Patent Publication Number: US-2012044729-A1

Title: Bridgeless coupled inductor boost power factor rectifiers

Description:
CROSS REFERENCES 
     This application claims priority to U.S. provisional patent application Ser. No. 61/376,178 entitled “BRIDGELESS COUPLED INDUCTOR BOOST POWER FACTOR RECTIFIERS,” filed on Aug. 23, 2010, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure is directed to power factor correction rectifier electronic circuits, and new circuits and methods for power factor correction in the absence of a bridge rectifier. 
     Power factor is a measurement that is commonly used in ac circuits to represent differences in the phases of voltage and current ac waveforms. In reactive ac circuits, the current waveform may lead, or lag, the voltage waveform. Zero or near zero phase differences between the voltage and current waveforms result in a power factor at or near one, while increasing phase differences between the current and voltage waveforms result in a lower power factor. Active power factor correction (PFC) techniques have been used for increasing the power factor in reactive ac circuits. Increasing the power factor in such systems can have the effect of reducing the total harmonic distortion in ac line currents, reducing the load of the power generating station, and increasing the real power delivered to the circuit thereby reducing the cost of the power consumed by the circuit. 
     SUMMARY 
     Methods, systems, and devices are described for new bridgeless active PFC converters that achieve relatively high efficiency. Various exemplary circuit topologies are provided based on coupled and tapped inductor boost converters utilizing one or more bi-directional voltage blocking switch, which achieve relatively low conduction losses. Zero voltage switching implementations that achieve both comparatively low conduction losses and reduction or elimination of first order drain circuit turn on switching losses are also provided. 
     The present disclosure provides, in various aspects, a bridgeless power factor correction apparatus, comprising, an AC input having a first input terminal and a second input terminal, an inductor module coupled with the first input terminal, and a switching module coupled between the second input terminal and the inductor module. The switching module may comprise a bi-directional voltage blocking switch that is configured to selectively couple the inductor module with the AC input based on an output voltage and a phase difference between an input voltage waveform and an input current waveform. An output module may be coupled with the inductor module, and provide an output to a load that may be coupled with the output module. The inductor module may comprise a first winding coupled with the AC input and the switching module, and a second winding inductively coupled with the first winding and coupled with the output module. The inductor module may also comprise a tapped inductor, and the second winding is common to a portion of the primary winding. The output module may include a full or half bridge rectifier. PFC systems disclosed herein may also include zero voltage switching circuits through an auxiliary switch and auxiliary capacitor coupled between the first input terminal and the inductor module, the auxiliary switch configured to accomplish a reversal of current in the inductor module during an off time of the main switch to direct current in the inductor module towards the main switch to drive the main switch to zero volts during a turn on transition of the main switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the following drawings. In the appended figures, similar components or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
         FIG. 1  is a block diagram illustration of a PFC system. 
         FIG. 2  illustrates a bridgeless isolated coupled inductor boost active PFC system with a full bridge secondary circuit according to an embodiment. 
         FIG. 3  illustrates exemplary first and second operational states for the circuit of  FIG. 2  during a positive half cycle of the input ac power supply. 
         FIG. 4  illustrates exemplary first and second operational states for the circuit of  FIG. 2  during a negative half cycle of the input ac power supply. 
         FIG. 5  illustrates a bridgeless isolated coupled inductor boost active PFC system with a half bridge secondary circuit according to an embodiment. 
         FIG. 6  illustrates exemplary first and second operational states for the circuit of  FIG. 5  during a positive half cycle of the input ac power supply. 
         FIG. 7  illustrates exemplary first and second operational states for the circuit of  FIG. 5  during a negative half cycle of the input ac power supply. 
         FIG. 8  illustrates a bridgeless isolated coupled inductor boost active PFC system with a half bridge secondary circuit and MOSFET switches according to an embodiment. 
         FIG. 9  illustrates a bridgeless isolated coupled inductor boost active PFC system with a full bridge secondary circuit and MOSFET switches according to an embodiment. 
         FIG. 10  illustrates a bridgeless isolated coupled inductor boost active PFC system with a full bridge secondary circuit and MOSFET switches according to an embodiment. 
