Selecting a Littelfuse Varistor

Transcription

1 Selecing a Lielfuse Varisor Applicaion Noe July 1999 AN Inroducion The varisor mus operae under boh a coninuous operaing (sandby) mode as well as he prediced ransien (normal) mode. The selecion process, herefore, requires a knowledge of he elecrical environmen. When he environmen is no fully defined, some approximaions can be made. For mos applicaions, he selecion is a five-sep process: 1. Deermine he necessary seady-sae volage raing (working volage) 2. Esablish he ransien energy absorbed by he varisor 3. Calculae he peak ransien curren hrough he varisor 4. Deermine power dissipaion requiremens 5. Selec a model o provide he required volage-clamping characerisic A final consideraion is o choose he appropriae package syle o sui he applicaion. Seady-Sae Volage Raing Consider he maximum coninuous volage ha will be applied o he varisor including any high line condiions (i.e., 110% or more of nominal volage). Raings are given for coninuous sinusoidal AC and DC volages. If a nonsinusoidal waveform is applied, he recurren peak volage should be limied o 2x V M(AC). Specificaions for he UlraMOV Series varisor, for example, are shown in Table 1 for 140V AC raed devices o illusrae he use of he raings and specificaions able. V M(AC) - These models can be operaed coninuously wih up o 140V RMS a 50Hz - 60Hz applied. They would be suiable for 120V AC nominal line operaion and would allow for abou a 120% high line condiion. V M(DC) - Operaion up o 180V DC applied coninuously is allowed. Energy Transien energy raings are given in he W TM column of he specificaions in joules (wa-second). The raing is he maximum allowable energy for a single impulse of 10/µs curren waveform wih coninuous volage applied. Energy raings are based on a shif of V N of less han ±10% of iniial value. When he ransien is generaed from he discharge of an inducance (i.e., moor, ransformer) or a capacior, he source energy can be calculaed readily bu, in mos cases he ransien is from a source exernal o he equipmen and is of unknown magniude. For his siuaion an approximaion echnique can be used o esimae he energy of he ransien absorbed by he varisor. The mehod requires finding he ransien curren and volage applied o he varisor. To deermine he energy absorbed he following equaion applies: E = V C ( )I( ) = KV 0 C I where I is he peak curren applied, V C is he clamp volage which resuls, is he impulse duraion and K is a consan. K values are given in Figure 1 for a variey of waveshapes frequenly encounered. The K value and pulse widh correspond o he curren waveform only, assuming he varisor volage waveform is almos consan during he curren impulse. For complex waveforms, his approach also can be used by dividing he shape ino segmens ha can be reaed separaely. TABLE 1. ULTRAMOV RATINGS AND SPECIFICATIONS EXAMPLE MAXIMUM RATING (85 o C) CHARACTERISTICS (25o C) MODEL NUMBER DEVICE MODEL NUMBER BRAND- ING CONTINUOUS RMS VOLTS DC VOLTS ENERGY 2ms TRANSIENT V M(AC) V M(DC) W TM 2 x PULSE I TM PEAK CURRENT 8 x 20µs I TM 1 x PULSE VARISTOR VOLTAGE AT 1mA DC TEST CURRENT V NOM MIN MAXIMUM CLAMPING VOLTAGE 8 x 20µs TYPICAL CAPACI- TANCE V NOM MAX V C f = 1MHz (V) (V) (J) (A) (A) (V) (V) (A) (pf) V07E140 7V V10E140 10V V14E140 14V V20E140 20V UlraMOV is a rademark of Lielfuse, Inc or Copyrigh Lielfuse, Inc. 1999

