Abstract:
A DC-DC converter apparatus comprising half or full bridge, two-stage resonant converter, which may include series resonant (inductor, capacitor) devices. An isolated transformer having primary and secondary winding supplies current to full-wave secondary stage-bridge through the use of primary winding resonant devices employing primary stage-bridge. The magnetizing of said devices employs zero-current, zero-voltage resonant-transition switching technology, which reduces switching losses at all switching frequencies to almost zero. The regulation of output voltage at all loads and input voltages achieved by the control of the switching frequency and the phase between signals for primary and secondary stages. The proper intermittent of the frequency and the phase allows achieving the value of efficiency up to 97%.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to a switching power converter and, more specifically, to a two-stage converter with transformer separation having improved zero voltage switching (ZVS) and zero current switching (ZCS) operation modes. 
       BACKGROUND OF THE INVENTION 
       [0002]    The increased demand in the field of power conversion-size reduction and high-efficiency is difficult to meet with the present available power converters. Some features inherent in devices of relevant technology militate against achievement of the desired goals. Some converters are specifically designed to handle a wide range of output loads and variations of input voltage, resulting in less efficiency. Reducing the size of components (capacitors and magnetic component) related to high switching frequency and as results to concomitant high switching losses. 
         [0003]    Common approach of ZVS and ZCS topology disclosed in the prior art is series resonant converter described in e.g. USOORE33866E, U.S. Pat. No. 6,178,099B1 that achieves nearly lossless soft switching using pulse width modulation and frequency super-resonant control of commutation in full or half bridge converter. Soft switching converters attempt to take advantage of the parasitic effect of components within the converter in order to reduce the voltage potentials across (and current flows through) the switches before effecting a switching operation. More specifically, soft switching converters adjust the switching timing in order to charge and to discharge the parasitic switch capacitances of the transistors through the use of current supplied by the magnetizing inductance of the winding of the transformer, thereby reducing the voltage across Off or Open transistors, and current flow through On or Closed transistors. During the time switching which transistors takes, the soft switching reduces the power losses during the switching operation, thereby enabling the converter to operate at high frequency, high efficiency and with reduced electromagnetic interference. 
         [0004]    However, the operation with typical pulse width modulation or the variation of switching frequency is a deficient in full volume of line and maximal load alternation which leads to stresses on the switching devices which are similar to those of a hard switched converter. 
         [0005]    At the series resonant converters disclosed in, e.g., U.S. Pat. No. 4,855,888 and EP 0503862B1 there is presented the topology of two-stage (full or half bridge) circuitry connected to input voltage. In the similar devices, such as those disclosed in U.S. Pat. No. 5,999,417 or U.S. Pat. No. 6,930,893 two stages are floating relatively to one another, where the output voltages are algebraic sum voltages of the output voltages of the two stages. The regulation of the output voltages is achieved by alteration of the phase between the two stages of the converter. However, the operation in full volume (from 0 to max) of loads and line variation leads to inevitable hard switching, e.g., excessive switching losses, excessive EMI and the like. 
         [0006]    Quasi-resonant bidirectional converters were introduced in U.S. Pat. No. 4,663,699 and  Natural ZVS Medium - Power Bidirectional DC - DC Converters with Minimum Numbers of Devices , Hui Li Fang Zheng Peng, J. S. Lawlee, IEEE, Vol. 39, No. 2, March/April 2003. Presented two stage active half-bridge topology featured ZVS in full volume of line and load variation. However, high current switching leads to high level conductive and switching losses, and causes the limitation to power, obstacles to decrease the cost, volume and EMI. 
         [0007]    From this prior art other challenge is presented to minimize losses during the switch transition time of two stage active full bridge or half-bridge converter. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention comprises two stages active full or half bridge switching circuits. The circuits comprise of four or two switches, respectively. Two stage connected by transformers where primary winding through series resonant tank applied to first stage, secondary winding to second stage. First stage is supplied by input voltages, second derives output voltage, DC or AC. The stages may be controlled by signals produced by a controller. The simultaneous changing of frequency and phase between signals attributed to first and second stage may feature the variation of output voltage from 0 to max at full alternation of line (input voltages, for example, rectified 50 Hz “Voltage Main”) voltages and loads. 
         [0009]    The present invention, in embodiments described hereinafter, may offer one, some, or all of the following advantages and features:
   1. ZVS, ZCS in every combination of input voltages, output voltages and load from light (no-load) to max.   2. Load switching losses related to ZVS, ZCS.   3. Low conductive losses due to minimized value and sine shape of the current conducted through resonant tank and switching stages.   4. High efficiency due to low switching and conductive losses.   5. Means to distribute DC and AC voltage which are fully adjusted to the value of DC output voltages and to feature efficiency of up to 97%.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present invention and its advantages, the description of the invention should be read in conjunction with the following drawings in which: 
           [0016]      FIGS. 1A and 1B  are schematic diagrams of four-way and two-way converter according to some embodiments of the present invention; 
           [0017]      FIG. 2A  is a schematic illustration of a two stage embodiment of a converter according to some embodiments of  FIG. 1 ;  FIG. 2B  is a schematic illustration of a converter with control unit according to some embodiments of the present invention 
           [0018]      FIGS. 3A-3F , are illustrations of a converter according to the present invention and indications of current flow in branches of the converter in various steps of the working cycle; 
           [0019]      FIG. 3G  is simplified equivalent scheme of a converter according to embodiments of the present invention; 
           [0020]      FIGS. 4A-4D  are simplified illustrations of wave forms in the time-domain of current through and voltage across switching devices at both sides of a converter according to some embodiments of the present invention; 
           [0021]      FIGS. 5A-5D  are simplified illustrations of switching timing diagrams of control signals at the gate terminals of switching devices of a converter according to some embodiments of the present invention; 
           [0022]      FIG. 6  illustrates a diagram of output power versus the phase shift of the secondary stage of a converter with respect to its primary stage, according to some embodiments of the present invention; 
           [0023]      FIG. 7  illustrates a diagram of output power (Gain) versus the frequency and load; and 
           [0024]      FIG. 8  illustrates a diagram of efficiency versus the phase according to some embodiments of the invention. 
       
