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Varistors: Ideal Solution to Surge Protection

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Varistors: Ideal Solution to Surge Protection Varistors: Ideal Solution to Surge Protection By Bruno van Beneden, Vishay BCcomponents, Malvern, Pa. If you’re looking for a surge protection device that delivers high levels of performance while address- ing pressures to reduce product size and compo- nent count, then voltage dependent resistor or varistor technologies might be the ideal solution. ew regulations concerning surge protection limit the voltage to a defined level. The crowbar group in- are forcing engineers to look for solutions cludes devices triggered by the breakdown of a gas or in- that allow such protection to be incorpo- sulating layer, such as air gap protectors, carbon block de- rated at minimal cost penalty, particularly tectors, gas discharge tubes (GDTs), or break over diodes in cost-sensitive consumer products. In the (BODs), or by the turn-on of a thyristor; these include automotiveN sector, surge protection is also a growing ne- overvoltage triggered SCRs and surgectors. cessity—thanks to the rapid growth of electronic content One advantage of the crowbar-type device is that its very in even the most basic production cars combined with the low impedance allows a high current to pass without dissi- acknowledged problems of relatively unstable supply volt- pating a considerable amount of energy within the protec- age and interference from the vehicle’s ignition system. tor. On the other hand, there’s a finite volt-time response Another growing market for surge protection is in the as the device switches or transitions to its breakdown mode, telecom sector, where continuously increasing intelligence during which the load may be exposed to damaging over- in exchanges and throughout the networks leads to greater voltage. Another limitation is power-follow, where a power use of sensitive semiconductors, and the stringent demands current from the voltage source follows the surge discharge. on uptime and availability mean that high susceptibility to This current may not be cleared in an ac circuit—and clear- disturbances in supply is intolerable. ing is even more uncertain in dc applications. Zener—or avalanche diodes—and voltage-dependent Surge Protection Solutions resistors (varistors) display a variable impedance, depend- Surge protection devices protect against surges generated by electromag- Surge protection devices protect against surges generated netic effects, such as light- ning or electrostatic dis- by electromagnetic effects, such as lightning or electrostatic charge caused by a variety of effects. As such, surge discharge caused by a variety of effects. protection may be applied at the mains input to com- bat disturbances on the mains supply external to the oper- ing on the current flowing through the device or the volt- ating equipment or internally generated overvoltages usu- age across its terminals. They use this property to clamp ally caused by high inductive load switching. the overvoltage to a level dependent on the design and A surge protector may either attenuate a transient by construction of the device. The impedance characteristic, filtering or divert the transient to prevent damage to the although nonlinear, is continuous and displays no time load. Those that divert the transient fall into two broad delay such as that associated with the spark-over of a gap categories: crowbar devices that switch into a very low im- or the triggering of a thyristor. The clamping device itself pedance mode to short circuit the transient until the cur- is transparent to the supply and to the load at a steady state rent is brought to a low level; and clamping devices that voltage below the clamping level. Power Electronics Technology May 200326 www.powerelectronics.com VARISTORS FOR SURGE PROTECTION Low-Cost, High-Performance Varistors where C is also a geometry-dependent device constant. The main function of the clamp is to absorb the overvolt- Fig. 1 also compares the varistor characteristic with that age surge by lowering its impedance to such a level that the of the ideal voltage clamping device, which would display voltage drop on an always-present series impedance is sig- a slope of zero, as well as a Zener diode characteristic. The nificant enough to limit the overvoltage on “critical parts” Zener diode comparison highlights the extended protect to an acceptable level. Modern Zener diodes are very ef- region the varistor also offers for a comparable current and fective and come closest to the ideal constant voltage clamp. power capability. However, the avalanche voltage is maintained across a thin junction area, leading to substantial heat generation. There- Selection Criteria fore, the energy dissipation capability of a Zener diode is For most applications, you can determine the selection by quite limited. assessing four aspects of the desired application: A varistor, by contrast, displays a nonlinear, variable 1. The normal operating conditions of the apparatus or impedance. The varistor designer can control the degree of system, and whether ac