         FIG. 11  illustrates a bridgeless non-isolated tapped inductor boost active PFC system with a half bridge secondary circuit according to an embodiment. 
         FIG. 12  illustrates a zero voltage switching bridgeless non-isolated tapped inductor boost active PFC system with a half bridge secondary circuit according to an embodiment. 
         FIG. 13  illustrates a zero voltage switching bridgeless isolated coupled inductor boost active PFC system with a full bridge secondary circuit according to an embodiment. 
         FIG. 14  illustrates a zero voltage switching bridgeless isolated coupled inductor boost active PFC system with a half bridge secondary circuit according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     This description provides examples, and is not intended to limit the scope, applicability or configuration of the invention. Rather, the ensuing description will provide those skilled in the art with an enabling description for implementing embodiments of the invention. Various changes may be made in the function and arrangement of elements. 
     Thus, various embodiments may omit, substitute, or add various procedures or components as appropriate. For instance, it should be appreciated that various of the described operations may be performed in an order different than that described, and that various steps may be added, omitted or combined. Also, aspects and elements described with respect to certain embodiments may be combined in various other embodiments. It should also be appreciated that the following exemplary embodiments may individually or collectively be components of a larger system, wherein other procedures may take precedence over or otherwise modify their application. 
     Systems, devices, and methods are described for isolated and non-isolated bridgeless active power factor correction circuits with low conduction losses. A new single stage isolated bridgeless active power factor correction circuit with low conduction losses is provided. In some embodiments, power factor correction circuits are provided with both low conduction losses and zero voltage switching. Exemplary PFC circuits provide reduced conduction losses in a bridgeless configuration through the use of a bi-directional voltage blocking switch. Other exemplary PFC circuits provide an isolated system through the use of a coupled inductor with a bi-directional voltage blocking switch coupled between a primary winding of the inductor and an ac power source. An output module may provide rectification of the signal induced at a secondary winding of the coupled inductor to provide a rectified output voltage to a load that is couplable with the output module. 
     In traditional PFC rectifiers the ac line voltage is rectified with a bridge rectifier. The output of the bridge rectifier is a dc voltage. The active PFC circuit in such traditional rectifiers is a dc circuit that sees only one polarity of line voltage. The bridge rectifier in such circuits incurs conduction losses due to the forward voltage drop of the diodes that comprise the bridge rectifier. These losses can be on the order of 2% of the total power processed by the PFC rectifier. Active PFC circuits that eliminate the bridge rectifier have been developed, and are referred to as bridgeless PFC rectifiers. Accomplishing bridgeless PFC in some cases requires some extra components and, in some cases, creates some additional problems, such as a high degree of common mode noise. Furthermore, traditional bridgeless PFC circuits generally do not offer isolation and require more than one conversion stage to achieve an isolated output. 
     With reference first to  FIG. 1 , a block diagram of an exemplary PFC system  100  is described. In this example, an ac line  110  provides input alternating current (ac) power to the system  100 . The ac line  110  provides input ac power at first input terminal  115  and second input terminal  120 . The alternating current power may be any suitable alternating current type of supply, such as commonly available sinusoidally varying 120 Volt, 60 Hz, power commonly available in Japan and North America, or 230 Volt, 50 Hz, power commonly available in Europe, and other parts of the world. Of course, the ac power may also include non-sinusoidally varying input power, such as input power in which current and voltage waveforms have a triangle waveform, for example. As is well understood, and as discussed above, the input current and input voltage waveforms from ac line  110  may be out of phase, resulting in a decrease in power factor for power delivered to the system  100 . Power factor may be increased in various embodiments through inductor module  125  and switching module  130 . The inductor module  125  in the example of  FIG. 1  is coupled with the first input terminal  115  and the switching module  130 . The switching module  130 , in turn, is coupled with the inductor module  125  and the second input terminal  120 . An output module  135  is coupled with the inductor module  125 . A load  140  is couplable with the output module  135 . 
