Improving current sharing between parallel varistors

Metal oxide varistors are widely used in many power electronics circuits to protect against transient over voltages. Certain applications can be very demanding on the energy handling capability of the varistors. Consequently, varistors are connected in parallel to achieve the required energy rating. This paper presents a method to improve the current sharing between parallel varistors when used in power electronic systems. In the conduction region, the dynamic resistance of the varistor is very small. Hence, a small resistance connected in series with each varistor can control the current sharing. The clamping voltage is unaffected and the energy dissipation in the series resistance is modest. Experimental results confirm the effectiveness of this method.