Patent Publication Number: US-2015085534-A1

Title: Regenerative and ramping acceleration (rara) snubbers for isolated and tapped-inductor converters

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional and claims priority of U.S. provisional application No. 61/880,759, filed Sep. 20, 2013, which is incorporated herein as though set forth in full. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to voltage converter circuits having coupled inductors, and in particular to a regenerative and ramping acceleration (RARA) snubber circuit for switching converters with either isolation transformer(s) or tapped inductor(s). A snubber circuit according to the present disclosure can reduce the stress of the switching devices in a switching converter, can accelerate the output current ramping, and can improve the overall efficiency of the hosting switching converter. A snubber circuit according to the present disclosure can assist the output rectifier to achieve zero voltage turn on and zero current turn off, can recycle the absorbed leakage energy back to the hosting switching converters, can provide fast output current ramping, and can improve the overall efficiency. 
     BACKGROUND 
     Numerous voltage converters, or voltage converter circuits, use magnetic components with multiple coupled windings such as transformers and coupled inductors. These magnetic components practically include an equivalent leakage inductance in series with each winding. The leakage inductance can cause several problems in switching converters. 
     As the winding current is interrupted by a switch, the leakage inductance has to discharge its energy into the switch and surrounding stray capacitances in the circuit. This may result in a large voltage overshoot and ringing across the switch. Generally, the overshoot and ringing may shorten the lifetime of the switch and in severe cases may exceed the switch rating causing destruction. The ringing may also emit electro-magnetic interference (EMI) and can disturb the operation of nearby systems. 
     Further, as a switch or diode is turned on, the leakage inductance can impede the ramping of the current in a winding. The delay of the secondary current ramping may shorten the conduction time of the output rectifier. As a result, a considerable amount of energy can be prevented from being delivered to the output. Consequently, the practical voltage conversion ratio may fall short from that of the expected. To compensate for this effect, the converter may have to be operated at higher duty cycle, which can elevate conduction losses and impair the efficiency. At higher power the problem may be more severe, since current ramping delay can become longer as the output current needs to be ramped to a higher value. 
     This output current ramping problem may become acute in transformer isolated or tapped inductor converters with high step-up ratio. This is because in these applications the transformer or tapped inductor may be designed with high turns ratio and can have a substantial secondary leakage that can severely restrict the output current build-up and may impair energy transfer to the output. Hence, the performance of the converter with multi-winding magnetic structure can be profoundly affected by the leakage inductances. 
     A common industry practice is using a RC clamp circuit to absorb the leakage energy and so limit the voltage stress across the main switch of the flyback transformer. However, RC clamp dissipates the absorbed energy which is lost to heat. Thus, the converter efficiency is impaired. Typically, efficiency may be in the 75-80% range. 
     To handle the transients caused by the primary winding leakage inductance, snubber circuit may be utilized to absorb the leakage energy while preventing overvoltage providing controllable rate of voltage rise dV/dt across the switch, and alleviating switching loss of the semiconductor devices. Known snubber circuits, such as disclosed in “K. M. Smith, C. Ji, and K. M. Smedley, “Energy regenerative clamp for flyback Converter”, VCI, invention disclosure, September 1998” or in “C. Liao, K. Smedley, “Design of high efficiency Flyback converter with energy regenerative snubber,” in Proc. IEEE App. Power Electron. Conf. and Expo. APEC′08, 2008”, are typically designed to capture the energy stored in the leakage inductance of the primary winding of a transformer and recycle it to the circuit while suppressing the voltage spike and ringing across the active power switch. However, known snubber circuits provide no solution to the problem of the output current ramping delay caused by the secondary leakage inductance and its impact on converter performance. 
     SUMMARY 
     The present disclosure relates to a snubber circuit for a voltage converter, the snubber circuit being provided to charge a capacitor with the current flowing through the secondary inductance (or inductor) of the converter after a rectifier diode of the converter is turned off by said current; the snubber circuit being arranged to discharge the capacitor by complementing the current in the secondary inductor after the flow of the current in the secondary inductor is inverted. 
     An embodiment of the present disclosure relates to a voltage converter circuit comprising: a primary inductor; a secondary inductor, at least a portion of the second inductor being mutually coupled to the primary inductor; a rectifier diode connected to the secondary inductor such that the rectifier diode turns off when current flows in the secondary inductor in a first direction; and a snubber circuit arranged to charge a first snubber capacitor with the current flowing through the secondary inductor after the rectifier diode turns off; the snubber circuit being arranged to discharge the first snubber capacitor by complementing the current in the secondary inductor after the flow of the current in the secondary inductor is inverted. 
     According to an embodiment of the present disclosure, the secondary inductor comprises an inductor portion mutually coupled to the primary inductor and a leakage inductor in series with said inductor portion. 
