Despite its many benefits, semiconductor technology also has a few drawbacks, one of which is the vulnerability of solid state devices to overvoltages. Even voltage pulses of very low energy can produce interference and damage, sometimes with severe consequences. Optimum overvoltage or transient suppression is a crucial design factor. Varistors have been proven to be excellent protective devices due to their application flexibility and high reliability. The metal oxide varistor, with its extremely attractive price/performance ratio, is an ideal component for limiting surge voltage and current and for absorbing energy. Overvoltage protection devices like varistors are often referred to in international publications as a TVSS (transient voltage surge suppressors).
Figure 1. Circuit symbol for varistor
Varistors (variable resistors) are voltage-dependent resistors with a symmetrical V-I characteristic curve (Figure 2) whose resistance decreases with increasing voltage. Connected in parallel with the electronic device or circuit that is to be guarded, they form a low-resistance shunt when voltage increases and thus prevent any further rise in surge overvoltage.
The voltage dependence of varistors may be approximately characterized by
where I is the current through the varistor, K is the ceramic constant (depending on varistor type), V is the voltage across the varistor, and α is the nonlinearity exponent (measure of nonlinearity of curve). Metal oxide varistors can produce α values that are higher than 30. This puts their protection levels in the same category as those provided by zener diodes and suppressor diodes. Exceptional current handling capability combined with response times of < 25 ns make them an almost perfect protective device.
Microstructure and conduction mechanism
Sintering zinc oxide together with other metal oxide additives under specific conditions produces a polycrystalline ceramic whose resistance exhibits a pronounced dependence on voltage. This phenomenon is referred to as the varistor effect. Figure 3 shows the conduction mechanism of a varistor element in simplified form.
The zinc oxide grains themselves are highly conductive, while the intergranular boundary formed by other oxides is highly resistive. Only at those points where zinc oxide grains meet does sintering produce “microvaristors”, comparable to symmetrical zener diodes (protection level approx. 3.5 V). The electrical behavior of the metal oxide varistor, as indicated by figure 3, results from the number of microvaristors connected in series or in parallel. This implies that the electrical properties are controlled by the physical dimensions of the varistor:
- Twice the ceramic thickness produces twice the protection level because then twice the number of microvaristors are arranged in series.
- Twice the area produces twice the current handling capability because then twice the number of current paths are arranged in parallel.
- Twice the volume produces almost twice the energy absorption capability because then there are twice as many absorbers in the form of zinc oxide grains.
The series and parallel connection of the individual microvaristors in the sintered body of a varistor also explains its high electrical load capacity compared to semiconductors. While the power in semiconductors is dissipated almost entirely in one thin p-n junction area, in a varistor it is distributed over all the microvaristors, i.e. uniformly throughout the component’s volume. Each microvaristor is provided with energy absorbers in the form of zinc oxide grains with optimum thermal contact. This permits high absorption of energy and thus exceptionally high surge current handling capability.
For matching very different levels of protection to ceramic thicknesses that are suitable for fabrication, varistors have to be produced from ceramics with different voltage gradients. The variation of raw materials and sintering processes influence the growth of grain size (grain diameter approx. 10 to 100 μm and thus produce the required specific ceramic voltage (approx. 30 to 250 V/mm). The V-I characteristic of the individual microvaristors is not affected by this. Ceramics with a small specific voltage (40 V for low-voltage types) cannot handle the same current density as high-voltage types. That explains the differences in surge current, energy absorption and mechanical dimensions within various types. Sintered metal oxide ceramics are processed on different production lines:
The varistor disk is fitted with leads of tinned copper wire and then the ceramic body is coated with epoxy resin in a fluidized bed.
Disk varistors in housing
The disk varistors are fitted into a housing for special overvoltage field application.
ThermoFuse (ETFV) types
These are designed for self-protection under abnormal overvoltage conditions.
Fail-safe (SFS) types
No flame or rupture under specified test conditions (see “Reliability data”, “Overvoltage test” in the data sheet).
The large electromagnetic forces involved in handling currents between 10 kA and 100 kA call for solid contacting with special electrodes and potting in a plastic housing. Block varistors are electrically and mechanically connected by screw terminals.
After contacting of the varistor ceramics with special bolt-holed electrodes, these components are coated with epoxy resin in a fluidized bed.
Figure 4 shows the simplified equivalent circuit of a metal oxide varistor. From this, the behavior of the varistor can be interpreted for different current ranges.
Leakage current region (< 10–4 A)
In the leakage current region, the resistance of an ideal varistor goes towards ∞ so it can be ignored since the resistance of the intergranular boundary will predominate. Therefore RB << RIG. This produces the equivalent circuit in Figure 5. The ohmic resistance RIG determines behavior at low currents, and the V-I curve becomes linear (downturn region). RIG shows a distinct temperature dependence, so a marked increase in leakage current must be expected as temperature increases.
Normal operating region (10–5 to 103 A)
With RVAR << RIG and RB << RVAR , RV determines the electrical behavior (Figure 6). The V-I curve follows to a good approximation the simple mathematical description by a power function where α > 30, i.e. the curve appears more or less as a straight line on a log-log scale (see V-I Characteristics - Varistors).
High-current region (I> 103A)
Here the resistance of the ideal varistor approaches zero. This means that RVAR << RIG and RVAR < RB (Figure 7). The ohmic bulk resistance of ZnO causes the V-I curve to resume a linear characteristic (upturn region).
Equivalent circuits 4 and 5 indicate the capacitance of metal oxide varistors (see product specifications for typical values). In terms of overvoltage suppression, a high capacitance is desirable because, with its lowpass characteristic, it smooths steep surge voltage edges and consequently improves the protection level.
The response time of the actual varistor ceramics is in the picosecond range. In the case of leaded varistors, the inductance of the connecting leads causes the response time to increase to values of several nanoseconds. For this reason, all attempts must be made to achieve a mounting method with the lowest possible inductance, i.e. shortest possible leads.
L - Lead Inductance (≈ nH/mm)
C - Capacitance
RIG - Resistance of intergranular boundary (ρ ≈ 10 12 to 10 13 Ω cm)
RVAR - Ideal varistor (0 to ∞ Ω)
RB - Bulk resistance of ZnO (ρ ≈ 1 to 10 Ω cm)
For more information, please read: