Accurate feed-forward sensing in flyback-transformer based secondary controller

摘要

A secondary side controller for an AC-DC converter and method for operating the same are provided. Generally, the controller includes a single synchronous rectifier sense (SR-SNS) pin coupled to a drain of a SR on a secondary of a transformer to sense a voltage (V_SRD). In feed-forward (FF) mode V_SRD is a sum of a voltage (V_IN) on a primary divided by a turn-ratio (N) of the transformer and an output bus voltage (V_BUS). A voltage-to-current (V2I) converter coupled to the SR-SNS pin and to the output bus removes V_BUS from V_SRD. A sample and hold (S/H) module coupled to the SR-SNS pin samples a voltage (V_SAMP) including information on V_IN/N. A V_IN/N V2I converter coupled to the S/H module converts V_SAMP to a feed-forward current (I_FF), and a cancellation and signal module coupled thereto extracts information on V_IN from I_FF and generates signals to control the AC-DC converter.