On the reliability of DC-DC converters

Switching power converters for high voltage and current applications such as the full bridge converter (FBC) with phase-shift control impose large stresses on the switching devices used. Field failures are observed in the MOSFETs, which are used as switching devices. These failures show specific patterns in fault location and devices failing. Since failure is not observed in all the cases for the given conditions, failures must be caused due to some error or defect in the circuit and/or device which does not occur in every specimen. In this paper, the phenomenon underlying the failure is identified to be reverse recovery of the anti-parallel diodes across the MOSFETs. It is shown that under conditions of high di/dt, dynamic avalanching can occur in these MOSFETs. The reverse recovery of these devices is simulated using a 2-D device simulator. High electric field and impact ionization is observed at the junction, substantiating the claim of avalanche process. The diode design for the power MOSFET is found to be incompatible with the circuit requirements making it susceptible to failure due to reverse recovery.