Degradation phenomena of ZnO varistors

The dc degradation behaviour of ZnO varistors as a function of the variables current, temperature, and time is investigated. The degradation rate is phenomenologically presented by an equation reflecting the observed aging behaviour in dependence on the influencing variables. Accordingly, the degradation rate consists additively of the fraction of degradation during current load and the fraction of recovery. The degradation rate during current load is proportional to current strength. The temperature dependence can be described by an Arrhenius factor (activation energy EA). The degradation rate of recovery is proportional to a nonlinear function of degradation degree. It is directed against degradation rate by current load and is also dependent on temperature (Arrhenius factor with activation energy ER). The varistor-dependent constants of the equation can relatively rapidly be determined at higher currents (in the mA range) and temperatures (125 to 165°C). The applicability of the equation to other loading cases is discussed.