Design procedure for compact pulse transformers with rectangular pulse shape and fast rise times

Pulse modulators based on solid state technology and for pulses in the μs-range often utilize a pulse transformer, since it could offer an inherent current balancing for parallel connected power semiconductors and the turns ratio of the pulse transformer allows to adapt the modulator design to the available switch technology. The applications like radar systems, linear accelerators or klystron/magnetron modulators usually require a nearly rectangular pulse shape with a fast rise time and a as small as possible overshoot. In reality however, parasitic elements of the pulse transformer as leakage inductance and capacitances limit the achievable rise time and define the resulting overshoot. Therefore, in modulators based on pulse transformers, the design of the pulse transformer is crucial. In this paper, a step by step design procedure of a pulse transformer for rectangular pulse shape with fast rise time is presented. Different transformer topologies are compared with respect of the parasitic elements, which are then calculated analytically depending on the mechanical dimensions of the transformer. Additionally, the influence of the limited switching speed of semiconductors and the nonlinear impedance characteristic of a klystron is analyzed.