Abstract:
A single switch PFC power supply (including forward and fly-back power supply) in single stage has two transformers: one forward transformer, one main transformer. The main transformer transfers electrical power from the primary circuit to secondary circuit. The forward transformer is used to correct input current waveform. The two transformer&#39;s primary windings are connected in series. An extra winding of the forward transformer, a capacitor and two diodes are formed a no loss snubber circuit to enhance the efficiency of the power supply.

Description:
[0001]    The present invention relates to a power converter, and more particularly, to a high efficiency power factor correction (PFC) power 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]    Referring to  FIG. 1 , a power converter having two stages, according to prior art is illustrated. A first stage is PFC stage, which includes an inductor L 1 , a rectifier D 1  and a transistor Q 1  is driven by a PFC control signal from the PFC stage. A second stage includes a transistor Q 2  controlled by a control signal PWM, a transformer T 1  and secondary circuitry, thus output voltage is regulated and output ripple noise is reduced. However, the PFC stage configuration increases the cost and 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 present invention provides a single stage PFC converter with no loss snubber circuit that reduces the cost and the size, i.e. device counts, and to improve the converter efficiency. The present invention can further provide a power converter operating in lower stress to obtain higher reliability. 
       SUMMARY OF THE INVENTION 
       [0004]    The first objective of the present invention is to provide a switching power supply that operates from AC line voltage having a power factor correction and output isolation. 
         [0005]    The second objective of this invention is to provide for a one stage power factor correction in an AC to DC converter. 
         [0006]    The third objective of the present invention is to provide a simple circuit of PFC power supply to reduce the manufacture cost. 
         [0007]    The fourth objective of the present invention is to provide a more efficient PFC power supply circuit. 
         [0008]    The fifth objective of the present invention is to provide a snubber circuit without loss. 
         [0009]    Further objects and advantages of this 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 of the prior art, the two stage AC to DC converter. 
           [0011]      FIG. 2  is an embodiment of the present invention of the primary circuit of single switch power supply. 
           [0012]      FIG. 2   a  is another arrangement of present invention of the PFC single switch power supply. 
           [0013]      FIG. 2   b  is yet another arrangement of present invention of the PFC single switch power supply. 
           [0014]      FIG. 3  is the present invention of the fly-back or forward power supply of the  120   v  AC input. 
           [0015]      FIG. 3   a  is another arrangement of the present invention of the fly-back or forward power supply of the  120   v  AC input. 
           [0016]      FIG. 3   b  is separated windings arrangement of the present invention of the fly-back or forward power supply of the  120   v  AC input. 
           [0017]      FIG. 3   c  is a circuit arrangement of 120 or 240 of the present invention of the fly-back or forward power supply. 
           [0018]      FIG. 4  is an input current waveform of  50   w  fly-back power supply. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    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 arrangement shown since the invention is capable of other embodiments. 
         [0020]    The topology of the present invention is that a (PFC) forward transformer primary winding is connected in series with the main transformer primary winding. The main transformer transfers power from the primary circuit to the secondary circuit, the forward transformer transfers power to its second winding of the forward transformer to correct the input current waveform. When switch is off, the no loss snubber circuit stores the electrical energy in the capacitor; when the switch is on, it relieves the energy to forward transformer. 
         [0021]    Refer to  FIG. 2 : The circuit diagram is a power factor correction for a single switch power supply unit. The arrangement is following: 
         [0022]    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. A first capacitor C 10  is coupled to the output terminals of the full bridge rectifier BD 20 . 
         [0023]    An inductor L 10  has two terminals, the first terminal and the second terminal which is coupled to the positive output terminal of the full bridge rectifier BD 10 . 
         [0024]    A diode D 10  has a cathode and an anode which is coupled to the first terminal of the inductor L 10 . 
         [0025]    A second diode D 12  has a cathode and an anode which is coupled to the first terminal of the inductor L 10 . 
         [0026]    A forward transformer T 10  has three windings. The second winding T 10   p   2  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 first winding T 10   p   1  has a first terminal and a second terminal which is coupled to the second terminal of the second winding. The third winding T 10   p   3  has a first terminal and a second terminal which is coupled to the negative output of the full-bridge rectifier BD 10 ; The second terminals of three windings have the same electrical polarity; 
         [0027]    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 10   p   2  of the forward transformer T 10  and the negative terminal is coupled to the negative output of the full bridge rectifier BD 10 . 
         [0028]    A main transformer T 12  has a primary winding and a secondary winding. The first terminal of the primary winding is coupled to the first terminal of the first winding T 10   p   1  of the forward transformer T 10 . The secondary winding of the main transformer T 12  is coupled to the secondary circuit. The main transformer can be a forward transformer and fly-back transformer. 
         [0029]    A switch Q 10  has three terminals, a first terminal, a secondary terminal and a control terminal. The first terminal is coupled to the second terminal of the primary winding of the main transformer T 12 . The second terminal is coupled to the negative output of the full bridge rectifier BD 10 . The control terminal is coupled to the PWM or PFM control circuit. 
         [0030]    The third capacitor C 14  has a first terminal and a secondary terminal. The first terminal is coupled to the first terminal of the switch Q 10 . 
         [0031]    The third diode D 14  has an anode coupled the second terminal of the third capacitor C 14  and a cathode coupled to the positive terminal of the second capacitor. 
         [0032]    The fourth diode D 16  has a anode coupled to the terminal of the first winding T 10   p   3  of the forward transformer T 10 , cathode coupled to the second terminal of the third capacitor C 14 . 
         [0000]    The operation of the  FIG. 2  is following: 
         [0033]    When switch Q 10  is on, the third capacitor C 14  discharges its electrical energy through the third winding T 10   p   3  of the forward transformer T 10  and fourth diode D 16 . A current conducts through the primary winding T 10   p   1  of the forward transformer T 10  and the primary winding of the main transformer T 12  and the switch Q 10  and at the same time there is an induced voltage in the second winding T 10   s  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 10   p   2  of the forward transformer T 10 . When switch Q 10  is off, the inductance has a induced voltage and this induced voltage and input voltage force a current charging the second capacitor C 12  through the inductor L 10  and second diode D 12 . The leakage inductances of the main transformer T 12  primary winding and forward transformer primary T 10   p   1  charge the third capacitor C 14  through the third diode D 14 . 
         [0034]    The number of the second winding T 10   p   2  and the third winding T 10   p   3  of the forward transformer T 10  is more than two times of the number of the primary winding of the forward transformer T 10 . 
         [0035]    The number of the primary winding T 10   p  of the forward transformer T 10  and value of the inductor L 10  is adjusted to certain value to correct the input current waveform in the best shape. 
         [0036]    The circuit, comprising of the second winding T 10   p   2  of the forward transformer T 10 , the inductor D 10 , the second capacitor C 12 , the first diode D 10  and the second diode D 12 , can have several different arrangements. These arrangements have the same working principle. For example, the circuit arrangement in  FIG. 2   a  and  FIG. 2   b.    
         [0037]    The snubber circuit, including the third winding T 20   p   3  of the forward transformer T 20 , the third capacitor C 14 , the third diode D 24  and fourth diode D 26 , recovers the the energy of leaking inductance of the main transformer T 12  and the forward transformer T 10 , when the switch Q 10  is off. It relieves the energy, when the switch Q 20  is on. The series connection of the third winding T 20   p   3  and the fourth diode can be insert a inductor of small inductance value. The series circuit can have difference arrangement from the  FIG. 2 . By using this snubber circuit the efficiency of the fly-back power supply can reach as high as 92% for 22v DC output. 
         [0038]    The control signal can be a PWM or a PFM signal and the PFM signal has better advantage for designing single stage PFC power supply of a fly-back type power supply. 
         [0039]    For 110v AC power the circuit can be arranged as  FIG. 3 . 
         [0040]    The inductor L 20  has two terminals and the first terminal is connected to a first power line; 
         [0041]    A first diode D 20  has an anode and a cathode, the anode is coupled to the second power line. 
         [0042]    A third diode D 24  has an anode and a cathode, the cathode is coupled to the second power line. 
         [0043]    A first transformer T 20  has three windings, a first winding T 20   p   1 , a second winding T 20   p   2  and a third winding T 20   p   3 . The second winding T 20   p   2  has a first terminal and a second terminal and a central tap which is coupled to the second power line. A first winding T 20   p   1  has a first terminal and a second terminal and a third winding T 20   p   3  has a first terminal and a second terminal. The first terminals of the three windings have the same electrical polarity. 
         [0044]    A first capacitor C 20  has a positive terminal and a negative terminal, the positive terminal is coupled to cathodes of the firs diode D 20  and the first terminal of the first winding of the forward transformer, the negative terminal is coupled to the second terminal of the inductor L 20 . 
         [0045]    A second capacitor C 22  has a positive terminal and negative terminal, the positive terminal is coupled to the second terminal of the inductor L 20  and the negative terminal is coupled to anode of third diode D 24 . 
