Methods and power controllers for primary side control

Abstract

Power controllers and related primary-side control methods are disclosed. A disclosed power controller has a comparator and an ON-triggering controller. The comparator compares a feedback voltage with an over-shot reference voltage. Based on an inductance-coupling effect, the feedback voltage represents a secondary-side voltage of a secondary winding. Coupled to the comparator, the ON-triggering controller operates a power switch at about a first switching frequency when the feedback voltage is lower than the over-shot reference voltage. The ON-triggering controller operates the power switch at about a second switching frequency when the feedback voltage exceeds the over-shot reference voltage. The second switching frequency is less than the first switching frequency.

What is claimed is: 
     
         1 . A primary-side control method comprising:
 providing a feedback node, coupled to an auxiliary winding;   generating a feedback voltage by sampling the feedback node during discharging of the secondary winding;   controlling a power switch in a first switching frequency;   comparing the feedback voltage and an over-shot reference voltage; and   controlling the power switch in a second switching frequency when the feedback voltage is greater than the over-shot reference voltage;   wherein the second switching frequency is lower than the first switching frequency, and the feedback voltage represents a secondary-side voltage of the secondary winding through an inductance-coupling effect.   
     
     
         2 . The primary-side control method of  claim 1 , further comprising:
 turning on the power switch if an auxiliary winding voltage of the auxiliary winding is approximately within a voltage valley, when the feedback voltage is lower than the over-shot reference voltage; and   turning on the power switch after the power switch turned off for a maximum OFF time, when the feedback voltage is higher than the over-shot reference voltage.   
     
     
         3 . The primary-side control method of  claim 1 , further comprising:
 comparing the feedback voltage and a target voltage and a target voltage to control a compensation voltage; and   controlling ON time of the power switch according to the compensation voltage.   
     
     
         4 . The primary-side control method of  claim 1 , further comprising:
 determining the first switching frequency according to the compensation voltage.   
     
     
         5 . A power supply controller for performing primary-side control, comprising:
 a comparator for comparing a feedback voltage and an over-shot reference voltage, wherein the feedback voltage represents a secondary-side voltage of a secondary winding through an inductance-coupling effect;   a sampler coupled between the comparator and a feedback node coupled to an auxiliary winding, the sampler sampling the feedback node to generate the feedback voltage; and   an ON-triggering controller coupled to the comparator, wherein when the feedback voltage is lower than the over-shot reference voltage, the ON-triggering controller causes a power switch to operate at approximately a first switching frequency, and when the feedback voltage is higher than the over-shot reference voltage, the ON-triggering controller causes the power switch to operate at approximately a second switching frequency;   wherein the second switching frequency is lower than the first switching frequency.   
     
     
         6 . The power supply controller of  claim 5 , further comprising:
 a pulse generator for providing a pulse during discharging of the secondary winding for causing the sampler to sample the feedback node.   
     
     
         7 . The power supply controller of  claim 6 , further comprising:
 a transconductor for comparing the feedback voltage and a target voltage to control a compensation voltage.   
     
     
         8 . The power supply controller of  claim 7 , wherein the ON-triggering controller is an oscillator for providing a periodic signal to trigger turning on of the power switch, and the compensation voltage determines a switching frequency of the periodic signal. 
     
     
         9 . The power supply controller of  claim 6 , wherein:
 the ON-triggering controller is an OFF time controller coupled to a feedback node;   the OFF time controller triggers turning on of the power switch if an auxiliary winding voltage of the auxiliary winding is approximately in a voltage valley, when the feedback voltage is lower than the over-shot reference voltage; and   the OFF time controller triggers turning on of the power switch after the power switch turned off for a maximum OFF time, when the feedback voltage is larger than the over-shot reference voltage.