    
    
       [0025]    It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The invention described herein after pertains to an electrically isolated DC-DC converter that may be used to deliver power of DC voltage or AC voltage (high frequency of up to, for example, 1 MHz yet the operational frequency high range may get higher as the technology of production of switching transistors improves) from a DC source such as a battery or rectifier utility. In such a converter, a transformer may be used to provide electrical isolation and a step-down or step-up in voltage level, according to its ratio, as may be required. Switching means (such as transistors, preferably MOSFET) and diodes, preferably of the fast recovery type may be used in conjunction with capacitors and inductors to induce non-DC current (i.e. AC or pulsating current) in the primary circuit to activate a transformer to create the required conversion. A control circuit may typically be included to provide the required driving signals to the transistor control terminals. The ability to provide a desired regulation performance is usually derived from the power circuit&#39;s topology and the manner in which its switching devices are controlled. 
         [0027]    Attention is made now to  FIGS. 1A and 1B  which are schematic diagrams of four-way converter  2  and two-way converter  4  respectively according to some embodiments of the present invention.  FIG. 1A  depicts converter  2  with a two-stage structure coupled by a transformer TR, where the power flows through the first stage (primary stage) and then through the second stage (secondary stage). Both stages may be provided with regulation units. Additionally, the secondary stage may be electrically isolated from the primary stage by transformer TR which may also provide a step-down or step-up function of DC and AC output voltages. In converter  2  both primary and secondary stages of converter  2  may comprise quad switching topology with switching means Q 1   a -Q 1   d  and Q 2   a -Q 2   d  in the first and second stages respectively. Switching means Q 1   a -Q 1   d  and Q 2   a -Q 2   d  may be realized by any suitable controllable switch that will stand the switching conditions developing during the operation of converter  2 , such as switching frequency, switching current and switching voltage. Switching means Q 1   a -Q 1   d  and Q 2   a -Q 2   d  may be controlled by a control unit (not shown). The purpose of the control unit may be to determine when each of the switching means will be turned ON or OFF, in each stage respectively.  FIG. 1B  depicts converter  4  with a two-stage structure coupled by a transformer TR, where the power flows through the first stage (primary stage) and then through the second stage (secondary stage). Similarly to converter  2  of  FIG. 1A , the secondary stage may be electrically isolated from the primary stage by transformer TR which may also provide a step-down or step-up function of DC and AC output voltages. Both primary and secondary stages of converter  4  may comprise dual switching topology with switching means Q 10   a -Q 10   b  and Q 20   a -Q 20   b  respectively. It would be apparent to one skilled in the art that any other topology of switching means, such as a full bridge (four-way) switching assembly in the primary stage and a half bridge (two-way) switching assembly in the secondary stage, or the like may be used. 
         [0028]    Attention is made now to  FIG. 2A  which is a schematic illustration of a two stage DC-DC converter  6  according to some embodiments of the invention.  FIG. 2A  illustrates an embodiment implementing two power stages. In this circuit, each of the two stages comprises a half-bridge converter, yet it would be appreciated by a person of ordinary skill in the art that a full-bridge converter may similarly be used. Primary stage may comprise switching transistors T 1 , T 2 ; fast-recovery diodes D 1 , D 2 ; filtering capacitor C 2 ; auxiliary capacitors C 3 , C 4  and parasitic or implemented capacitors C 5 , C 6 . The secondary stage may substantially be a mirror picture of the primary stage: switching transistor T 3 , T 4 , diodes D 3  and D 4 , parasitic or implemented capacitors C 7  and C 8 ; auxiliary capacitors C 9 , C 10  and filtering capacitor C 11 . The two stages may be coupled through transformer TR 1  which may comprise magnetizing inductance represented by inductor M and leakage inductances represented by inductors Ls 1 , Ls 2 ; resonant inductance L 1  and resonant capacitor C 1 . 
