Pulse transformers

Pulse transformers capable of transmitting substantially rectangular voltage pulses, with durations of less than one microsecond, were developed for radar applications during World War II. Their primary functions were to match the impedances of high-power microwave radio-frequency electron-tube generators to electronic pulse generators and coaxial transmission cables, and to provide polarity reversal and impedance matching functions within pulse generator circuitry. The principle contributor to the development of satisfactory pulse transformers was the development of cores of thin-gauge magnetic materials having 1 μs pulse permeabilities in the range of 500 to 3500 for flux-density changes of 10 000 G. The development of void free dry-type insulation systems made it possible to produce dry-type pulse transformers for operation at pulse voltages below 12 kV. The pulse transformer development work was paralleled by analytical work which enabled pulse transformer designers to make designs to meet the requirements of the radar circuit designers. The analysis relates the three principle pulse transformer parameters, which are magnetizing inductance, leakage inductance, and effective distributed capacitance, in combination with the circuit parameters which are source resistance, effective load resistance, and load shunt capacitance, to the rise time, top ripple and droop, and fall time of the output pulse. Pulse transformers now cover a wide range of sizes and power levels and pulse widths. Small units in computer applications may deliver only a few volts at pulse widths less than 0.1 μs. Large units in linear accelerators, however, may deliver pulse powers of 70 MW at 270 kV with a pulse width of about 3 μs.