Neutron-induced failure tests of 3300-V IGBTs for the Spallation Neutron Source accelerator

The RF transmitters for the 1-GeV Spallation Neutron Source (SNS) linear accelerator are powered by 15 converter-modulator stations. Each converter-modulator provides pulses up to 11 MW peak power with a 1.1 MW average power to a multiple-klystron load. A low-voltage switching network, comprised of three sets of four IGBTs in an 'H-bridge' configuration, is used to generate the 20 kHz drive waveform components for the three step-up transformer primaries. This setup brings the total number of 3300-volt semiconductor IGBTs operating within the accelerator at 180. When biased to operating voltages of 1500 V or greater, all IGBTs are inherently sensitive to neutron interactions caused by background cosmic ray radiation. The project described used neutrons produced by proton spallation (with a similar spectrum as cosmic ray neutrons) to gain knowledge of failure mechanisms of the SNS IGBTs. These tests were performed using the highly accelerated neutron flux generated at the Weapons Neutron Research facility at the Los Alamos National Laboratory. The results of these experiments have shown the existence of a critical threshold voltage that lies within the operational range beyond which the IGBTs are no longer effective. This threshold varied with each brand of IGBT tested. Statistical failure times have also been projected for all brands of IGBT tested under normal biased operation with the SNS accelerator given natural neutron flux in Oak Ridge, TN.