Abstract:
A single-stage power factor correction power supply has two transformers: a main transformer and an auxiliary transformer (forward transformer). The main transformer transfers energy form the primary circuit to the secondary circuit. The auxiliary transformer is used to correct input current waveform. An extra winding of the auxiliary transformer is used to reduce the voltage stress of the switch component. The advantage of this design over the two-stage power supply is that the voltage across the storage capacitor can be designed to be only slightly higher than the peak value of the rectified input voltage. Therefore, it uses less energy to correct input current waveform and results in less of an Electromagnetic Compatibility (EMC) problem because it has a lower input current amplitude through the inductor than that of the two-stage PFC power supply. It increases power supply unit reliabilty and efficiency.

Description:
[0001]    The present invention relates to a power converter, and more particularly, to a power factor correction (PFC) converter in a single stage. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Power converters have widely served to convert an unregulated power source to a regulated voltage or current. A PFC (Power Factor Correction) technique is applied to make an input current follow the waveform of an input voltage. Adding a PFC stage to the front end of a power converter substantially avoids unnecessary power loss and heat dissipation in a power contribution system. 
         [0003]    A tranditional PFC power converter has two stages. A first stage is PFC stage, which includes an inductor, a rectifier and a transistor which is driven by a PFC control signal from the PFC stage. A second stage includes a transistor controlled by a control signal PWM, a transformer and a secondary circuitry, thus output voltage is regulated and output ripple noise is reduced. However, the PFC stage configuration increases the cost and the device counts of the converter, and hence the efficiency of power converter is reduced. Therefore, the development trend of a power converter is to build a single stage power converter with PFC function. The newly proposed topology of a single stage PFC power supply uses two transformers as  FIG. 1 : one forward transformer and one main transformer. The forward transformer transfers energy from its first winding to its second winding to correct the input current waveform, the main transformer transfers energy from the primary circuit to the secondary circuitry—output circuit ( FIG. 4  in PCT—CN2012000948). This topology simplifies the control circuits by using one switch and one control circuit, and at the same time, it improves the efficiency of the power converter compared to the two stage PFC power converter, but adding a forward transformer to a power supply unit increases voltage stress of the switching component. 
         [0004]    The present invention provides a simple method by adding an extra winding of the forward transformer and a diode to reduce voltage stress of the switching component of the power supply unit. The present invention also provides a method to recover the energy stored in the leaking inductance of the primary winding of the main transformer and to improve the converter efficiency. The present invention can further provide a power converter operating in lower stress to obtain higher reliability. 
       SUMMERY OF THE INVENTION 
       [0005]    The first objective of the present invention is to provide a switching power supply that operates from AC line voltage and has power factor correction and output isolation. 
         [0006]    The second objective of the present invention is to provide a one stage power factor correction in an AC to DC converter. 
         [0007]    The third objective of the present invention is to provide a simple circuit of PFC power supply to reduce the manufacture cost. 
         [0008]    The fourth objective of the present invention is to provide a more efficient PFC power supply circuit. 
         [0009]    Further objects and advantages of the present invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated, schematically, in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  is a schematic diagram of the prior art ( FIG. 4  in PCT—CN201200948). 
           [0011]      FIG. 2  is a schematic diagram of the present invention of a single stage power factor correction converter. 
           [0012]      FIG. 3  is another schematic diagram of the present invention of a single stage power factor correction converter. 
           [0013]      FIG. 4  is a secondary circuit of a fly-back converter in accordance with the present invention. 
           [0014]      FIG. 5  is a secondary circuit of a forward converter in accordance with the present invention. 
           [0015]      FIG. 6  is an input current waveform of a 50w a fly-back converter of present invention prototype. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Before explaining the disclosed embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of other embodiments. The topology of the present invention is that a forward transformer&#39;s first winding is connected in series with a main transformer&#39;s primary winding and an extra winding of the forward transformer connected with a diode is coupled to output terminals. The main transformer transfers power from the primary circuit to the secondary circuit and the forward transformer transfers power to its second winding of the forward transformer to correct the input current waveform. The adding of an extra winding of the forward transformer and a diode is used to reduce the voltage stress of the switching component of a converter. 
         [0017]    Refer to  FIG. 2 : 
         [0018]    It illustrates the circuit diagram of a single switch power factor correction fly-back converter or a single switch power factor correction forward converter according to the present invention. The circuit is configured as following: 
         [0019]    A full bridge rectifier BD 10  has output terminals (a positive terminal and a negative terminal) and input terminals which are coupled to AC power lines. 
         [0020]    A first capacitor C 10  is coupled to the output terminals of the full bridge rectifier BD 10 . 
         [0021]    An inductor L 10  has a second terminal and a first terminal which is coupled to the positive output terminal of the full bridge rectifier BD 10 . 
