Varistors: Ideal Solution to Surge Protection

Transcription

1 aristors: deal Soltion to Srge Protection By Brno van Beneden, ishay BCcomponents, Malvern, Pa. f yo re looking for a srge protection device that delivers high levels of performance while addressing pressres to redce prodct size and component cont, then voltage dependent resistor or varistor technologies might be the ideal soltion. New reglations concerning srge protection are forcing engineers to look for soltions that allow sch protection to be incorporated at minimal cost penalty, particlarly in cost-sensitive consmer prodcts. n the atomotive sector, srge protection is also a growing necessity thanks to the rapid growth of electronic content in even the most basic prodction cars combined with the acknowledged problems of relatively nstable spply voltage and interference from the vehicle s ignition system. Another growing market for srge protection is in the telecom sector, where continosly increasing intelligence in exchanges and throghot the networks leads to greater se of sensitive semicondctors, and the stringent demands on ptime and availability mean that high ssceptibility to distrbances in spply is intolerable. Srge Protection Soltions Srge protection devices protect against srges generated by electromagnetic effects, sch as lightning or electrostatic discharge cased by a variety of effects. As sch, srge protection may be applied at the mains inpt to combat distrbances on the mains spply external to the operating eqipment or internally generated overvoltages sally cased by high indctive load switching. A srge protector may either attenate a transient by filtering or divert the transient to prevent damage to the load. Those that divert the transient fall into two broad categories: crowbar devices that switch into a very low impedance mode to short circit the transient ntil the crrent is broght to a low level; and clamping devices that Srge protection devices protect against srges generated by electromagnetic effects, sch as lightning or electrostatic discharge cased by a variety of effects. limit the voltage to a defined level. The crowbar grop incldes devices triggered by the breakdown of a gas or inslating layer, sch as air gap protectors, carbon block detectors, gas discharge tbes (GDTs), or break over diodes (BODs), or by the trn-on of a thyristor; these inclde overvoltage triggered SCRs and srgectors. One advantage of the crowbar-type device is that its very low impedance allows a high crrent to pass withot dissipating a considerable amont of energy within the protector. On the other hand, there s a finite volt-time response as the device switches or transitions to its breakdown mode, dring which the load may be exposed to damaging overvoltage. Another limitation is power-follow, where a power crrent from the voltage sorce follows the srge discharge. This crrent may not be cleared in an ac circit and clearing is even more ncertain in dc applications. Zener or avalanche diodes and voltage-dependent resistors (varistors) display a variable impedance, depending on the crrent flowing throgh the device or the voltage across its terminals. They se this property to clamp the overvoltage to a level dependent on the design and constrction of the device. The impedance characteristic, althogh nonlinear, is continos and displays no time delay sch as that associated with the spark-over of a gap or the triggering of a thyristor. The clamping device itself is transparent to the spply and to the load at a steady state voltage below the clamping level. Power Electronics Technology May 23 26

2 Low-Cost, High-Performance aristors The main fnction of the clamp is to absorb the overvoltage srge by lowering its impedance to sch a level that the voltage drop on an always-present series impedance is significant enogh to limit the overvoltage on critical parts to an acceptable level. Modern Zener diodes are very effective and come closest to the ideal constant voltage clamp. However, the avalanche voltage is maintained across a thin jnction area, leading to sbstantial heat generation. Therefore, the energy dissipation capability of a Zener diode is qite limited. A varistor, by contrast, displays a nonlinear, variable impedance. The varistor designer can control the degree of nonlinearity over a wide range by exploiting new materials and constrction techniqes that extend the range of applications for varistors. For example, varistors now offer a cost-effective soltion for low-voltage logic reqiring a low protection level and low standby crrent, as well as for ac power line and high capacity, tility-type applications. Compared with transient sppressor diodes, varistors can absorb mch higher transient energies and can sppress positive and negative transients. Frthermore, against crowbar-type devices, varistor response time is typically less than a nanosecond, and devices can be bilt to withstand srges of p to a 7,A srge. They have a long lifetime compared with diodes, and the varistor failre mode is a short circit. This prevents damage to the load that may reslt if failre of the protection circit is ndetected. aristors typically offer cost savings over crowbar-type devices. aristor Operation Metal Oxide aristors, or MOs, are typically constrcted from sintered zinc oxide pls a sitable additive. Each intergranlar bondary displays a rectifying action and presents a specific voltage barrier. When these condct, they form a low ohmic path to absorb srge energy. Dring manfactre, the zinc oxide granles are pressed before being fired for a controlled period and temperatre ntil the desired electrical characteristics are achieved. A varistor s behavior is defined by the relation: = K α where K and α are device constants. K is dependent on the device geometry. On the other hand, α defines the degree of nonlinearity in the resistance characteristic and can be controlled by selection of materials and the application of manfactring processes. A high α implies a better clamp; zinc oxide technology has enabled varistors with α in the range 15 to 3 significantly higher than earlier generation devices sch as silicon carbide varistors. The - behavior of a varistor is shown in Fig. 1 highlighting the distinct operating zones of the varistor. The slope of the protect region is determined by the device parameter β, which bears an inverse relation to α. n fact, varistor behavior can also be described by the relation: = C β (the inverse of = K α ) where C is also a geometry-dependent device constant. Fig. 1 also compares the varistor characteristic with that of the ideal voltage clamping device, which wold display a slope of zero, as well as a Zener diode characteristic. The Zener diode comparison highlights the extended protect region the varistor also offers for a comparable crrent and power capability. Selection Criteria For most applications, yo can determine the selection by assessing for aspects of the desired application: 1. The normal operating conditions of the apparats or system, and whether ac or dc voltage is applied. Fig. 2 shows a flowchart that may be sed to determine the necessary steady-state voltage rating or working voltage. Yo can find DRs in varios sizes and voltages ranging from 8 p to 1rms or more. The higher the nominal voltage of the selected varistor compared with the normal circit operating voltage, the better its reliability is over time, as the device is able to withstand more srge crrents withot degrading performance. The disadvantage deal voltage-clamping device Zinc oxide DR Zener diode Fig behavior of a varistor. Clamping voltage Clamping voltage Zener voltage (reverse avalanche) 27 Power Electronics Technology May 23

