Degradation of low voltage metal oxide varistors in power supplies

The unprecedented proliferation of electronic devices in critical applications, such as smart grid and communication systems, has brought new challenges in system reliability. Electronic circuits and components are especially sensitive to transient overvoltages and lightning surges, and design must consider operation under these worst case situations. To address these transients, Metal Oxide Varistors (MOVs) are widely used in electronic circuits and power supplies. They absorb surges in the circuit input and maintain the voltage within acceptable limits. However, these surges and overvoltage transients can degrade MOVs over time. Reliability of an MOV depends on maintaining its Energy Absorption Capability (EAC) in an acceptable range. In this paper, for the first time, the effect of degradation is investigated on Energy Absorption Capability (EAC) of low voltage MOVs by experimental test in a UL-certified lab. The results show that EAC is decreasing with degradation due to surges. Furthermore, it is shown that Time-to-Failure (TtF) of the degraded MOVs is longer than that of new MOVs in some transient overvoltage (TOV) incidents. This helps explain the actual mechanism of MOV failures in real life applications and questions whether the widely accepted design guideline that a 10% change in varistor voltage at 1 mA is a degradation indicator. Finally, a nonlinear resistive - capacitor model is proposed for degraded MOVs.