         [0046]    A second diode D 22  has an anode and a cathode, the anode is coupled to the first terminal of the second winding T 20   p   2  of the forward transformer T 20  and the cathode is coupled to the positive terminal of the first capacitor C 20  and the cathode of the first diode D 20 . 
         [0047]    A fourth diode D 26  has an anode and a cathode, a cathode is coupled to the second terminal of the second winding T 20   p   2  of the forward transformer T 20  and the anode is coupled to the negative terminal of the second capacitor C 22 . 
         [0048]    A main transformer T 22  has a primary winding and a second winding, the first terminal of the primary winding is coupled to the second terminal of the first winding T 20   p   1  of the forward transformer T 20 . The secondary winding is coupled to secondary circuit. The main transformer can be a forward transformer or a fly-back transformer. 
         [0049]    A switch Q 20  has a first terminal, a second terminal and a control terminal. The control terminal is coupled to the control circuit which produces PWM or PFM control signal circuit. The first terminal is coupled to the second terminal of the primary winding of the main transformer T 22  and the second terminal is coupled to the negative terminal of the second capacitor C 22 . 
         [0050]    A third capacitor C 24  has a first terminal and a second terminal. The first terminal is coupled to the first terminal of the switch Q 20 . 
         [0051]    A fifth diode D 28  has an anode and a cathode, the anode is coupled to the second terminal of the third capacitor C 24  and the cathode is coupled to the positive terminal of the first capacitor C 20 . 
         [0052]    A sixth diode D 30  has an anode and a cathode, the anode is coupled to the second terminal of the third winding T 20   p   3  of the forward transformer T 20  and the cathode is coupled to the second terminal of the third capacitor C 24 . 
         [0053]    The number of the winding of the second winding T 20   p   2  and third winding T 20   p   3  of the forward transformer T 20  is more than two times the number of the winding of the first winding T 20   p   1  of the forward transformer. 
         [0000]    The operation of the  FIG. 3  is following: 
         [0054]    When voltage of the second power line is higher than the voltage of the first power line and when the switch Q 20  is switched on, the electrical energy in the third capacitor C 24  is discharged through the third winding T 20   p   3  of the forward transformer T 20 , the sixth diode D 30  and the switch Q 20 ; A current conducts through the primary winding T 20   p   1  of the forward transformer T 20  and the primary winding of the main transformer T 22  and the switch Q 20  and at the same time there is an induced voltage in the second winding T 20   p   2  of the forward transformer T 20 , therefore, there is a current drawn from the input to the second capacitor C 20  (charging the second capacitor C 20 ) through the inductor L 20 , the second diode D 22  and the second winding T 20   p   2  of the forward transformer T 20 . When switch Q 20  is off, the inductor L 20  has a induced voltage and this induced voltage and input voltage force a current charging the second capacitor C 20  conducts through the inductor L 20  and second diode D 20 . The leakage inductances of the main transformer T 22  primary winding and forward transformer primary T 20   p   1  charge the third capacitor C 24  through the third diode D 28 . 
         [0055]    When voltage of the second power line is lower than the voltage of the first power line and when the switch Q 20  is switched on, the electrical energy in the third capacitor C 24  is discharged through the third winding T 20   p   3  of the forward transformer T 20 , the sixth diode D 30  and the switch Q 20 . A current conducts through the primary winding T 20   p   1  of the forward transformer T 20  and the primary winding of the main transformer T 22  and the switch Q 20  and at the same time there is an induced voltage in the second winding T 20   p   2  of the forward transformer T 20 , therefore, there is a current drawn from the input to the second capacitor C 22  (charging the second capacitor C 22 ) through the inductor L 20 , the fourth diode D 26  and the second winding T 20   p   2  of the forward transformer T 20 . When switch Q 20  is off, the inductor L 20  has a induced voltage and this induced voltage and input voltage force a current charging the second capacitor C 22  through the inductor L 20  and third diode D 24 . The leakage inductances of the main transformer T 22  primary winding and forward transformer primary T 20   p   1  charge the third capacitor C 24  through the third diode D 28 . 
         [0056]    The PFC circuit consisted of second winding T 20   p   2  of the forward transformer, diodes D 20 , D 22 , D 24 , D 25  and the inductor can be have other arrangements for example circuit arrangement of  FIG. 3   a.    
         [0057]    If the second winding is separated into two equal parts, T 20   p   2  and T 20   p   3 , it can have another arrangement as  FIG. 3   b.    
         [0058]    If additional component  4  diodes, D 30 , D 31 , D 32 , D 33 , and a switch S 20  are added to above diagram, it can have  120   v  or  240   v  single switch power supply depended on the switch S 20  position as  FIG. 3   c.