Zinc Oxide Varistors Introduction
ZINC OXIDE VARISTORS.PROTECTION FUNCTION
APPLICATION
Definition of the varistor effect
The varistor effect is defined as being the property of anymaterial whose electrical resistance changes non-linearly
with the voltage applied to its terminals. In other words, within a given current range, the current-volt-age relationship can be expressed by the equation:I = KV In which K represents a constant depending on the geome-try of the part and the technology used and
IntergranularphaseZinc oxidegrains the non-lin-earity factor.The higher the value of this factor, the greater the effect. Theideal (and theorical) case is shown in Figure1 where = ∞ whereas a linear material has an equation of I = f(V) obeyingthe well-known Ohm’s law ( = 1).The relationship between these two extreme cases is shownin Figure 2. It should be pointed out that the I = f(V) curve issymmetrical with respect to zero in the case of zinc oxide
varistors.
CurrentVoltage0 = CurrentVoltage0 2 - Equivalent electrical circuit diagram
= 1 Figure 4 explains the behavior of a zinc oxide varistor. r rep-resents the equivalent resistance of all semiconductinggrains and that of the intergranular layer (the value of whichbasically varies with the applied voltage). Cp corresponds tothe equivalent capacitance of the intergranular layers.When the applied voltage is low, the resistivity of the inter-granular layer is quite high and the current passing through
the ceramic is low. When the voltage increases, the resis-tance
Figure 1Figure 2
Figure 4Figure 5
Figure 3 decreases (region II in Figure 5).When a certain voltage value is reached,
ZINC OXIDE VARISTORS
becomes lowerthan r and the I = f(V) characteristic tends to become ohmic
(region III).The equivalent capacitance due to the insulating layersdepends on their chemical types and geometries.
Values of a few hundred picofarads are usually found withcommonly used discs.Capacitance value decreases with the area of the ceramic.Consequently, this value is lower when maximum permissi-
ble energy and current values in the varistor are low, since
these latter parameters are related to the diameter of the
disc.Capacitance values are not subject to outgoing inspection.
1-Composition of the material
Zinc oxide varistors are a polycrystalline structured material
consisting of semiconducting zinc oxide
industrial crystals and a sec-ond phase located at the boundaries of the crystals.This second phase consists of a certain number of metallicoxides (Bi
2 O
3 ,MnO,Sb
2 O
3 , etc.). It forms the «heart»of thevaristor effect since its electrical resistivity is a non-linear
function of the applied voltage.Thus, a zinc oxide varistor consists of a large number ofboundaries (several millions) forming a series-parallel net-
work of resistors and
capacitors manufacturer, appearing somewhat like amultijunction semiconductor.Experimentally, it is found that the voltage drop (at 1mA) ateach boundary is about 3V. The total voltage drop for the
thickness of the material is proportional to the number N ofboundaries.V
r {
Zinc oxidegrainsgrainsboundaries Current Cp {
>r >r r> VoltageIIIIII Ï = f (V) 1mA 3 N where N = —in which L represents the average dimension of a zinc oxidegrain and t the thickness of the material.In other words: V tLtL
1mA 3 —Thus, with a thickness of 1 mm and average dimension ofL = 20 µ, we obtain a voltage of 150 V for a current of 1mA.The desired voltage at 1mA can thus be obtained either bychanging the thickness of the disc or by controlling the aver-
age dimension of the zinc oxide grain through heat treatment
or, yet again, by changing the chemical composition of thevaristor.The polycrystal is schematically represented in Figure 3. Atroom temperature the semiconducting grains have very low
resistivity (a fews ohms/cm).
On the contrary, the resistivity of the second phase (or inter-granular layer) basically depends on the value of the applied
voltage.If the voltage value is low, the phase is insulating (region I ofthe I = f(V) curve). As the voltage increases this phase
becomes conductive (region II). At very high current valuesthe resistivity of the grain can become preponderant and theI = f(V) curve tends towards a linear law (region III).The curve I = f(V) for the different types can be found in cor-responding data sheets.
3 3