secondary side  112  being activated or switched on, which are implemented, for example, by MOSFET transistors. The part of the freewheeling phase II, during which the synchronous rectifiers SR 1  and SR 2  are switched on at the same time, is located at the end of the freewheeling phase II and designated I IA or I IB. If the ZVS transformer T ZVS  is present, the current I T1A  decreases during this end phase II B , as shown in  FIG.  4   b   , while without ZVS transformer T ZVS  there is an increase in the current I T1A  in the end phase II A , as can be seen in  FIG.  4     a.    
     In phase II, a closed circuit is formed from the series circuit of the ZVS secondary coil T 3   B , the primary coil T 1   A , and the two switches B and D by activating the low-side switches B and D. While this primary-side circuit is formed by the first low-side switch B and the second low-side switch D simultaneously connecting the negative potential to the bridge terminals  105  and  106 , the two switches SR 1 , SR 2  of the secondary-side rectifier are activated shortly before the control of the switches A, B of the lagging branch  107  during the end phase II A , II B . According to  FIGS.  4   a  and  4   b   , only the first synchronous rectifier SR 1  is activated in the end phase II A , II B , since the second synchronous rectifier SR 2  is already activated. 
     The second synchronous rectifier SR 2  is switched off before the further transition phase  404   a,    404   b , that is to say before the switches A, B of the lagging branch  107  are actuated. As a result of this control of the two switches SR 1 , SR 2  in the end phase II A , II B , the two terminals of the secondary coil T 1   B  are simultaneously connected to the same potential, for example, to the ground potential, before the transition phase  404   a,    404   b  of the lagging branch  107 , and in this way a circuit made up of the secondary coil T 1   B  and the two switches SR 1 , SR 2  is formed on the secondary side. 
     In the circuit formed on the primary side by the switches B and D and the coils T 3   B  and T 1   A , freewheeling is generated during phase II, since the collapsing magnetic field in the coils T 3   B  and T 1   A  maintains the primary current I T1A  and the leakage inductance L lk  of main transformer T 1  receives its current from T ZVS , in particular from T 3   B , and the current through T 1   A  continues to increase. The slope of the current I T1A  is calculated according to: 
     
       
         
           
             
               
                 
                   
                     di 
                     dt 
                   
                   = 
                   
                     
                       
                         V 
                         in 
                       
                       
                         
                           n 
                           ZVS 
                         
                         ⁢ 
                         
                           L 
                           lk 
                         
                       
                     
                     - 
                     
                       
                         I 
                         p 
                       
                       ⁢ 
                       
                         e 
                         
                           
                             ( 
                             
                               
                                 2 
                                 ⁢ 
                                 
                                   r 
                                   
                                     ds 
                                     , 
                                     on 
                                   
                                 
                               
                               
                                 L 
                                 lk 
                               
                             
                             ) 
                           