     The switching module  130  is configured to selectively couple the inductor module  125  with the second input terminal  120  in a manner that increases the power factor of the power provided from the inductor module  125  to the output module  135 . In various examples, the switching module  130  receives a voltage level of the signal output from output module  135 , as well as the phase difference between the input current and input voltage waveforms, and selectively couples the inductor module  125  with the second input terminal  120  in order to maintain a desired output voltage level and decrease the phase difference between the input current and input voltage waveforms. In various embodiments, the inductor module  125  includes a coupled inductor, thereby providing electrical isolation between the output module  135  and the ac line input  110 . The switching module  130 , in various embodiments, includes a bi-directional voltage blocking switch and a controller. As used herein, the term “switch” refers to an electrical circuit element that can have two electrical states, one of which substantially blocks current flow through the element and the other of which allows current flow through the element substantially unimpeded. Examples of switches include, for example, rectifier diodes, transistors, relays, and thyristors. The output module  135 , in various embodiments, includes half or full bridge rectifiers, along with coupling and output capacitors. 
     With reference now to  FIG. 2 , another exemplary power factor correction system  200  is illustrated. In this example, the inductor module  125   a  includes a coupled inductor  205  that includes a primary winding  210  and a secondary winding  215 . The primary winding  210  has a dotted terminal and an undotted terminal. Similarly, the secondary winding  215  has a dotted terminal and an undotted terminal. In this embodiment, the dotted terminal of primary winding  210  is coupled with the first terminal  115   a  of AC Line  110   a,  and the undotted terminal is coupled with switch  220  of switching module  130   a.  The switch  220  operates to selectively couple the undotted terminal of the primary winding  210  with the second input terminal  120   a.  The output module  135   a  of this example includes a coupling capacitor  225  coupled between the dotted terminal of the secondary winding and a full bridge rectifier. The full bridge rectifier includes a first diode  230 , a second diode  235 , a third diode  240 , and a fourth diode  245 . The coupling capacitor  225  is coupled with the dotted terminal of secondary winding  215 , and is coupled with the anode of first diode  230  and the cathode of second diode  235 . The undotted terminal of secondary winding  215  is coupled with the anode of third diode  240  and the cathode of the fourth diode  245 . An output capacitor  250  is coupled between the output terminals of output module  135   a,  in parallel with load  140   a.  In this example, the switch  220  is operated to selectively couple the undotted terminal of primary winding  210  with the second input terminal  120   a  based on a voltage level of the signal output from output module  135   a,  as well as the phase difference between the input current and input voltage waveforms. The switch  220  is operated to provide voltage and current waveforms on the primary winding  210  that have little or no phase difference, while maintaining a desired voltage difference at the output terminals of output module  135   a . Similarly as discussed above, the switch  220  of this example may be a bi-directional voltage blocking switch, which can eliminate the need for a bridge on the primary side in inductor  205 . In various traditional PFC circuits, one or more of a bridge or a flyback converter may be utilized. The exemplary system of  FIG. 2  may have efficiencies compared to such systems, with the elimination of the bridge rectifier on the primary side of the inductor  205  enhancing the efficiency of the circuit of  FIG. 2  by up to about 2% as compared to a system that utilizes a bridge rectifier. Furthermore, the use of coupled inductor  205  in a coupled inductor boost converter as illustrated in  FIG. 2  may increase efficiency by up to about 5% as compared to a system that utilizes a traditional flyback converter. 
     In operation of the PFC system of  FIG. 2 , there are four operating states. There are two positive half cycle operating states and two negative half cycle operating states. With reference now to  FIG. 3 , the two positive half cycle operating states are illustrated. In particular,  FIG. 3  illustrates a first positive half cycle operating state  300  during an on time of the switch  220 , and the second positive half cycle operating state  305  during an off time of the switch  220 . In these positive half cycle operating states, AC line input  110   b  is positive, as illustrated in  FIG. 3 . According to the first positive half cycle operating state  300 , switch  220  is closed, and current I p  is present through the primary winding  210 , which induces current I s  in the secondary winding  215 . In this example, coupling capacitor  225  accommodates voltages of two polarities. During the first positive half cycle  300 , the first diode  230  and fourth diode  245  are forward biased. While in the first positive half cycle operating state  300 , current I p  flows in the primary winding  210  as magnetizing current in inductor  205  ramps up. In addition to the magnetizing current in inductor  205 , there are additional currents. A current I s  flows in secondary winding  215  induced through the magnetic coupling of the primary winding  210  and secondary winding  215 . In addition to the magnetizing current in primary winding  210  an additional current related to I s  flows in primary winding  210 . During the first positive half cycle operating state  300  the coupling capacitor  225  is charged and the output capacitor  250  is discharged as it supplies current to the load  140   a.  During the second positive half cycle operating state  305 , the switch  220  is open, and thus no current flows through the primary winding  210 . The magnetizing current I s  flows in the secondary winding  215  and the second diode  235  and third diode  240  are forward biased, with current in the second and third diodes  235 ,  240  ramping down as coupling capacitor  225  is discharged and output capacitor  250  is charged. 
