A parallel resonant circuit for use in a series resonant converter. The parallel resonant circuit is located between the clamp diodes and the dominant series resonant capacitors of the converter. The parallel resonant circuit has a predetermined resonant frequency which is set to be above the telephone voice band. The circuit limits the operating frequency of the converter for light loads to be no lower than the parallel resonant frequency.

A d.c. voltage is produced from a single-phase electric mains for supplying an X-ray tube. This supply is pulsed under the control of a synchronizing circuit with which it is further possible to adjust the duration of the operating pulses, as well as their phase blocking with respect to the mains. By acting in this way excessive current is not taken during the voltage troughs between two successive half-waves. The losses by the Joule effect in the mains are reduced and the quality of the pulsed high voltage is improved because of the increased stability. There is only a slight increase in the exposure time.

A high frequency converter of push-pull form comprises a center tapped D.C. input supply 16, 17, 18, two solid state switching devices (20, 21) connected in series across the D.C. input terminals, a load circuit including a supersonic frequency transformer 30 and an output rectifier 35, and an oscillatory circuit including at least one capacitor 40 and a parallel inductor 34 connected in parallel with the primary winding 37 of the transformer between the center tapping 19 of the supply and the junction 25 of the switching devices. The switching devices are synchronized to the oscillation of the oscillatory circuit to allow the circuit to oscillate freely for part of a cycle to transfer energy from the inductor 34 to the capacitor 40 to charge the latter to a certain condition in which the voltage across the parallel inductor is substantial and that across the switching device is negligible and only then connect the parallel inductor across the D.C. input terminals whereupon the parallel inductor receives and stores energy from the said supply. Means such as a saturable reactor 33 are connected in series in the load circuit to restrict load current to a low value during a part of the cycle in which the capacitor receives energy from the parallel inductor and until the capacitor is charged to the said condition and the switching means has connected the load circuit across the supply.

A dual-mode system for operating electrically powered devices, such as hand-held power tools and appliances, includes a cordless battery pack that supplies the power and current demands of the device in a cordless mode or a corded voltage converter that supplies the necessary power and current demands in a physical envelope commensurate in size and interchangeable with that of the battery pack. The corded voltage converter is provided with a high efficiency power supply that allows the converter to generate the power and current required by the driven device and accommodate widely varying loads consequent to normal operation of the device and do so in a physical envelope that is commensurate in size with and interchangeable with the cordless battery pack. The corded voltage converter includes a power conditioner that conditions input source current, such as 120 or 240 volts AC or DC, to provide a relatively high DC voltage that is chopped at a comparatively high frequency, e.g., 25 KHz, by switching devices. The high-frequency, high voltage current is presented to a step-down transformer with the stepped-down output rectified to provide a low-voltage DC current to a drive motor of the device.

Switching stress on the switches of a DC-to-AC inverter bridge are reduced, and fall time losses are reduced, by driving the inverter from a source of constant current, by operating the bridge in a mode in which all the switches are periodically rendered conductive simultaneously, and by tuning the load to produce resonant currents which reduce the currents through the switches of the inverter at the moment of turn-off.