                           ⁢ 
                           t 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     This additional increase in current (in absolute value) induced by the leakage inductance L lk  and the secondary-side short-circuiting can be seen in  FIG.  4   b   , at the point  409   b  in the region of the end of the freewheeling phase II B , while SR 1  and SR 2  are on. This increase in current occurs before the transition phase  404   b , while in the same region II of  FIG.  4   a   , without T ZVS , a decrease in the current I T1A  can be seen. As a result of the additionally injected current increase due to the discharge of coils T 3   B  and T 1   A , the magnetic energy, which is stored in the leakage inductance L lk  of T 1 , increases until finally all the charge stored in the switches A, B of the lagging branch  107  is converted into magnetic energy of the leakage inductance L lk , so that the switches A, B are essentially free of charges and the transition of the lagging branch A/B (lagging leg)  107  can be carried out under ZVS conditions. The current circulates through the leakage inductance. If the leakage inductance is too small, the current has to be increased to ensure sufficient energy for ZVS conditions. 
     4. In the transition phase  404   a,    404   b  between phase II and III, the switch B  104   b  is switched off and the switch A  104   a  is switched on (turn-off B/turn-on A). This phase  404   a,    404   b  “turn-off B/turn-on A” is referred to as the transition of the lagging branch  107  (lagging leg transition) A/B. 
     During the energizing phases I and III, the current increases continuously, but in the opposite direction, so that the current at points  405   a,    405   b  differs from 0 A. The magnitude of the increase in current depends on the output power of the converter  100 . This current ensures the ZVS conditions when the switches C, D of the leading branch  108  are switched. 
     During each of the freewheeling phases II and IV, the current decreases in the opposite direction. As can be seen in phases II A  and/or IV A , the conditions for ZVS switching of A and B are not achieved. Only if, as can be seen in phases II B  and/or IV B , a ZVS transformer is present and the switches SR 1 , SR 2  are switched on at the identical time or at the same time, can ZVS conditions be achieved for the switching of A and B. Alternatively or additionally, the leakage inductance of T 1  could also be increased, but this can result in losses in the output capability and is therefore only carried out to a small extent when it is carried out. 
     Thus, after a sudden increase in the current I T1A  in the transition phase  406   a,    406   b , the current flow I T1A  may decrease during the freewheeling phase II until the end region II A , II B  is reached. Up to the end region II A , II B , the curves of the current I T1A  of  FIGS.  4   a  and  4   b    correspond regardless of whether the ZVS transformer T ZVS  is present or not. 
     Essentially, only the linearly increasing current I T1A  during phases II B  and IV B  is used for the ZVS switching of A and B when the ZVS transformer is present. The current pulse of the current I T1A , immediately after the switching of A and B, relates to interactions with the parasitic capacitances of the circuit and it can be neglected. The different influencing of the current flow I T1A  with and without ZVS transformer T ZVS  in the end region II A , II B  of the freewheeling phase II is responsible for the different behavior of a circuit with ZVS transformer and without ZVS transformer. 
     4.a) If the PSFB is used without a transformer T ZVS  being present, as shown in  FIG.  4   a   , the phase  404   a  of the transition of the lagging branch  107  is a critical phase, since it follows the freewheeling phase II. This is because, as can be seen both in  FIG.  4   a    and in  FIG.  5   a    at reference number  407   a , voltage is still present across switch A in the region of transition  404   a  of lagging branch  107  while switch A is actuated. The actuation of the switch is shown at the reference sign  408   a . In the example that the switch A is implemented by a MOSFET, the switch A is activated at its gate in the region  408   a , wherein the entire voltage VDSA is still applied to its drain-source terminal in this time range. 
     As a result of the low current I T1A  during phase II A , the magnetic energy stored in inductance T 1   A  is not sufficient to completely discharge the parasitic capacitances of the switches A and B and the primary coil T 1   A  when a low load is connected to the output  110   a,    110   b . Only in the case of a large load would a current occur in the leakage inductance which would be large enough to cause ZVS conditions in the switches A, B of the lagging branch  107 . In  FIG.  4   a   , however, a low load is assumed and therefore hard switching of the switches A and B occurs. Admittedly, the time range in which soft (ZVS) switching can take place can be increased, by the value of the inductance of the primary coil T 1   A  being increased. However, from a certain value onwards there is the risk that the output capability of the DC-DC converter  100  or the efficiency of the converter output  110   a,    110   b  is endangered. 
       FIG.  5   a    shows the detail  404   a  in the region of the transition of the lagging bridge branch  107  (lagging leg transition) for the case that no transformer T ZVS  is provided. 
     4.b) If the ZVS transformer T ZVS  is provided according to the example embodiment according to  FIG.  1    of the invention, as shown in  FIGS.  4   b ,  5   b   , by activating the switches SR 1 , SR 2  during the end phase II B , an increased current I T1A  can flow during the end phase II B . Because of the increased current flow in the end phase II B , a soft transition can be created when switching A and B. The current I T1A  does not stop at the point  405   b , but rather continues to flow, in particular, it continues to rise during the end phase II B  of the freewheeling phase II up to the point  409   b . This increase in current flow is caused by the ZVS transformer, which increases the effects of switching the secondary-side switches SR 1 , SR 2  or the secondary-side rectifiers SR 1 , SR 2  on the primary side. 
     In this case, the ZVS transformer T ZVS , i.e., the combination of the coils T 3   A  or T 3   B , has the function during the freewheeling phase II or IV and in particular in an end region II B  or IV B , i.e., during the time interval during which the switches SR 1 , SR 2  are activated at the same time and short-circuit the secondary coil T 1   B , of increasing the primary current I T1A . Since the low-side switches B and D are switched on during the freewheeling phase II, the voltage across T 3   B  is kept at 0V during the freewheeling phase II. The voltage reflected from the secondary side into the primary coil is thus also zero. However, the voltage across the coil T 3   A  is half the input voltage ½ V in . The voltage across T 3   B  is kept at 0V during the freewheeling phase II. While the voltage of the T 3   A  winding is not equal to zero, the current increases linearly through the ZVS transformer T ZVS . This current is proportional to the time at which SR 1  and SR 2  are activated and inversely proportional to the leakage inductance of T ZVS . 
     Since the voltage via the low-side switch B is kept at 0V during the freewheeling phase II, the switch B can be switched over during the transition phase  404   b  immediately after the freewheeling phase II under ZVS conditions; in particular, the low-side switch B can be switched off under ZVS conditions. After the switching of the switch B, the current additionally injected by the coil T 3   A  into the coil T 3   B  is conducted to the connection node  105  between the switches A and B and helps all the charge in the parasitic elements of the high-side switch A and/or all voltage across the high-side switch A to be dissipated and to ensure ZVS conditions for switching the switch A. Thus, the high-side switch A of the lagging branch  107  can be switched by a small dead time after the low-side switch B of the lagging branch  107  under ZVS conditions, as shown in  FIG.  5     b.  As the detail in  FIG.  5   b    shows, A is actuated after V DS =0 applies and thus the voltage across A is essentially zero volts. 
     In one example, it may be the case that the two switches A, B of the lagging branch  107  are switched essentially simultaneously during the transition phase  406   b . In another example, it may be the case that the high-side switch A of the lagging branch  107  is switched in time after the low-side switch B of the lagging branch  107 . In yet another example, it may be the case that the high-side switch A of the lagging branch  107  is switched during phase III after the low-side switch B of the lagging branch  107 , which is switched during the freewheeling phase II. In yet another example, it may be the case that the low-side switch B of the lagging branch  107  is switched before the second synchronous rectifier SR 2  and the high-side switch A of the lagging branch  107  is switched after the second synchronous rectifier SR 2 . 
     The same applies when the switches A, B of the lagging branch  107  are switched after the freewheeling phase IV. However, the current I T1A  flows during the energizing phases III and the freewheeling phase IV in the opposite direction compared to that of the energizing phase I and the freewheeling phase II. Coming out of the energizing phase III, the switch A is switched on and the switch B is switched off. The freewheeling phase IV begins with the switching of the switches C, D of the leading branch  108 . A loop or freewheeling loop is formed in the upper region of the circuit at the high terminal  109 . The freewheeling loop has the switch A, the coil T 3   B , the coil T 1   A , the high-side node  102 , and the switch C. The voltage is also kept at 0V in this freewheeling loop. When leaving the freewheeling phase IV in the end phase IV B , the switch A of the lagging branch  107 , which is located in the freewheeling loop, is therefore again switched first. Since the voltage in this freewheeling loop is kept at 0V, the switch A of the lagging branch can be switched under ZVS conditions. When this switch is switched, the additional current generated by switching the synchronous rectifiers SR 1 , SR 2  can also be used to switch the second switch B under ZVS conditions. 
     The switching behavior of the switches A  104   a , B  104   b , C  104   c , D  104   d  is the same in  FIGS.  4   a ,  4   b ,  5   a ,  5   b   , regardless of whether the ZVS transformer T ZVS  is present, as assumed in  FIGS.  4   b ,  5   b   , or not present, as assumed in  FIGS.  4   a   ,  5   a.  This switching behavior corresponds to the switching behavior of a phase-shifted full bridge (PSFB), so that the ZVS transformer T ZVS  can be retrofitted in every PSFB without changing the switching behavior. 
     The ZVS transformer T ZVS  ensures the current increase  409   a  in the end phase II B  of the freewheeling phase II or the current increase with the opposite sign in the end phase IV B  of the freewheeling phase IV. 
     During the freewheeling phase II and also during the freewheeling phase IV, the input voltage is the sum of the drain voltages Vds A  and Vds B  if the switches A and B are implemented as MOSFETs. 
     