     With reference now to  FIG. 4 , the two negative half cycle operating states are illustrated. In particular,  FIG. 4  illustrates a first negative half cycle operating state  400  in which switch  220  is closed, and a second negative half cycle operating state  405  during which switch  220  is open. In these operating states, AC line input  110   c  is negative, as illustrated in  FIG. 4 . During a first negative half cycle operating state  400  the switch  220  is closed and current flows in the primary winding  210  of coupled inductor  205  as magnetizing current ramps up in coupled inductor  205 . In the output module  135   a,  second and third diodes  235 ,  240  are forward biased, coupling capacitor  225  is discharged and output capacitor  250  is charged. The primary winding current I p  comprises both the magnetizing current and a current related to current I s  that flows in secondary winding  215 . During the second negative half cycle operating state  405 , the switch  220  is off, and no current flows in the primary winding  210  of the coupled inductor  205 . During this operating state, the first diode  230  and the fourth diode  245  are forward biased as the magnetizing current in coupled inductor  205  ramps down and charges coupling capacitor  225 . During the second negative half cycle operating state  405 , output capacitor  250  discharges into the load. 
     The exemplary circuit and operating states of  FIGS. 2-4  provide an efficient PFC system because the secondary winding  215  voltage is at or below the output voltage, thus resulting in a relatively small number of secondary winding turns, as compared to a comparable flyback converter in which the secondary winding voltage may exceed many times the output voltage and requires many more turns. Similarly, rectifier diodes  230 ,  235 ,  240 , and  245 , the diode voltage stresses remain at or below the output voltage, whereas in a flyback converter a diode with a voltage rating many times the output voltage would be implemented due to winding voltage that may exceed the output voltage by a significant amount. The switching module  130   a  in various examples includes a controller that controls the state of switch  220 . The types of control that can be used in such embodiments include appropriate control modes used in active power factor correction, as will be readily understood by one of skill in the art, including average current mode control, voltage mode control, boundary mode control, and ZVS boundary mode control, to name a few examples. 
     With reference now to  FIG. 5 , another exemplary PFC system  500  is illustrated. In this embodiment, inductor module  125   b  includes a magnetically coupled inductor  505  having a primary winding  510  and a secondary winding  515 . The primary winding  510  includes a dotted terminal and an undotted terminal. Similarly, secondary winding  515  includes a dotted terminal and an undotted terminal. Switch module  130   b  includes a switch  520 . Output module  135   b  in this example includes a half-bridge rectifier with a first diode  525  and a second diode  530 , a coupling capacitor  535 , and an output capacitor  540 . A first terminal of an AC Line  110   d  is connected to the dotted terminal of primary winding  510  of the coupled inductor  505 . The undotted terminal of the primary winding  510  of coupled inductor  505  is connected to a first terminal of switch  520 . Switch  520  may include a bi-directional voltage blocking switch. A second terminal of switch  520  is coupled with a second terminal of the AC line  110   d.  The dotted terminal of secondary winding  515  of coupled inductor  505  is coupled with a positive terminal of coupling capacitor  535 . The undotted terminal of the secondary winding  515  of coupled inductor  505  is coupled with an anode terminal of a first diode  525  and to a cathode terminal of second diode  530 . An anode terminal of second diode  530  is connected to the negative terminal of coupling capacitor  535 , to a negative terminal of a output capacitor  540 , and to a first terminal of a load  545 . A cathode terminal of first diode  525  is connected to a positive terminal of output capacitor  540  and to a second terminal of load  545 . 
