SWITCHING CONTROL CIRCUIT HAVING OFF-TIME MODULATION TO IMPROVE EFFICIENCY OF PRIMARY-SIDE CONTROLLED POWER SUPPLY

摘要

A voltage-waveform detector produces a voltage-feedback signal and a discharge-time signal by multi-sampling a voltage signal of a transformer. The discharge-time signal represents a discharge time of a secondary-side switching current. A voltage-loop error amplifier amplifies the voltage-feedback signal and generates a control signal. An off-time modulator generates a discharge-current signal and a standby signal in response to the control signal and an under-voltage signal indicating a low supply voltage of the controller. An oscillator produces a pulse signal determining the off-time of the switching signal in response to the discharge-current signal. A PWM circuit generates the switching signal in response to the pulse signal and the standby signal. The standby signal further controls the off-time of the switching signal and maintains a minimum switching frequency.