Varistors: Ideal Solution to Surge Protection - Vishay

1 Power Electronics Technology May regulations concerning Surge protectionare forcing engineers to look for solutionsthat allow such Protection to be incorpo-rated at minimal cost penalty, particularlyin cost-sensitive consumer products. In theautomotive sector, Surge Protection is also a growing ne-cessity thanks to the rapid growth of electronic contentin even the most basic production cars combined with theacknowledged problems of relatively unstable supply volt-age and interference from the vehicle s ignition growing market for Surge Protection is in thetelecom sector, where continuously increasing intelligencein exchanges and throughout the networks leads to greateruse of sensitive semiconductors, and the stringent demandson uptime and availability mean that high susceptibility todisturbances in supply is Protection SolutionsSurge Protection devicesprotect against surgesgenerated by electromag-netic effects, such as light-ning or electrostatic dis-charge caused by a varietyof effects.

2 As such, surgeprotection may be appliedat the mains input to com-bat disturbances on the mains supply external to the oper-ating equipment or internally generated overvoltages usu-ally caused by high inductive load Surge protector may either attenuate a transient byfiltering or divert the transient to prevent damage to theload. Those that divert the transient fall into two broadcategories: crowbar devices that switch into a very low im-pedance mode to short circuit the transient until the cur-rent is brought to a low level; and clamping devices thatlimit the voltage to a defined level. The crowbar group in-cludes devices triggered by the breakdown of a gas or in-sulating layer, such as air gap protectors, carbon block de-tectors, gas discharge tubes (GDTs), or break over diodes(BODs), or by the turn-on of a thyristor; these includeovervoltage triggered SCRs and advantage of the crowbar-type device is that its verylow impedance allows a high current to pass without dissi-pating a considerable amount of energy within the protec-tor.

3 On the other hand, there s a finite volt-time responseas the device switches or transitions to its breakdown mode,during which the load may be exposed to damaging over-voltage. Another limitation is power-follow, where a powercurrent from the voltage source follows the Surge current may not be cleared in an ac circuit and clear-ing is even more uncertain in dc or avalanche diodes and voltage-dependentresistors (varistors) display a variable impedance, depend-ing on the current flowing through the device or the volt-age across its terminals. They use this property to clampthe overvoltage to a level dependent on the design andconstruction of the device. The impedance characteristic,although nonlinear, is continuous and displays no timedelay such as that associated with the spark-over of a gapor the triggering of a thyristor.

4 The clamping device itselfis transparent to the supply and to the load at a steady statevoltage below the clamping : Ideal Solutionto Surge ProtectionIf you re looking for a Surge Protection device thatdelivers high levels of performance while address-ing pressures to reduce product size and compo-nent count, then voltage dependent resistor orvaristor technologies might be the Ideal Bruno van Beneden, V i s h ay B C co m p onents, Ma l ve r n, Protection devices protect against surges generatedby electromagnetic effects, such as lightning or electrostaticdischarge caused by a variety of Electronics Technology May 200327 VARISTORS FOR Surge PROTECTIONFig. 1. V-I behavior of a varistor. Low-Cost, High-Performance VaristorsThe main function of the clamp is to absorb the overvolt-age Surge by lowering its impedance to such a level that thevoltage drop on an always-present series impedance is sig-nificant enough to limit the overvoltage on critical parts to an acceptable level.

5 Modern Zener diodes are very ef-fective and come closest to the Ideal constant voltage , the avalanche voltage is maintained across a thinjunction area, leading to substantial heat generation. There-fore, the energy dissipation capability of a Zener diode isquite varistor, by contrast, displays a nonlinear, variableimpedance. The varistor designer can control the degree ofnonlinearity over a wide range by exploiting new materialsand construction techniques that extend the range of ap-plications for varistors. For example, varistors now offer acost-effective Solution for low-voltage logic requiring a lowprotection level and low standby current, as well as for acpower line and high capacity, utility-type with transient suppressor diodes, varistorscan absorb much higher transient energies and can sup-press positive and negative transients.