         [0029]    Attention is made now further to  FIG. 2B , which is a schematic illustration of a converter  20  according to some embodiments of the present invention. Converter  20  may comprise two stages  21 ,  23  coupled by coupling means such as a transformer  30 , resonant circuitry which may comprise resonant capacitor C 1  and resonant inductor L 1  and a controller  36 . Input stage  21  may comprise input terminals for receiving input power having voltage Vin, filtering capacitor C 2 , auxiliary capacitors C 3 , C 4  and switching assemblies  22  and  24 , each may comprise switching means, fast recovery diode and parasitic/implemented capacitance. Output stage  23  may comprise switching assemblies  26 ,  28  each of which may comprise switching means, fast recovery diode and parasitic/implemented capacitance, auxiliary capacitors C 9  and C 10  and filtering capacitor C 11 . The output voltage Vout develops on output terminals of secondary stage  23 . Primary stage  21  and secondary stage  23  may be coupled by a coupling transformer  30  which may comprise magnetizing inductance represented by inductor M and leakage inductances represented by inductors Ls 1 , Ls 2 ; resonant inductance  32  and resonant capacitor  34 . Switching assemblies  22 ,  24 ,  26  and  28  may be controlled by controller  36  which may be adapted to provide control signals to the control terminals of switching assemblies  22 ,  24 ,  26  and  28  according to any desired switching scheme. Controller  36  may further receive V REF AC  representing actual voltage on the input terminals of converter  20 ; V REF DC  to provide reference for the desired DC voltage and V FB OUT  to provide feedback for the operation of converter  20  by providing voltage from the output terminals of converter  20 . 
         [0030]    Attention is made now to  FIGS. 3A-3F  which are partial schematic illustrations of a converter  10  according to embodiments of the present invention with indications of the current flow in branches of the converter in various steps of the working cycle. Attention is also made to FIG.  3 A 1  which depicts in more details optional topology and components comprised in switching assembly  11 . Switching assembly  11  may be regarded as a generic topology of a switching means SWx (where x may have the values 1, 2, 3, or 4). Generic switching assembly  11  may comprise of transistor Tswx connected in parallel to diode Dswx and to capacitor Cswx. In the circuits discussed herein below, a switching means will be referred to as a switching assembly of the generic type discussed above, yet it would be apparent to a person skilled in the art that switching assembly SWx may be any other appropriate device and circuitry. The control terminal denoted VgSWx is the control terminal through which switching assembly SWx may be turned (or gated) ON or OFF by a control circuitry  36  ( FIG. 2B ).  FIGS. 3A-3F  are shown without a control unit, such as controller  36  of  FIG. 2B , for more clarity yet it should be apparent to a person with ordinary skill in the art that a control unit, such as controller  36 , may be used to control the operation of converter  10  by means of gating its switching devices SW 1 -SW 4  ON or OFF at the desired timing, as will be described in more details below. The description relating to  FIGS. 3A-3F  intends mainly to describe the flow of currents in the branches of a converter in various phases of a work cycle, according to embodiments of the invention. The description of the flow of currents in a converter according to embodiments of the present invention is followed by a description of time graphs of voltages and currents in a converter according to the invention and respective control signals at the control gates of the switching devices. 
         [0031]    As depicted in  FIG. 3A  (and may be applicable to converters presented in other drawings as well, with the required changes) converter  10  may comprise a first stage comprising capacitors C 2 , C 3  and C 4  and switching means SW 1  and Sw 2 , all connected between two poles of Vin, denoted Vin+ and Vin− respectively. Switching means SW 1  and Sw 2  may be connected in a totem-pole arrangement with a central tap denoted “ 1 ”. Similarly, converter  10  may comprise a second stage comprising switching means SW 3  and SW 4  and capacitors C 9 , C 10  and C 111 , all connected between two poles of Vout, denoted Vout+ and Vout− respectively. Switching means SW 3  and SW 4  may be connected in a totem-pole arrangement with a