       
      
       V 
       i 
       =Vds 
       A 
       +Vds 
       B  
      
     
       FIG.  5   b    shows the detail in the region of the switching interval  404   b  or the transition phase  404   b . At the switching time  407   b  of switches A and B of the lagging branch  107  (lagging leg transition), the voltage across switch A has dropped to essentially 0V for the case that a transformer T ZVS  is provided, so that ZVS switching is possible. 
     In contrast, without transformer T ZVS , no ZVS switching is possible at the switching time  407   a  of the switch B, as shown in  FIG.  5     a.    
     The switches A, B are connected in series in the first bridge terminal  105  and the switches C, D are connected in series in the second bridge terminal  106 . The first bridge connection  105  and the second bridge connection  106  are also connected via a series circuit of the coil T 3   B  and the primary coil T 1   A  of the main transformer T 1 . The coil T 3   B  of the main transformer can be considered to be an additional coil T 3   B  to the primary coil T 1   A , since it can be used to increase the total inductance of the series circuit made up of T 1   A  and T 3   B . A high total inductance between the nodes  105  and  106  can improve the ZVS behavior of bridge circuit  101 . 
     The additional coil T 3   B  can be coupled to a current injection device T 3   A  or a current injection device T 3   A . In the example in  FIG.  1   , the current injection device T 3   A  is also a coil T 3   A . The coil T 3   B  can be coupled to the additional coil T 3   B  with low magnetic coupling and thus form the additional transformer T ZVS  or the ZVS transformer T ZVS . Using a small transformer T ZVS  with low magnetic coupling, the ZVS switching of the switches A and B can be achieved. The current injection device T 3   A  forms the primary coil T 3   A  of the additional transformer T ZVS  and the additional coil T 3   A  forms the secondary coil T 3   A  of the additional transformer T ZVS . 
     Due to the series circuit of the auxiliary transformer T ZVS  with the main transformer T 1 , a good output capability may be achieved for the phase-shifted full bridge. If the input voltage V in  falls below a predeterminable value, the DC-DC converter cannot generate a voltage that is able to supply a load connected to the output  110   a,    110   b , for example, the output of the DC-DC converter cannot charge a 12V battery if the input voltage V in  is too low. If a current injection device T 3   A  or a primary winding T 3   A  is provided, which is coupled to a secondary coil T 3   B , which is connected in series with the primary coil T 1   A  of the main transformer, this performance of the output  110   a ,  110   b  can be increased. The good output capability can therefore be characterized in that even at a low input voltage V in , a load at the output of the DC-DC converter can still be supplied with power, which may then also be low because of the low input voltage V in . 
     Due to this good output capability, efficient battery applications can be made possible, for example, auxiliary DC-DC converters for electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV) in which the voltage range V in  can be large as a function of the state of charge of the high-voltage battery (HV battery) connected to the high-side node  102  and the low-side node  103 . 
     All switches A, B, C, D connected to the primary coil T 1   A  of the main transformer T 1  are referred to as primary switches. These can be implemented with the aid of MOSFETs A, B, C, D. In order to enable a ZVS for all primary switches A, B, C, D, the series circuit of the additional coil T 3   B  with the primary coil T 1   A  of the main transformer T 1  is provided. The additional voltage at the additional coil T 3   B  makes it possible to achieve the good output capability at the output  110 . 
     The voltage drop across T 3   B  influences the performance of the output  110  and the addition of a primary coil T 3   A , which is magnetically coupled to T 3   B , increases the performance of the output by increasing the voltage that is applied to the primary side of the main transformer T 1 . T ZVS  has a twofold effect on increasing the performance of the output. On the one hand, the voltage applied to the primary coil T 1   A  of the main transformer T 1  increases by a value given by the formula 
     
       
         
           
             
               
                 V 
                 in 
               
               
                 n 
                 ZVS 
               
             
             . 
           
         
       
     
     The output voltage V out =V o  results from the increased input voltage of the transformer as: 
     
       
         
           
             
               
                 
                   
                     V 
                     o 
                   
                   = 
                   
                     
                       
                         V 
                         in 
                       
                       
                         n 
                         tr 
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         1 
                         + 
                         
                           1 
                           
                             n 
                             ZVS 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     D 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Here n tr  is the number of turns of the main transformer T 1 . 
     A ZVS transformer T ZVS  can reduce the switching losses. With or without ZVS transformer T ZVS , it takes a predetermined time until a primary voltage at the primary coil T 1   A  also appears at the secondary coil T 1   B  after this primary voltage has been applied to the primary coil T 1   A  of transformer T 1 . This delay occurs because the primary current through T 1   A  first has to pass from a freewheeling state to a state in which the output current is reflected at the primary coil T 1   A . It is true that it is desirable to increase the stored magnetic energy in order to enable long freewheeling in which the magnetic energy is dissipated. If this magnetic energy were increased by providing a high inductance of T 1A , this would result in high switching losses (duty loss). By using the ZVS transformer, the magnetic energy can be increased by providing a current without increasing the inductance of T 1A . 
     In other words, by providing the ZVS transformer T ZVS , a high level of magnetic energy can be stored in the system, but with a low leakage inductance of T 1A  and thus the switching losses (duty loss) can be reduced. The storage of a high level of magnetic energy is necessary in order to establish ZVS conditions for the transition of the switches A, B of the lagging branch  107 , in particular if the switches A, B are implemented as MOSFETs. The ZVS transformer T ZVS  is dimensioned in such a way that, in particular, the magnetic energy that enables the ZVS transition of the lagging branch  107  is stored. Storing a high level of magnetic energy does not essentially improve the switching behavior. 
     Since the ZVS transformer also has a leakage inductance, the ZVS transformer stores the magnetic energy in its leakage inductance. This magnetic energy is proportional to the peak current which flows through the ZVS transformer T ZVS . This peak current is in turn proportional to the time interval of the freewheeling phase during which the switches SR 1  and SR 2  are switched on at the same time. The leakage inductance of the ZVS is established during the design of the circuit so that it can absorb enough energy for inducing ZVS conditions, and thereafter can be difficult to change, therefore the current which is required to provide ZVS conditions is controlled by the period during which the switches SR 1  and SR 2  are switched on at the same time during the freewheeling phase. 
     The time required for the transition between the two freewheeling states in the freewheeling phases II and IV can be viewed as a switching loss (duty loss), which can be quantified as follows: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     D 
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         I 
                       