     According to an embodiment of the present disclosure, one of the anode and the cathode of the rectifier diode is connected to a first terminal of the secondary inductor, a first terminal of the first snubber capacitor being connected to said first terminal of the secondary inductor; the snubber circuit comprises a first snubber diode connected between a second terminal of the first snubber capacitor and the other of the anode and the cathode of the rectifier diode, the first snubber diode and the rectifier diode being connected in opposition; and the snubber circuit comprises a second snubber diode connected to the second terminal of the first snubber capacitor, the first and second snubber diodes being connected in series. 
     According to an embodiment of the present disclosure, the voltage converter circuit comprises an output filter capacitor connected between first and second output terminals. 
     According to an embodiment of the present disclosure, a second terminal of the secondary inductor is connected to a first terminal of the primary inductor, wherein the first output terminal is connected to the other of the anode and the cathode of the rectifier diode and wherein the second output terminal is connected to a ground of the voltage converter circuit. 
     According to an embodiment of the present disclosure, the first and second snubber diodes in series are connected in parallel with the output filter capacitor. 
     According to an embodiment of the present disclosure, a power source is connected between a second terminal of the primary inductor and said ground, and a switch is connected between the first terminal of the primary inductor and said ground; the snubber circuit comprising a third snubber diode connected in series between the first terminal of the primary inductor and the second snubber diode; and a second snubber capacitor having a first terminal connected between the third and second snubber diodes. 
     According to an embodiment of the present disclosure, a second terminal of the second snubber capacitor is connected to the second output terminal. 
     According to an embodiment of the present disclosure, a second terminal of the second snubber capacitor is connected to the first output terminal. 
     According to an embodiment of the present disclosure, a second terminal of the second snubber capacitor is connected to the first terminal of the primary inductor. 
     According to an embodiment of the present disclosure, the voltage converter circuit comprises an output filter capacitor connected between first and second output terminals, wherein the first output terminal is connected to the other of the anode and the cathode of the rectifier diode and the second output terminal is connected to a second terminal of the secondary inductor. 
     According to an embodiment of the present disclosure, the voltage converter circuit comprises an output filter capacitor connected between first and second output terminals, wherein the first output terminal is connected to the first terminal of the secondary inductor and the second output terminal is connected to the other of the anode and the cathode of the rectifier diode via a charge inductor, a second terminal of the secondary inductor being coupled to said other of the anode and the cathode of the rectifier diode via a transfer capacitor. 
     Embodiments of the present disclosure consist of an electronic component comprising at least the snubber circuit as detailed in the embodiments above. 
     An embodiment of the present disclosure relates to a method of converting voltage comprising: providing a voltage converter circuit having a primary inductor and a secondary inductor, at least a portion of which is mutually coupled to the primary inductor; and a rectifier diode connected to the secondary inductor such that the rectifier turns off when current flows in the secondary inductor in a first direction; providing a first snubber capacitor; charging said first snubber capacitor with the current flowing through the secondary inductor after the rectifier diode turns off; and discharging the first snubber capacitor by complementing the current in the secondary inductor after the flow of the current in the secondary inductor is inverted. 
     According to an embodiment of the present disclosure, the secondary inductor comprises an inductor portion mutually coupled to the primary inductor and a leakage inductance in series with said inductor portion. 
     According to an embodiment of the present disclosure, one of the anode and the cathode of the rectifier diode is connected to a first terminal of the secondary inductor, a first terminal of the first snubber capacitor being connected to said first terminal of the secondary inductor; wherein the snubber circuit comprises a first snubber diode connected between a second terminal of the first snubber capacitor and the other of the anode and the cathode of the rectifier diode, the first snubber diode and the rectifier diode being connected in opposition; and wherein the snubber circuit comprises a second snubber diode connected to the second terminal of the first snubber capacitor, the first and second snubber diodes being connected in series; wherein the current charging said first snubber capacitor flows through the second snubber diode; and wherein the current discharging said second snubber capacitor flows through the first snubber diode. 
     According to an embodiment of the present disclosure, the method comprises turning on the rectifier diode after the first snubber capacitor is discharged. 
     According to an embodiment of the present disclosure, a second terminal of the secondary inductor is connected to a first terminal of the primary inductor, wherein a first output terminal is connected to the other of the anode and the cathode of the rectifier diode and wherein a second output terminal is connected to a ground of the voltage converter circuit; wherein a power source is connected between a second terminal of the primary inductor and said ground, and wherein a switch is connected between the first terminal of the primary inductor and said ground; the snubber circuit comprising a third snubber diode connected in series between the first terminal of the primary inductor and the second snubber diode; and a second snubber capacitor having a first terminal connected between the third and second snubber diodes; the method further comprising: charging the second snubber capacitor with the current that flows in the primary inductor after the switch is turned off; and discharging the second snubber capacitor into the first snubber capacitor through the second snubber diode after the rectifier diode is turned off; said charging said first snubber capacitor with the current flowing through the secondary inductor after the rectifier diode turns off comprising charging the first snubber capacitor through the third and second snubber diodes with the current flowing through the secondary inductor after the first snubber capacitor is discharged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention(s) may be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1(   a ) is a schematic diagram of the structure of an embodiment of a RARA snubber according to the present disclosure. 