central tap denoted “ 2 ”. Further converter  10  may comprise a resonance circuit comprising inductor L 1  and capacitor C 1  connected between said first central tap “ 1 ” and said second central tap “ 2 ”, either directly (not shown in  FIG. 3A  but exemplified in other drawings) or via a coupling transformer TR 1 . Additionally converter  10  may comprise a return path between said first and said second stages connected between point “ 3 ”, being a third central tap between capacitor C 3  and capacitor C 4  and point “ 4 ” being a fourth central tap between capacitor C 9  and capacitor C 10 . the return path of resonance circuit may be connected directly between points “ 3 ” and “ 4 ” (not shown in  FIG. 3A  but exemplified in other drawings) or via a coupling transformer TR 1 . It will be noted that in other embodiments of the present invention the node of capacitors C 3  and C 4 , as well as the node of C 9  and C 10  may be replaced by an additional arrangement of two switching means arranged in totem-pole arrangement with their central tap at point “ 3 ” or “ 4 ” respectively, such as in a converter occupying a full-bridge at each stage. 
         [0032]    When switching assemblies SW 1  and SW 3  are conducting (that is are switched ON), switching assemblies SW 2  and SW 4  are gated OFF; capacitor C SW1  of switching assembly SW 1  is charged to 0 volts, capacitor C SW2  of switching assembly SW 2  is charged to Vin, capacitor C SW3  of switching assembly SW 3  is charged to 0 volts, and capacitor C 10  to Vout. Resonant inductor L 1  and leakage inductances Ls 1 , Ls 2 , magnetized by currents Isw 1,2  and Isw 3,4  are flowing from Vin to Vout (the description of currents Isw 1,2  and Isw 3,4  as flowing ‘through’ transformer TR 1  is a common way of describing a simplified transformer coupled circuitry, specifically if the transformation ratio is 1:1). 
         [0033]      FIG. 3B  shows the next step in the sequence of four steps of the presently described embodiment. After the switching assembly SW 1  is gated OFF by control signal V GSW1 , current I SW1,2  begins to charge capacitors C SW1  and C SW2 . Because the initial voltage on capacitor C SW1  is substantially zero, the mode of the switching is substantially ZVS, because the switching of switching assembly SW 1  is done when the voltage across its terminals is substantially zero. Because the shape of the current I SW1,2  is sine, and the value of the current is minimum (efficient for recharge of capacitors C SW1 , C SW2 ), the implemented mode might be named ZVS. 
         [0034]    The recharge of the respective capacitors Csw 1  and Csw 2  forms the roll-off of voltage in point  1  (see  FIG. 3A ). At the end of that recharge step the voltage at point  1  (in  FIG. 3A ), and in accordance also the current through the diode turns diode D SW2  ON. The transition time of change of voltage across SW 1  is the time between t 2  and t 3  (see  FIGS. 4A-5D ). Only after that, at time t 4 , control signal V GSW2  is applied to SW 2  to turn the switching assembly ON. The currents are described in  FIG. 3B . At this time the changes in voltage has finished 
         [0035]      FIG. 3C  shows a next step in the sequence of operation of converter  10  according some embodiments of the invention. After current I SW1,2  changed its direction in the current loop of the first stage of converter  10  (see t 5 ,  FIG. 4A ) the direction of current I SW3,4  in the current loop of the second stage is also changed (see t 5 ,  FIG. 4C ). Only after that control signal V GSW3  is applied to switching assembly SW 3  and by that it will turn the switching means OFF and current I SW3,4  will start recharging capacitors C SW3 , C SW4 . See t 6  in  FIG. 5C . Because the initial voltage on capacitor C SW3  is now substantially zero, the switching mode is substantially purely ZVS. Because the value of current I SW3,4  is minimum (enough for efficient recharging of capacitors C SW3 , C SW4 ), the implemented mode is substantially ZCS. The recharge of the respective capacitors C SW3  and C SW4  is forming the roll-off of voltage in point  2 . The corresponding time points are t 6  and t 7  respectively. The end of the recharge cycle turns diode D 4  ON. Only after that, at time t 8 , control signal V GSW4  is applied to transistor SW 4  to turn it ON. The time of this recharge cycle, t 6 -t 7 , is a transition time. The currents of this step are as depicted in  FIG. 3D . By the end of this time the change in all voltages in this cycle fraction has finished. A signal that is applied to V GSW2  turns assembly SW 2  now to OFF (at t 9 ). Current I SW1,2  begins to recharge capacitors C SW1 , C SW2 . Because initial voltage of capacitor C SW2  is substantially zero, and the current I SW1,2  charges C sw1  and C sw2  according to the slope of V SW1-2  between t 9  to t 10 . The implemented mode of this switching is substantially ZVS. Because the shape of the current as function of time is substantially a sine and is approaching crossing zero but not exactly zero—which is important to support charging of the capacitors, the mode of switching is substantially ZCS. 