6 Furthermore, againstcrowbar-type devices, varistor response time is typically lessthan a nanosecond, and devices can be built to withstandsurges of up to a 70,000A Surge . They have a long lifetimecompared with diodes, and the varistor failure mode is ashort circuit. This prevents damage to the load that mayresult if failure of the Protection circuit is undetected. Varis-tors typically offer cost savings over crowbar-type OperationMetal Oxide Varistors, or MOVs, are typically constructedfrom sintered zinc oxide plus a suitable additive. Each in-tergranular boundary displays a rectifying action and pre-sents a specific voltage barrier. When these conduct, theyform a low ohmic path to absorb Surge energy. Duringmanufacture, the zinc oxide granules are pressed beforebeing fired for a controlled period and temperature untilthe desired electrical characteristics are achieved.

7 Avaristor s behavior is defined by the relation:I = KV where K and are device is dependent on the device geometry. On the otherhand, defines the degree of nonlinearity in the resistancecharacteristic and can be controlled by selection of mate-rials and the application of manufacturing processes. A high implies a better clamp; zinc oxide technology has en-abled varistors with in the range 15 to 30 significantlyhigher than earlier generation devices such as silicon car-bide varistors. The V-I behavior of a varistor is shown inFig. 1 highlighting the distinct operating zones of the varis-tor. The slope of the protect region is determined by thedevice parameter , which bears an inverse relation to .In fact, varistor behavior can also be described by therelation:V = CI (the inverse of I = KV )where C is also a geometry-dependent device 1 also compares the varistor characteristic with thatof the Ideal voltage clamping device, which would displaya slope of zero, as well as a Zener diode characteristic.

8 TheZener diode comparison highlights the extended protectregion the varistor also offers for a comparable current andpower CriteriaFor most applications, you can determine the selection byassessing four aspects of the desired application:1. The normal operating conditions of the apparatus orsystem, and whether ac or dc voltage is applied. Fig. 2 showsa flowchart that may be used to determine the necessarysteady-state voltage rating or working can find VDRs in various sizes and voltages rang-ing from 8V up to 1000 Vrms or more. The higher the nomi-nal voltage of the selected varistor compared with the nor-mal circuit operating voltage, the better its reliability is overtime, as the device is able to withstand more Surge cur-rents without degrading performance. The disadvantagePower Electronics Technology May 2.

9 Flowchart used to determine the necessary steady state voltage rating or working voltage. !" # $ # $ " % & '& ()* + , - ' , - . # $ # $ " % & '& ()(/% $ % & '& )0 0% $ % & '& ()* + , - ' , - . 1 . is a reduction in the level of Protection offered by an over-specified varistor. Hence, you should maintain the follow-ing relation:Maximum withstand voltage of protected device > clamping voltage > max. continuous Determine the repetitive peak current. Fig. 3 shows aflowchart that may be used to determine the repetitive peakcurrent. Maximum Surge currents are related to the size ofthe component and start from a few hundred amperes upto several tens of kiloamperes (at standard waveforms of 8/20 s). Once the repetitive peak current is known, then youcan calculate the necessary energy absorption, in Joules( or Ws), for the Calculate the energy absorption.

10 There are two cases one for dc and one for ac energy. Energy ratings for avail-able varistors start at a few Joules up to several 1 Calculating dc Dissipation: The power dissi-pated in a varistor is equal to the product of the voltageand current, and may be written:W = I V = C I +1 When the coefficient = 30 ( = ), the power dis-sipated by the varistor is proportional to the 31st power ofthe voltage. A voltage increase of only will, in thiscase, double the dissipated power. Consequently, it s im-portant that the applied voltage doesn t rise above a cer-tain maximum value, or the permissible rating will be ex-ceeded. Moreover, since varistors have a negative tempera-ture coefficient, at a higher dissipation (and accordingly ata higher temperature) the resistance value will decrease andthe dissipated power will increase 2 Calculating ac Dissipation: When a sinusoidalalternating voltage is applied to a varistor, the dissipation iscalculated by integrating the VI product.