         FIG. 1(   b ) is a schematic diagram of the structure of another embodiment of a RARA snubber according to the present disclosure. 
         FIG. 2(   a ) is a schematic diagram of an application of the RARA snubber of  FIG. 1(   a ) to a diode rectifier with a capacitive filter with positive voltage polarity. 
         FIG. 2(   b ) is a schematic diagram of an application of the RARA snubber of  FIG. 1(   a ) to a diode rectifier with a capacitive filter with negative voltage polarity. 
         FIG. 2(   c ) is a schematic diagram of application of the RARA snubber of  FIG. 1(   a ) to a voltage converter having a transformer isolated diode rectifier with capacitive filter. 
         FIG. 2(   d ) is a schematic diagram of application of the RARA snubber of  FIG. 1(   a ) to a voltage converter having a coupled inductor with diode rectifier and capacitive filter. 
         FIG. 2(   e ) is a schematic diagram of application of the RARA snubber of  FIG. 1(   b ) to a coupled inductor boost converter. 
         FIG. 2(   f ) is a schematic diagram of another application of the RARA snubber of  FIG. 1(   b ) to a coupled inductor boost converter. 
         FIG. 2(   g ) is a schematic diagram of another application of the RARA snubber of  FIG. 1(   b ) to a coupled inductor boost converter. 
         FIG. 3(   a ) is a schematic diagram of an application of the RARA snubber of  FIG. 1(   a ) to a Flyback converter. 
         FIG. 3(   b ) is a schematic diagram of an application of the RARA snubber of  FIG. 1(   a ) to an isolated SEPIC converter. 
         FIG. 3(   c ) is a schematic diagram of an application of the RARA snubber of  FIG. 1(   a ) to an isolated Zeta converter. 
         FIG. 3(   d ) is an application of the RARA snubber of  FIG. 1(   a ) to an isolated Cuk converter. 
         FIG. 3(   e ) is an application of the RARA snubber of  FIG. 1(   a ) to a coupled inductor boost converter. 
         FIG. 3(   f ) is an application of the RARA snubber of  FIG. 1(   a ) to a current fed push-pull converter. 
         FIG. 3(   g ) is an application of the RARA snubber of  FIG. 1(   b ) to a coupled inductor boost converter showing also the leakage inductances of the coupled inductor. 
         FIG. 4(   a ) is a schematic diagram of a converter with diode rectifier with capacitive filter employing the RARA snubber of  FIG. 1(   a ). 
         FIG. 4(   b ) is a schematic diagram showing the current path within the converter of  FIG. 4(   a ) towards the zero current turn-off of the rectifier. 
         FIG. 4(   c ) is a schematic diagram showing the current path within the converter of  FIG. 4(   a ) during the snubber charging. 
         FIG. 4(   d ) is a schematic diagram showing the current path within the converter of  FIG. 4(   a ) during the main switch conduction. 
         FIG. 4(   e ) is a schematic diagram showing the current path within the converter of  FIG. 4(   a ) during the rectifier current ramping. 
         FIG. 4(   f ) is a schematic diagram showing the current path within the converter of  FIG. 4(   a ) during the zero voltage turn-on and conduction of the rectifier. 
         FIG. 5(   a ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the zero voltage turn off of the main switch. 
         FIG. 5(   b ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the first phase of the secondary current ramping. 
         FIG. 5(   c ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the second phase of the secondary current ramping. 
         FIG. 5(   d ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the rectifier diode, conduction of the secondary current. 
         FIG. 5(   e ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the zero current turn on of the main switch, and secondary current falling towards zero current turn off of the rectifier diode. 
         FIG. 5(   f ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during a first charging phase of the first snubber capacitor and discharging of the second snubber capacitor. 
         FIG. 5(   g ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during a second charging phase of the first Cs snubber capacitor. 
         FIG. 5(   h ) is a schematic diagram of the coupled inductor boost converter of  FIG. 2(   e ) showing the current path during the main switch conduction. 
         FIG. 6  illustrates a method according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A snubber circuit according to embodiments of the present disclosure can help alleviate the above described problems caused by the secondary leakage inductance in transformer isolated or tapped inductor switching converters (isolated or non isolated coupled inductor converters) and can improve their performance. Henceforth, a snubber according to an embodiment of the present disclosure is referred to as Regenerative and Ramping Acceleration (RARA) Snubber. 