         [0036]    The recharge process forms the roll-off of voltage in point  1 , as shown in  FIG. 3E . When diode D SW1  is turned ON it indicates the end of the recharge stage of C SW1 . Only after that signal V GSW1  is applied to switching assembly SW 1  to turn it ON. The time of this recharge is a transition time lasting between t 9  to t 10 . By the end of this time the voltage changing in point  1  has finished. In accordance with the change of direction of the voltage applied to inductance L 1  the direction of currents I SW1,2  and I SW3,4  alternates respectively as indicated in  FIG. 3F . Switching off transistor SW 4 , at time t 13 , initiates recharge process of capacitors C SW3 , C SW4 . 
         [0037]    At time t 13  SW 4  is switched off and in accordance with the direction of current I SW3,4 , recharges Csw 3  and Csw 4  until Vsw  3 - 4  reaches, during transition time, maximum voltage between t 13  to t 14 . The switching conditions are ZVS and ZCS, similarly to the previous stages. Substantially with the conditions satisfying ZVC and ZCS at SW 4 . The recharge time of the capacitors is a transition time and it finishes when diode D SW3  is turned ON (see t 14 ,  FIGS. 4A-5D ). After it, by means of control signal V GSW3  switching assembly SW 3  is turned ON, at time t 15 . at the end of this stage the circuit is ready to arrive at the initial state, as described above, at the end of the cycle. 
         [0038]    As may be clearly seen above, all changes in the direction of currents I SW1,2 , I SW3,4  are followed by the value and the polarity of voltages applied to inductor L 1  and whole resonant tank—L 1  and C 1 . All the transition times relate to the values of I SW1,2 , I SW3,4  and values of capacitors C SW1 , C SW2 , C SW3 , C SW4  that include the parasitic capacitors of the transistors, and optionally also the auxiliary capacitors. 
         [0039]    Attention is made now to  FIG. 3G , which is a simplified equivalent scheme  300  of a converter according to embodiments of the present invention. Equivalent scheme  300  may faithfully represent a two-stage converter according to the present invention under several reasonable assumptions. In an ideal transformer the leakage inductance (which is represented in  FIGS. 2A and 2B , for example, by LS 1  and LS 2 ) is equal substantially to zero. Therefore the coils representing the leakage inductances may be removed in this scheme. Assuming that the transfer ratio of the transformer is 1:1 then the voltage at the terminals of the primary stage of the transformer is equal to that on the terminals of the secondary stage and same applies to currents in the primary and secondary windings. Therefore the transformer may be removed completely from equivalent scheme  300 . Thus, input stage  302  may be connected, under these assumptions to output stage  304  directly by Li—Cl resonant tank. As seen in  FIG. 3G  input stage  302  may comprise a set of switching assemblies SW 1 , SW 2  and output stage  304  may comprise a set of switching assemblies SW 3 , SW 4 . SW 1 -SW 4  may be controlled (i.e. gated) ON or OFF via their respective control terminals Vgsw 1 -Vgsw 4  respectively by applying control signals from, for example, controller  36 . Equivalent scheme  300  may be used for simplified analysis of the operation of a converter operating in accordance with embodiments of the present invention. Based on equivalent scheme  300  it will be apparent that the currents in the input stage  302  and in the output stage  304  are the same. 
         [0040]    From equivalent scheme  300  of  FIG. 3G  it is evident that the current of input stage  302  (or I SW1,2 ) which is presented in  FIG. 4A  is equal to the current of the output stage  304  (or I SW3,4 ) which is presented in  FIG. 4C . 
