Abstract:
Several inversion circuits used to convert a DC input to an AC output comprise two series circuits, at least one clamp capacitor, and at least one transformer. Each of the series circuits is in parallel with the DC input. The first series circuit includes one switch network and at least one transformer primary. The second series circuit includes one voltage-clamp network and at least one transformer primary. At least one clamp capacitor couples the first and the second series circuits, and is attached to each series circuit at a node between the respective transformer primary winding. The voltage-clamp network may be implemented with two of the three sub-circuits connected in series: a diode, a resister-capacitor-diode, and a MOSFET-capacitor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention is related to the field of power converter, and more specifically, to a voltage-clamp method for DC/DC power converters. 
         [0003]    2. Description of Related Art 
         [0004]    Achieving a higher power density is an endless goal of modern power converter engineers for the crucial applications wherein the allocated space of the power converter is limited. In addition to being highly compact, the power converter has to be able to minimize the power dissipation. 
         [0005]    In low-to-medium level power conversion applications, single-ended power converter topology, such as a single-switch forward converter or a single-switch flyback converter, is widely used. It includes an isolation transformer, a switch on a primary side of the transformer, a rectifier and an output filter on a secondary side of the transformer. By way of the on/off control of the power switch, an AC voltage is generated in the transformer primary from input DC voltage and converted to another value in the transformer secondary. After being rectified and filtered, DC output power with different voltage/current combinations can be obtained. 
         [0006]    An issue of concern regarding aforementioned converters is that a magnetizing and the leakage energies stored in the transformer must be taken into consideration during the design of the converter. Otherwise, these magnetic energies stored in the transformer may cause the failure of the converter. 
         [0007]    Another issue of concern regarding aforementioned converters is to alleviate the electromagnetic interference EMI problems. Part of the EMI problems is caused by the pulsating current ripples, di/dt, in the power converters. Also, the lower the pulsating current ripples, the lower the RMS value of the current. As a result, conduction losses can be reduced to improve the efficiency. Therefore, a power converter with a low input current ripple becomes one of the design criteria of concern. 
         [0008]    To achieve a low current ripple as well as to recycle the transformer&#39;s magnetizing and leakage energies, several power converters have been proposed in the literatures and become the prior art of the present invention. 
         [0009]    One of which shown in  FIG. 1  is the power converter proposed for low power level applications in “Design Tricks, Techniques and Tribulation at High Conversion Frequencies,” Bruce Carsten, HFPC 1987, pp. 139-152 and is also described in “Snubber Circuits: Theory, Design and Application,” Philip C. Todd, TI seminar 900. Topic 2, 1993. Recently, its input current ripple reduction property has been explored by the inventor of the present invention in “Improved Forward Topologies for DC-DC Applications with Built-in Input Filter,” Ph.D. dissertation, Virginia Polytechnic &amp; State University, Blacksburg, Va., U.S.A, 2006. 
         [0010]    However, this circuit contains a single switch which is selected to withstand twice the input voltage. In some applications, ample voltage-rating semiconductor switches may be available at the cost of increasing the conduction losses due to the higher voltage-rating semiconductor switch accompanied with a higher R DSon . On the contrary, voltage stress may be too high for available semiconductor switches in many other applications. 
         [0011]    By series-connecting two semiconductor switches, the voltage stress on each device can be reduced. Using low-voltage rating semiconductor switch, the equivalent R DS(ON)  is reduced. As a result, the conduction losses can be significantly reduced and improve the converter&#39;s efficiency. As shown in  FIG. 2 , an invented power converter has been filed with the U.S. patent Ser. No. 11/812,339 application number on Jun. 18, 2007 by the inventor of the present invention, which is incorporated herein by reference. Each of the two series-connected semiconductor switches has been designed to accommodate rated for approximately the input voltage. 
         [0012]    To further reduce the input/output current ripple by means of the ripple cancellation mechanism, another one of which is shown in  FIG. 3 . It was invented in U.S. Pat. No. 5,523,936, issued on Jun. 4, 1996, to the inventor of the present invention. 