       FIG. 1(   a ) illustrates a snubber circuit, or RARA snubber  10 , according to an embodiment of the present disclosure. RARA snubber  10  comprises a first capacitor  12  having a first terminal provided to be connected to a terminal of a secondary inductor of a voltage converter circuit (not shown), the secondary inductor comprising a leakage inductor  14 . RARA snubber  10  comprises diode elements  16  and  18  connected in series and also connected each to the second terminal of first capacitor  12 . 
     An embodiment of the present disclosure provides for connecting RARA snubber  10  to a voltage converter circuit (not shown in  FIG. 1A ) having a primary inductor and a secondary inductor; at least a portion of the second inductor being mutually coupled to the primary inductor; and having a rectifier diode connected to the secondary inductor such that the rectifier diode turns off when current flows in the secondary inductor in a first direction. According to an embodiment of the present disclosure, leakage inductor  14  is the leakage inductor of the secondary inductor of such voltage converter circuit. According to an embodiment of the present disclosure, inductor  14  does not have to be a physical component. According to an embodiment of the present disclosure inductor  14  can be the secondary inductor itself. 
     According to an embodiment of the present disclosure, RARA snubber  10  is arranged such that first capacitor  12  is charged with the current flowing through the secondary inductor of the converter after the rectifier diode of the converter is turned off; and RARA snubber circuit  10  is arranged to discharge first capacitor  12  by complementing the current in the secondary inductor after the flow of the current in the secondary inductor is inverted. 
       FIG. 1(   b ) shows a RARA snubber  20  according to an embodiment of the present disclosure, comprising a second capacitor  22  having a first terminal connected to the second terminal of first capacitor  12  via diode  18 ; and comprising a third diode element  24  connected in series with diode  18  at the first terminal of second capacitor  22 . 
     According to an embodiment of the present disclosure, the voltage converter (not shown in  FIG. 1(   b )) to which RARA snubber  20  is provided for being connected to, has a primary switch connected to the primary inductor, wherein the free terminal of diode  24  in  FIG. 1(   b ) is connected between the primary switch and the primary inductor. According to an embodiment of the present disclosure, RARA snubber  20  is arranged such that: a/second capacitor  22  is charged with the current that flows in the primary inductor after the primary switch is turned off; b/second snubber capacitor  22  is discharged into the first snubber capacitor  12  via snubber diode  18  after the rectifier diode is turned off; and c/ first snubber capacitor  12  is charged through snubber diodes  24  and  18  with the current flowing through the secondary inductor after the first snubber capacitor  22  is discharged. 
       FIG. 2(   a ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a diode rectifier  30  with a capacitive filter with positive voltage polarity. According to an embodiment of the present disclosure, diode rectifier  30  can form part of a voltage converter (not shown), driven by a coupled inductor or transformers secondary. According to an embodiment of the present disclosure, diode rectifier  30  comprises a rectifier diode  32  and an output filter capacitor  34 . 
       FIG. 2(   b ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a diode rectifier  36  with a capacitive filter with negative voltage polarity. According to an embodiment of the present disclosure, diode rectifier  30  can form part of a voltage converter (not shown), driven by a coupled inductor or transformers secondary. According to an embodiment of the present disclosure, the diode rectifier  36  differs from diode rectifier  30  in that its rectifier diode  32  is inverted with respect to rectifier diode  32  of diode rectifier  30 . 
       FIG. 2(   c ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a rectifier  30  as in  FIG. 2(   a ), in a voltage converter  40  having a transformer isolated diode rectifier with capacitive filter, comprising a transformer  42  in output of which rectifier  30  is formed. According to an embodiment of the present disclosure, inductor  14  is the leakage inductance of the secondary inductor  44  of transformer  42 , wherein inductance  46  is the inductance of the primary inductor  48  of transformer  42 , and inductance  50  the leakage inductance of the primary inductor  48  of transformer  42 . 
       FIG. 2(   d ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a rectifier  30  as in  FIG. 2(   a ), in a voltage converter having a coupled inductor with diode rectifier and capacitive filter, comprising a coupled inductors connected in series  52 , in output of which rectifier  30  is formed. According to an embodiment of the present disclosure, inductor  14  is the leakage inductance of the secondary inductor  54  of the coupled inductors  52 , wherein inductance  56  is the inductance of the primary inductor  58  of coupled inductors  52 , and inductance  60  the leakage inductance of the primary inductor  58  of coupled inductors  52 . 
       FIG. 2(   e ) is a schematic diagram of application of the RARA snubber  20  of  FIG. 1(   b ) to a non-isolated coupled inductor converter, in particular a coupled inductor boost converter  70 . Boost converter  70  comprises coupled inductors  72  having a secondary inductor output terminal connected to the anode of a rectifier diode  74 , the cathode of diode  74  being connected to a first output terminal  76 . A primary inductor of coupled inductors  72 , coupled in series with the secondary inductor, has an input terminal connected to a power source  78 , the power source being connected to a ground of the circuit, itself connected to a second output terminal  80 . A switch or power switch  82 , such as a power transistor or transistor, connects the output terminal of the primary inductor to the ground and a filter capacitor  84  is connected between first and second output terminals  76 ,  80 . A load  86  is represented connected to first and second output terminals  76 ,  80 . 
