A DC voltage up-converter circuit having an inductor in series between the relatively low value of DC voltage and the stepped up output of the circuit, wherein the output end of the inductor is periodically switched to ground potential and an output storage capacitor stores the energy surges which result when the ground connection is disconnected. The cathode of a zener diode is connected to the output to provide a limit to the output of the circuit and the anode of the zener diode is connected to means for control limiting the amount of current through the switching means when said ground potential is connected.

A circuit for regulating the voltage across a capacitor which is charged by a pulse driven voltage converter. In the present circuit the voltage across the capacitor is sampled during each pulse period. The width of the pulse employed to drive the converter is then adjusted in response to this voltage to the lowest value required to replace the charge lost by the capacitor since the preceding pulse.

When the oscillator which supplies the alternating drive voltage for a liquid crystal display fails, the circuit of the present application removes the direct current operating voltage from the liquid crystal driver transistors. The oscillator drives a voltage converter which produces the operating voltage. A transistor in series with the line carrying the operating voltage is controlled by the operating voltage developed by the converter so that when the oscillator fails and the converter goes off, the transistor cuts off.

A voltage supply circuit comprises an input for receiving a DC input voltage having one of plural possible levels; a DC-AC inverter for inverting the DC input voltage and which includes a boosting transformer having a winding with end terminals and an intermediate terminal and an inverter drive circuit for providing an AC current to the transformer; a connection between the input and the intermediate terminal of the boosting transformer for applying the DC input voltage to the latter; and a control circuit for controlling the inverter drive circuit, and which selects between a first condition, in which the inverter drive circuit is made operative so that a boosted voltage is provided at one end terminal of the boosting transformer, and a second condition, in which the inverter drive circuit is made inoperative, so that the DC input voltage introduced at the intermediate terminal is provided as a DC output voltage at the end terminal of the booster transformer. The control circuit can include a zener diode as a level detector to control a switching transistor so that the first and second conditions are selected on the basis of the level of the DC input voltage.

A power supply system for use in conjunction with digital systems and particularly adapted to avoid impairment of digital system performance by power supply generated noise. The clock frequency of the digital system is monitored, divided and then applied to control the switching frequency of a D.C. to D.C. power converter. The converter thus switches at a synchronized submultiple of the digital clock frequency such that switching transients are confined to transition times within the digital system.