Start circuit for generation of pulse width modulated switching pulses for switch mode power supplies

摘要

A start circuit for starting the generation of pulse width modulated switching pulses is disclosed for use in a DC/DC switching converter power supply. The starting circuit is responsive to the initiation of a DC voltage source and starts the operation of a pulse generator. The starting circuit also includes a relaxation oscillator connected to the DC voltage source for periodically generating a start pulse at a predetermined frequency. Ongoing power is coupled to the pulse generator once the generator begins operation. The relaxation oscillator is selectively disabled when the pulse generator is powered by the ongoing power supplied by the power supply during normal operation. The start circuit further includes a delay function for delaying the disabling of the relaxation oscillator for a preselected period after shutdown of the power supply and includes a rapid start function for reducing the time between restoration of input power and the operation of the start circuit.