         [0013]    Again, to take the advantage of reducing the voltage stress, the circuit diagram of its two-switch version is shown in  FIG. 4 . It has been filed with the U.S. patent Ser. No. 11/812,339 application number on Jun. 18, 2007, by the inventor of the present invention. 
         [0014]    Because the transformer reset voltage of the aforementioned power converters is equal to the input voltage, a maximum duty cycle is limited to 50%. The turns ratio of the transformer is thus restricted to a smaller value resulting in accompanying with a higher RMS input current and higher rectifier&#39;s voltage stress. Consequently, the conduction losses are increased. 
         [0015]    Accordingly, those skilled in the art understand that one of the effects of increasing the duty cycle of the power switch is that an overall efficiency of the power converter can be increased. 
         [0016]    A system and method is thus needed to maximize the converter&#39;s efficiency by means of recovering the magnetic energies, decreasing the current ripple, reducing voltage stress, and allowing above 50% duty cycle operation. 
       SUMMARY OF THE INVENTION 
       [0017]    Accordingly, an object of the present invention is to provide inversion circuits having reduced input current ripple thereby to alleviate the EMI problems and to improve the converter&#39;s efficiency. 
         [0018]    A further object of the present invention is to provide inversion circuits employing clamped capacitor to recycle the magnetic energies thereby to improve the converter&#39;s efficiency. 
         [0019]    A further object of the present invention is to provide inversion circuits using low voltage-rating semiconductor switch thereby to improve the converter&#39;s efficiency. 
         [0020]    A further object of the present invention is to provide inversion circuits surpassing 50% duty cycle thereby to improve the converter&#39;s efficiency. 
         [0021]    The present invention therefore introduces the broad concept of resetting a transformer by transferring energy to reset windings via at least two capacitors of the power converter circuit. In one embodiment of the present invention, a power converter comprises two series circuits, one capacitor, and one transformer. The transformer has at least two identical primary windings and at least one secondary winding. Both series circuits are connected in parallel with the DC input source Vi. The first series circuit includes the first transformer primary winding and one switch network; while the second series circuit includes the voltage-clamp network and the secondary transformer primary winding. The switch network comprises at least one semiconductor switch and the voltage-clamp network comprises at least one active or one passive voltage-clamp cell. The active voltage-clamped cell is formed by a MOSFET series-connected with a capacitor (MOSFET-Capacitor) while the passive voltage-clamp cell is formed by a diode or a resistor parallel-connected to a capacitor with series-connecting to a diode. The capacitor is used to couple the first and the second series circuits by connecting a first node and a second node, wherein the first node is a node between the switch network and the first transformer primary, and the second node is a node between the voltage-clamp network and the second transformer primary. One driver signal is issued by the gate drive to turn on/off the semiconductor switch within the switch network. Consequently, an AC voltage is thus generated in the transformer secondary winding. After being rectified and filtered (not shown), the output of the power converter provides an output voltage Vo to a load. 
         [0022]    The capacitor voltage and the voltage across the voltage-clamp network are summed together to be the transformer reset voltage. Because the voltages across the two transformer primary windings are canceled each other due to their opposite-parity, the capacitor voltage is the same level as the input voltage. Thus, the reset voltage is higher than the input voltage and the maximum duty cycle of the power switch can be exceeded 50%. Those skilled in the art understand that one of the effects of increasing the duty cycle of the power switch is that an overall efficiency of the power converter can be increased. 
         [0023]    To accomplish desired function, two series-connected semiconductor switches may be substituted for the switch network and two series-connected active and/or passive cells may be substituted for the voltage-clamp network. Moreover, the center nodes between two active and/or passive cells and two series-connected semiconductor switches are connected together to provide an individual clamped voltage on each of the two series-connected semiconductor switches. In addition, two driver signals are issued by the gate drive to turn on/off the two semiconductor switches within the switch network simultaneously. Also, at least one complementary signal issued by the gate drive is necessarily provided to drive the semiconductor switch within the voltage-clamp network. Moreover, two capacitors and/or two transformers may be used instead of using a single capacitor and/or a single transformer, respectively. 