     According to an embodiment of the present disclosure, a first terminal of the first snubber capacitor  12  is connected to the anode of rectifier diode  74 ; first snubber diode  16  is connected between the second terminal of the first snubber capacitor  12  and the cathode of rectifier diode  74 , first snubber diode  16  and rectifier diode  74  being connected in opposition; and second snubber diode  18  is connected to the second terminal of first snubber capacitor  12 , first and second snubber diodes  16 ,  18  being connected in series. According to an embodiment of the present disclosure, third snubber diode  24  is connected in series between the output terminal of the primary inductor and second snubber diode  18 ; and second snubber capacitor  22  has a first terminal connected between the third and second snubber diodes  24 ,  18 . According to an embodiment of the present disclosure, a second terminal of second snubber capacitor  22  is connected to the ground. 
       FIG. 2(   f ) is a schematic diagram of another application of the RARA snubber  20  of  FIG. 1(   b ) to a coupled inductor boost converter  90 , which differs from the boost converter  70  of  FIG. 2(   e ) in that the second terminal of second snubber capacitor  22  is connected between the input of the primary and the power supply instead of being connected to the ground. 
       FIG. 2(   g ) is a schematic diagram of another application of the RARA snubber  20  of  FIG. 1(   b ) to a coupled inductor boost converter  92 , which differs from the boost converter  70  of  FIG. 2(   e ) in that the second terminal of second snubber capacitor  22  is connected to the cathode of rectifier diode  74  instead of being connected to the ground. 
     According to an embodiment of the present disclosure, RARA snubber  10  or  20  can limit voltage ringing across the rectifier, limit the reverse recovery current of the rectifier diode, provide lossless zero voltage turn-on and lossless zero current turn-off switching conditions for the rectifier, accelerate the secondary winding current build-up, recycle the absorbed energy and/or improve the overall converter&#39;s efficiency. 
     In addition to the above mentioned features RARA snubber  20  can also provide lossless zero voltage turn off of the power switch, lossless zero current turn on of the power switch, capturing and recycling of the primary leakage energy, controlled voltage rate of rise and peak voltage across the switch. 
     According to an embodiment of the present disclosure, RARA snubber  10  can be employed on the secondary winding of an isolating transformer in, for example, the Flyback, SEPIC, ZETA, Cuk, tapped inductor topologies, and current fed push-pull converters, as shown hereafter. The application of the disclosure is not limited to these topologies/converters as it can be employed in other topologies/converters with multi-winding magnetic devices as well. Also, in the given examples shown herein, it is understood that the leakage inductance of the transformer or tapped inductor may be utilized as the snubber inductance, Ls, similarly to the described above and as illustrated for example in  FIG. 2  ( c ) and in  FIG. 2  ( d ). 
       FIG. 3(   a ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a rectifier  30  as in  FIG. 2(   a ), in a Flyback converter  94 . Flyback converter  94  comprises a transformer  96  having a primary inductor and a secondary inductor, at least a portion of the second inductor being mutually coupled to the primary inductor. Rectifier  30  is arranged such that the anode of rectifier diode  32  is connected to a first terminal of the secondary inductor; a first terminal of snubber capacitor  12  is connected to the first terminal of the secondary inductor; first snubber diode  16  is connected between a second terminal of snubber capacitor  12  and the cathode of rectifier diode  32 , first snubber diode  16  and rectifier diode  32  being connected in opposition. According to an embodiment of the present disclosure, second snubber diode  18  is connected to the second terminal of snubber capacitor  12 , the first and second snubber diodes  16 ,  18  being connected in series; output filter capacitor  34  is connected between first and second output terminals of converter  94 , wherein the first output terminal is connected to the cathode of rectifier diode  34  and the second output terminal is connected to the second terminal of the secondary inductor of transformer  96 . In  FIG. 3(   a ), a load  98  is connected between the first and second output terminals of converter  94 . According to an embodiment of the present disclosure, the second output terminal of converter  98  is connected to a ground. According to an embodiment of the present disclosure, the primary inductor of transformer  96  has an input terminal connected to a power supply  100  and the primary inductor of transformer  96  has an output terminal connected to a ground via a switch or power switch  102 . According to an embodiment of the present disclosure, a snubber circuit  104  is connected between said ground and the input and output terminals of the primary inductor to protect the primary of converter  98 . 