     In operation, similarly as described above with respect to PFC system  200 , there are four operating states. There are two positive half cycle operating states and two negative half cycle operating states. With reference now to  FIG. 6 , a first positive half cycle operating state  600 , and a second positive half cycle operating state  605  are illustrated. During the first positive half cycle operating state  600 , the switch  520  is closed and second diode  530  is forward biased. During the first positive half cycle operating state  600  current I p  flows in the primary winding  510  of coupled inductor  505  as magnetizing current ramps up in the coupled inductor  505 . In addition to the magnetizing current in coupled inductor  505 , there is an additional current I s  induced in secondary winding  515  due to the fact that the primary  510  and secondary  515  windings are magnetically coupled. During the first positive half cycle operating state  600  the coupling capacitor  535  is charged and the output capacitor  540  is discharged as it supplies current to the load  545 . During a second positive half cycle operating state  605 , switch  520  is open, thereby resulting in no current flow through primary winding  510 . Magnetizing current I s  flows in the secondary winding  515  of coupled inductor  505  and in the first diode  525  and ramps down as coupling capacitor  535  is discharged and output capacitor  540  is charged. 
     With reference now to  FIG. 7 , a first negative half cycle operating state  700 , and a second negative half cycle operating state  705  are illustrated. During first negative half cycle operating state  700 , the switch  520  is closed and current I p  flows in the primary winding  510  of coupled inductor  505 , as magnetizing current ramps up in coupled inductor  505 . In the output module  135   b,  first diode  525  is forward biased, coupling capacitor  535  is discharged and output capacitor  540  is charged. During the second negative half cycle operating state  705 , the second diode  530  is forward biased and conducts current as the magnetizing current Is in secondary winding  515  of coupled inductor  505  ramps down and charges coupling capacitor  535 . During the second negative half cycle operating state  705 , switch  520  is open thereby resulting in no current flow through primary winding  510  of coupled inductor  505 . Current I s  in secondary winding  515  flows to forward bias the second diode  530 , and output capacitor  540  discharges into the load  545 . 
     The exemplary PFC system  500  illustrated in  FIGS. 5-7  is relatively efficient, as the secondary winding  515  voltage remains at or below the voltage level of the output voltage, resulting in relatively few secondary winding  515  turns, as compared to a comparable flyback converter in which the secondary winding voltage can exceed many times the output voltage and requires many more turns. Similarly, the rectifier diodes  525  and  530  have diode voltage stresses that remain at or below the output voltage level. In a typical flyback converter, a diode with a voltage rating many times the output voltage may be used. The switching module  130   b  in various examples includes a controller that controls the state of switch  520 . The types of control that can be used in such embodiments include appropriate control modes used in active power factor correction, as will be readily understood by one of skill in the art, including average current mode control, voltage mode control, boundary mode control, and ZVS boundary mode control, to name a few examples. 
       FIG. 8  illustrates a PFC system  800 , similar to the system  500  of  FIG. 5 , in which the switching module  130   c  includes a control module  805  and a pair of source and gate connected MOSFETS  810  and  815 . The pair of source and gate connected MOSFETS  810  and  815  in this example form a switch with bi-directional voltage blocking capability. The control module  805  is coupled with the input terminals and either side of AC line input  110   g , an output terminal of output module  135   c,  and a sense resistor  820 . The control module  805  controls the state of MOSFETs  810  and  815  based on the detected level of input current and voltage, output voltage, and a voltage present at the connection to sense resistor  820 . Types of control that can be used in such embodiments include appropriate control modes used in active power factor correction, as will be readily understood by one of skill in the art, including average current mode control, voltage mode control, boundary mode control, and ZVS boundary mode control, to name a few examples. The inductor module  125   c  of this example includes coupled inductor  825  with primary winding  830  and secondary winding  835 . The output module  135   c  includes a half bridge active rectifier with first rectifier switch  840  and second rectifier switch  845 . The output module includes coupling capacitor  850 , and output capacitor  855 , similarly as described above with respect to  FIGS. 5-7 . Output module  135   c  also includes control module  860  coupled with the dotted terminal of secondary winding  835 , and configured to turn on and off switches  840  and  845  to achieve appropriate rectification of the output voltage provided to load  865 . The operating states for PFC system  800  are similar to those described above with respect to  FIGS. 6-7 . 
