A high-precision constant current primary side controller with inductance compensation

Abstract

A primary-side regulation AC-DC converter operating in PFM (Pulse frequency modulation) mode with highly accurate output current is designed, which employs a novel inductance compensation technique to improve the precision of output current, diminishing the bad impact of the big tolerance of the transformer primary inductance in the same batch. An improved method to detect the demagnetization time precisely is proposed for the inductance compensation module. In this paper, the output current is regulated by the OSC charging current, which is controlled by CC (constant current) controller. Meantime, for different primary inductors, the inductance compensation module adjusts the OSC charging current finely to improve the accuracy of output current. The operation principle and design of demagnetization time detector and CC controller with inductance compensation module are analyzed and illustrated herein. The control chip is implemented based on TSMC 0.35μm 5V/40V BCD process, and a 12V/1.1A prototype has been built to prove the proposed control method. The deviation of output current is within ±3% and the variation of output current is less than 1% when the inductance of primary windings vary by 10%.