Self-starting carrier subscriber power supply

Abstract

A power supply in carrier subscriber terminal equipment includes a switched DC-to-DC converter that is powered by a DC line voltage and driven by a pulse width modulator for producing a first DC voltage on a bus line that powers the modulator and an oscillator which keys the modulator. Circuitry for starting the converter, modulator, and oscillator includes a first transistor and Zener diode that are resistively connected in series across the line voltage for producing a second DC voltage on the Zener, which is less than the first voltage. A second transistor is turned on by the second DC voltage for coupling it to the bus line to initially energize the oscillator and modulator, which then drives the converter to cause it to produce the first voltage on the bus line. A second output of the oscillator is also coupled through a voltage doubler to the first transistor for turning it and the Zener off. A third transistor is responsive to conduction of the second transistor for clamping the second output of the oscillator to approximately ground for blocking it from the first transistor until the converter produces the first voltage on the bus line, which reverse biases the base-emitter junction of the second transistor for cutting it and the third transistor off to make it possible for the first transistor and Zener diode, and thus the starter circuit, to be turned off.