Effect of material and design parameters on the life and operating voltage of a ZnO varistor
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I. INTRODUCTION When a ZnO varistor 1 ^ is connected to a continuous ac voltage stress, a leakage current constantly flows through the device.5^12 This leakage current consists of a capacitive component (Ic) and a resistive component (IR ). The latter is responsible for the Joule heating of the device13 and also increases faster with time than the capacitive component.14 The situation worsens with increasing applied voltage and/or ambient temperature.15 The voltage and temperature dependence of IR is believed to be the result of a field-activated, thermally assisted diffusion of zinc interstitial, a native defect in ZnO. This diffusion occurs at the depletion layer of the grain boundary junction, resulting in a decreased junction voltage that effectively lowers the junction impedance and increases IR at a constant applied voltage.16 In the limiting case, the current increases so rapidly with time that the device "runs away" thermally, and ends its useful life (Fig. 1). A convenient approach for predicting "life" is to assume that life terminates when the current or power reaches a critical value at a given temperature and applied voltage. This approach has been taken by several workers17"19 who have assumed, rather arbitrarily, that this critical value is reached when the current or power is "doubled." In practice, however, there is always a thermal balance, and only when the generated watts exceed the dissipated watts can the varistor be considered technically "dead." Thus, for every design, there is a "limiting power" that a varistor is allowed to generate before it thermally runs away. Based on this concept, a procedure has been developed for estimating the lifetime of gapless metal oxide surge arresters for ac application.20 In this article this concept of limiting power has been further expanded to determine the effect of material and design parameters on a varistor's life. Furthermore, this paper provides a methodology for establishing the useful life of all ZnO-based varistors, arrestJ. Mater. Res. 2(2), Mar/Apr 1987
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ers, and surge suppressors under a variety of device performance and design conditions. II. THERMAL RUNAWAY AND LIMITING POWER Before developing the equations governing a varistor's life, it is worthwhile to establish the conditions for thermal runaway and define a limiting power that a varistor is allowed to generate under a given set of conditions. The importance of limiting power is that when the generated watts exceed that power, the varistor can be considered technically dead. The condition for thermal runaway is that PG, the quantity of heat generated by the device when subjected to a steady-state voltage stress, exceeds PD, the quantity of heat dissipated from the body: P •*>P
When a sinusoidal peak voltage is applied to the device, the power generated per unit volume of the material can be expressed as20 PG^VIR-
(2)
T = Constant V = Constant
Steady state
t (Time) FIG. 1. Current rise with time at constant temperature and voltage indicating schematical
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