         [0024]    Several embodiments of the present invention can thus be obtained. However, in one embodiment of the present invention, the voltage-clamp network formed by one single diode or multiple diodes is not necessary to the present invention. 
         [0025]    In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, several preferred embodiments accompanied with figures are described in detail below. 
         [0026]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0028]      FIG. 1 ,  FIG. 2 ,  FIG. 3 , and  FIG. 4  are the circuit diagrams of the power converter as prior art of the present invention. 
           [0029]      FIG. 5  is the first circuit diagram of the present invention. 
           [0030]      FIG. 5A  and  FIG. 5J  illustrate ten embodiments of the power converter accordance with the present invention. 
           [0031]      FIG. 6  is the second circuit diagram of the present invention. 
           [0032]      FIG. 6A  and  FIG. 6J  illustrate another ten embodiments of the power converter accordance with the present invention. 
           [0033]      FIG. 7  is the third circuit diagram of the present invention. 
           [0034]      FIG. 7A  and  FIG. 7J  illustrate another ten embodiments of the power converter accordance with the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    As illustrated in  FIG. 5  is a circuit diagram of the power converter  100  to introduce the broad concept of resetting a transformer by transferring energy to reset winding via the capacitors of the present invention. The circuit used to convert a DC input to an AC output comprises two series circuits, one capacitor C 1 , and one transformer T 1 . The transformer T 1  has two identical primary windings Lp 1  and Lp 3  and at least one secondary winding Ls. Both series circuits are connected in parallel with the DC input source Vi. The first series circuit comprises the first transformer primary winding Lp 1  and one switch network  120 ; while the second series circuit comprises the voltage-clamp network  110  and the secondary transformer primary winding Lp 3 . The switch network  120  comprises at least one semiconductor switch and the voltage-clamp network  110  comprises at least one active or one passive voltage-clamp cell. The active voltage-clamp cell is formed by a MOSFET Sc series-connected with a capacitor Cc (MOSFET-Capacitor) while the passive voltage-clamp cell is formed by a resistor Rc parallel-connected to a capacitor Cc with series-connecting to a diode Dc. The capacitor C 1  is used to couple the first and the second series circuits by connecting a first node N 1  and a second node N 2 , wherein the first node N 1  is a node between the switch network  120  and the first transformer primary Lp 1 , and the second node N 2  is a node between the voltage-clamp network  110  and the second transformer primary Lp 3 . Because the voltages across the first and second transformer primary windings are cancelled each other, the capacitor C 1  voltage level is equal to the input voltage Vi. At least one driver signal  131  is issued by the gate drive  130  to turn on or turn off the semiconductor switch within the switch network  120 . Consequently, an AC voltage is generated in the secondary winding Ls. After being rectified and filtered (not shown), the output of the power converter provides an output voltage Vo to a load. 
         [0036]    The power converter  100  operates as follows. During a first interval, at least one gate drive signal  131  is issued to turn on the semiconductor switch within the switch network  120 . In addition to the input voltage Vi applied to the primary winding Lp 1 , the capacitor voltage V C1  is also applied to the second winding Lp 3 . A magnetizing current associated with the transformer T 1  increases linearly. Then, during a complementary interval, the gate drive signal  131  turns off the semiconductor switch within the switch network  120 . The energy stored in the leakage inductance of the transformer T 1  is absorbed by the capacitor C 1  and the capacitor Cc within the voltage-clamp network  110 . Therefore, the voltage across the switch network  120  has no voltage spike and is limited to the sum of the three voltages provided by the capacitor C 1 , the capacitor Cc within the voltage-clamp network  100 , and the input voltage Vi. The magnetizing and leakage energies are then recovered to the input via the second winding Lp 3  and the voltage-clamp network  110 , thereby resetting the transformer T 1 . 
         [0037]    The transformer reset voltage is equal to the sum of the voltages across the capacitor C 1  and the capacitor Cc within the voltage-clamp network  100 . Because the voltage across the capacitor C 1  is clamped to input voltage Vi, the reset voltage is higher than the input voltage. The duty cycle of the semiconductor switch within the switch network  120 , therefore, can be above 50%. 
