Effects of ZnO varistor degradation on the overvoltage protection mechanism of electronic boards

In this study, we investigate degradation of ZnO varistors and their effects on electronic boards. We propose a model showing how the varistor clamping voltage Vv changes by time for different stress levels. To evaluate the model, we exploit measured experimental test data as well as field return data with over 1000 board failures. Varistors in-use are usually subjected to a long-term AC or DC voltage stress and surge which may lead to degradation of the varistors with an increase of the Vv parameters. We show that degraded varistors cause dramatic failures of other components in the same power block of the electronic board. We perform experiments on varistor degradation and failing mechanisms by applying 8/20 us, 2 ms and accelerated AC voltage test methodologies. We also simulate the relation of the varistor degradation with other components in time domain.