Varistors Structure, Function and Design

As the symbol hints the varistor is a voltage dependent resistor. Primarily it is used for taking care of voltage transients of different shapes. Unlike thermistors it has no practical time delay of its function.

R 5.3.1 Function and design

Just as thermistors the varistor is fabricated from a compressed and sintered powder compound that consists of silicon carbide (SiC) or zinc oxide (ZnO). The ZnO varistor has superior characteristics and predominates today all manufacture. This section deals with ZnO varistors only. If we sinter zinc oxide together with other metal oxide additives there is formed a polycrystalline ceramic. In the intergranular boundaries of the zinc oxide grains are formed P-N junctions with semiconductor characteristics – so-called Double Schottky Barriers – with rectifying characteristics. Each ZnO grain with adherent boundaries acts as a miniature varistor with asymmetrical V/I characteristic. The grain compound can be regarded as a conglomerate of “microvaristors” connected in series and parallel. Thus the load and energy absorption capability in that way will be totally superior to conventional semiconducting elements. There the power generation exclusively will occur in the thin P-N boundary while in the varistor it takes place in all the microvaristors distributed homogeneously in the whole body.

Of the different designs discs are predominant, with radial leads and a variety of encapsulations, or with whole, metallized surfaces intended for series connection by piling the separate varistors. Encapsulations, for example lacquer or the like, require high tensional stability and resistance against washing agents. Of the uncovered types a certain tubular design intended for connectors should also be mentioned At manufacturing the goal is a grain compound with a regular structure, especially in high voltage/ high energy varistors.

An irregular structure impedes the heat conduction and may lead to an uneven current distribution and occurring Hot Spots and local degradation of the material. In low voltage applications homogeneity is of vital importance. The combination of a multi-electrode construction and small, uniform grains has created the multilayer varistor (MLV), preferably used as SMD, intended for low voltage applications.

R 5.3.2 Some definitions

Maximum working voltage

Maximum working voltage = the maximum voltage which continuously may be applied across the varistor.

Varistor voltage

The varistor voltage = the voltage across the varistor when a current of 1 mA is passed through the body.

Maximum clamping voltage

By maximum clamping voltage we mean the peak voltage across the varistor when it is subjected to a specified peak pulse current with a specified waveform. A common test pulse is the IEC specified so called 8/20 pulse with a specified current intensity.

Figure R5-26. Standard current pulse according to the IEC 60 with t1 = 8 µs and t2 = 20 µs.

Maximum transient peak current

The maximum current that may be allowed through a varistor depends on pulse shape and pulse width, repetition frequency and number of cycles. A starting point for the pulse capability is given by the maximum peak current of 8/20 shape that changes the varistor voltage with maximum 10 %. R

5.3.3 V / I characteristic

The V/I curve of a varistor has tolerances. When the leakage current is specified one presupposes the highest possible current at a certain voltage. The clamping voltage – the one occurring across the varistor when subjected to transients – is specified as the highest possible at a certain current. Thus the specifications are controlled by the unbroken line in Figure R5-28.

R 5.3.4 Principles of voltage limitation

If the varistor is to operate as a voltage limiter a series impedance is required in the circuit. Common straight conductors have both resistance and inductance (≈ 1nH/mm) and provide in themselves a certain protection. Practically, however, the problem is solved by a series resistor of some hundred(s) of ohms (Figure R5-30).

Figure R5-30. An example of voltage limitation by a varistor + series resistor combination.

R 5.3.5 Response time

When a varistor is struck by a voltage transient the voltage rises across the component until the cutting characteristics start. There is, however, a short delay, a dynamic effect, that produces a certain overshoot, according to Figure R5-31. As an example there is shown an incoming pulse with a rise time of 10 kV/µs. The response of the varistor time is specified as the rise time t₂ – t₁ in the figure. The response time seldom exceeds 20 ns; response time of multilayer varistor SMDs usually is less than 1 ns. The response time sometimes is called turn on time.

Figure R5-31. Response time t₂-t₁ of a varistor.

R 5.3.6 Choice of type

Choosing the correct varistor presupposes a thorough determination of working voltage, including tolerances, expected transients, their duration, pulse energy and repetition frequency. In addition we have to take into consideration environmental requirements and mounting conditions. Connecting two varistors in parallel in order to increase the current load capability should be done only if the varistor voltages (at 1 mA) differ from each other by less than 1 %. Otherwise the unbalance of the load may be considerable and in the most unfortunate cases amounts to 1000:1.

R 5.3.7 Failure modes

The most common failure mode is short-circuiting which may occur after unacceptably high and energy-rich voltage pulses or after an operation at a steady-state voltage beyond the voltage rating. In the latter case, the failure mode might change into open-circuit due to melting effects in the terminal joints or rupture of the package when the large amount of energy generated causes expulsion of the packaging material.

R 5.3.8 Reliability

The reliability of non-linear resistors, including varistors, is today regarded as relatively good. Not least it depends on serious manufacturers using SPC based production programs and having verified type approvals. Examples of climatic category classified products 40/125/56 are other indications of a good quality. Furthermore, if we choose glass/metal packages the quality will be even better.

Nuclear radiation

Tests with radioactive radiation in the form of neutron, β and γ radiation show that varistors are capable of withstanding high intensities without any effects on the characteristics.

Table R5-3. VARISTORS

* Occurring capacitance leads to a capacitive shunting that makes the varistor unusable at higher frequencies.

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[1] EPCI European Passive Components Institute experts original articles
[2] CLR Passive Components Handbook by P-O.Fagerholt*

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