       FIG. 3(   b ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to a rectifier  30  as in  FIG. 2(   a ), in a SEPIC converter  106  that differs from Flyback converter  94  in that the input of the primary inductor is connected to the power supply  100  by a LC circuit and the output of the primary inductor is connected directly to the ground; the LC circuit comprising an inductor  108  connected between the power supply  100  and a middle point and a capacitor  110  connected between the middle point and the input of the primary inductor; the switch  102  being connected between the middle point and the ground and the snubber circuit  104  having one terminal coupled to the ground and two terminals coupled to each side of inductor  108 . 
       FIG. 3(   c ) is a schematic diagram of an application of the RARA snubber  10  of  FIG. 1(   a ) to an isolated Zeta converter  112 . According to an embodiment of the present disclosure, Zeta converter  112  comprises a transformer  96  having a primary inductor and a secondary inductor, at least a portion of the second inductor being mutually coupled to the primary inductor. The anode of a rectifier diode  114  is connected to a first terminal of the secondary inductor; a first terminal of snubber capacitor  12  is connected to the first terminal of the secondary inductor; first snubber diode  16  is connected between a second terminal of snubber capacitor  12  and the cathode of rectifier diode  32 , first snubber diode  16  and rectifier diode  32  being connected in opposition. According to an embodiment of the present disclosure, second snubber diode  18  is connected to the second terminal of snubber capacitor  12 , the first and second snubber diodes  16 ,  18  being connected in series; an output filter capacitor  116  is connected between first and second output terminals, wherein the first output terminal is connected to the first terminal of the secondary inductor and the second output terminal is connected to the cathode of the rectifier diode  114  via a charge inductor  118 ; a second terminal of the secondary inductor being coupled to the cathode of the rectifier diode via a transfer capacitor  120 . In  FIG. 3(   c ), a load  122  is connected between the first and second output terminals of converter  112 . According to an embodiment of the present disclosure, the first output terminal of converter  112  is connected to a ground. According to an embodiment of the present disclosure, the primary inductor of transformer  96  has an input terminal connected to a power supply  100  and the primary inductor of transformer  96  has an output terminal connected to a ground via a switch or power switch  102 . According to an embodiment of the present disclosure, a snubber circuit  104  is connected between said ground and the input and output terminals of the primary inductor to protect the primary of converter  98 . 
       FIG. 3(   d ) is an application of the RARA snubber of  FIG. 1(   a ) to an isolated Cuk converter that differs from Zeta converter  112  in that the input of the primary inductor is connected to the power supply  100  by a LC circuit and the output of the primary inductor is connected directly to the ground; the LC circuit comprising an inductor  108  connected between the power supply  100  and a middle point and a capacitor  110  connected between the middle point and the input of the primary inductor; the switch  102  being connected between the middle point and the ground and the snubber circuit  104  having one terminal coupled to the ground and two terminals coupled to each side of inductor  108 . 
       FIG. 3(   e ) is an application of the RARA snubber  10  of  FIG. 1(   a ) to a coupled inductor boost converter  126  as shown in  FIG. 2(   d ). According to an embodiment of the present disclosure, the primary inductor of coupled inductors  52  has an input terminal connected to a power supply  100  and the primary inductor of coupled inductors  52  has an output terminal connected to a ground via a switch or power switch  102 . According to an embodiment of the present disclosure, a snubber circuit  104  is connected between said ground and the input and output terminals of the primary inductor to protect the primary of converter  126 . According to an embodiment of the present disclosure, a load  128  is connected in output of converter  126  to the terminals of capacitor  34 . 
       FIG. 3(   f ) is an application of the RARA snubber of  FIG. 1(   a ) to a current fed push-pull converter  130 , comprising essentially two voltage converters  40  as in  FIG. 2(   c ) sharing a single output filter capacitor  34 , wherein the transformers  42  of the two voltage converters share a common magnetic core. According to an embodiment of the present disclosure, a power supply  100  is connected between a ground and a supply node, the supply node being connected to an input terminal of the primary inductor of each of the transformers  42  via a snubber circuit  104 . According to an embodiment of the present disclosure, the input terminal of the primary inductor of each of the transformers  42  is connected to the ground via a switch  102 . According to an embodiment of the present disclosure, the output terminals of the primary inductor of each of the transformers  42  are connected to a common point, connected to the supply node via an inductor  132 . 
       FIG. 3(   g ) is the coupled inductor boost converter  70  of  FIG. 2(   e ), showing the leakage inductances of the coupled inductors  72  consistently with the coupled inductors  52  of  FIG. 2(   d ). 
     The operation of an embodiment of the present disclosure will now be described in relation with  FIGS. 4(   a ) to  4 ( f ). Application of a RARA snubber circuit  10  as shown in  FIG. 1(   a ), to a generalized Switching Network  140  having a transformer isolated diode rectifier with capacitive filter  30  such as illustrated in  FIG. 2(   a ), connected to a transformer  42  such as illustrated in  FIG. 2(   c ), is illustrated in  FIG. 4(   a ). In this discussion the details of the switching network  140  on the transformer&#39;s primary  142  are omitted. Switching network  140  may have diverse practical implementations, for example as shown in  FIGS. 3(   a ) and  3 ( b ), and therefore is expected to introduce some variations in the sequence of events in the RARA snubber circuit, however, a principle of the operation according to an embodiment of the disclosure is as described below. 
