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

Microseconds range pulse modulators based on solid state technology often utilize a pulse transformer, since it could offer an inherent current balancing for parallel connected switches and with the turns ratio the modulator design could be adapted to the available semiconductor switch technology. In many applications as e.g. radar systems, linear accelerators or klystron/magnetron modulators a rectangular pulse shape with a fast rise time and a as small as possible overshoot is required. In reality, however, parasitic elements of the pulse transformer as leakage inductance and capacitances limit the achievable rise time and result in overshoot. Thus, the design of the pulse transformer is crucial for the modulator performance. 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 core material, the limited switching speed of semiconductors and the nonlinear impedance characteristic of a klystron are analyzed.