Passive clamp for on/off control of a capacitor charger

Claim 1

In a D.C. to D.C. converter adapted to be energized by a power supply, the converter comprising a transformer including a primary winding and a secondary winding, a capacitor coupled to the secondary winding for accumulating electric energy, first transistor switching means having a base electrode and an emitter electrode, the switching means being coupled in series with the power supply and the primary winding for periodically interrupting power supply current to the primary winding, and a control circuit for detecting the charge status of the capacitor, the control circuit being arranged to produce a converter ON signal and a converter OFF signal, the improvement comprising: second transistor switching means having a base electrode, a collector electrode and an emitter electrode; current regulating means connected between the control circuit and the base electrode of said second transistor switching means, said current regulating means being adapted to assume a first state in response to the converter ON signal and a second state in response to the converter OFF signal; semiconductor means connected between the base electrode of the first transistor switching means and the emitter electrode of said second transistor switching means, and providing a first predetermined voltage at the emitter electrode of said second transistor switching means when said current regulating means assumes its first state; and a resistor connected between the power supply and the junction between the base electrode of said second transistor switching means and said current regulating means, the size of said resistor being such that (1) when said current regulating means is in said first state a second predetermined voltage is developed at the base electrode of said second transistor switching means, the difference between said first and second voltages rendering said second switching means non-conductive, and (2) when said current regulating means is in said second state, the first transistor switching means is turned off and said second transistor switching means assumes a stand-by state in which electrical noise of a particular polarity and sufficient magnitude may cause the first and second transistor switching means turn on, but in which the voltage drop across the emitter-base junction of the first transistor switching means is clamped by the collector-emitter voltage of the second transistor switching means at a low level that is insufficient to drive the first transistor switching means into heavy conduction, whereby the first transistor switching means turns off once such noise terminates.