Abstract:
This invention discloses apparatus and methods for increasing the duty cycle of the single ended power converters surpass 50 percent limitation by adding active switch-capacitor network to the primary circuit and several inversion circuits can be realized to convert a DC input to an AC output. The circuits comprise two series circuits, at least one clamp clamping capacitor, and at least one transformer. The first series circuit includes one active switch paralleled with a diode, one capacitor and at least one transformer primary. The second series circuit includes at least one active switch and at least one transformer primary. At least one clamp clamping 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation-in-part application of and claims the priority benefit of U.S. patent application Ser. No. 12/102,877, filed Apr. 15, 2008, now pending. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention is related to the field of power converter, and more specifically, to single ended power converters operate beyond the 50% duty cycle limitation. 
         [0004]    2. Description of Related Art 
         [0005]    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. 
         [0006]    In low-to-medium level power conversion applications, single-ended power converter topology, such as a forward converter or a flyback converter, is widely used. It includes an isolation transformer, at least one active switch on the primary side of the transformer, a rectifier and an output filter on the secondary side of the transformer. By way of the on/off control of the power active 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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    To achieve a low current ripple as well as to recycle the transformer&#39;s magnetizing and leakage energies, several single ended power converters have been proposed in the literatures and become the prior art of the present invention. 
         [0010]    One of which shown in  FIG. 1  is the single ended 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 plotted in “Snubber Circuits: Theory, Design and Application,” Philip C. Todd, TI seminar 900. Topic 2, 1993. However, no detailed descriptions are offered in both papers. 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. 
         [0011]    This circuit contains a single active switch which is selected to withstand higher than the input voltage. In some applications, ample voltage-rating MOSFETs as the active switches may be available at the cost of increasing the conduction losses due to the higher voltage-rating MOSFET accompanied with a higher R DSon . On the contrary, voltage stress may be too high for available active switches in many other applications. 
         [0012]    By series-connecting two active switches, the voltage stress on each device can be reduced. Using low-voltage rating MOSFET as an example, 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 , it was invented in U.S. Pat. No. 7,515,439, issued on Apr. 7, 2009, to the inventor of the present invention. Each of the two series-connected active switches has been designed to accommodate rated for approximately the input voltage. 
         [0013]    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. 
         [0014]    Again, to take the advantage of reducing the voltage stress, the circuit diagram of its two active switch version is shown in  FIG. 4 . It was invented in U.S. Pat. No. 7,515,439, issued on Apr. 7, 2009, to the inventor of the present invention. 
         [0015]    Because the transformer reset voltage of the aforementioned single ended 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. 
         [0016]    Accordingly, those skilled in the art understand that one of the effects of increasing the duty cycle of the power active switch is that an overall efficiency of the single ended power converter can be increased. 
         [0017]    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 
       [0018]    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. 
         [0019]    A further object of the present invention is to provide inversion circuits employing clamping capacitor to recycle the magnetic energies thereby to improve the converter&#39;s efficiency. 
         [0020]    A further object of the present invention is to provide inversion circuits using low voltage-rating active switch thereby to improve the converter&#39;s efficiency. 
         [0021]    A further object of the present invention is to provide inversion circuits surpassing 50% duty cycle thereby to improve the converter&#39;s efficiency. 
         [0022]    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. 
         [0023]    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 
         [0024]    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. 
           [0025]      FIG. 1 ,  FIG. 2 ,  FIG. 3 , and  FIG. 4  are the circuit diagrams of the single ended power converter as prior art of the present invention. 
           [0026]      FIG. 5A   FIG. 5B  and  FIG. 5C  illustrate three embodiments of the single ended power converter accordance with the present invention. 
           [0027]      FIG. 6A  and  FIG. 6B  illustrate another two embodiments of the single ended power converter accordance with the present invention. 
           [0028]      FIG. 7A ,  FIG. 7B  and  FIG. 7C  illustrate another three embodiments of the single ended power converter accordance with the present invention. 
           [0029]      FIG. 8A ,  FIG. 8B  and  FIG. 8C  illustrate another three embodiments of the single ended power converter accordance with the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    As illustrated in  FIG. 5A  is a circuit diagram of the single ended power converter to introduce the concept of resetting a transformer via the clamping capacitors of the present invention. The circuit used to convert a DC input to an AC output comprises two series circuits, one clamping capacitor C 2 , 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 a first active switch S 1  paralleled with a first diode DS 1 , a first clamping capacitor C 1 , and the first transformer primary winding Lp 1 ; while the second series circuit comprises the second transformer primary winding Lp 3 , a second active switch S 2 . Wherein the first diode DS 1  is the body diode of the first active switch S 1  or an external diode. The second clamping capacitor C 2  is used to couple the first series circuit and the second series circuit by connecting a first node N 1  and a second node N 2 , wherein the first node N 1  is a node between the first clamping capacitor C 1  and the first transformer primary Lp 1  in the first series circuit, and the second node N 2  is a node between the second transformer primary Lp 3  and the second active switch S 2 . A driver signal is issued by the control circuit (not shown) to turn on/off the active switch S 2  of the second series circuit. On the other hand, a complementary driver signal is also issued by the same control circuit to turn on/off the active switch S 1  of the first series circuit. Consequently, an AC voltage is thus generated in the transformer secondary winding Ls. After being rectified and filtered (not shown), the output of the power converter provides an output voltage V 0  to a load. 