         [0041]    Attention is made now also to  FIGS. 4A-4D  and  5 A- 5 D which are wave forms in the time-domain of current through and voltage across switching devices at both sides of a converter and simplified illustrations of switching timing diagrams of control signals at the gate terminals of switching devices of a converter, according to some embodiments of the present invention. Substantially when transistors SW 1  and SW 3  are conducting (that is are switched ON), transistors SW 2  and SW 4  are gated OFF. Capacitor Csw 1  of transistor SW 1  is then charged to 0 volts, and capacitor Csw 2  to Vin. Capacitor Csw 3  is charged to 0 volts, and capacitor Csw 4  to Vout. Resonant inductor L 1  and leakage inductances Ls 1 , Ls 2  are then magnetized by currents Isw 1 - 2  and Isw 3 - 4  which are flowing from Vin terminals to Vout terminals and then back from Vout terminals to Vin terminals. The switching scheme described above begins at time t 2  in  FIGS. 4A-4D  and  5 A- 5 D. 
         [0042]      FIGS. 4B and 4D  present the voltages V SW1,2  at point  1  and V SW3,4  at point  2 . The switching timing schemes of V GSW1 -V GSW4  are presented in  FIGS. 5A to 5D . The switching is done in roll-off mode signals and not in roll-on mode signals. With respect to the momentarily values of I SW1,2  and I SW3,4  ( FIGS. 4A and 4C  respectively) the switching schemes of  FIGS. 5A to 5D  present the switching ON or OFF of V GSW1 -V GSW4  in each step of the various steps described above with respect to  FIGS. 3A-3F .
       t 1 —switching assembly SW 1  is turned ON ( FIG. 3B ). The transition time must be completed first.   t 2 —switching assembly SW 1  is gated OFF.   t 3 —switching assembly SW 1  and SW 2  is turned OFF. The transition time is t 2 -t 3  must be completed.   t 4 —switching assembly SW 2  is gated ON.   t 5 —switching assembly SW 2  is turned ON. The current Isw 3 - 4  changes polarity.   t 6 —switching assembly SW 3  is gated OFF.   t 7 —switching assembly SW 3  is turned OFF.   t 8 —SW 4  is turned ON.   t 9 —switching assembly SW 2  is turned OFF   t 10 —switching assembly SW 2  is turned ON         
         [0053]    As shown in  FIG. 4A  to  FIG. 5D , the amplitudes of the resonant tank comprising inductance L 1  and capacitance C 1 , currents I SW1,2,3,4  and the value of output voltage Vout may vary according to the variation of frequency/time of the cycle period. This is one of the embodiments for regulation of the values of the currents I SW1,2,3,4  and the voltage Vout. It should be noted that graphs of  FIGS. 4A-5D  reflect the values related to the chosen phase difference between the voltages of the input (primary) stage, i.e. V SW1,2  and the output (secondary) stage, i.e., V SW3,4 . The time between t 2  to t 6 . is the phase shift δ. Related to the period (T). the phase is δ/T. 
         [0054]    Attention is made now to  FIGS. 6 ,  7  and  8 , which are a diagram of output power versus the phase of the secondary stage of a converter with respect to its primary; a diagram of output power (Gain) versus the frequency and load and a diagram of efficiency versus the phase according to some embodiments of the invention. As shown in  FIG. 6 , the variation of the phase may impose variations of the output voltage Vout, of the output power Pout and the currents I SW1,2  and I SW3,4 . The dynamic range of change of phase from zero to π may be divided to three main sub-regions, denoted I, II and III in  FIG. 6 . From a phase value of substantially zero to phase of approximately 0.2π (sub-region I) no control of Pout by means of phase shift changes is done. From phase value of 0.2π to 0.5π (sub-region II) the value of Pout and Vout increase to a maximum, and then, in sub-region III, decrease to substantially zero when the phase value reaches π. Sub-region I is practically not an operational range of control by phase change because only in phase values above 0.2π does soft-switching begins to be effective. Yet, the range of phase above 0.2π is preferable for exercising some embodiments of the invention. The cause is that higher efficiency is achieved at this range (from approximately 96% at 0.2π to approximately 90% at π), as shown in  FIG. 8 . Thus it would be clear that the regulation of a converter circuit according to the present invention by means of controlling the phase has to be featured substantially from phase value of 0.2π to π. This is another embodiment for regulation of the converter parameters. 
