Fast-Rise-Time Pulse Transformers Built From Rotated Stacked 1:1 Transformers

A new type of fast-rise-time pulse transformer based on transmission-line transformer (TLT) techniques is described. The transformer is wound using loosely coupled wire pairs or a two-wire twin cable rather than transmission lines in order to avoid high-voltage (HV) cable-failure problems, which are experienced with commercial coaxial transmission lines commonly used in TLTs. Although the high-frequency response associated with conventional TLTs is not quite as good, when using wire pairs, it is still quite possible to design high-turns-ratio transformers with rise times of below 100 ns. However, pulse rise times can be a little longer than this due to the switching-speed limitations of the switches used in the pulse generators that drive the transformers. The frequency response of the transformer can be predicted by a relatively simple transformer model, which can also be used to predict the rise-time performance with a variety of loads. The model can be further used to determine how much inductance is required at each stage of the transformer to achieve the specified low-frequency performance. It also shows that mutual coupling of windings within the transformers can significantly improve their low-frequency response and pulse droop. The results are presented on both low-voltage test transformers and medium-scale HV pulse transformers driven by an insulated-gate-bipolar-transistor switched-pulse generator