         [0031]    The single ended power converter operates as follows. Before the first time interval, both active switches S 2  and S 1  are turned off. During a first time interval, one gate drive signal is issued to turn on the active switch S 2 . In addition to the input voltage Vi applied to the second primary winding Lp 3 , the clamping capacitor voltage V C2  is also applied to the first primary winding Lp 1 . A magnetizing current associated with the transformer T 1  increases linearly. At the end of the first time interval, the gate drive signal turns off the active switch S 2 . The energy stored in the leakage inductance of the transformer T 1  is absorbed by the second clamping capacitor C 2  and the first clamping capacitor C 1 . Therefore, the voltage across the active switch S 2  has no voltage spike and is limited to the sum of the three voltages provided by the voltage across the second clamping capacitor C 2 , the voltage across the first clamping capacitor C 1 , and the input voltage Vi. 
         [0032]    Due to the forward biased, DS 1  is turned on. The transformer reset voltage is thus equal to the sum of the voltages across the clamping capacitors, C 1  and C 2 . This operation condition is still valid because a complementary gate driver signal is applied to turn on the active switch S 1  before DS 1  is turned off. Since the voltage across the clamping capacitor C 2  is clamped to input voltage Vi, the reset voltage can be thus higher than the input voltage. The duty cycle of the active switch, therefore, can be above 50%. 
         [0033]    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 single ended power converter&#39;s efficiency can be achieved. 
         [0034]    Turning now to  FIG. 5B  is another embodiment of the single ended power converter constructed according to the foregoing principles of the present invention. Two series-connected active switches S 2  and S 3  are used to replace the active switch S 2  in  FIG. 5A . Moreover, a clamping diode D 1  is connected between the DC input and center node of the second active switch and the third active switch to clamp the voltage across active switches S 2  and S 3 . The active switches, S 2  and S 3 , are turned on simultaneously. To assure the voltage-clamping function performed by the clamping diode D 1 , however, the turn-off timing of the gate drivers between the switches has to be designed properly. The active switch S 3  should not be turned off before the active switch S 2  in the circuit  FIG. 5B . The voltages across the active switch S 2  and S 3  are thus clamped to Vi, and Vi+V C1 , respectively. As a result, lower voltage rating active switch can be used for S 2  and S 3  and decrease the conduction loss. 
         [0035]      FIG. 5C  is another embodiment of the single ended power converter constructed according to the foregoing principles of the present invention. Two series-connected active switches S 2  and S 3  are used to replace the active switch S 2  in  FIG. 5A . Moreover, a clamping diode D 1  is connected between the said first node N 1  and center node of the second active switch and the third active switch to clamp the voltage across active switches S 2  and S 3 . The active switches, S 2  and S 3 , are turned on simultaneously. To assure the voltage-clamping function performed by the clamping diode D 1 , however, the turn-off timing of the gate drivers between the switches has to be designed properly. The active switch S 3  should be turned off before the active switch S 2  in the circuit  FIG. 5C . The voltages across the active switch S 2  and S 3  are thus clamped to Vi+V C1 , and Vi, respectively. As a result, lower voltage rating active switch can be used for S 2  and S 3  and decrease the conduction loss. 
         [0036]    As illustrated in  FIG. 6A  is another circuit diagram of the single ended power converter to introduce the concept of resetting a transformer via the clamping 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 clamping capacitors (C 2  and C 3 ), and one transformer T 1 . The input inductor, L in  represented the parasitic inductor or an external inductor 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. The first series circuit comprises the first transformer primary Lp 1 , a first active switch S 1  paralleled with a first diode DS 1 , a first clamping capacitor C 1 , and a second transformer primary Lp 2 ; while the second series circuit comprises a third transformer primary Lp 3 , a second active switch S 2 , and the fourth transformer primary Lp 4 . Wherein the diode DS 1  is the body diode of the first active switch S 1  or an external diode. The second clamping capacitor C 2  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 first transformer primary Lp 1  and the first active switch S 1 , and the second node N 2  is a node between the active switch S 2  and the fourth transformer primary Lp 4 . The third clamping capacitor C 3  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 first clamping capacitor C 1  and the second transformer primary Lp 2 , and the fourth node N 4  is a node between the third transformer primary Lp 3  and the second active switch S 2 . Because the voltages across the transformer primary windings Lp 1  and Lp 4  (Lp 2  and Lp 3 ) are cancelled each other, the clamping capacitor voltages, V C2  and V C3 , are equal to the input voltage. One driver signal is issued by the gate drive controller (not shown) to turn on/off the second active switch S 2 ; while one complementary driver signal is also issued by the gate drive controller to turn on/off the first active switch S 1 . Consequently, an AC voltage is generated in the secondary winding Ls. After being rectified and filtered (not shown), the output of the single ended power converter provides an output voltage V 0  to a load. 