     With reference now to  FIG. 9 , a PFC system  900 , similar to the system  200  of  FIG. 2 , in which the switching module  130   d  includes a control module  905  and a pair of source and gate connected MOSFETS  910  and  915 . The pair of source and gate connected MOSFETS  910  and  915  in this example form a switch with bi-directional voltage blocking capability. The control module  905  is coupled with either side of AC line input  110   h,  an output terminal of output module  135   d,  and a sense resistor  920 . The control module  905  controls the state of MOSFETs  910  and  915  based on the detected level of input current and voltage, output voltage, and a voltage present at the connection to sense resistor  920 . Types of control that can be used in such embodiments include appropriate control modes used in active power factor correction, as will be readily understood by one of skill in the art, including average current mode control, voltage mode control, boundary mode control, and ZVS boundary mode control, to name a few examples. The inductor module  125   d  of this example includes coupled inductor  925  with primary winding  930  and secondary winding  935 . The output module  135   d  includes coupling capacitor  940  and full bridge diode rectifier with a first diode  945 , a second diode  950 , a third diode  955 , and a fourth diode  960 . The output module  135   d  includes an output capacitor  965  and is couplable to load  970 . The operating states for PFC system  900  are similar to those described above with respect to  FIGS. 3-4 . 
       FIG. 10  illustrates another exemplary PFC system  1000 , in which the switching is accomplished through switching modules  1005  and  1010 . Switching modules are coupled to inductor module  1015 , which is in turn coupled with output module  1020  and a load  1025 . The switching module  1005  includes a first control module  1030 , a MOSFET switch  1035 , and a sense resistor  1040 . Similarly, switching module  1010  includes a second control module  1045 , a MOSFET switch  1050 , and a sense resistor  1055 . Inductor module  1015  includes a coupled inductor  1060  with primary winding  1065  and secondary winding  1070 . Output module  1020  includes a full bridge rectifier, similar to output modules  135   a  and  135   d . Output module  1020  includes coupling capacitor  1075  and full bridge diode rectifier with a first diode  1080 , a second diode  1085 , a third diode  1090 , and a fourth diode  1095 . The output module  135   d  includes an output capacitor  1097  and is couplable to load  1025 . In the examples of  FIGS. 8 and 9 , the switches ( 130   c  and  130   d ) require a level shifting circuit or magnetically coupled driver to drive the main switch pair of MOSFETs. In the PFC system  1000 , two control circuits  1030  and  1045  are provided, and a level shifting circuit would not be required. In this example, the first control module  1030  modulates switch  1035  during the positive half cycle and maintains switch  1035  on during the negative half cycle. During the negative half cycle, the second control module  1045  modulates switch  1050  and maintains switch  1050  on during the positive half cycle. The output module  1020  operates in a manner similarly as described for the operating states of  FIGS. 3 and 4 . 
       FIG. 11  illustrates a bridgeless active PFC tapped inductor boost converter system  1100  for non-isolated applications. In this example, inductor module  125   e  includes a tapped inductor with primary winding  1105  and a secondary winding  1110  that is realized by tapping the primary winding  1105  so that the secondary winding  1110  is shared with the primary winding  1105 . During the on time of the switch  1115  both secondary current and primary current flow in the common winding  1105 , but the two currents flow in opposite directions so that the total current in the common winding is reduced compared to the similar isolated circuit  500  of  FIG. 5 . Output module  135   e  of this example, includes a half bridge rectifier with first diode  1125 , second diode  1130 , coupling capacitor  1120 , and output capacitor  1135 . Output module  135   e  is couplable with load  1140 . The result of the common winding  1105  is significantly reduced winding conduction losses in the common winding, relative to the isolated equivalent circuit of  FIG. 5 . The operating states of the system  1100  are similar to those described with respect to  FIGS. 6 and 7 . 
