Flyback power supply with forced primary regulation

Abstract

A power supply includes an energy transfer element having first, second and third windings. An input of the first winding is coupled to an input of the power supply and an output of the second winding is coupled to an output of the power supply. A secondary control circuit is coupled across the second winding to switch a switched element coupled to the second winding in response to a difference between an actual output value and a desired output value to force a current in the third winding that is representative of the difference between the actual output value and the desired output value. A primary control circuit is coupled to a primary switch and to the third winding. The primary control circuit is coupled to switch the primary switch in response to the current forced in the third winding by the secondary control circuit.

1 . A power supply, comprising:
 an energy transfer element having first, second and third windings, wherein an input of the first winding is coupled to an input of the power supply and an output of the second winding is coupled to an output of the power supply;   a switched element coupled to the second winding;   a secondary control circuit coupled across the second winding to switch the switched element in response to a difference between an actual output value at the output of the second winding and a desired output value to force a current in the third winding that is representative of the difference between the actual output value at the output of the second winding and the desired output value;   a primary switch coupled to a first winding; and   a primary control circuit coupled to the primary switch and to the third winding, the primary control circuit coupled to switch the primary switch to regulate the output of the power supply in response to the current forced in the third winding by the secondary control circuit.   
     
     
         2 . The power supply of  claim 1  wherein the energy transfer element comprises coupled inductors. 
     
     
         3 . The power supply of  claim 1  wherein the second winding is galvanically isolated from the first and third windings. 
     
     
         4 . The power supply of  claim 1  wherein the switched element is coupled to be switched to have a first impedance or a second impedance, wherein the first and second impedances are not equal. 
     
     
         5 . The power supply of  claim 4  wherein the first impedance is substantially zero and the second impedance is substantially infinite. 
     
     
         6 . The power supply of  claim 1  wherein the switched element is unidirectional. 
     
     
         7 . The power supply of  claim 1  wherein the switched element is bidirectional. 
     
     
         8 . The power supply of  claim 1  wherein the primary control circuit comprises a shunt regulator coupled to receive the current forced in the third winding by the secondary control circuit to extract an error signal from the forced current, wherein the primary control circuit is coupled to switch the primary switch in response to the error signal. 
     
     
         9 . The power supply of  claim 1  wherein the secondary control circuit comprises a secondary error amplifier coupled to a voltage divider coupled across the output of the second winding and a reference voltage to determine the difference between the actual output value at the output of the second winding and the desired output value. 
     
     
         10 . The power supply of  claim 1  wherein the secondary control circuit comprises a shunt regulator coupled to a voltage divider coupled across the output of the second winding, the shunt regulator having an internal reference voltage, the shunt regulator coupled to determine the difference between the actual output value at the output of the second winding and the desired output value. 
     
     
         11 . The power supply of  claim 1  wherein the switched element comprises a synchronous rectifier coupled to the second winding. 
     
     
         12 . The power supply of  claim 1  wherein the secondary control circuit is coupled to control an output current and an output voltage of the power supply with a drive signal coupled to the switched element. 
     
     
         13 . The power supply of  claim 1  wherein the secondary control circuit is coupled to cause the switched element to reduce a current from the second winding during a portion of an off time of the primary switch to force the current in the third winding. 
     
     
         14 . The power supply of  claim 1  wherein the power supply is a flyback converter.