         [0038]    Obviously, a higher than 50% operating duty cycle results in increasing transformer turns ratio accompanied with a low primary current and lower voltage stresses on the secondary rectifiers. Consequently, further improvements of the power converter&#39;s efficiency can be achieved. 
         [0039]    Turning now to  FIG. 5A  and  FIG. 5B  are two embodiments of power converter constructed according to the foregoing principles of the present invention. The voltage-clamp network  110 A is a passive voltage-clamp cell formed by a R C -C C -D C  sub-circuit, as shown in  FIG. 5A , and the voltage-clamp network  110 B is an active voltage-clamp cell formed by a Sc-Cc sub-circuit, as shown in  FIG. 5B , respectively. One complementary signal  132  issued by the gate drive  130  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  110 B. 
         [0040]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 5C  and  FIG. 5D . The voltage-clamp networks  110 C and  110 D comprise two series-connected passive voltage-clamp cells formed by the combination of a diode D a  and a R C -C C -D C  sub-circuits. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  120  is clamped to Vi or Vi+V CC . 
         [0041]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 5E . The voltage-clamp network  110 E comprises two series-connected passive voltage-clamp cells formed by a R C -C C -D C  sub-circuit and a Ra-Ca-Da sub-circuit. The voltage across the switch S 1  or S 2  within the switch network  120  is clamped to Vi+V Ca  and Vi+V CC , respectively. 
         [0042]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 5F  and  FIG. 5G . The voltage-clamp networks  110 F and  110 G comprise two series-connected voltage-clamp cells formed by the combination of a diode Da and a MOSFET-Capacitor (Sc-Cc) sub-circuit. One complementary signal  132  issued by the gate drive  130  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  110 F or  110 G. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  120  is clamped to Vi or Vi+V CC . 
         [0043]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 5H  and  FIG. 5I . The voltage-clamp networks  110 H and  110 I comprise two series-connected voltage-clamp cells formed by the combination of a R C -C C -D C  sub-circuit and a Sc-Cc sub-circuit. One complementary signal  132  issued by the gate drive  130  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  110 H or  110 I. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  120  is clamped to Vi+V CC  or Vi+V Ca . 
         [0044]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention is shown in  FIG. 5J . The voltage-clamp networks  110 J comprise two series-connected voltage-clamp cells formed by a Sc-Cc sub-circuit and a Sa-Ca sub-circuit. Two complementary signals  132  issued by the gate drive  130  are necessarily provided to drive the semiconductor switch Sc and Sa within the voltage-clamp network  110 J. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  120  is clamped to Vi+V CC  or Vi+V Ca . 
         [0045]    As illustrated in  FIG. 6  is another circuit diagram of the power converter  200  to introduce the broad concept of resetting a transformer by transferring energy to reset winding via the capacitors as well as to further reduce the current ripple of the present invention. The circuit used to convert a DC input to an AC output comprises two series circuits, two capacitors (C 1  and C 2 ), and one transformer T 1 . The input inductor, L in  represented the parasitic inductor or an external inductor, as designed, is inserted between the DC input Vi and the two series circuits. The transformer T 1  has four identical primary windings Lp 1 , Lp 2 , Lp 3  and Lp 4  and has at least one secondary winding Ls. Both series circuits are connected in parallel with the DC input source Vi. The first series circuit comprises the first and the second transformer primaries Lp 1  and Lp 2  and one switch network  220 . The second series circuit comprises a voltage-clamp network  210  and the third and the fourth transformer primaries Lp 3  and Lp 4 . The switch network  220  comprises at least one semiconductor switch and the voltage-clamp network  210  comprises at least one active or one passive voltage-clamp cell. The active voltage-clamp cell is formed by a MOSFET Sc series-connected with a capacitor Cc while the passive voltage-clamp cell is formed by a diode Da or a resistor Rc parallel-connected to a capacitor Cc with series-connecting to a diode Dc. The first capacitor C 1  is used to couple the first and the second series circuits by connecting a first node N 1  and a second node N 1 , wherein the first node N 1  is a node between the switch network  220  and the first transformer primary Lp 1 , and the second node N 2  is a node between the voltage-clamp network  210  and the fourth transformer primary Lp 4 . The second capacitor C 2  is used to couple the first and the second series circuits by connecting a third node N 3  and a fourth node N 4 , wherein the third node N 3  is a node between the switch network  220  and the second transformer primary Lp 2 , and the fourth node N 4  is a node between the voltage-clamp network  210  and the third transformer primary Lp 3 . Because the voltages across the transformer primary windings Lp 1  and Lp 3  (Lp 2  and Lp 3 ) are cancelled each other, each capacitor voltage level is equal to the input voltage. At least one driver signal  231  is issued by the gate drive  230  to turn on or turn off the at least one semiconductor switch within the switch network  220 . Consequently, an AC voltage is generated in the secondary winding Ls. After being rectified and filtered (not shown), the output of the power converter provides an output voltage Vo to a load. 