3 What is the voltage sorce? AC voltage DC voltage oltage is sinsoidal oltage is not sinsoidal Tolerance on known Tolerance on not known Maximm crest Maximm crest voltage not known Tolerance on nominal Tolerance on nominal Add tolerance vale to Mltiply nominal voltage by 1.15 Mltiply maximm crest voltage by.77 Mltiply nominal voltage by 1.2 Add tolerance vale to Mltiply nominal voltage by 1.2 Select next ac voltage greater than calclated voltage sing maximm continos ac voltage colmn in table electrical characteristics of the data sheet Select next dc voltage greater than calclated voltage sing maximm continos dc voltage colmn in table electrical characteristics of the data sheet Go to mltichoice selection of repetitive peak crrent Fig. 2. Flowchart sed to determine the necessary steady state voltage rating or working voltage. is a redction in the level of protection offered by an overspecified varistor. Hence, yo shold maintain the following relation: Maximm withstand voltage of protected device > max. varistor clamping voltage > max. continos operating voltage. 2. Determine the repetitive peak crrent. Fig. 3 shows a flowchart that may be sed to determine the repetitive peak crrent. Maximm srge crrents are related to the size of the component and start from a few hndred amperes p to several tens of kiloamperes (at standard waveforms of 8/ 2 µs). Once the repetitive peak crrent is known, then yo can calclate the necessary energy absorption, in Joles (Watt.second or Ws), for the varistor. 3. Calclate the energy absorption. There are two cases one for dc and one for ac energy. Energy ratings for available varistors start at a few Joles p to several hndred Joles. Case 1 Calclating dc Dissipation: The power dissipated in a varistor is eqal to the prodct of the voltage and crrent, and may be written: W = = C β+1 When the coefficient α = 3 (β =.33), the power dissipated by the varistor is proportional to the 31 st power of the voltage. A voltage increase of only 2.26% will, in this case, doble the dissipated power. Conseqently, it s important that the applied voltage doesn t rise above a certain maximm vale, or the permissible rating will be exceeded. Moreover, since varistors have a negative temperatre coefficient, at a higher dissipation (and accordingly at a higher temperatre) the resistance vale will decrease and the dissipated power will increase frther. Case 2 Calclating ac Dissipation: When a sinsoidal alternating voltage is applied to a varistor, the dissipation is calclated by integrating the prodct. A sitable expression is as follows: P = 1 2 (a + 1)/2 π (sinωt) α + 1 π dt Transient energy ratings are qoted in Joles. t s important to ensre the varistor is able to absorb this energy throghot the planned prodct lifetime or replacement interval withot failing. When the device is being sed to protect against transients reslting from an indctive or capacitive discharge, sch as switching a motor, the transient energy is easily calclated. However, if the varistor is expected to protect against transients originating from external sorces, the magnitde of the transient is typically nknown and an approximation techniqe mst be applied. This involves calclating the energy absorbed after finding the transient crrent and voltage applied to the varistor. The following eqation may be applied: E = ntegral of (everything p to the c (t) (t) t) from to τ = K c τ Where is the peak crrent, c is the reslting clamp voltage, τ is the implse dration, and K is an energy form factor constant dependent on the crrent waveform. 4. Package size and style. Electrical and mechanical considerations mst be taken into accont when selecting the package size and style. This incldes determining the reqired energy rating and srge crrent amplitdes, and Power Electronics Technology May 23 28

