Analysis of a voltage controlled frequency foldback technique that improves short circuit protection for buck derived converters

The output VI characteristics of most high frequency buck-derived PWM power converters exhibit a current tail during fault conditions. A current tail is actually an increase in the average output current as the output voltage decreases toward zero. Ideally, the controller's peak current limit function should prevent the average output current from exceeding a specified limit. In reality, the tail is due to the turn-off propagation delay inherent in the controller and in the power switch. Here, a frequency foldback scheme is proposed to reduce the effects of the propagation delay during an output overload or fault condition. Frequency foldback refers to decreasing the PWM's oscillator frequency during short circuit or overload conditions and allows the duty cycle to decrease below the value normally limited by the total propagation delay.