         [0046]    The power converter  200  operates as follows. During a first interval, a gate drive signal  231  is issued to turn on the semiconductor switch within the switch network  220 . In addition to the input voltage Vi applied to the primary windings Lp 1 -Lp 2 , each capacitor voltage is also applied to its individual pair of primary winding Lp 2 -Lp 4  or Lp 1 -Lp 3 , respectively. A magnetizing current associated with the transformer T 1  increases linearly. Then, during a complementary interval, the gate drive signal  231  turns off the semiconductor switch within the switch network  220 . The energy stored in the leakage inductance of the transformer T 1  is absorbed by the capacitors C 1  and C 2  as well as the capacitor within the voltage-clamp network  210 . Therefore, the voltage across the switch network  220  has no voltage spike and limited to the sum of the three voltages provided by the capacitor C 1 , the capacitor C 2 , and the capacitor within the voltage-clamp network  200 . The magnetizing and leakage energies are then recovered to the input via the third primary winding Lp 3 , the fourth primary windings Lp 4 , and the voltage-clamp network  210 , thereby resetting the transformer T 1 . 
         [0047]    The transformer reset voltage is equal to the sum of the capacitor voltage (C 1  or C 2 ) and the capacitor voltage within the voltage-clamp network  210 . Because the voltage across each capacitor (C 1  or C 2 ) is clamped to input voltage Vi, the reset voltage is higher than the input voltage. The duty cycle of the semiconductor switch within the switch network  220 , therefore, can be above 50%. 
         [0048]    Obviously, a higher than 50% operating duty cycle results in increasing transformer turns ratio accompanied with a low primary current and lower voltage stresses on the secondary rectifiers. Consequently, further improvements of the power converter&#39;s efficiency can be achieved. 
         [0049]    Turning now to  FIG. 6A  and  FIG. 6B  are two embodiments of power converter constructed according to the foregoing principles of the present invention. The voltage-clamp network  210 A is a passive voltage-clamp cell formed by a R C -C C -D C  sub-circuit and the voltage-clamp network  210 B is an active voltage-clamp cell formed by a Sc-Cc sub-circuit, respectively. One complementary signal  232  issued by the gate drive  230  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  210 B. 
         [0050]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 6C  and  FIG. 6D . The voltage-clamp networks  210 C and  210 D comprise two series-connected passive voltage-clamp cells formed by the combination of a diode D a  and a R C -C C -D C  sub-circuit. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  220  is clamped to Vi or Vi+V CC . 
         [0051]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 6E . The voltage-clamp network  210 E comprises two series-connected passive voltage-clamp cells formed by a R C -C C -D C  sub-circuit and a Ra-Ca-Da sub-circuit. The voltage across the switch S 1  or S 2  within the switch network  220  is clamped to Vi+V Ca  and Vi+V CC , respectively. 