         [0022]    A first diode D 10  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
         [0023]    A second diode D 12  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
         [0024]    A forward transformer T 10  has three windings: a first winding T 101 , a second winding T 102  and a third winding T 103 . The second winding T 102  has a first terminal which is coupled to the cathode of the first diode D 10  and a second terminal which is coupled to the cathode of the second diode D 12 . The second terminals of the three winding of the forward transformer have the same polarity. 
         [0025]    A second capacitor C 12  has a positive terminal and a negative terminal. The positive terminal is coupled to the second terminal of the second winding T 102  of the forward transformer T 10  and the negative terminal is coupled to the negative output of the full bridge rectifier BD 10 . 
         [0026]    A secondary circuitry has a positive output terminal and a negative output terminal. 
         [0027]    A main transformer T 12  has two windings: a primary winding and a secondary winding. The primary winding of the main transformer has a first terminal which is coupled to the positive terminal of the second capacitor C 12  and a second terminal which is coupled to a second terminal of the first winding T 101  of the forward transformer T 10 .The secondary winding of the main transformer T 12  is coupled to a secondary circuitry. The main transformer T 10  can be a fly-back transformer or a forward transformer. 
         [0028]    A switch has three terminals: a first terminal, a second terminal and a control terminal. 
         [0029]    The first terminal is coupled to the first terminal of the first winding T 101  of the forward transformer T 10 . The second terminal of the switch is coupled to the negative output of the bridge rectifier BD 10 . The control terminal is coupled to a PWM control signal. 
         [0030]    A third diode has a cathode which is coupled to the positive output of the secondary circuitry and an anode which is coupled to a first terminal of the third winding T 103  of the forward transformer T 10 . The second terminal of the third winding T 103  of the forward transformer is coupled to the negative output terminal of the secondary circuitry. 
         [0031]    A third capacitor C 14  has a first terminal which is coupled to the second terminal of the primary winding of the main transformer T 12  and a second terminal which is coupled to the negative output of the bridge-rectifier BD 10 . 
         [0032]    The Operation of the  FIG. 2  is Following: 
         [0033]    When switch Q 10  is on, a current discharged from the capacitor C 14  conducts through the first winding T 101  of the forward transformer T 10  and the switch Q 10 ; a current drawn from the second capacitor C 12  conducts through the primary winding of the main transformer T 12  and the first winding T 101  of the forward transformer T 10 ; at the same time, there is an induced voltage in the second winding T 102  of the forward transformer T 10 , therefore, there is a current drawn from the input to the second capacitor C 12  (charging the second capacitor C 12 ) through the inductor L 10 , the first diode D 10  and the second winding T 102  of the forward transformer T 10 . When switch Q 10  is off, there is an induced voltage in the inductor L 10 , this voltage and input voltage force a current to charge the second capacitor C 12  through the inductor L 10  and the second diode D 12 . The energy stored in the leaking inductance of the primary winding of the main transformer T 12  is released to the third capacitor C 14 . The energy stored in the forward transformer is released to the secondary circuitry through the third winding T 103  of the forward transformer T 10  and the third diode D 14  and the induced voltage across the first winding of the forward transformer is reduced to ratio of the output voltage of the secondary circuitry, so that the voltage stress of the switch Q 10  is reduced. 
         [0034]    The number of the windings of the second winding of the forward transformer T 10  is about two times of that of the first winding of the forward transformer T 10 . The number of the windings of the third winding of the forward transformer T 10  is less than half of that of the first winding of the forward transformer, depending on the voltage of the output of the secondary circuitry. 
         [0035]    The number of the windings of the first winding T 101  of the forward transformer T 10  and the value of the inductor L 10  are adjusted to a certain value to correct the input current waveform in the best shape. 
         [0036]    The circuit, comprising of the second winding T 102  of the forward transformer T 10 , the inductor L 10 , the second capacitor C 12 , the first diode D 10  and the second diode D 12 , can have numerous different arrangements. These arrangements have the same working principles. For example, the inductor L 10  can be coupled between the negative terminal of the second capacitor C 12  and the negative output terminal of the full bridge rectifier BD 10 . The anodes of the first diode D 10  and the second diode D 12  are coupled to the positive output terminal of the full bridge rectifier BD 10 . The second winding T 102  of the forward transformer T 10  is coupled between the cathode of the first diode D 10  and the junction of the cathode of the second diode D 12  and the positive terminal of the second capacitor C 12 . 
         [0037]    The control signal can be a PWM or a PFM signal. 
         [0038]    Refer to  FIG. 3   
         [0039]    It is another circuit diagram of a single switch power factor correction fly-back converter or a single switch power factor correction forward converter according to the present invention. The circuit is configured as following: 
         [0040]    A full bridge rectifier BD 10  has output terminals (a positive terminal and a negative terminal) and input terminals which are coupled to AC power lines. 