                       
                         V 
                         in 
                       
                     
                     ⁢ 
                     
                       L 
                       lk 
                     
                     ⁢ 
                     f 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Here, ΔI is the current difference between the current through T 1   A  after the transition phase  404   a,    404   b  “turn-off B/turn-on A” i.e., after the phase  404   a,    404   b  of the transition of the lagging branch  107  (lagging leg transition) as described in 4. and the current through the primary coil T 1   A  after the phase when the primary voltage generated by the output current appears at the secondary coil T 1   B , i.e., the point in time that the output current is reflected at T 1   A  (reflected output current). 
     ΔD is a time value corresponding to a region along a time axis and f is the frequency of the PWM. ΔD Is the period of time it takes for the current to change. This period of time ΔD is to be as short as possible in order to achieve a good output voltage capability. 
     The time interval ΔD increases with increasing load at output  110 , since the current difference ΔI increases. This increase in the switching losses ΔD can only take place in a limited range, since from a certain value they are so severe that the output  110  is no longer capable of providing the required output voltage V out , for example, for charging a 12V battery. 
     In a standard PSFB without T ZVS , the ZVS region, i.e., the range of input voltages yin at which ZVS is possible, can be increased by increasing the inductance of the primary coil T 1   A , but then the switch-on losses (duty cycle losses) increase, since it is necessary to wait longer and longer until the high level of stored magnetic energy has dissipated the parasitic voltages of switches A, B, C, D to enable switching under ZVS conditions. This is because if higher voltages v in  are applied to switches A, B, C, D, higher parasitic voltages are also stored in the switches. However, it is desirable to operate the bridge circuit  101  with the highest possible PWM switching frequency f and thus with the lowest possible ΔD. 
     By providing T ZVS , the ZVS region, i.e., the range of input voltages V in  at which the DC-DC converter circuit  100  can still be operated efficiently, can be increased by increasing the current I T1A , which flows during the freewheeling phase II through T 1   A , while at the same time the leakage inductance L lk  of the transformer T 1  is kept low. Although ΔI is also increased, which increases switch-on losses, more magnetic energy can also be stored at the same time. If the current is increased, more magnetic energy can be stored, but power losses and/or line loss (RMS (Root Mean Square) losses) also increase. Soft switching or ZVS switching, however, reduces the line losses. 
     In the circuit of  FIG.  1   , the primary side of the DC-DC converter is configured as a phase-shifted full bridge (PSFB) with an additional small transformer T ZVS , to assist the zero voltage switching (ZVS) of the primary-side switches A and B of the lagging branch  107 . By providing the transformer T ZVS , the stored magnetic energy can be increased by injecting a current; in particular, the current can be used to neutralize parasitic charges on the switches A, B, C, D and in particular on the switches A, B of the lagging branch  107 . This neutralization can take place very quickly, so that the DC-DC converter  100  can work at a high switching frequency f. 
     For the primary-side switches C and D of the leading branch  108  (leading leg), in a PSFB soft switching or ZVS switching can be implemented essentially always, thus regardless of whether the ZVS transformer T ZVS  is present or not. 
     The DC input voltage V in  corresponds to the voltage of the HV battery. The voltage V in  can be in a range from 240V to 470V or at 400V to 800V for applications with more powerful HV batteries, such as those used in electric buses or high-performance electric cars. The fluctuation of the input voltage V in  can depend on the state of charge of the HV battery. The duty cycle of the PWM used depends on the applied input voltage V in . However, it may be necessary to provide other types of switching devices A, B, C, D and other transformers T 1 , T ZVS  if different voltage ranges are to be supplied, for example, 240V to 470V or 400V to 800V. 
     The same voltages are applied to the primary switches A, B, C, D as to the points  105 ,  106 . Since V in  can fluctuate over a wide range due to the change in the state of charge of the HV battery, for example, in the range from 240V to 470V, a control loop is provided in the DC-DC converter (not shown in  FIG.  1   ) that controls the duty cycle of the control signal of the switches A, B, C, D to keep the output voltage V out  at a constant value, for example, V out =14.5V or V out =12V. However, if, for example, the input voltage drops from 470V to 240V, the duty cycle and/or the frequency of switches A, B, C, D has to be increased to ensure a stable and/or constant output voltage V out . The duty cycle is determined from the quotient of the duration of the energizing phase I and the sum of the duration of the energizing phase I and the duration of the freewheeling 
     
       
         
           
             phase 
             ⁢ 
                 
             II 
             ⁢ 
                 
             
               
                 ( 
                 
                   
                     Duty 
                     ⁢ 
                         
                     Cycle 
                   
                   = 
                   
                     
                       Duration 
                       ⁢ 
                           
                       of 
                       ⁢ 
                           
                       the 
                       ⁢ 
                           
                       Energizing 
                       ⁢ 
                           
                       Phase 
                       ⁢ 
                           
                       I 
                     
                     
                       