     In the example illustrated, it is assumed that the switch in the primary, such as switch  102  in  FIGS. 3(   a ) and  3 ( b ), is controlled by a high frequency switching signal. At the start of the switching cycle the snubber inductor  14  and the rectifier diode  32  conduct a positive secondary current to the output filter capacitor,  34 , as illustrated in  FIG. 4(   b ). At the instant when the switching network  140  imposes a voltage across transformer&#39;s primary that causes the voltage of the transformer&#39;s secondary to change polarity, the current through inductor  14  and rectifier diode  32  starts ramping down. The snubber inductance  14  can limit the rate of fall of the rectifier diode  32  current. As the current through the rectifier diode  32  falls to zero, zero-current turn-off of the rectifier diode  32  is accomplished. 
     Upon the rectifier diode  32  cut off, the secondary winding voltage V 2  via the diode  18 , starts charging the snubber capacitor  12  through resonant action with the inductance  14  as shown in  FIG. 4  ( c ). As the resonant current through capacitor  12  decays to zero, diode  18  turns off at zero current. 
     After the current ceases, the snubber capacitor  12  remains charged and stores a certain voltage as illustrated in  FIG. 4(   d ), until the switching network  140  initiates a change in the polarity of the transformer&#39;s primary voltage. 
     When, due to action of the switching network  140 , the primary voltage, V 1 , changes polarity, as illustrated in  FIG. 4  ( e ), the secondary voltage, V 2 , also changes polarity. The stored snubber capacitor voltage then adds to the secondary winding voltage, V 2 , and develops a resonant current pulse through capacitor  12 , inductor  14  and diode  16  into the output filter capacitor  34 , as illustrated in  FIG. 4(   e ). According to an embodiment of the present disclosure this resonant discharge of capacitor  12  can help to rapidly ramp up the secondary winding current and results in fast current switch-over from the primary to the secondary winding. 
     According to an embodiment of the present disclosure, since the switching network  140  can typically include snubbers, fast current switch-over from the primary winding to the secondary winding can reduce energy transfer to the primary snubbers of the switching network  140 . The reduced energy circulation in the primary snubbers of the switching network can lower the peak voltage across the switches of switching network as well as improve the switching network efficiency. 
     As the voltage across the snubber capacitance  12 , is discharged to zero, zero voltage turn-on condition is provided for the rectifier diode  32  turn-on, as illustrated in  FIG. 4(   f ). 
     Whereas diode  16  is turned off at zero current, conduction interval of the rectifier diode  32  can continue until the switching network  140  repeats its switching cycle. 
     The operation of an embodiment of the present disclosure will now be described in relation with  FIGS. 5(   a ) to  5 ( h ). Application a RARA snubber circuit  20  as shown in  FIG. 1(   b ) according to an embodiment of the present disclosure to a coupled inductor boost converter  70  such as illustrated in  FIG. 2(   e ), is illustrated in  FIG. 5(   a ). 
     According to an embodiment of the present disclosure, switch  82  is controlled by a high frequency switching signal. Upon turn off of the switch  82 , as illustrated in  FIG. 5(   a ), a primary current continues to flow out of a central tap of the coupled inductor  72  via the snubber diode  24  into second snubber capacitor  22 . Since, at this state, second snubber capacitor  22  is typically totally discharged and voltage across it is zero, lossless zero voltage turn off of the switch  82  is accomplished. Furthermore, the voltage rise across the switch  82  is limited by the rate of charge of second snubber capacitor  22 , as is the switch peak voltage. 
     According to an embodiment of the present disclosure, certain instant voltage of the central tap of the coupled inductor  72  can become sufficiently high to forward bias the snubber diode  16 , via positively charged snubber capacitor  12 , as illustrated in  FIG. 5(   b ). At this instant secondary current commences to flow. The high voltage of snubber capacitor  12  adds to the voltage across the secondary. As a result, the higher voltage across the secondary leakage significantly speeds up the rising of the secondary current. 
     According to an embodiment of the present disclosure, when all or almost all of the energy of the primary leakage inductance is captured by second snubber capacitor  22 , the central tap current ceases, as illustrated in  FIG. 5(   c ), whereas the secondary current continues flowing through capacitor  12  and diode  16  to the output filter capacitor  84  and load  86 R. 