         [0052]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 6F  and  FIG. 6G . The voltage-clamp networks  210 F and  210 G comprise two series-connected voltage-clamp cells formed by the combination of a diode Da and a Sc-Cc sub-circuit. One complementary signal  232  issued by the gate drive  210  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  210 F or  210 G. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  220  is clamped to Vi or Vi+V CC . 
         [0053]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 6H  and  FIG. 6I . The voltage-clamp networks  210 H and  210 I comprise two series-connected voltage-clamp cells formed by the combination of a R a -C a -D a  sub-circuit and a Sc-Cc sub-circuit. One complementary signal  232  issued by the gate drive  210  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  210 H or  210 I. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  220  is clamped to Vi+V Cc  or Vi+V Ca . 
         [0054]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention is shown in  FIG. 6J . The voltage-clamp networks  210 J comprise two series-connected voltage-clamp cells formed by a Sc-Cc sub-circuit and a Sa-Ca sub-circuit. Two complementary signals  232  issued by the gate drive  230  are necessarily provided to drive the semiconductor switch Sc and Sa within the voltage-clamp network  210 J. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  220  is clamped to Vi+V CC  or Vi+V Ca . 
         [0055]    As illustrated in  FIG. 7  is another circuit diagram of the power converter  300  to introduce the broad concept of resetting a transformer by transferring energy to reset winding via the capacitors and to further reduce the current ripple as well as to alleviate the thermal stress of the transformer of the present invention. The power converter  300  used to convert a DC input to an AC output comprises one input inductor, two series circuits, two capacitors C 1  and C 2 , and two transformers T 1  and T 2 . The input inductor, L in  represented the parasitic inductor or an external inductor, as designed, is inserted between the DC input Vi and the two series circuits. The transformer T 1  has two identical primary windings Lp 1  and Lp 4  and has at least one secondary winding Ls 1 ; while the transformer T 2  has two identical primary windings Lp 2  and Lp 3  and has at least one secondary winding Ls 2 . Each series circuit is connected in parallel with the DC input source Vi. The first series circuit comprises the first primary Lp 1  of the first transformer T 1 , the first primary Lp 2  of the second transformer T 2 , and one switch network  320 . The second series circuit comprises the second primary Lp 4  of the first transformer T 1 , the second primary Lp 3  of the second transformer T 2 , and the voltage-clamp network  310 . The switch network  320  comprises at least one semiconductor switch and the voltage-clamp network  310  comprises at least one active or one passive voltage-clamp cell. The active voltage-clamp cell is formed by a MOSFET Sc series-connected with a capacitor Cc; while the passive voltage-clamp cell is formed by a diode Da or a resistor Rc parallel-connected to a capacitor Cc with series-connecting to a diode Dc. The first capacitor C 1  is used to couple the first and the second series circuits by connecting a first node N 1  and a second node N 2 , wherein the first node N 1  is a node between the switch network  320  and the first primary Lp 1  of the first transformer T 1 , and the second node N 2  is a node between the voltage-clamp network  310  and the first primary Lp 4  of the first transformer T 1 . The second capacitor C 2  is used to couple the first and the second series circuits by connecting a third node N 3  and a fourth node N 4 , wherein the third node N 3  is a node between the switch network  320  and the first primary Lp 2  of the second transformer T 2 , and the fourth node N 4  is a node between the voltage-clamp network  310  and the second primary Lp 3  of the transformer T 2 . Because the voltages across the transformer primary windings Lp 1  and Lp 3  (Lp 2  and Lp 3 ) are cancelled each other, each capacitor voltage level is equal to the input voltage. At least one driver signal  331  is issued by the gate drive  330  to turn on/off the at least one semiconductor switch within the switch network  320 . Consequently, two AC voltages are generated in the secondary windings (Ls 1  and Ls 2 ). After series-connecting or paralleled-connecting Ls 1  and Ls 2  and being rectified and filtered (not shown), the power converter provides an output voltage Vo to a load. 