4 whether the device is intended to protect against exceptional srges or those cased by repetitive events will feed into the selection process. The amont of energy expected to be dissipated will also inflence this, and designers mst ensre the package dimensions are appropriate to the physical and mechanical design of the prodct. Conventional form factors typically range from disc types of a few millimeters in diameter p to 5 mm, or block and rectanglar types for high-energy handling parts. Other important selection considerations are the effects of lead indctance and device capacitance, which also impact the performance of the varistor in circit, and mst be considered when choosing to se a varistor. n conventional leaded devices, the indctance of the lead can slow the fast action of the varistor to the extent that protection is negated. Modeling the varistor presents a shnt capacitance that may range from a few tens of pf p to several nf, depending on size and voltage range of the device. Depending on the application, the presence of this capacitance can be of little conseqence, a desirable property, or, at worst, problematical. For example, in dc applications a large capacitance is desirable and can provide a degree of filtering and transient sppression. On the other hand, it may preclde the se of a varistor to protect high-freqency circits. Sample Applications Looking at Fig. 4, yo can see how a varistor may be sed to protect a generic load against power srges originating from the spply. The power spply s own otpt impedance combines with that of the varistor to create a potential divider whose ratio varies with the varistor impedance to protect the load. Yo can see an alternative application in Fig. 5, on page 32. Withot varistor protection, the measred peak crrent throgh the pmp motor when S is closed is 1A. The energy expended in establishing the electromagnetic field in the indctance of the motor is therefore: 2 L =.4 = 2 mj 2 2 Withot varistor protection, an initial crrent of 1A will flow throgh the thyristor bridge when S is opened, and a voltage sfficient to damage or destroy the thyristors will be developed. Arcing will occr across the opening contacts of the switch. Bt with a varistor inserted in the circit, the peak voltage developed across the varistor on opening switch S is: = C MAX β = 6. The thyristors in the bridge can withstand this voltage withot dam- Which parameter of line is known? Origins of the plses not known Origins of the plses known Lightning or indstrial indctive load on line Electrostatic discharge (ESD) Solenoid (e.g. transformer, electromagnetic etc.) Short circit crrent vale known RLC line impedance known Short circit crrent vale not known RLC line impedance not known crrent < 5 A crrent eqals vale of peak crrent passing throgh solenoid (don t forget to calclate the dissipation when the recrrent time is short. i.e.<5 mintes) ale of repetitive peak crrent eqals short circit crrent vale Mlitply nominal voltage by 1, divide reslt by RLC line impedance vale to find the repetitive peak crrent Line conforms to category A of ANS / EEE C or category of EC 6664 (Long branch circit and otlets) crrent is 2 A Line conforms to category B of ANS / EEE C or category of EC 6664 (Long branch circit and otlets) crrent is 4 A Sbscribers lines crrent is 75 A Telecom lines Trnk carrier systems crrent is 15 A Repeaters crrent is 8 A When the repetitive peak crrent is: max. 5 A max. 12 A max. 25 A max 5 A max. 1 A The correct series to se is: 5mm 7mm 1mm 14mm 2 mm Fig. 3. Flowchart sed to determine the repetitive peak crrent. 29 Power Electronics Technology May 23

5 S Heater Electronic circit Rp 33W RH = 24W Fig. 4. Sppression directly across mains. age. The total energy retrned to the circit is 2 mj. Of this 2 mj, 15.1 mj is dissipated in the heater, and mj is dissipated in the varistor. The varistor can withstand more than 1 5 transients containing this amont of energy. For frther reference, Fig. 6 shows how varistors may be sed to sppress internally generated spikes in a T application. New Paths of Development aristors offer cost savings and performance advantages over crowbartype srge protectors and Zener diode clamp devices in a wide range of applications. Enhanced materials and optimized component design particlarly in the field of Zinc Oxide varistors have opened p new applications for varistors, especially those reqiring low protective level and a low standby crrent. n line with this indstry s overriding drive toward miniatrization and srface-mont technology, DRs in a 22 5 Hz L.4H back e.m.f. Pmp motor Fig. 5. Protection of a thyristor bridge in a washing machine. single-layer SMD package are emerging to satisfy medim energy handling capabilities within a relatively small volme. Also, where disc-type varistors occpy relatively large space within an enclosre, new low-profile varistors redce the maximm height above the board for sch a device, while maintaining eqivalent crrent handling capabilities. n addition to these, ltrahigh srge varistors are also more widely sed in the market, capable of offering an improved srge crrent/size ratio and allowing replacement of large components by smaller devices with similar performance and reliability. Other new varistor types incorporate a thermo fse to provide a predictable fail-safe behavior in case of To drm motor Fig. 6. aristors sed to sppress internally generated spikes in a T application. abnormal se. Frther avenes of development inclde varistors capable of handling ambient temperatres above 125 C over the fll voltage/ srge capability range. PETech Power Electronics Technology May 23 3