         [0055]    The operational frequency range for a converter of the present invention may also be divided to three sub-regions. First frequency sub-region, denoted I in  FIG. 7  extends substantially from f 0  to f min , where f 0  is the natural resonance frequency of the resonant tank of a converter according to the present invention and f min  is the frequency at which for a given load of the converter the phase is approx. 0.2π when control signals of the primary and secondary stages of the converter are in-phase of the resonant tank and the reactive current is just large enough to enable soft switching. Second frequency sub-region, denoted II, extends substantially from f min , to f 1  where f 1  is the frequency at which for a given load of the converter the phase is approx. 0.5π when control signals of the primary and secondary stages of the converter are in-phase of the resonant tank. A third frequency sub-region, denoted III, extends substantially from f 1  to f 2  where f 2  is the maximum available frequency at which soft switching is still possible for a given converter circuitry. This frequency highly depends on the switching rate of the switching assemblies of the converter. It should be noted that in a preferred embodiment the control of the phase in a converter of the present invention when in sub-region II is achieved by means of change of the frequency of the converter which imposes change of the phase as a result of the change of the reaction of the secondary stage of the converter to the operation of the resonant tank. When operating in phase sub-region III the control of the phase is achieved by means of change of the phase directly by means of control of the switching ON or OFF of the switching assemblies of the secondary stage of the converter relative to the switching of the primary stage of the converter. 
         [0056]    When planning a control scheme for a converter according to the present invention it should be noted that working in sub-regions II and III of phase, that is from phase of 0.2π to 0.5π and from 0.5π to π respectively, imposes requirements on the regulation of a converter of the present invention that may be contradicting. A reason for that is that one-sided variation of the phase (i.e.—changing the phase of the secondary stage of a converter with respect to the primary stage) results two-sided variation of Vout. This drawback may easily be compensated by simultaneous variations of the frequency at a first range of the frequency, from frequency f min  to f 1  (see  FIG. 7 ), where the value of f min  depends on the actual load of the converter and the required efficiency and wherein at f min  Vout and Pout are substantially at maximum. In the frequency range of f min  to f 1  the controller may issue control signals so as to keep the value of phase at substantially 0.2π and regulate Pout and Vout only by means of changing the frequency. The goal of this process is to operate the converter, as much as possible, in one-sided regulation mode and achieve high efficiency. In a second range of phase values, where the frequency equals f 1  and higher, when the phase between the voltage and current of the resonant tank exceeds 0.5π the system of the present invention performs the variation of phase by means of the control signals. In this range the regulation is one-sided, which enables to regulate the output of the converter either by control of the phase or by control of the frequency or by both. The latter is better because simultaneous regulation (i.e. by control of both the phase and the frequency) may increase the efficiency when at low or non load and at high input voltage, or when output voltage is very low. 
         [0057]    Control scheme of a converter built and operating according to embodiments of the present invention may be presented with respect a two-dimensional space of working conditions of the converter, having the phase angle as one axis and the frequency as its other axis. When applying the division of the phase axis into the three sub-regions detailed above and the frequency axis divided into its three sub-regions also detailed above that two-dimensional space is partitioned into nine regions, which may be denoted, for example, region f I -p II , which defines the region where the frequency of the converter is in its I sub-region and the phase is in its II sub-region. Based on this notation method it is now possible to define the required working conditions of a converter according to the present invention by denoting the value of each of the variables frequency (denoted “f” in Table 1) and phase (denoted “p” in Table 1) expressed by a sub-region identifier. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 p 
                   