         [0056]    The power converter  300  operates as follows. During a first interval, a gate drive signal  331  is issued to turn on the semiconductor switch within the switch network  320 . In addition to the input voltage Vi applied to the primary windings Lp 1 -Lp 2 , each capacitor voltage is also applied to its individual pair of the primary winding Lp 2 -Lp 4  or Lp 1 -Lp 3 , respectively. Then, during a complementary interval, the gate drive signal  331  turns off the semiconductor switch within the switch network  320 . The energy stored in the leakage inductance of the transformer T 1  is absorbed by the capacitors C 1  and C 2  as well as the capacitor within the voltage-clamp network  310 . Therefore, the voltage across the switch network  320  has no voltage spike and limited to the sum of the three voltages provided by the capacitor C 1 , the capacitor C 2 , and the capacitor within the voltage-clamp network  310 . The magnetizing and leakage energies are then recovered to the input via the third primary winding Lp 3 , the fourth primary windings Lp 4 , and the voltage-clamp network  310 , thereby resetting the transformer T 1 . 
         [0057]    The transformer reset voltage is equal to the sum of the capacitor voltage (C 1  or C 2 ) and the capacitor voltage within the voltage-clamp network  310 . Because the voltage across each capacitor (C 1  or C 2 ) is clamped to input voltage Vi, the reset voltage is higher than the input voltage. The duty cycle of the semiconductor switch within the switch network  320 , therefore, can be above 50%. 
         [0058]    Obviously, a higher than 50% operating duty cycle results in increasing transformer turns ratio accompanied with a low primary current and lower voltage stresses on the secondary rectifiers. Consequently, further improvements of the power converter&#39;s efficiency can be achieved. 
         [0059]    Turning now to  FIG. 7A  and  FIG. 7B  are two embodiments of power converter constructed according to the foregoing principles of the present invention. The voltage-clamp network  310 A is a passive voltage-clamp cell formed by a R C -C C -D C  sub-circuit and the voltage-clamp network  310 B is an active voltage-clamp cell formed by a Sc-Cc sub-circuit, respectively. One complementary signal  332  issued by the gate drive  330  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  310 B. 
         [0060]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 7C  and  FIG. 7D . The voltage-clamp networks  310 C and  310 D comprise two series-connected passive voltage-clamp cells formed by the combination of a diode D a  and a R C -C C -D C  sub-circuit. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  320  is clamped to Vi or Vi+V CC . 
         [0061]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 7E . The voltage-clamp network  310 E comprises two series-connected passive voltage-clamp cells formed by a R C -C C -D C  sub-circuit and a Ra-Ca-Da sub-circuit. The voltage across the switch S 1  or S 2  within the switch network  320  is clamped to Vi+V Ca  and Vi+V CC , respectively. 
         [0062]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 7F  and  FIG. 7G . The voltage-clamp networks  310 F and  310 G comprise two series-connected voltage-clamp cells formed by the combination of a diode Da and a Sc-Cc sub-circuit. One complementary signal  332  issued by the gate drive  330  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  310 F or  310 G. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  320  is clamped to Vi or Vi+V CC . 
         [0063]    Another two embodiments of power converter constructed according to the foregoing principles of the present invention are shown in  FIG. 7H  and  FIG. 7I . The voltage-clamp networks  31  OH and  310 I comprise two series-connected voltage-clamp cells formed by the combination of a R a -C a -D a  sub-circuit and a Sc-Cc sub-circuit. One complementary signal  332  issued by the gate drive  330  is necessarily provided to drive the semiconductor switch Sc within the voltage-clamp network  310 H or  310 I. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  320  is clamped to Vi+V CC  or Vi+V Ca . 
         [0064]    Another one embodiment of power converter constructed according to the foregoing principles of the present invention is shown in  FIG. 7J . The voltage-clamp networks  310 J comprise two series-connected voltage-clamp cells formed by a Sc-Cc sub-circuit and a Sa-Ca sub-circuit. Two complementary signals  332  issued by the gate drive  330  are necessarily provided to drive the semiconductor switch Sc and Sa within the voltage-clamp network  310 J. Depends on the configuration, the voltage across the switch S 1  or S 2  within the switch network  320  is clamped to Vi+V CC  or Vi+V Ca . 
         [0065]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.