Switching control circuit with variable switching frequency for primary-side-controlled power converters

摘要

The invention presents a switching control circuit for a primary-side-controlled power converter. A pattern generator produces a digital pattern to control a programmable capacitor that is connected to an oscillator, which produces frequency hopping to reduce the EMI. A voltage-waveform detector produces a voltage-feedback signal and a discharge-time signal by multi-sampling a voltage signal of a transformer. A current-waveform detector and an integrator generate a feedback signal. The integration of a current-waveform signal with a timing signal generates the average-current signal. Time constant of the integrator is correlated to the switching frequency. The oscillator generates the timing signal and a pulse signal in response to the output of a current-loop error amplifier. A PWM circuit generates the switching signal in response to the pulse signal and the output of a voltage-loop error amplifier for switching the switching device and regulating the output of the power converter.