Patent Publication Number: US-2013229833-A1

Title: Feedback circuit and control method for an isolated power converter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of co-pending application Ser. No. 12/838,988, filed on Jul. 19, 2010, for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 098124622 filed in Taiwan, R.O.C. on Jul. 21, 2009 under 35 U.S.C. §119, the entire contents of all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to an isolated power converter and, more particularly, to a feedback circuit and control method for an isolated power converter. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1 , a conventional isolated power converter  10  includes a rectifier circuit  12  to convert an alternating-current (AC) voltage VAC to a direct-current (DC) voltage Vin applied to a primary coil Lp of a transformer T 1  via a snubber  16 , a power switch  18  serially connected to the primary coil Lp, and a controller  14  to generate a control signal Vgate according to a feedback signal Vcomp and a sense signal Vcs proportional to the current Ip flowing through the primary coil Lp, to switch the power switch  18  to convert the voltage Vin to an output voltage Vout. The controller  14  has a power input pin VDD to receive a supply voltage Vcc. A feedback circuit  20  detects the output voltage Vout to generate the feedback signal Vcomp for the controller  14 . The feedback circuit  20  includes an opto-coupler  22  and a Zener diode  28  functioning as a shunt regulator. The opto-coupler  22  generates a current Icomp according to the output voltage Vout to determine the feedback signal Vcomp. The opto-coupler  22  includes a light-emitting diode (LED)  24  as an input device and a transistor  26  as an output device. A current Id proportional to the output voltage Vout flows to the ground via the LED  24  and the Zener diode  28 , The opto-coupler  22  amplifies the current Id flowing through the LED  24  to generate the current Icomp flowing through the transistor  26 . The Zener diode  28  is connected to the LED  24  to limit the maximum voltage at the cathode of the LED  24 . 
     When the loading of the power converter  10  becomes lighter, the output voltage Vout increases and thereby causes an increase in the current Id flowing through the LED  24 . As a result, the current Icomp flowing through the transistor  26  increases accordingly. The increased current Icomp pulls the feedback signal Vcomp lower and thereby shortens the on time of the power switch  18 . However, the increase in the currents Id and Icomp also implies higher power consumption, which lowers the efficiency of the power converter  10  at light loading. 
     Therefore, it is desired an apparatus and method for improving the light load efficiency of an isolated power converter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a feedback circuit and control method for improving the light load efficiency of an isolated power converter. 
     According to the present invention, a feedback circuit for an isolated power converter using a controller to switch a power switch to convert an input voltage to an output voltage, includes an opto-coupler coupled to an output of the isolated power converter to amplify a first current related to the output voltage to generate a second current, a current-to-voltage converter connected to the opto-coupler to generate a first voltage according to the second current, a reversed polarity regulator connected to the opto-coupler to decrease the first current in response to an increase in the output voltage during a light load period, a voltage source to provide a second voltage, and a start up circuit coupled to the current-to-voltage converter and the voltage source to select one of the first voltage and the second voltage as a feedback signal for the controller. 
     According to the present invention, a control method for an isolated power converter using a controller to switch a power switch to convert an input voltage to an output voltage, includes amplifying a first current related to the output voltage to generate a second current by an opto-coupler, decreasing the first current in response to an increase in the output voltage during a light load period, generating a first voltage according to the second current, and selecting one of the first voltage and a preset second voltage as a feedback signal for the controller. 
     Due to the first and second currents in the opto-coupler decreasing in response to an increase in the output voltage during the light load period, the light load efficiency of the isolated power converter is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a conventional isolated power converter; 
         FIG. 2  is a circuit diagram of a feedback circuit according to the present invention; and 
         FIG. 3  is a circuit diagram of another embodiment for the reversed polarity regulator shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a circuit diagram of a feedback circuit  30  according to the present invention. Referring to  FIG. 2  in conjunction with  FIG. 1 , in the feedback circuit  30 , an opto-coupler  40  includes a transistor  42  as an output device of the opto-coupler  40  connected between the power input pin Vcc and a current-to-voltage converter  46 , and an LED  44  coupled to the output of the isolated power converter  10 , the current Id flowing through the LED  44  and related to the output voltage Vout is amplified by the opto-coupler  40  to produce a current Icomp flowing through the transistor  42 , a reversed polarity regulator  48  is connected to the opto-coupler  40  to control the current Id such that the current Id decreases or increases in response to an increase or decrease in the output voltage Vout, respectively, the current-to-voltage converter  46  includes a resistor Rco to generate a voltage VA according to the current Icomp provided by the opto-coupler  40 , and a start up circuit  32  operates to ensure start of the isolated power converter  10 . More specifically, while the isolated power converter  10  is being started, the start up circuit  32  selects a voltage Vbias as the feedback signal Vcomp for the controller  14 ; after the isolated power converter  10  is started, the start up circuit  32  selects the voltage VA as the feedback signal Vcomp for the controller  14 . 
     In the start up circuit  32 , a switch SW 1  is connected between the voltage source Vbias and the controller  14 , a switch SW 2  is connected between the current-to-voltage converter  46  and the controller  14 , a comparator  34  receives and compares the voltages Vbias and VA to generate a comparison signal Sc 1 , a hysteresis comparator  36  receives and compares the supply voltage Vcc and a reference voltage Vref 1  to generate a comparison signal Sc 2 , and a flip-flop  38  has a set terminal S and a reset terminal R to receive the comparison signals Sc 1  and Sc 2 , respectively, to switch the switches SW 1  and SW 2  according to the comparison signals Sc 1  and Sc 2 . When the isolated power converter  10  is started, both the voltages VA and Vcc are zero and consequently, the comparison signal Sc 1  is low and the comparison signal Sc 2  is high. Hence, the output signal Q of the flip-flop  38  is low and thus turns on the switch SW 1  and turns off the switch SW 2 . At this time, the voltage Vbias is selected as the feedback signal Vcomp for the controller  14 , causing the output voltage Vout, the voltage VA, and the supply voltage Vcc all to increase. When the voltage VA becomes higher than the voltage Vbias, the comparison signal Sc 1  transits to high, and thus the output signal Q of the flip-flop  38  is set high, thereby turning off the switch SW 1  and turning on the switch SW 2 . At this time, the voltage VA is selected as the feedback signal Vcomp for the controller  14 . 
     In the reversed polarity regulator  48 , a bipolar junction transistor (BJT)  50  has its collector and emitter coupled to the output of the isolated power converter  10  and the LED  44 , respectively, and a Zener diode  52  is connected between the base of the BJT  50  and ground GND to limit the maximum voltage at the base of the BJT  50 . When the load of the isolated power converter  10  becomes lighter, the output voltage Vout increases, so the voltages at the collector and the emitter of the BJT  50  increase accordingly. Furthermore, due to the voltage at the base of the BJT  50  limited by the Zener diode  52 , the voltage VBE between the base and the emitter of the BJT  50  decreases in response to the increase in the output voltage Vout. According to the current formula of the BJT  50 , the BJT  50  conducts the current 
         Id=Is×e   (VBE/VT)    [Eq-1]
 
     where Is is a scale current and VT is the thermal voltage. From the equation Eq-1, the current Id decreases with a decrease in the voltage VBE. In other words, at light load, the current Id decreases in response to the increase in the output voltage Vout, and hence the current Icomp decreases as well. Accordingly, the voltage VA as the feedback signal Vcomp also decreases and thereby shortens the on time of the power switch  18 . 
       FIG. 3  is a circuit diagram of another embodiment for the reversed polarity regulator  48  shown in  FIG. 2 , which includes a PMOS transistor  54 , an operational amplifier  56 , and resistors Rd 1  and Rd 2 . Referring to  FIG. 3  in conjunction with  FIG. 1 , the PMOS transistor  54  is connected between the output of the isolated power converter  10  and the LED  44  of the opto-coupler  40 , the resistors Rd 1  and Rd 2  divide the output voltage Vout to generate a voltage Vd, and the operational amplifier  56  controls the channel thickness of the PMOS transistor  54  according to a reference voltage Vref 2  and the voltage Vd. When the load of the isolated power converter  10  becomes lighter, the voltage Vd increases with the output voltage Vout. Accordingly, the operational amplifier  56  provides a higher voltage to the gate of the PMOS transistor  54  and thereby reduces the channel thickness of the PMOS transistor  54 . As a result, the currents Id and Icomp are decreased. 
     When the isolated power converter  10  employing the feedback circuit  30  of the present invention operates at light load, the currents Id and Icomp in the opto-coupler  40  decrease in response to an increase in the output voltage Vout, thus improving the light load efficiency of the isolated power converter  10 . 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.