               
             
          
           
               
                   
                   
                   
                 Sub-region 
                 Sub-region 
               
               
                   
                 f 
                 Sub-region I 
                 II 
                 III 
               
               
                   
                   
               
               
                   
                 Sub-region I 
                 f I -p I   
                 f I -p II   
                 f I -p III   
               
               
                   
                 Sub-region 
                 f II -p I   
                 f II -p II   
                 f II -p III   
               
               
                   
                 II 
               
               
                   
                 Sub-region 
                 f III -p I   
                 f III -p II   
                 f III -p III   
               
               
                   
                 III 
               
               
                   
                   
               
             
          
         
       
     
         [0058]    According to the present invention preferable regions for operation of the converter, in order to achieve, inter alia, the stated goals of high efficiency at large ranges of load variation and input voltage variations are regions f II -p II  and f III -p III . Operating the converter of the present invention in these regions will also ensure stability of the working parameters. It will be noted that the control of the working conditions of a given converter circuitry having a given resonance frequency may fully be controlled by, for example, controller  36  of  FIG. 2B  merely by applying a desired switching scheme to switching assemblies for example, switching assemblies  22 ,  24 ,  26  and  28  ( FIG. 2B ). Control of the frequency may be achieved by changing the switching rate of all four switching assemblies  22 ,  24 ,  26  and  28  substantially in the same extent. Control of the phase of the converter of the present invention may be achieved by changing the switching timing scheme of switching assemblies  26  and  28  of the secondary stage of the converter  20  with respect to the timing of switching assemblies  22  and  24  of the primary stage of the converter. As much as the switching scheme of the secondary stage is delayed with respect to that of the primary stage the phase angle grows. 
         [0059]    The ability of a converter built and operating according to the present invention to convert AC input voltage into the desired value of DC output voltage. Additional advantage feature of the converter is to use available DC power at the output terminals and convert it into AC power at, for example, a AC high frequency output terminals, as shown in  FIG. 1A . When the input voltage drops down sharply the second stage may derive power from a output capacitor (for example C 11  in  FIG. 2 ) to provide AC voltage to the AC output  5  in converter  4  of  FIG. 1B . It may happen when the input is supplied with AC voltage at low frequency of, for example, 50 Hz, and the output voltage AC at high frequency of up to, for example, 1 MHz. Moreover, because the power of AC output voltage in this case does not pass through the first stage of the converter, the efficiency of this embodiment may go up to 97%. 
         [0060]    Other aspects and features of the presented invention can be obtained from study of the drawings, the disclosure, and the appended claims.