Post Pulse Behaviour of Active Modulators Using Pulse Transformers for Magnetron Applications

Active modulators using pulse transformers are an option for pulsing magnetrons. In this context, active implies a single or a number of paralleled or a number of series connected or a series/parallel combination of semiconductor switches used to pulse a magnetron via a step up pulse transformer. The pulse is delivered by a large capacitor bank (pulse energy is very much less than the capacitor stored energy) and the switches must be of a type that can be turned both on and off such as MOSFETs or IGBTs. A problem can arise because of the biased diode nature of the magnetron load (typically a magnetron that works at 20 kV/20 A will draw very little current at 16 kV, ref figure 1). Thus when the active switches are turned off the current pulse in the magnetron is terminated. The stray capacitance on the output of the pulse transformer however remains charged to almost full voltage. This can give rise to a complex series of events which can damage the active switches. This paper will carefully define the problem and describe a solution