Apparatus and method for sensing of isolated output

Abstract

A switched-mode power supply includes an energy transfer element coupled between a primary side and a secondary side. A first main terminal of a switch is coupled to the energy transfer element and a second main terminal of the switch is coupled to an input of the primary side. A driver circuit is coupled to drive the switch to be open at a first one of a plurality of levels and closed at a second one of the plurality of levels. The driver circuit is coupled to drive the switch to be substantially independent of a voltage between the first and second main terminals at a third one of the plurality of levels. A current conducted between the first and second main terminals at the third one of the plurality of levels is sufficient to only partially discharge a capacitance that is coupled to the first main terminal.

What is claimed is: 
     
         1 . A switched-mode power supply comprising:
 an energy transfer element coupled to transfer energy between a primary side of the switched-mode power supply and a secondary side of the switched-mode power supply;   a switch having a first main terminal, a second main terminal, and a control terminal, wherein the first main terminal is coupled to the energy transfer element and the second main terminal coupled to an input of the primary side of the switched-mode power supply;   a driver circuit coupled to drive the switch at one of a plurality of levels, wherein the driver circuit is coupled to drive the switch to be open between the first and second main terminals at a first one of the plurality of levels, wherein the driver circuit is coupled to drive the switch to be closed between the first and second main terminals at a second one of the plurality of levels, wherein the driver circuit is coupled to drive the switch to be substantially independent of a voltage between the first and second main terminals at a third one of the plurality of levels, and wherein a current conducted between the first and second main terminals at the third one of the plurality of levels is sufficient to only partially discharge a capacitance that is coupled to the first main terminal; and   a controller coupled to receive a current sense signal representative of the current conducted between the first and second main terminals, the controller further coupled to receive a feedback signal representative of a regulated output of the switched-mode power supply, wherein the controller is coupled to output a selection signal to the driver circuit to drive the switch at one of the plurality of levels in response to the current sense signal and the feedback signal.   
     
     
         2 . The switched-mode power supply of  claim 1  wherein the driver circuit is coupled to drive the switch at one of the plurality of levels to control a transfer of energy from the primary side of the switched-mode power supply to the secondary side of the switched-mode power supply. 
     
     
         3 . The switched-mode power supply of  claim 1  wherein the switch comprises a transistor coupled to the energy transfer element and the input of the primary side of the switched-mode power supply. 
     
     
         4 . The switched-mode power supply of  claim 3  wherein the transistor is coupled to be OFF in response to the driver circuit driving the switch at the first one of the plurality of levels. 
     
     
         5 . The switched-mode power supply of  claim 3  wherein the transistor is coupled to be ON in response to the driver circuit driving the switch at the second one of the plurality of levels. 
     
     
         6 . The switched-mode power supply of  claim 1  wherein the transistor comprises a MOSFET coupled to operate in a saturation region of the MOSFET in response to the driver circuit driving the switch at the third one of the plurality of levels. 
     
     
         7 . The switched-mode power supply of  claim 1  wherein the feedback signal is coupled to be generated in response to a reflected signal representative of the regulated output of the switched-mode power supply. 
     
     
         8 . A switched-mode power supply comprising:
 an energy transfer element coupled to transfer energy between a primary side of the switched-mode power supply and a secondary side of the switched-mode power supply, the energy transfer element having a bias winding coupled to produce a bias winding voltage responsive to an output voltage of the switched-mode power supply when a rectifier coupled to the secondary side conducts a secondary current;   one or more switches coupled between the energy transfer element and an input of the primary side of the switched-mode power supply;   switch driver circuitry coupled to drive the one or more switches in a plurality of modes, wherein in a first one of the plurality of modes, current conduction across the one or more switches is substantially zero, wherein in a second one of the plurality of modes, current conduction across the one or more switches is sufficient to drive the bias winding voltage to a magnitude that is representative of the output voltage of the switched-mode power supply, and wherein in a third one of the plurality of modes, current conduction across the one or more switches has a sufficiently low magnitude and duration that the rectifier coupled to the secondary side conducts the secondary current without the bias winding voltage being driven to a magnitude that is representative of an input voltage of the switched-mode power supply; and   feedback circuitry coupled to receive a feedback signal representative of the bias winding voltage and a current sense signal representative of the current conduction across the one or more switches, wherein the feedback circuitry is coupled to generate a mode select signal to which the switch driver circuitry is responsive to drive the one or more switches in the plurality of modes to control the transfer of energy between the primary side of the switched-mode power supply and the secondary side of the switched-mode power supply.   
     
     
         9 . The switched-mode power supply of  claim 8  wherein the feedback circuitry includes a first feedback circuit coupled to the energy transfer element to generate a first signal in response to a reflected signal representative of the output voltage of the switched-mode power supply. 
     
     
         10 . The power converter of  claim 8  wherein the feedback circuitry includes a second feedback circuit coupled to the energy transfer element to generate a second signal in response to a portion of a decaying oscillation in a reflected signal representative of the output voltage of the switched-mode power supply. 
     
     
         11 . The switched-mode power supply of  claim 8  wherein in the third one of the plurality of modes, the current conduction across the one or more switches is sufficient to only partially discharge a capacitance that is coupled to the one or more switches. 
     
     
         12 . The switched-mode power supply of  claim 8  wherein switch driver circuitry is coupled to drive the one or more switches in the plurality of modes to control the transfer of energy between the primary side of the switched-mode power supply to the secondary side of the switched-mode power supply. 
     
     
         13 . An integrated circuit for use in a switched-mode power converter, the integrated circuit comprising:
 a first terminal to be coupled to an energy transfer element that is coupled between a primary side of the power converter and a secondary side of the switched-mode power converter;   a second terminal to be coupled to an input of the switched-mode power converter;   a third terminal to be coupled to receive a current sense signal representative of a current conduction between the first terminal and the second terminal;   a fourth terminal to be coupled to receive a feedback signal representative of a regulated output of the switched-mode power converter;   a current control circuit coupled to the first terminal, the second terminal, the third terminal, and the fourth terminal, the current control circuit coupled to operate in one of a first mode, a second mode, and a third mode in response to the current sense signal and the feedback signal, wherein in the first mode the current control circuit is coupled to conduct substantially no current between the first terminal and the second terminal, wherein in the second mode the current control circuit is coupled to conduct current between the first terminal and the second terminal that is sufficient to fully discharge a capacitance coupled to the first terminal, and wherein in the third mode the current control circuit is coupled to conduct current between the first terminal and the second terminal that is sufficient to only partially discharge the capacitance coupled to the first terminal.