Design of an oscillator with off-time modulation for AC-DC converter

An oscillator with off-time modulation (OTM) for AC-DC converter is designed in this paper. For the oscillator, capacitor and current sources are the key elements. The charging current source is constant while discharging current decreases in OTM mode. A feedback voltage, which is the output of voltage-loop error amplifier, is taken as an index of load. In light load and no load conditions, the feedback voltage modulates the discharge current, and then the off-time and the frequency of clock signal changes linearly with load. The function verification and the electrical characteristics simulation are performed with HSPICE. From the simulation results, the frequency of oscillator linearly decreases from 42.5 kHz to 455 Hz according to the output load, which means the function of OTM is realized.