Abstract:
A power factor correction system is comprised of a inductor and a forward transformer. The inductor is connected to the first winding of the forward transformer. In series. The inductor is used to boost the input voltage to the design level and correct input current waveform. The forward transformer is used to prevent the large reversed current through rectifier diode and the switch. The second winding of the forward transformer is used to receive the energy through the first winding of the forward transformer to correct input current waveformer and the third winding is used to reduce the switch voltage stress.

Description:
FIELD OF THE INVENTION 
     The present invention relates to AC to DC converters, and more particularly, to a high power factor correction with improved efficiency. 
     BACKGROUND 
     AC-to-DC converters are used to convert an AC input voltage, such as a fully-rectified AC line voltage from a power outlet, to a regulated DC output voltage at a desired output voltage level. To meet regulatory requirements, AC to DC convertes are designed with power factor correction (PFC) to achieve a high power factor while reducing total harmonic distortion (THD). Various topologies have been proposed for AC-To-DC converters incorporating power factor correction (PFC). For example, a conventional boost topology useds a bridge rectifier (also referred to as a diode bridge) to rectify the AC input voltage to DC followed by a boost converter functioning as an active PFC circuit  FIG. 1 . The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that always in phase with and at the same frequency as the line voltage. However, the boost converter suffers significant power loss due to rectified diode and the boost switch power loss. One method uses a critical boundary boost power factor correction, in this design, the inductor current is allowed to completely go to zero before the next switching cycle of the mosfet is initiated and all diode losses are due to forward conduction. But it is difficult to design a controller with stable operation and fast transient response for both modes, the boost converter will be unstable between DCM and CCM. The present invention overcomes the above problems by using a forward transformer to prevent the rectifier reversed conduction loss and mosfet turn on loss. 
     SUMMARY 
     In one embodiment, the present invention provides a power factor correction system comprising two inductors, a forward transformer, four diodes, three capacitors, a switch and a control unit. The power factor correction system is coupled in series to a rectifier bridge. The rectifier bridge receives an input current from an AC power source and provides a rectified input current to the power factor correction system. The control unit is configured to operate the controllable switch. 
     In another embodiment, the present invention provides a power factor correction comprising two coupled inductors, a forward transformer, four diodes, three capacitors, a switch and a control unit. 
     Other aspects of the present invention will become apparent by consideration of the detailed description and the accompanying drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the prior art. 
         FIG. 2  is a schematic of one embodiment of the present invention. 
         FIG. 3  is a schematic of another embodiment of the present invention. 
         FIG. 4  is a schematic of yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the present invention are explained in detail, it is to be understood that the present invention is not limited in its application to the details of the construction and the arrangements of components set forth in the following description or illustrated in the drawings. The present invention is capable of the other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  illustrates the traditional PFC topology. This boost converter usually operates in discontinous current (DCM) which is well known in the switching converters. In DCM, the output current (which equals the inductor current during time periods in which the switch Q 1  is off) drops to zero and remains zero for at leas a short delay time. This definition distinguishes DCM from continuous current mode (CCM) in which the inductor current would not drop to zero during a switching cycle. In this case, when the switch is on, there is large reversed current conducting through the diode D 1  and the switch Q 1 , resulting significant power loss. 
       FIG. 2  illustrates one example of a rectifier circuit that includes a boost PFC system of the present invention. The bridge rectifier BD 201  receives current from an AC power source AC  201 . The rectifier circuit includes a capacitor C 201  that smoothes the ripple current before it is supplied to the boost PFC system. The boost PFC system includes two inductors, a forward transformer, four diodes, two capacitors, a switch, and a control circuit unit. The control circuit unit generates control signal to control the switch according to the input voltage and the output voltage of the boost PFC system. 
     The configuration of the PFC system is as following: 
     The forward transformer T 20  have three windings: a first winding T 201 , a second winding T 202  and a third winding T 203 . The first inductor L 201 , the first winding T 201  of the forward transformer T 20  and the switch Q 201  are connected in series and then is coupled to the outputs of the bridge rectifier BD 201  with the inductor L 201  to the positive output of the bridge rectifier BD 201  and the switch Q 201  to the negative output of the bridge rectifier BD 201 . The anode of the first diode D 201  is connected to the first inductor L 201 . The third capacitor C 203  has a positive terminal which is connected to the cathode of the first diode D 201 , and a negative terminal which is connected to the negative output of the bridge rectifier BD 201 . The second winding T 202  of the forward transformer T 20  and the third diode D 203  are connected in series and then connected to the second diode D 202  in parallel and then connected to the second inductor L 202  in series; and then this circuit is coupled between the positive output of the bridge rectifier BD 201  and the positive terminal of the third capacitor C 203  with the cathodes of the two diode D 202 , D 203  coupled toward the positive terminal of the third capacitor C 203 . The third winding T 203  of the forward transformer T 20  and the fourth diode D 204  are connected in series then coupled to the third capacitor C 203  in parallel with the cathode of the fourth diode D 204  toward the positive terminal of the third capacitor C 203 . The load  8201  is connected to the third capacitor in parallel. The load R 201  is a DC to DC converter or a DC to AC converter. 
     The operation of the PFC system is as following: 
     When the switch Q 201  is on, a current from the positive output of the bridge rectifier BD 201  conducts through the first inductor L 201 , the first winding T 201  of the forward transformer T 20  and the switch Q 201 , to the negative output of the bridge rectifier BD 201 ; and at the same time there is an induced voltage in the second winding T 202  of the forward transformer T 20  and this voltage and the input rectified voltage force a current through the second inductor L 202 , the second winding T 202  of the forward transformer T 20  and the third diode D 203 , to charge the third capacitor C 203 . 
     When the switch Q 201  is off, there is an induced voltage in the first inductor L 201  and the induced voltage in the first inductor and the input rectified voltage force a current through the first inductor L 201  and the first diode D 201  to charge the third capacitor C 203 ; and at the same time there is an induced voltage in the second inductor L 202  and the induced voltage in the second inductor and the input rectified voltage force a current through the second inductor L 202  and the second diode D 202  to charge the third capacitor C 203 . The energy stored in the forward transformer T 20  is released through the third winding T 203  of the forward transformer T 20  and the fourth diode D 204  to charge the third capacitor C 203 . 
     The number of the windings of the third winding T 203  of the forward transformer T 20  is designed three times more than the number of the windings of the first winding T 201  of the forward transformer T 20  to reduce the switch Q 201  voltage stress. 
     The second capacitor C 202  is used to reduce the switching power loss of the switch Q 201 . When the switch Q 201  is off, the voltage across the switch Q 201  is limited by the voltage across the second capacitor C 202 . 
     The first winding T 201  of the forward transformer T 20  is used to prevent large reversed current through the second first diode D 201  and the switch Q 201 . The current through the first inductor L 201  and the second inductor L 202  is able to be operated in DCM or CCM mode without significant power loss. 
       FIG. 3  illustrates another example of a rectifier circuit that includes boost PFC system of the present invention. In this circuit the first inductor L 201  and the second inductor L 202  have the same magnetic core. 
       FIG. 4  illustrates yet another example of a rectifier circuit that includes boost PFC system of the present invention. In this circuit the second inductor L 202  is part of the first inductor L 201 . When the inductance value of the second inductor L 202  equals to the inductance value of the first inductor L 201 , there is only one inductor required for this circuit. When the inductance value of the second inductor equals to zero, there is only one inductor required for this circuit. 
     The present invention provides, among other things, a power factor correction system and methods of the operating the same to reduce power loss. Various features and advantages of the present invention are set forth in the following claims.