         [0041]    A first capacitor C 10  is coupled to the output terminals of the full bridge rectifier BD 10 . 
         [0042]    An inductor L 10  has a second terminal and a first terminal which is coupled to the positive output terminal of the full bridge rectifier BD 10 . 
         [0043]    A first diode D 10  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
         [0044]    A second diode D 12  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
         [0045]    A forward transformer T 10  has three windings: a first winding T 101 , a second winding T 102  and a third winding T 103 . The second winding T 102  has a first terminal which is coupled to the cathode of the first diode D 10  and a second terminal which is coupled to the cathode of the second diode D 12 . A second terminal of the third winding T 103  of the forward transformer T 10  is coupled to positive output of the bridge rectifier BD 10 . The second terminals of the three windings of the forward transformer have the same polarity. 
         [0046]    A second capacitor C 12  has a positive terminal and a negative terminal. The positive terminal is coupled to the second terminals of the second winding T 102  and the first winding T 101  of the forward transformer T 10 . The negative terminal of the second capacitor C 12  is coupled to the negative output of the full bridge rectifier BD 10 . 
         [0047]    A main transformer T 12  has two windings: a primary winding and a secondary winding. The primary winding of the main transformer has a first terminal which is coupled to the first terminal of the first winding T 101  of the forward transformer T 10  and a second terminal. The secondary winding of the main transformer is coupled to a secondary circuitry. 
         [0048]    The main transformer can be a fly-back transformer or a forward transformer. 
         [0049]    A switch Q 10  has three terminals: a first terminal, a second terminal and a control terminal. The first terminal of the switch is coupled to the second terminal of the primary winding of the main transformer T 12 . The second terminal of the switch is coupled to the negative output of the bridge rectifier BD 10 . The control terminal of the switch is coupled to a PWM control signal. 
         [0050]    A third diode D 14  has an anode which is coupled to the first terminal of the third winding T 103  of the forward transformer T 10  and a cathode which is coupled to positive terminal of the second capacitor C 12 . 
         [0051]    A third capacitor C 14  has a first terminal which is coupled to the second terminal of the primary winding of the main transformer T 12  and a second terminal which is coupled to the anode of the third diode D 14  and the first terminal of the third winding T 103  of the forward transformer T 10 . 
         [0052]    The Operating Principle as Following Refer to  FIG. 3   
         [0053]    When switch Q 10  is on, a current from the input conducts through the third winding T 103  of the forward transformer T 10 , the capacitor C 14  and the switch Q 10 ; a current drawn from the second capacitor C 12  conducts through the first winding T 101  of the forward transformer T 10 , the primary winding of the main transformer T 12  and the switch Q 10 . At the same time there is an induced voltage in the second winding T 102  of the forward transformer T 10 , therefore, there is a current drawn from the input to the second capacitor C 12  (charging the second capacitor C 12 ) through the inductor L 10 , the first diode D 10  and the second winding T 102  of the forward transformer T 10 . When switch Q 10  is off, there is an induced voltage in the inductor L 10 , this voltage and input voltage force a current to charge the second capacitor C 12  through the inductor L 10  and the second diode D 12 . The energy stored in the leaking inductance of the primary winding of the main transformer T 12  is released to the third capacitor C 14 . The energy stored in the forward transformer is released to the second second capacitor C 12  through the third winding T 103  of the forward transformer T 10  and the third diode D 14  and the induced voltage across the first winding T 101  of the forward transformer T 10  is reduced, so that the voltage stress of the switch Q 10  is reduced. The number of the windings of the second winding of the forward transformer T 10  is about two times of that of the first winding T 101  of the forward transformer T 10 . The number of the windings of the third winding T 103  of the forward transformer T 10  is about one and half times of that of the first winding T 101  of the forward transformer. 
         [0054]    The number of the windings of the primary winding T 101  of the forward transformer T 10  and the value of the inductor L 10  are adjusted to a certain value to correct the input current waveform in the best shape. 
         [0055]    The circuit, comprising of the second winding T 102  of the forward transformer T 10 , the inductor L 10 , the second capacitor C 12 , the first diode D 10  and the second diode D 12 , can have numerous different arrangements. These arrangements have the same working principles. For example, the inductor L 10  can be coupled between the negative terminal of the second capacitor C 12  and the negative output terminal of the full bridge rectifier BD 10 . The anodes of the first diode D 10  and the second diode D 12  are coupled to the positive output terminal of the full bridge rectifier BD 10 . The second winding T 102  of the forward transformer T 10  is coupled between the cathode of the first diode D 10  and the junction of the cathode of the second diode D 12  and the positive terminal of the second capacitor C 12 . 
         [0056]    The third capacitor C 14  can be not employed and use a snubber circuit instead. 
         [0057]    The control signal can be a PWM or a PFM signal. 
         [0058]    The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.