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Zinc oxide varistor microstructure and properties

Zinc oxide varistor is a zinc oxide as the main body, add a polycrystalline semiconductor ceramic element more metal oxides, classic electronic ceramics made. Its microscopic structure shown in Figure 1. Zinc oxide ceramics are made of zinc oxide grains and grain boundary material composition, wherein the zinc oxide crystal doped with donor impurities and N-type semiconductor material contains large amounts of grain boundary formation of large metal oxide interface states, so that each microscopic unit is a back to back Schottky barrier, the entire ceramic is composed of many back to back Schottky barrier mat combination of series and parallel. Figure 2 is a varistor equivalent circuit.
  
Zinc oxide varistors typical VI characteristic curve shown in Figure 3:


Pre-breakdown region: In this region, is applied to both ends of the varistor voltage is less than its breakdown voltage, the conductivity is hot excited electronic conductivity mechanism. Therefore, varistors equivalent to a more than 10MΩ insulation resistance (Rb much larger than Rg), then through the varistor resistive current of only microampere level, can be seen as open. The area is a state in which the varistor circuit during normal operation.

Breakdown zone: When applying a voltage greater than the breakdown voltage over both ends of the varistor, which belong to the tunnel breakdown conducting electronic conductivity mechanism (Rb and Rg equivalent), the voltage characteristics were excellent non-linear conductance characteristic, namely :
I = CVα
I  -- current through the varistor
C  -- formulation and process-related constants
V  -- varistor voltage across
α  -- is nonlinear coefficient, generally greater than 30
Seen from the above equation, the breakdown area, small changes varistor terminal voltage can cause dramatic changes in current, varistors it is with this feature to suppress over-voltage amplitude and to absorb or release too energy voltage surges caused.

Upslope: When overvoltage great so by varistor current is greater than about 100A / cm2, the voltage characteristics of the varistor voltage characteristics mainly by the grain resistance to decide. At this voltage characteristics of the varistor linear conductance characteristics, namely:
I = V / Rg
Upslope current and voltage almost linear relationship, varistors has deteriorated in the region, lost its suppression overvoltage surge absorb or release energy and other characteristics.
The conductive mechanism varistor, its over-voltage fast response, such as with leaded and special electrode products, the general response time of less than 25 nanoseconds. So long as the proper selection and use, varistors on the line appearing transient overvoltage has excellent inhibition thereby protecting the circuit components against overvoltage damage other purpose.


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