Method and apparatus for controlling the maximum output power of a power converter

Abstract

An example control circuit for use in a power converter includes an input voltage sensor, a current sensor, and a drive signal generator. The input voltage sensor generates a first signal representative of an input voltage (Vin) of the power converter. The current sensor generates a second signal representative of a switch current through a power switch of the power converter. The drive signal generator generates a drive signal to control switching of the power switch in response to the first and second signals. The drive signal generator adjusts a duty cycle of the drive signal based on a product K×Vin×t to control a maximum output power of the power converter, where K is a fixed number and t is a time it takes the second signal to change between two values of the switch current when the power switch is in an on state.

What is claimed is: 
     
         1 . A control circuit for use in a power converter, the control circuit comprising:
 an input voltage sensor coupled to generate a first signal representative of an input voltage (Vin) of the power converter;   a current sensor coupled to generate a second signal representative of a switch current through a power switch of the power converter; and   a drive signal generator configured to generate a drive signal to control switching of the power switch in response to the first and second signals, wherein the drive signal generator adjusts a duty cycle of the drive signal based on a product K×Vin×t to control a maximum output power of the power converter, wherein K is a fixed number and t is a time it takes the second signal to change between two values of the switch current when the power switch is in an on state.   
     
     
         2 . The control circuit of  claim 1 , wherein the fixed number K is substantially equal to 1 to compensate for variances in an inductance value of an energy transfer element of the power converter. 
     
     
         3 . The control circuit of  claim 1 , wherein the fixed number K is substantially equal to 2 to compensate for variances in a protective current limit threshold of the power switch. 
     
     
         4 . The control circuit of  claim 1 , wherein the fixed number K is a number between the values of 1 and 2 to compensate for variances in both an inductance value of an energy transfer element of the power converter and a protective current limit threshold of the power switch. 
     
     
         5 . The control circuit of  claim 4 , wherein the fixed number K is substantially equal to 1.3. 
     
     
         6 . The control circuit of  claim 1 , wherein one of the two values of switch current is substantially zero. 
     
     
         7 . The control circuit of  claim 1 , wherein one of the two values of switch current is a protective current limit threshold of the power switch. 
     
     
         8 . The control circuit of  claim 1 , wherein the drive signal generator is configured to adjust the duty cycle of the drive signal by adjusting a power switch protective current limit threshold of the power switch. 
     
     
         9 . The control circuit of  claim 1 , wherein the drive signal generator is configured to adjust the duty cycle of the drive signal by controlling a period of time that the power switch is in the on state during each switching cycle period. 
     
     
         10 . The control circuit of  claim 1 , wherein the drive signal generator generates the drive signal to control switching of the switch to regulate an output of power converter in response to a feedback signal. 
     
     
         11 . A power converter, comprising:
 an energy transfer element coupled between an input and an output of the power converter;   a power switch coupled to the energy transfer element; and   a control circuit coupled to control switching of the switch to regulate the output of the power converter, wherein the control circuit includes:
 an input voltage sensor coupled to generate a first signal representative of an input voltage (Vin) of the power converter; 
 a current sensor coupled to generate a second signal representative of a switch current through a power switch of the power converter; and 
 a drive signal generator configured to generate a drive signal to control switching of the power switch in response to the first and second signals, wherein the drive signal generator adjusts a duty cycle of the drive signal based on a product K×Vin×t to control a maximum output power of the power converter, wherein K is a fixed number and t is a time it takes the second signal to change between two values of the switch current when the power switch is in an on state. 
   
     
     
         12 . The power converter of  claim 11 , wherein the fixed number K is substantially equal to 1 to compensate for variances in an inductance value of an energy transfer element of the power converter. 
     
     
         13 . The power converter of  claim 11 , wherein the fixed number K is substantially equal to 2 to compensate for variances in a protective current limit threshold of the power switch. 
     
     
         14 . The power converter of  claim 11 , wherein the fixed number K is a number between the values of 1 and 2 to compensate for variances in both an inductance value of an energy transfer element of the power converter and a protective current limit threshold of the power switch. 
     
     
         15 . The power converter of  claim 14 , wherein the fixed number K is substantially equal to 1.3. 
     
     
         16 . The power converter of  claim 11 , wherein one of the two values of switch current is substantially zero. 
     
     
         17 . The power converter of  claim 11 , wherein one of the two values of switch current is a protective current limit threshold of the power switch.