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