or dc voltage is applied. Fig. 2 shows nonlinearity over a wide range by exploiting new materials a flowchart that may be used to determine the necessary and construction techniques that extend the range of ap- steady-state voltage rating or working voltage. plications for varistors. For example, varistors now offer a You can find VDRs in various sizes and voltages rang- cost-effective solution for low-voltage logic requiring a low ing from 8V up to 1000Vrms or more. The higher the nomi- protection level and low standby current, as well as for ac nal voltage of the selected varistor compared with the nor- power line and high capacity, utility-type applications. mal circuit operating voltage, the better its reliability is over Compared with transient suppressor diodes, varistors time, as the device is able to withstand more surge cur- can absorb much higher transient energies and can sup- rents without degrading performance. The disadvantage press positive and negative transients. Furthermore, against crowbar-type devices, varistor response time is typically less than a nanosecond, and devices can be built to withstand surges of up to a 70,000A surge. They have a long lifetime V compared with diodes, and the varistor failure mode is a Protect region short circuit. This prevents damage to the load that may Ideal voltage-clamping Clamping result if failure of the protection circuit is undetected. Varis- device voltage tors typically offer cost savings over crowbar-type devices. Working voltage Varistor Operation Metal Oxide Varistors, or MOVs, are typically constructed 0 from sintered zinc oxide plus a suitable additive. Each in- I tergranular boundary displays a rectifying action and pre- sents a specific voltage barrier. When these conduct, they V form a low ohmic path to absorb surge energy. During manufacture, the zinc oxide granules are pressed before Protect region Clamping being fired for a controlled period and temperature until Zinc voltage oxide the desired electrical characteristics are achieved. A Working voltage varistor’s behavior is defined by the relation: VDR I = KVα where K and α are device constants. 0 K is dependent on the device geometry. On the other I hand, α defines the degree of nonlinearity in the resistance characteristic and can be controlled by selection of mate- rials and the application of manufacturing processes. A high V α implies a better clamp; zinc oxide technology has en- α abled varistors with in the range 15 to 30—significantly Protect region higher than earlier generation devices such as silicon car- Zener voltage Zener (reverse avalanche) bide varistors. The V-I behavior of a varistor is shown in diode Fig. 1 highlighting the distinct operating zones of the varis- Working voltage tor. The slope of the protect region is determined by the device parameter β, which bears an inverse relation to α. 0 In fact, varistor behavior can also be described by the I relation: V = CIβ (the inverse of I = KVα) Fig. 1. V-I behavior of a varistor. www.powerelectronics.com27 Power Electronics Technology May 2003 VARISTORS FOR SURGE PROTECTION What is the voltage source? AC voltage DC voltage Voltage is Voltage is sinusoidal not sinusoidal Tolerance on Tolerance on Maximum crest Maximum crest Tolerance Tolerance nominal voltage nominal voltage voltage known voltage not known on nominal on nominal known not known voltage known voltage known Add tolerance Multiply nominal Multiply maximum Multiply nominal Add tolerance Multiply nominal value to crest voltage value to voltage by 1.20 nominal voltage voltage by 1.15 by 0.707 voltage by 1.20 nominal voltage Select next ac voltage greater Select next dc voltage greater than calculated voltage using than calculated voltage using maximum continuous ac voltage maximum continuous dc voltage column in table column in table electrical characteristics electrical characteristics of the data sheet of the data sheet Go to multichoice selection of repetitive peak current Fig. 2. Flowchart used to determine the necessary steady state voltage rating or working voltage. is a reduction in the level of protection offered by an over- a higher temperature) the resistance value will decrease and specified varistor. Hence, you should maintain the follow- the dissipated power will increase further. ing relation: Case 2—Calculating ac Dissipation: When a sinusoidal Maximum withstand voltage of protected device > max. alternating voltage is applied to a varistor, the dissipation is varistor clamping voltage > max. continuous operating calculated by integrating the VI product. A suitable expres- voltage. sion is as follows: 2. Determine the repetitive peak current. Fig. 3 shows a 1 (a + 1)/2 π α + 1 P = π × 2 × ∫0 (sinωt) × dt flowchart that may be used to determine the repetitive peak current. Maximum surge currents are related to the size of Transient energy ratings are quoted in Joules. It’s im- the component and start from a few hundred amperes up portant to ensure the varistor is able to absorb this energy to several tens of kiloamperes (at standard waveforms of 8/ throughout the planned product lifetime or replacement 20 µs).
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