                         
                           
                             
                               Duration 
                               ⁢ 
                                   
                               of 
                               ⁢ 
                                   
                               the 
                               ⁢ 
                                   
                               Energizing 
                               ⁢ 
                                   
                               Phase 
                               ⁢ 
                                   
                               I 
                             
                             + 
                           
                         
                       
                       
                         
                           
                             Duration 
                             ⁢ 
                                 
                             of 
                             ⁢ 
                                 
                             the 
                             ⁢ 
                                 
                             Freewheeling 
                             ⁢ 
                                 
                             Phase 
                             ⁢ 
                                 
                             II 
                           
                         
                       
                     
                   
                 
                 ) 
               
               . 
             
           
         
       
     
     The frequency for the control signals for A, B, C, D, SR 1 , SR 2  remains constant and is not varied. 
     Since the energy transmission and/or power transmission via the main transformer T 1  depends on the primary voltage, only a low power and/or energy could be transmitted via the main transformer T 1  due to the reduced primary voltage and the power that can be provided with the voltage V out  would be reduced. 
     In other words, a high inductance is desired in the connecting circuit between  105  and  106 , to provide a high level of magnetic energy for discharging the switches A, B of the lagging branch  107  at a high input voltage V in  and to thereby enable ZVS switching. However, if the inductance provided by the inductance of the primary coil T 1   A  were to be increased more and more by a series circuit of an additional inductance T 3   B , the performance of the output voltage V out  or the output power would be reduced more and more, since the discharging of the switches A, B of the lagging branch  107 , in particular at high voltages V in , could either not take place quickly enough or could not take place completely. This means that the DC-DC converter could only be operated in very low voltage ranges. 
     While the primary side of the main transformer T 1  is essentially applied at the high voltage V in  of 240V-470V, at the secondary side T 1   B  of the main transformer T 1 , a DC voltage of 14.5V or a voltage in the range of approximately 12V to 15V is applied, which is provided as the output voltage V out , for example, a radio or other consumer of the vehicle electrical system. 
     The provision of the additional transformer T ZVS  compensates for the loss of the output power by increasing the voltage that is applied to the primary coil T 1   A  of the main transformer T 1 . Since the primary coil T 1   A  of the main transformer T 1  is connected in series with the secondary coil T 3   B  of the ZVS transformer, the output voltage capability increases. To compensate for this influence, the primary coil T 3   A  is provided, which is connected between the switching node  105  and the fixed potential  111 . A voltage that is applied on the primary coil T 3   A  generates a voltage on the secondary coil T 3   B . This voltage on the secondary coil T 3   B  increases the voltage on the primary coil T 1   A  and ensures good output voltage capability. 
     In this way, the single-stage DC-DC converter can be operated with a large input voltage range and ZVS can still be guaranteed for all primary-side MOSFETs A, B, C, D. A single-stage DC-DC converter is a DC-DC converter that converts a first voltage level into a second voltage level only once without generating further intermediate voltage levels. 
     One side or one terminal of the primary coil T 3   A  of the additional transformer T ZVS  is connected to the first bridge terminal  105  and to one side of the additional coil T 3   B  or the secondary coil T 3   B  of the additional transformer T ZVS . The other side or the other terminal of the primary coil T 3   A  of the additional transformer T ZVS  is connected to a third bridge branch  109 , which is formed as a series circuit of two capacitors C 1  and C 2 . This other side of the primary coil T 3   A  of the additional transformer T ZVS  is connected between the first capacitor C 1  and the second capacitor C 2 . The third bridge branch  109  is connected to the first input terminal  102  and the second input terminal  103  and is connected in parallel to the first  107  and second  108  bridge branches. The third bridge branch  109  ensures that a connection of the coil T 3   A  is kept at a fixed or constant potential. A voltage change in the primary coil T 3   A  of the ZVS transformer, which injects a current into the secondary coil T 3   B , thus depends on a change in potential at the bridge points  105  and  106 . The pulse reflected by the switching of the synchronous rectifiers SR 1 , SR 2  in the primary coil T 1   A  thus also has an effect on the additional transformer T ZVS . 
     On the output side, the series circuit of a first synchronous rectifier (Synchronous Rectifier, SR) SR 1  and a second synchronous rectifier (Synchronous Rectifier, SR) SR 2  is connected in parallel to the secondary coil T 1   B  of the main transformer T 1 . 
     These are connected via a first output coil L 1  and a second output coil L 2  as well as an output capacitor C 0  to the output  110  of the DC-DC converter  100 , via which the output voltage V out  is provided. The synchronous rectifier SR 1 , SR 2  is operated in such a way that the positive or negative half-wave induced in the secondary coil T 1   B  is passed on to the smoothing capacitor Co with the same polarity, so that a DC output voltage V out  is generated. 
       FIG.  2    shows a block diagram of a DC-DC converter  200  having a bridge circuit  101  and two main transformers T 1 , T 2  according to one example embodiment of the present invention. In this configuration, the current on the secondary side of the main transformer  1  is divided between four coils L 1 , L 2 , L 3 , L 4  and four synchronous rectifiers SR 1 , SR 2 , SR 1 ′, SR 2 ′, whereby the efficiency of the circuit and the treatment of the current may be simplified. In addition, the main transformer T 1  from  FIG.  1    is divided into the two main transformers T 1 , T 2 . The primary coil T 1   A  of the first main transformer is coupled to the secondary coils T 1   B  and T 1   C  of the first main transformer. The primary coil T 1   A  of the first main transformer is coupled to the secondary coils T 1   B  and T 1   C  of the first main transformer. The output circuits  112   a ,  112   b  essentially correspond to the output circuit  112  from  FIG.  1   . However, a secondary side of the two transformers T 1 , T 2  is used in each of the output circuits  112   a,    112   b . In this case, the synchronous rectifiers SR 1  and SR 1 ′ are operated in the same way and the synchronous rectifiers SR 2 , SR 2 ′ are operated in the same way. 
