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Home , Overvoltage, Surge protector, Varistor, Zener diode

: 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 dependent or technologies might be the ideal solution.

ew regulations concerning surge protection limit the voltage to a defined level. The 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 in cost-sensitive consumer products. In the (BODs), or by the turn-on of a ; these include automotiveN sector, surge protection is also a growing ne- 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 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 (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 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 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 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 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 characteristic. The nificant enough to limit the overvoltage on “critical parts” 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 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). Once the repetitive peak current is known, then you interval without failing. When the device is being used to can calculate the necessary energy absorption, in Joules protect against transients resulting from an inductive or (Watt.second or Ws), for the varistor. capacitive discharge, such as switching a motor, the tran- 3. Calculate the energy absorption. There are two cases— sient energy is easily calculated. However, if the varistor is one for dc and one for ac energy. Energy ratings for avail- expected to protect against transients originating from able varistors start at a few Joules up to several hundred external sources, the magnitude of the transient is typi- Joules. cally unknown and an approximation technique must be Case 1—Calculating dc Dissipation: The power dissi- applied. This involves calculating the energy absorbed af- pated in a varistor is equal to the product of the voltage ter finding the transient current and voltage applied to the and current, and may be written: varistor. The following equation may be applied: β+1 ∆ W = I × V = C × I E = Integral of (everything up to the Vc (t) I (t) t) from 0 α β τ τ When the coefficient = 30 ( = 0.033), the power dis- to = KVcI st sipated by the varistor is proportional to the 31 power of Where I is the peak current, Vc is the resulting clamp τ the voltage. A voltage increase of only 2.26% will, in this voltage, is the impulse duration, and K is an energy form case, double the dissipated power. Consequently, it’s im- factor constant dependent on the current waveform. portant that the applied voltage doesn’t rise above a cer- 4. Package size and style. Electrical and mechanical con- tain maximum value, or the permissible rating will be ex- siderations must be taken into account when selecting the ceeded. Moreover, since varistors have a negative tempera- package size and style. This includes determining the re- ture coefficient, at a higher dissipation (and accordingly at quired energy rating and surge current amplitudes, and

Power Electronics Technology May 200328 www.powerelectronics.com VARISTORS FOR SURGE PROTECTION whether the device is intended to pro- Modeling the varistor presents a ies with the varistor impedance to tect against exceptional surges or capacitance that may range protect the load. You can see an alter- those caused by repetitive events will from a few tens of pF up to several native application in Fig. 5, on page feed into the selection process. The nF, depending on size and voltage 32. Without varistor protection, the amount of energy expected to be dis- range of the device. Depending on measured peak current through the sipated will also influence this, and the application, the presence of this pump motor when S is closed is 1A. designers must ensure the package di- capacitance can be of little conse- The energy expended in establishing mensions are appropriate to the quence, a desirable property, or, at the in the induc- physical and mechanical design of the worst, problematical. For example, in tance of the motor is therefore: product. Conventional form factors dc applications a large capacitance I2 × L = 0.4 = 200 mJ typically range from disc types of a is desirable and can provide a de- 2 2 few millimeters in diameter up to 50 gree of filtering and transient sup- Without varistor protection, an mm, or block and rectangular types pression. On the other hand, it may initial current of 1A will flow through for high-energy handling parts. preclude the use of a varistor to pro- the thyristor bridge when S is opened, Other important selection consid- tect high-frequency circuits. and a voltage sufficient to damage or erations are the effects of lead induc- destroy the will be devel- tance and device capacitance, which Sample Applications oped. Arcing will occur across the also impact the performance of the Looking at Fig. 4, you can see how a opening contacts of the switch. But varistor in circuit, and must be con- varistor may be used to protect a ge- with a varistor inserted in the circuit, sidered when choosing to use a varis- neric load against power surges origi- the peak voltage developed across the tor. In conventional leaded devices, nating from the supply. The power varistor on opening switch S is: β the inductance of the lead can slow supply’s own output impedance com- V = CMAX × I = 600V. the fast action of the varistor to the bines with that of the varistor to cre- The thyristors in the bridge can extent that protection is negated. ate a potential divider whose ratio var- withstand this voltage without dam-

Which parameter of line is known?

Origins of the Origins of the pulses not known pulses known

Lightning or Solenoid industrial inductive Electrostatic (e.g. , load on line discharge (ESD) electromagnetic etc.) Short circuit Short circuit current current Repetitive peak Repetitive peak value known value not known current < 50 A current equals value of peak current passing through solenoid RLC line RLC line (don’t forget to calculate the dissipation when the impedance known impedance not known recurrent time is short. i.e.<5 minutes)

Line conforms to Line conforms to Telecom lines category A of category B of Value of Mulitply nominal ANSI / IEEE C 62.41 ANSI / IEEE C 62.41 repetitive peak voltage by 10, or category II or category III current equals divide result of IEC 60664 of IEC 60664 Subscribers Trunk carrier Repeaters (Long branch (Long branch lines systems short circuit by RLC line circuit and outlets) circuit and outlets) current value impedance value to find the Repetitive peak Repetitive peak Repetitive peak Repetitive peak Repetitive peak repetitive current is 75 A current is 150 A current is 800 A peak current current is 200 A current is 400 A

When the repetitive The correct series peak current is: to use is: max. 50 A 5mm max. 120 A 7mm max. 250 A 10mm max 500 A 14mm max. 1000 A 20 mm

Fig. 3. Flowchart used to determine the repetitive peak current. www.powerelectronics.com29 Power Electronics Technology May 2003 VARISTORS FOR SURGE PROTECTION

S Heater

Electronic RH = 24W circuit u Rp 33W

220V L 50 Hz 0.4H u Fig. 4 . Suppression directly across mains. back e.m.f. Pump age. The total energy returned to the motor circuit is 200 mJ. Of this 200 mJ, 15.1 mJ is dissipated in the heater, and 184.3 mJ is dissipated in the varis- To drum motor tor. The varistor can withstand more than 105 transients containing this Fig. 5. Protection of a thyristor bridge in a washing machine. amount of energy. For further refer- single-layer SMD package are emerg- ence, Fig. 6 shows how varistors may ing to satisfy medium energy handling be used to suppress internally gener- capabilities within a relatively small ated spikes in a TV application. volume. Also, where disc-type varis- tors occupy relatively large space New Paths of Development within an enclosure, new low-profile Varistors offer cost savings and per- varistors reduce the maximum height formance advantages over crowbar- above the board for such a device, type surge protectors and Zener di- while maintaining equivalent current u ode clamp devices in a wide range of handling capabilities. In addition to applications. Enhanced materials and these, ultrahigh surge varistors are also optimized component design—par- more widely used in the market, ca- pable of offering an improved surge ticularly in the field of Zinc Oxide Fig. 6. Varistors used to suppress internally varistors—have opened up new appli- current/size ratio and allowing re- generated spikes in a TV application. cations for varistors, especially those placement of large components by requiring low protective level and a smaller devices with similar perfor- abnormal use. Further avenues of de- low standby current. mance and reliability. velopment include varistors capable In line with this industry’s overrid- Other new varistor types incorpo- of handling ambient temperatures ing drive toward miniaturization and rate a thermo to provide a pre- above 125°C over the full voltage/ surface-mount technology, VDRs in a dictable “fail-safe” behavior in case of surge capability range. PETech

Power Electronics Technology May 200330 www.powerelectronics.com