     According to an embodiment of the present disclosure, when the secondary current discharges snubber capacitor  12  and voltage across it falls to zero or near zero, the power diode, or rectifier diode,  74 , turns on at zero or near-zero voltage as illustrated in  FIG. 5(   d ). Diode  74  then starts carrying the secondary current and allows the coupled inductor to discharge its energy to the output filter capacitor  84  and the load  86 . 
     According to an embodiment of the present disclosure, when the switch  82  is turned on as illustrated in  FIG. 5(   e ), the turn-on occurs at lossless zero current condition. From this moment or instant the coupled inductor primary current starts ramping up, whereas the secondary current starts ramping down. When the secondary current falls to zero, the power diode  74  is turned off at lossless zero or near zero current conditions. 
     According to an embodiment of the present disclosure, after the rectifier diode  74  turns off, the secondary current flows through the switch  82  and snubber diode  18 , so that snubber capacitor  12  is charged, whereas snubber capacitor  22  is discharged, as illustrated in  FIG. 5(   f ). 
     According to an embodiment of the present disclosure, the charge stored by capacitor  22  is removed and transferred to capacitor  12 . Hence, the leakage energy captured earlier by snubber capacitor  22  is recycled. 
     According to an embodiment of the present disclosure, upon total or nearly total discharge of snubber capacitor  22 , the secondary current flows through diodes  24  and  18 , as illustrated in  FIG. 5(   g ), and by resonance with the secondary leakage inductance, snubber capacitor  12  continues to pre-charge to its maximum voltage. 
     According to an embodiment of the present disclosure, then, the switch  82  remains in the on state and continues charging the coupled inductor primary, as illustrated in  FIG. 5(   h ), until the controller commands it to off. Henceforth, the described above cycle of events can then repeat. 
       FIG. 6  illustrates a method according to the present disclosure, comprising providing a voltage converter circuit having a primary inductor and a secondary inductor, at least a portion of which is mutually coupled to the primary inductor; and a rectifier diode connected to the secondary inductor such that the rectifier turns off when current flows in the secondary inductor in a first direction, such as illustrated in  FIGS. 1-5 . 
     According to an embodiment of the present disclosure, the method further comprises providing a first snubber capacitor such as capacitor  12  as illustrated in  FIGS. 1-5 ; charging said first snubber capacitor with the current flowing through the secondary inductor after the rectifier diode turns off; and discharging the first snubber capacitor by complementing the current in the secondary inductor after the flow of the current in the secondary inductor is inverted. 
     According to an embodiment of the present disclosure, the topology of a Regenerative Snubber with Fast Output Current Ramping for Isolated Step-up Converters can, for example, recycle the absorbed energy, facilitate lossless switching conditions, and limit the switch voltage stress. Some benefits of a snubber circuit according to an embodiment of the present disclosure include, but are not limited to, a reduced switch voltage stress and higher efficiency. For example only, preliminary experiments showed that when fitted with a snubber circuit according to an embodiment of the present disclosure, the efficiency of a flyback converter can exceed 90%. 
     The circuits and methods according to embodiments of the present disclosure can be used to increase the efficiency of transformer isolated DC-DC power processing units. The circuits and methods according to embodiments of the present disclosure can be used in a wide range of commercial, industrial and military applications, and include, but are not limited to, applications which require generation of high DC voltage from low DC voltage source or vice versa. Circuits according to embodiments of the present disclosure can include, but are not limited to, for example, power processors for solar power generation, high voltage laser chargers, copiers and flashlights. 
     While inductors, capacitors, diodes and resistors are discussed, these may be substituted with one or more circuit elements having similar or equivalent features and/or characteristics. For example only, any inductor disclosed herein may be substituted with any inductive element that exhibits inductive characteristics, capacitors may be substituted with any capacitive element that exhibits capacitive characteristics, diodes may be substituted with any a diode element that exhibits diode characteristics, and resistors may be substituted with any resistive element that exhibits resistive characteristics. For example only, any of the circuit elements disclosed herein may be implemented by transistors or other elements. 
     The foregoing description of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. 
     It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather means “one or more.” Moreover, no element, component, nor method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the following claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . . ” 
     It should be understood that the figures illustrated in the attachments, which highlight the functionality and advantages of the present invention, are presented for example purposes only. The architecture of the present invention is sufficiently flexible and configurable, such that it may be utilized (and navigated) in ways other than that shown in the accompanying figures. 
     Furthermore, the purpose of the foregoing Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is not intended to be limiting as to the scope of the present invention in any way. It is also to be understood that the steps and processes recited in the claims need not be performed in the order presented. 
     Also, it is noted that the embodiments may be described as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
     The various features of the invention described herein can be implemented in different systems without departing from the invention. It should be noted that the foregoing embodiments are merely examples and are not to be construed as limiting the invention. The description of the embodiments is intended to be illustrative, and not to limit the scope of the claims. As such, the present teachings can be readily applied to other types of apparatuses and many alternatives, modifications, and variations will be apparent to those skilled in the art