       FIG. 12  illustrates an exemplary PFC system  1200  similar to the  FIG. 11  example, that includes an active reset network  1205  as compared to the  FIG. 11  example. The active reset network  1205  comprises an auxiliary switch  1215  and an auxiliary capacitor  1210 . The auxiliary switch  1215  is operated substantially in anti-synchronization to the main switch  1220 , except for brief dead times during the switching transitions when both switches  1215  and  1220  are off. In the steady state the current in the auxiliary switch  1215  reverses during the on time of the auxiliary switch  1215  so that the switch current is directed towards driving a zero voltage turn on transition for main switch  1220 . The energy for driving the zero voltage turn on transition for main switch  1220  derives from series or leakage inductance stored energy and/or magnetizing energy of the coupled inductor in the inductor module  125   f . At light loads the magnetizing energy of inductor module  125   f  will be the main source of zero voltage switching (ZVS) drive energy, but at heavy loads most of the energy will be derived from the series or leakage inductance. In this example, similar to the example of  FIG. 11 , inductor module  125   f  includes a tapped inductor with primary winding  1225  and a secondary winding  1230  that is realized by tapping the primary winding  1225  so that the secondary winding  1230  is shared with the primary winding  1225 . During the on time of the main switch  1220  both secondary current and primary current flow in the common winding  1225 , but the two currents flow in opposite directions so that the total current in the common winding  1225  is reduced compared to the similar isolated circuit  500  of  FIG. 5 . Output module  135   f  of this example, includes a half bridge rectifier with first diode  1240 , second diode  1245 , coupling capacitor  1235 , and output capacitor  1250 . Output module  135   f  is couplable with load  1255 . The operating states of the system  1200  are similar to those described with respect to  FIGS. 6 and 7 . 
       FIG. 13  illustrates another exemplary PFC system  1300  similar to the system of  FIG. 2 , but with an active clamp network  1305  for providing a ZVS drive mechanism through auxiliary capacitor  1310  an auxiliary switch  1315 .  FIG. 14  illustrates another PFC system  1400  similar to the  FIG. 5  example, but with an active clamp network  1405  for providing a ZVS drive mechanism through auxiliary capacitor  1410  an auxiliary switch  1415 . In the active clamp networks of the examples of  FIGS. 12 ,  13 , and  14 , the auxiliary switch may comprise a switch with bi-directional voltage blocking capability. 
     While the above description contains many examples of PFC systems, these should not be construed as limitations on the scope of the invention, but rather, as exemplifications thereof. Many other variations are possible. For example, PFC systems may include circuits similar to the circuits shown but with polarity of the input or output reversed from that illustrated. PFC systems may also include circuits similar to those shown, but having coupled magnetic circuit elements with more than two windings and circuits with more than one output. In many of the illustrated circuits there are series connected networks. The order of placement of circuit elements in series connected networks is inconsequential in the described examples, so that series networks in the illustrated circuits with circuit elements reversed or placed in an entirely different order within series connected networks are equivalent to the circuits illustrated, as will be readily recognized by one skilled in the art. Also, some of the embodiments show N channel MOSFET switches, but the operation revealed and the benefits achieved may also be realized in circuits that implement the switches using P channel MOSFETs, IGBTs, JFETs, bipolar transistors, junction rectifiers, or schottky rectifiers. 
     These components may, individually or collectively, be implemented with one or more Application Specific Integrated Circuits (ASICs) adapted to perform some or all of the applicable functions in hardware. Alternatively, the functions may be performed by one or more other processing units (or cores), on one or more integrated circuits. In other embodiments, other types of integrated circuits may be used (e.g., Structured/Platform ASICs, Field Programmable Gate Arrays (FPGAs) and other Semi-Custom ICs), which may be programmed in any manner known in the art. The functions of various modules may also be implemented, in whole or in part, with instructions embodied in a memory, formatted to be executed by one or more general or application-specific processors. 
     It should be noted that the systems and devices discussed above are intended merely to be examples. It must be stressed that various embodiments may omit, substitute, or add various procedures or components as appropriate. Also, features described with respect to certain embodiments may be combined in various other embodiments. Different aspects and elements of the embodiments may be combined in a similar manner. Also, it should be emphasized that technology evolves and, thus, many of the elements are exemplary in nature and should not be interpreted to limit the scope of the invention. 
     Specific details are given in the description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, well-known circuits, processes, algorithms, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the embodiments. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. For example, the above elements may merely be a component of a larger system, wherein other rules may take precedence over or otherwise modify the application of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention.