Control circuit and method for a flyback converter to compensate for an entry point of a burst mode

Abstract

A control circuit and method are provided for a flyback converter converting an input voltage to an output voltage, to compensate for an entry point of a burst mode of the flyback converter, so that the entry point is not affected by the input voltage, and audible noise resulted from a higher input voltage is reduced without impacting the light load efficiency of the flyback converter.

What is claimed is: 
     
         1 . A control circuit for a flyback converter including a transformer connected with a power switch switched by a control signal for the transformer to convert an input voltage into an output voltage, the control circuit comprising:
 a compensator compensating a feedback signal with a sawtooth wave to generate a compensated feedback signal, the sawtooth wave independent of the input voltage, the feedback signal being a function of the output voltage; and   a pulse width modulation circuit connected to the compensator to generate the control signal according to the compensated feedback signal and a current sense signal, the current sense signal being a function of a current flowing through the power switch.   
     
     
         2 . The control circuit of  claim 1 , wherein the pulse width modulation circuit comprises:
 a comparator connected to the compensator to compare the compensated feedback signal with the current sense signal to generate a comparison signal; and   a flip-flop connected to the comparator to generate the control signal according to the comparison signal and a clock.   
     
     
         3 . The control circuit of  claim 1 , wherein the compensator comprises an adder having a positive input to receive the feedback signal, a negative input to receive the sawtooth wave, and an output to provide the compensated feedback signal. 
     
     
         4 . The control circuit of  claim 1 , further comprising a burst circuit to determine whether to control the flyback converter to enter a burst mode according to the feedback signal and a preset value. 
     
     
         5 . The control circuit of  claim 4 , wherein the burst circuit comprises a comparator to generate a mask signal according to the feedback signal and the preset value to reduce the number of times the power switch is to be switched. 
     
     
         6 . The control circuit of  claim 5 , wherein the comparator is a hysteresis comparator. 
     
     
         7 . The control circuit of  claim 4 , wherein the sawtooth wave is used to stabilize an entry point of the burst mode.