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Posted on 10 May 2019

Terms and Descriptions - Varistors

 

 

 

 

 

 

 

Below is a guide to terms commonly found in product tables and data sheets for varistors.

Varistor operating voltage

Product tables generally specify maximum AC and DC operating voltages. These figures should only be exceeded by transients. Automotive types, however, are rated to withstand excessive voltage (jump start) for up to 5 minutes. Leakage current at specified operating voltage is negligible. The maximum permissible AC operating voltage is used to classify the individual voltage ratings within the type series. In most applications the operating voltage is a given parameter, so the varistors in the product tables are arranged according to maximum permissible operating voltage to facilitate comparison between the individual varistor sizes.

Varistor surge current (transient)

Short-term current flow – especially when caused by overvoltage – is referred to as surge current or transient. The maximum surge current that can be handled by a metal oxide varistor depends on amplitude, pulse duration, and number of pulses applied over device lifetime. The ability of a varistor to withstand a single pulse of defined shape is characterized by the maximum non-repetitive surge current specified in the product tables. If pulses of longer duration or multiple pulses are applied, the surge current must be derated according to specifications.

Maximum varistor surge current

The maximum non-repetitive surge current is defined by an 8/20 ms waveform (rise time 8 ms/decay time to half value 20 ms) according to IEC 60060. This waveform approximates a rectangular wave of 20 ms. The derating curves of the surge current, defined for rectangular waveforms, consequently show a knee between the horizontal branch and slope at 20 ms.

Varistor energy absorption

The energy absorption of a varistor is related to the surge current i(t) by

W=\int_{t_0}^{t_1} v(t)i(t)\,dt.

where v (t) is the voltage drop across the varistor during current flow.

Maximum varistor energy absorption

Surge currents of relatively long duration are required for testing maximum energy absorption capability. A rectangular wave of 2 ms according to IEC 60060 (figure 1) is commonly used for this test. In the product tables the maximum energy absorption is consequently defined for a surge current of 2 ms.

Figure 1. Waveform to IEC 60060 standard

 

Average varistor power dissipation

If metal oxide varistors are selected in terms of maximum permissible operating voltage, the resulting power dissipation will be negligible. However, the rated maximum power dissipation must be taken into account if the varistor has not enough time to cool down between a number of pulses occurring within a specified isolated time period.

Varistor voltage

The varistor voltage is the voltage drop across the varistor when a current of 1 mA is applied to the device. It has no particular electrophysical significance but is often used as a practical standard reference in specifying varistors.

Varistor tolerance

Tolerance figures refer to the varistor voltage at 25 °C. Also, the tolerance band be different for different values of current.

Note:
When the tolerance is examined, the current of 1 mA must only be applied briefly so that the results are not corrupted by warming of the varistor (see temperature coefficient). The current should only flow for 0.2 up to 2.0 s; a duration of 1 s is typical.

Varistor protection level (clamping voltage)

The protection level is the voltage drop across the varistor for surge currents > 1 mA.

The V-I characteristics  of a resistor show the maximum protection level as a function of surge current (ex. 8/20 ms waveform).

In the product tables, the protection level for surge currents should be specified. Protection level is also referred to as clamping voltage.

Varistor capacitance

Product tables specify typical capacitance figures for 1 kHz. The tabulated values show that metal oxide varistors behave like capacitors with a ZnO dielectric. The capacitance rises in proportion to disk area (and thus to current handling capability) and drops in proportion to the spacing of the electrodes, i.e. it decreases with increasing protection level. Capacitance values are not subject to outgoing inspection.

Varistor response behavior, response time

The response time of metal oxide varistor ceramics to transients is in the subnanosecond region, i.e. varistors are fast enough to handle even ESD (electrostatic discharge) transients extremely steep current rises of up to 50 A/ns. Similar results can be found silicon chips used in semiconductor protective devices like suppressor diodes. However, when the chip is mounted in its package, the response time increases to values > 1 ns due to the series inductance of its package. The varistors specified in data books often have response times of 1 mA with the standard 8/20 ms waveform (Figure 1). Therefore, they allow for the inductive voltage drop across the varistor for a particular di/dt. If surge currents with steep edges are to be handled, one should always design the circuit layout for as low an inductance as possible.

Varistor temperature coefficient

Metal oxide varistors show a negative temperature coefficient (TC) of voltage. Figure 2 shows typical varistor behavior. The temperature coefficient value drops markedly with rising currents and is completely negligible from roughly 1 mA upwards.

Figure 2. Typical temperature dependence of the V/I characteristic taking SIOV-S20K275 as an example

 

The temperature coefficient value drops markedly with rising currents and is completely negligible from roughly 1 mA upwards.An increase in leakage current is consequently noticeable at higher temperatures, especially in the μA region.

An increase in leakage current is consequently noticeable at higher temperatures, especially in the μA region.

The following equation describes the temperature coefficient TC of varistor voltage (at 1 mA):

|TC| < 0.05\% /K

Figure 3 shows results for SIOV-S20K275 as an example.

Varistor voltage temperature coefficient

Figure 3. Temperature coefficient of voltage at 1 mA for SIOV-S20K275

 

For more information please read:

Design Notes - Varistors

Introduction to Varistors

Varistor Operation - Derating, Temperature, Overload

Selection Guide - Varistors

Overvoltage Protection with Varistors

V-I Characteristics - Varistors

 

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