       FIG.  3    shows a block diagram of a DC-DC converter having a bridge circuit and a main transformer having center tap according to one example embodiment of the present invention. In this circuit variant, only one output coil L 1  is provided. 
     The converter circuits shown in  FIGS.  1 - 3    can be used both as a current doubler and as a center tap configuration on the secondary side. The center tap  301  is arranged on the secondary side of the main transformer T 1 ″ and is connected to the two partial secondary coils T 1   B ″ and T 1   C ″ and the coil L 1 . The two partial secondary coils T 1   B ″ and T 1   C ″ are also connected to the rectifiers SR 1 ″ and SR 2 ″. There is a ground terminal between the rectifiers SR 1 ″ and SR 2 ″, which is also connected to one of the output terminals. The capacitor C 0  is connected in parallel to the output. 
     The switching behavior with and without ZVS transformer is shown enlarged in  FIGS.  5   a   ,  5   b.  The voltage curves Vds A  across the high-side switch A of the lagging branch (lagging leg) A/B is included for a case in which no load is applied at output  110 , that is, for the load-free case or idling. In order to inject an additional current into the secondary coil T 3   B , the synchronous rectifiers SR 1  and SR 2  are switched on at the same time during the freewheeling phase II B  of the low-side switches B, D to generate a short pulse across the secondary coil T 1   B  due to the voltage drop to 0V output voltage, which is transferred to the primary coil T 1   A . The curve Vds A  of  FIG.  5   b    corresponds to the case in which the additional transformer T ZVS  in series with the main transformer T 1  is present, as shown in  FIG.  1   . 
     In phase III, switch A is closed and switch B is open. It can be seen that during the switching phase  404   a,    404   b , the initial voltage of approximately V in =400V across A and B for the case that T ZVS  is used, has already dropped to 0V before the switching phase  404   b , as indicated at point  407   b , whereas the voltage V in =400V for the case that T ZVS  is not used, still has a voltage at the end of the switching phase  404   a , as indicated at point  407   a . Thus, when using the ZVS transformer T ZVS , ZVS switching is also possible in the load-free case. This is because according to  FIG.  1   , the additional transformer T ZVS  is connected in series with the main transformer T 1  and helps to inject a current into the coil T 3   B  of the additional transformer. ZVS switching of the high switch A  104   a  can be achieved independently of the load at the output  110   a,    110   b . This is because if there is no load at the output, the output current is 0 A and the output load R load  is undefined. The output voltage V out  is regulated to a constant 14.5V regardless of the load, for example, by changing the frequency and/or the duty cycle of the PWM switches A, B, C, D. 
     The energizing phase III is followed by a further freewheeling phase IV, namely the freewheeling phase of the high-side switches A  104   a  and C  104   c . In this a freewheeling circuit is formed from switches A  104   a , C  104   c , additional coil T 3   B , and primary coil T 1   A . 
     With the phase-shifted full bridge topology (PSFB) having an additional inductance T 3   B , which is connected in series with the transformer T 1 , ZVS switching or soft switching may thus be achieved if the additional inductance T 3   B  is part of a transformer T ZVS . The additional transformer T ZVS  has a low coupling factor between the primary coil T 3   A  and the secondary coil T 3   B . The low coupling is achieved, for example, by a magnetic core having a slot. Energy that can be used for ZVS can be temporarily stored in the additional transformer T ZVS . Due to the low coupling of the ZVS transformer T ZVS , a leakage inductance is retained in T ZVS , because the part of the magnetic flux which does not couple into the secondary coil is noticeable as leakage inductance. This additional leakage inductance can be viewed as a further inductance which is in series with T 3   B , even if the leakage inductance is not a tangible component. The size of the leakage inductance can also be influenced via the coupling factor. The leakage inductance can also store magnetic energy, which can then be converted back into an electrical current flow to assist ZVS in that the bridge point  105  is discharged. 
     If a configuration having only one additional coil T 3   B  without primary coil T 3   A  or further coil T 3   A  is used, i.e., not a complete transformer T ZVS  but only a coil is connected in series with the main transformer, the output power of the DC-DC converter is reduced, since a voltage drops across the coil T 3   B  during switching, which then reduces the voltage on the primary coil T 1   A  of the main transformer. As shown in  FIGS.  4   a  and  5   a    in phase  404   a , a real ZVS of the high-side switch A cannot be achieved with such a configuration with only one additional coil T 3   B , even with a low load, since the current through the additional coil T 3   B  is too low. The coil T 3   B  without T 3   A  can only be used to achieve ZVS switching of the low-side switch B. 
     If, however, a complete ZVS transformer T ZVS  according to  FIG.  1    is used, an additional current can be injected into the coil T 3   B  by switching the synchronous rectifiers SR 1 , SR 2  and thus the effect of the magnetic energy in the leakage inductance of the transformer T ZVS  can be used. The energy E=½*(L*I 2 ) can be controlled via the current, which is determined by the length of time during which SR 1  and SR 2  are activated at the same time. The ZVS transformer T ZVS  thus contributes by activating the synchronous rectifiers SR 1 , SR 2  over a predetermined period of time II B  to increasing the primary current I T1A  by an amount as is required in the freewheeling phase II of the switch for ZVS switching of the lagging branch  107 . 
     By using an additional complete transformer T ZVS , a very efficient and cost-effective single-stage DC-DC converter can thus be implemented. In addition, ZVS switching can be achieved in the primary switches A, B, C, D, regardless of the load at the output  110 . In addition, a high output power can thus be provided at the output  110 , which can be important, in particular, for applications having a large input voltage range V in . 
     It may be considered to be a concept of the present invention to increase the magnetic energy stored in the transformer T ZVS  by increasing the primary current I T1A  instead of increasing the inductance of the secondary coil T 3   B , which would result in a reduction in the output power. Since the magnetic energy in the secondary coil T 3   B  is according to formula (3) 
     
       
         
           
             
               
                 E 
                 L 
               
               = 
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   L 
                   
                     T 
                     ⁢ 
                     3 
                     ⁢ 
                     B 
                   
                 
                 ⁢ 
                 
                   I 
                   2 
                 
               
             
             , 
           
         
       
     
     increasing the primary current I T1A  is more efficient than increasing the inductance of the secondary coil T 3   B . Since the duty cycle loss also depends on the inductance via the ratio 
     
       
         
           
             
               
                 V 
                 in 
               
               L 
             
             , 
           
         
       
     
     increasing me current by injecting an additional current helps to keep the ratio essentially unchanged, and increasing the stored magnetic energy without increasing the switch-on losses. 
     Each additional inductance connected in series, which is present as a real component or as a leakage inductance, reduces the output capability of the output  110  of the DC-DC converter  100 , for example, in relation to an output voltage v out  to be constantly provided as a function of a wide range of available input voltages V in . This reduction of the output capability can have a negative effect if the output voltage V out  of the converter  100  is to be regulated to a constant output voltage, for example, 14.5V, and the input voltage varies over a wide range, for example, in the range from 240V to 470V, depending on the state of charge of an HV battery. This is because the inductance connected in series may be necessary to enable soft switching under ZVS conditions. An inductance connected in series would degrade the output capability of the converter  100 , since it is no longer possible, for example, to generate a constant output voltage of 14.5V if the input voltage V in  is at a lower range limit, for example, at 240V of a range of 240V to 470V, and at the same time ZVS conditions are to be complied with. This is because it would actually be desirable to manage without any series inductance T 3   B . However, ZVS would not be possible and the efficiency of the converter would be low. 
     Since, in addition, large inductances are not required for T 3   B , the size of a DC-DC converter can be kept small, although it is operated with a high switching frequency f. The high switching frequencies are possible because of the rapid discharge of node  105  and are the same for the switches A, B, C, D and are determined by the duration of phases I, II, III, IV. 
     In addition, it is to be noted that “comprising” and “having” do not exclude any other elements or steps and that “one” or “a” does not exclude a plurality. Furthermore, it is to be noted that features or steps that have been described with reference to one of the above example embodiments can also be used in combination with other features or steps of other example embodiments described above. Reference signs in the claims are not to be regarded as a restriction.