Problems with transient voltage suppresors and their solutions

A number of modulators have been developed that use series connections of high voltage FET modules to provide pulses of up to 55 kV at 250 A. During preproduction testing a number of FET modules failed, which we established was due to load arcs. This was not expected since the arc testing programme conducted during the development had been completely successful. The major problem was traced to the silicon transient voltage suppressors (TVS) used to provide over voltage protection across the FET modules. It was found that the TVS devices on the FET modules had different characteristics than the initial samples evaluated despite being procured as the same part. Many devices from a selection of suppliers when tested with high peak power (>50 kW) and short duration pulse (<1 us) exhibited a wider parameter spread than our original evaluation suggested. FET avalanching and the nature of the magnetron arcs were also part of the problem. The paper will give an overview of the problems and how we established the root causes, and discuss the selection of TVS style devices and component issues. Comparison of the modulator arc fault tolerance before and after this programme will be presented showing a substantial improvement in reliability. Test fixtures used will be described with results from extensive pulse and life tests