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. 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 problem because it has lower input current amplitude through the inductor than that of the two stage PFC power supply.

Description:
The present invention relates to a power converter, and more particularly, to a power factor correction (PFC) power converter in a single stage. 
     DESCRIPTION OF THE RELATED ART 
     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. 
     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  which 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 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 a PFC function. The present invention provides a single stage PFC converter that reduces the cost and the size, i.e. device counts, and improves the converter efficiency. The present invention can further provide a power converter operating in lower stress to obtain higher reliability. 
     SUMMERY OF THE INVENTION 
     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. 
     The second objective of this invention is to provide for a one stage power factor correction in an AC to DC converter. 
     The third objective of the present invention is to provide a simple circuit of PFC power supply to reduce the manufacture cost. 
     The fourth objective of the present invention is to provide a more efficient PFC power supply circuit. 
     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 
         FIG. 1  is a schematic of the prior art, the two stage AC to DC converter. 
         FIG. 2  is a schematic of the present invention of a half-bridge, or a full-bridge, or a LLC power supply or an electronic ballast. 
         FIG. 2   a  is an arrangement of a half-bridge or a full-bridge or a LLC power supply or an electronic ballast in accordance with the present invention. 
         FIG. 2   b  is an embodiment of the present invention of the primary circuit of a half bridge power supply. 
         FIG. 3  is a schematic of the present invention of a single switch or a pull-push power supply. 
         FIG. 3   a  is an embodiment of the present invention of the primary circuit of single switch power supply. 
         FIG. 4  is a schematic of the present invention of a half-bridge or a LLC power supply of the 120 v AC input. 
         FIG. 5  is a schematic of the present invention of a fly-back or a forward power supply of the 120 v AC input. 
         FIG. 6  is an input current waveform of 250 w computer power supply. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 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. 
     Refer to  FIG. 2   
     The converter includes a bridge rectifiers BD 10 , two capacitors (C 10 , C 12 ), two diodes (D 10 , D 12 ), a forward transformer including two windings (T 10   p , T 10   s  with center-tap) and a main transformer T 12  including a primary winding and a secondary winding. The bridge rectifier BD 10  has input terminals, which are coupled to the input power lines, and two output terminals: a positive output terminal and a negative output terminal. 
     A first capacitor C 10  is coupled to output terminals of the full bridge rectifier BD 10 . 
     An inductor L 10  has a first terminal which is coupled to the positive output of the full-bridge rectifier and its a second terminal. 
     The first diode D 10  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
     A second diode D 12  has a cathode and an anode which is coupled to the second terminal of the inductor L 10 . 
     A forward transformer T 10  has a primary winding T 10   p , a second winding T 10   s  with a center tap. The second winding T 10   s  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 first diode D 12 . The primary winding T 10   p  is connected in series with the primary winding of the main transformer in the circuit of a half bridge or a full bridge or a LLC power supply or connected in series with the inductor of an electronic ballast. 
     The second capacitor C 12  has a negative terminal which is coupled to the negative output terminal of the bridge rectifier BD 10  terminal and a positive terminal which is coupled to the center tap of the second winding of the forward transformer T 10 . 
     The Operation of the  FIG. 2  (Half-Bridge Power Supply  FIG. 2   b  as Example): 
     When the first switch Q 10  is turned on, a current discharging from the second capacitor C 12  conducts through the first switch Q 10 , the primary winding T 10   p  of the forward transformer T 10 , the primary winding T 12   p  of the main transformer T 12  and the fourth capacitor C 16  and at the same time, the current from the output of the full bridge rectifier BD 10  conducts through the inductor L 10 , the first diode D 10  and the second winding T 10   s  of the forward transformer to charge the second capacitor C 12  and the third capacitor C 14  because there is an induced voltage in the second winding of the forward transformer. When the first switch Q 10  is turned off, the induced voltage in the inductor L 10  and the input voltage force a current which conducts through the second diode D 12  to charge the second and the third capacitor capacitors. 
     When the second switch is turned on, a current discharging from the third capacitor C 14  conducts through the fourth capacitor C 16 , the primary winding T 12   p  of the main transformer T 12 , the primary winding T 10   p  of the forward transformer T 10  and the second switch Q 12  and at the same time, the current from the output of the bridge rectifier BD 10  conducts through the inductor L 10  the second diode D 12  and the second winding T 10   s  of the forward transformer to charge the second capacitor C 12  and the third capacitor C 14 . When the second switch Q 12  is turned off, the induced voltage in the inductor L 10  and the input voltage force the current which conducts through the first diode D 10  to charge the second and the third capacitors. 
     The number of the windings of the second winding T 10   s  of the forward transformer T 10  is more than four times of that of the primary winding T 10   p  of the forward transformer T 10 . 
     The number of the windings of the primary winding 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. 
     For 120V AC power source, the circuit can be arranged as  FIG. 4 . The number of the windings of the second winding of the forward transformer T 30  is more than two times of that of the primary winding of the forward transformer. The number of the windings of the primary winding is designed to provide enough energy to correct the input current waveform. 
       FIG. 6  is one waveform of the half-bridge computer power supply prototype. 
     The circuit, comprising of the second winding T 10   s  of the forward transformer T 10 , the inductor L 10 , the first diode D 10 , the second diode D 12 , the second capacitor C 12  and the third capacitor C 14 , can have numerous different arrangements. These arrangements have the same working principle. For example, an arrangement of these components is: The inductor L 10  is coupled between the positive output terminal of the full bridge rectifier BD 10  and the positive terminal of the second capacitor C 12 ; The first diode D 10  has a cathode coupled to the negative output terminal of the full bridge rectifier BD 10 ; The second winding T 10   s  of the forward transformer T 10  has its center tap coupled to the negative terminal of the third capacitor C 14  and its first terminal coupled to the anode of the first diode D 10 ; The second diode D 12  has the cathode coupled to the negative output terminal of the full bridge rectifier BD 10  and the anode coupled to the second terminal of the second winding T 10   s  of the forward transformer T 10 . 
     Refer to  FIG. 3 : 
     The circuit diagram is a power factor correction for a single switch power supply unit or a push-pull power supply unit according the present invention. 
     A full bridge rectifier BD 20  has output terminals La positive terminal and a negative terminal) and input terminals which are coupled to AC power lines. 
     A first capacitor C 20  is coupled to the output terminals of the full bridge rectifier BD 20 . 
     An inductor L 20  has two terminals, the second terminal and the first terminal which is coupled to the positive output terminal of the full bridge rectifier BD 20 . 
     A first diode D 20  has a cathode and an anode which is coupled to the second terminal of the inductor L 20 . 
     A second diode D 22  has a cathode and an anode which is coupled to the second terminal of the inductor L 20 . 
     A forward transformer T 20  has two windings. The second winding T 20   s  has a first terminal which is coupled to the cathode of the first diode D 20  and a second terminal which is coupled to the cathode of the second diode D 22 . The primary winding T 20   p  of the forward transformer T 20  is connected to the primary winding of the main transformer T 22  in series. 
     A second capacitor has a positive terminal and a negative terminal. The positive terminal is coupled to the second terminal of the second winding T 20   s  of the forward transformer T 20  and the negative terminal is coupled to the negative output of the full bridge rectifier BD 20 . 
     A primary winding of the main transformer T 22  can be the primary winding of a fly-back transformer of a fly-back power supply unit, a forward transformer of a power supply unit or a push-pull power supply unit. 
     The Operation of the  FIG. 3   a  is Following: 
     When switch Q 20  is on, a current conducts through the primary winding of the main transformer T 22  and the primary winding T 20   p  of the forward transformer T 20  and at the same time there is an induced voltage in the second winding T 20   s  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 first diode D 20  and the second winding T 20   s  of the forward transformer T 20 . When switch Q 20  is off, the current charging the second capacitor C 22  conducts through the inductor L 20  and second diode D 22 . 
     The number of the windings of the second winding of the forward transformer T 20  is more than two times of that of the primary winding of the forward transformer T 20 . 
     The number of the windings of the primary winding T 20   p  of the forward transformer T 20  and the value of the inductor L 20  are adjusted to a certain value to correct the input current waveform in the best shape. 
     The circuit, comprising of the second winding T 20   s  of the forward transformer T 20 , the inductor L 20 , the second capacitor C 22 , the first diode D 20  and the second diode D 22 , can have several numerous different arrangements. These arrangements have the same working principle. For example, the inductor L 20  can be coupled between the negative terminal of the second capacitor C 22  and the negative output terminal of the full bridge rectifier BD 20 . The anodes of the first diode D 20  and the second diode D 22  are coupled to the positive output terminal of the full bridge rectifier BD 20 . The second winding T 20   s  of the forward transformer T 20  is coupled between the cathode of the first diode D 20  and the junction of the cathode of the second diode D 22  and the positive terminal of the second capacitor C 22 . 
     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. 
     For 120V AC power source, the circuit can be arranged as  FIG. 5 . The number of the windings of the second winding of the forward transformer is more than two times of that of the primary winding of the forward transformer.