Design of a Compact Epoxy Molded Pulsed Transformer

For some pulsed transformers, a primary coil should bear severe mechanical stresses caused by high-current (up to MA) pulses, and a secondary coil should be sufficiently insulated so that electrical breakdown would not occur owing to high voltage. To satisfy the above conditions, we applied two techniques in designing a relatively compact air-core foil-wound step-up pulsed transformer. A tapered secondary copper foil was sandwiched tightly between multi-layers of adhesive films and insulating polyester films, and the transformer assembly was molded with epoxy resin under low vacuum condition. The turn ratio of the transformer was designed to be 1:18, resulting in a coupling coefficient of 0.73, estimated based on measured inductance values. The size of the transformer was 140 mm in overall diameter and 400 mm in total length, and its weight was 13 kg. The transformer was tested in ambient air by using a capacitor bank and a fuse opening switch connected serially. Neither electric breakdown nor mechanical damages occurred at a pulse with peak current of 180 kA. Output voltage of 220 kV was obtained at input of 19 kV. The output pulse width was 400 ns FWHM when a resistive load of 300 ohm was connected to the secondary coil.