         [0037]    The single ended power converter operates as follows. Before the first time interval, both active switches S 2  and S 1  are turned off. During the first time interval, a gate drive signal is issued to turn on the active switch S 2 . In addition to the input voltage Vi applied to the primary windings Lp 3 -Lp 4 , the second and the third clamping capacitor voltages are also applied to its individual pair of primary winding Lp 1 -Lp 3  and Lp 4 -Lp 2 , respectively. A magnetizing current associated with the transformer T 1  increases linearly. At the end of the first time interval, the gate drive signal turns off the second active switch S 2 . The energies stored in the leakage inductance of the transformer T 1  are absorbed by the clamping capacitors (C 1 , C 2  and C 3 ). Therefore, the voltage across the active switch S 2  has no voltage spike and is limited to the sum of the three voltages provided by the voltage across the second clamping capacitor C 2 , the voltage across the third clamping capacitor C 3 , and the voltage across the first clamping capacitor C 1 . 
         [0038]    The magnetizing and leakage energies are then recovered to the input via the second primary winding Lp 2 , the first clamping capacitor C 1 , the diode DS 1 , and the first primary windings Lp 1 , thereby resetting the transformer T 1 . 
         [0039]    Due to the forward biased, DS 1  is turned on. The transformer reset voltage is equal to the sum of the first clamping capacitor voltage V C1  and the second or the third clamping capacitor voltage (V C2  or V C3 ). This operation condition is still valid because a complementary gate driver signal is applied to turn on the first active switch S 1  before DS 1  is turned off. Since the voltages across clamping capacitor, V C2  and V C3 , are clamped to input voltage Vi, the reset voltage can be thus higher than the input voltage. The duty cycle of the active switch S 2 , therefore, can be above 50%. 
         [0040]    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 single ended power converter&#39;s efficiency can be achieved. 
         [0041]    As illustrated in  FIG. 6B  is another circuit diagram of the single ended power converter to introduce the concept of resetting a transformer via the clamping capacitors and to further reduce the current ripple as well as to alleviate the thermal stress of the transformer of the present invention. Two transformers, T 1  and T 2 , are used to replace the transformer T 1  in  FIG. 6A . The transformer T 1  has two identical primary windings Lp 2  and Lp 3  and has at least one secondary winding LS 1 ; while the transformer T 2  has two identical primary windings Lp 1  and Lp 4  and has at least one secondary winding LS 2 . 
         [0042]    Another three embodiments of the single ended power converter constructed according to the foregoing principles of the present invention is shown in  FIG. 7A ,  FIG. 7B  and  FIG. 7C . Two series-connected active switches S 2  and S 3  are used to replace the active switch S 2  in  FIG. 6A . A clamping diode D 1  is used to clamp the second active switch S 2  or the third active switch S 3 . As shown, the clamping diode D 1  is connected between the fifth node N 5  and the first node N 1 , or between the fifth node N 5  and the third node N 3 , or between the fifth node N 5  and the sixth node N 6 , respectively. Wherein the fifth node N 5  is the center node of the third active switch S 3  and the second active switch S 2 ; while the sixth node N 6  is a the center node of the first active switch S 1  and the first clamping capacitor C 1 . The active switches, S 2  and S 3 , are turned on simultaneously. To assure the voltage-clamping function performed by the diode D 1 , however, the turn-off timing of the gate drivers between the two active switches has to be designed properly. For example, the active switch S 3  should not be turned off before the turning off of the active switch S 2  in the circuit  FIG. 7A . On the contrary, the active switch S 2  should not be turned off before the turning off of the active switch S 3  in the circuit  FIG. 7B  and  FIG. 7C . As a result, the voltages across the active switch S 2  and S 3  can be thus clamped to Vi, or Vi+V C1 , respectively. Lower voltage rating active switch can be used for S 2  and S 3  and decrease the conduction loss. 
         [0043]    As illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 8C  are another three circuit diagrams of the single ended power converter to introduce the concept of resetting a transformer via the clamping capacitors and to further reduce the current ripple as well as to alleviate the thermal stress of the transformer of the present invention. Two transformers, T 1  and T 2 , are used to replace the transformer T 1  in  FIG. 7A ,  FIGS. 7B , and  7 C, respectively. The transformer T 1  has two identical primary windings Lp 3  and Lp 2  and has at least one secondary winding LS 1 ; while the transformer T 2  has two identical primary windings Lp 1  and Lp 4  and has at least one secondary winding LS 2 . 
         [0044]    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.