2 Applicaion Noe 9771 WAVESHAPE EQUATION K I PK Π sin The waveform is divided ino wo pars ha are reaed separaely using he facors of Figure 1: curren waveform Secion (1) 0 o 5µs and (2) 5µs o 50µs. The maximum volage across he V130LA1 a A is found o be V from he V-I characerisics of he specificaion shee sin( π)e / Secion (1) E = kv C I = (0.5) () () (5) (10-6 ) = 0.13J Secion (2) E = kv C I = (1.4) () () (50-5) 10-6 ) = 3.15J 3.28J Toal Peak Curren The peak curren raing can be checked agains he ransien curren measured in he circui. If he ransien is generaed by an inducor, he peak curren will no be more han he inducor curren a he ime of swiching. Anoher mehod for finding he ransien curren is o use a graphical analysis. When he ransien volage and source impedance is known, a Thevenin equivalen circui can be modeled. Then, a load line can be drawn on he log - log, V-I characerisic as shown in Figure 3. The wo curves inersec a he peak curren value. 1.4 e -/ Based upon alpha of 25 o 40 FIGURE 1. ENERGY FORM FACTOR CONSTANTS The raed single pulse curren, I TM, is he maximum allowable for a single pulse of 8/20µs exponenial waveform (illusraed in Applicaion Noe AN9767, Figure 21). For longer duraion pulses, I TM should be deraed o he curves in he varisor specificaions. Figure 4 shows he deraing curves for 7mm size, LA series devices. This curve also provides a guide for deraing curren as required wih repeiive pulsing. The designer mus consider he oal number of ransien pulses expeced during he life of he equipmen and selec he appropriae curve. Where he curren waveshape is differen from he exponenial waveform of Figure 11 of AN9767, he curves of Figure 4 can be used by convering he pulse duraion on he basis of equivalen energy. This is easily done using he consans given in Figure 1. For example, suppose he acual curren measured has a riangular waveform wih a peak curren of 10A, a peak volage of 340V and an impulse duraion of µs. Consider he condiion where he exponenial waveform shown below is applied o a V130LA1 ype Lielfuse Varisor. Z S I V V OC V R A 50A FIGURE 3A. EQUIVALENT CIRCUIT 0 5µs 50µs FIGURE

3 Applicaion Noe 9771 RATED PEAK PULSE CURRENT (A) LOG VARISTOR (V) I V -VOC /Z S LOG VARISTOR CURRENT (A) FIGURE 3B. GRAPHICAL ANALYSIS TO DETERMINE PEAK I Then: V OC V C CLAMP VOLTAGE V R = V OC -IZ S VARISTOR V-I CHARACTERISTIC FIGURE 3. DETERMINING VARISTOR PEAK CURRENT FROM A VOLTAGE SOURCE TRANSIENT 2, , INDEFINITE NUMBER OF 2 PULSES 1 NUMBER OF PULSES MODEL SIZE 7mm V130LA1 - V300LA4 20 1,000 10,000 IMPULSE DURATION (µs) The equivalen exponenial waveform of equal energy is hen found from: The exponenial waveform is aken o have equal V C and I values. Then, Where: K* and * are he values for he riangular waveform and EXP is he impulse duraion for he equivalen exponenial waveform FIGURE 4. PEAK CURRENT DERATING BASED ON PULSE WIDTH AND NUMBER OF APPLIED PULSES E = (.5)(10)(340)()(10-6 ) = 850mJ E TRIANGULAR = E EXP 850mJ = 1.4 V C I EXP 850mJ EXP = 1.4 (340) (10) = 179µs Or: K* EXP = 1.4 The pulse rise porion of he waveform can be ignored when he impulse duraion is five imes or more longer. The maximum number of pulses for he above example would exceed 10 4 from he pulse deraing curves shown in Figure 4. Varisor Volage The varisor nominal volage (V NOM or V N ) represens he applied volage where he varisor ransiions from is sandby mode o is low impedance clamping mode. I is measured a he 1mA conducion poin. The minimum and maximum limi values are specified in he raings able. Power Dissipaion Requiremens Transiens generae hea in a suppressor oo quickly o be ransferred during he pulse inerval. Power dissipaion capabiliy is of concern for a suppressor if ransiens will be occurring in rapid succession. Under his condiion, he power dissipaion required is simply he energy (wa-seconds) per pulse imes he number of pulses per second. The power so developed mus be wihin he specificaions shown on he raings ables for he specific device ype. I is o be noed ha varisors can only dissipae a relaively small amoun of average power and are, herefore, no suiable for repeiive applicaions ha involve subsanial amouns of average power dissipaion (likewise, varisors are no suiable as volage regulaion devices). Furhermore, he operaing values need o be deraed a emperaures above he absolue maximum limis as shown in Figure 5. PERCENT OF RATED VALUE BA/BB, CA, DA/DB, 20 LA, C III, HA, NA, MA, 10 UlraMOV, PA, ZA SERIES AMBIENT TEMPERATURE ( o C) Volage Clamping Selecion CH, CP CS, RA SERIES FIGURE 5. CURRENT, ENERGY, POWER DERATING vs TEMPERATURE Transien V-I characerisics are provided in he specificaions for all models of varisors. Shown below in Figure 6 are curves for 130V AC raed models of he LA series. These curves indicae he peak erminal volage measured wih an applied 8/20µs impulse curren. For example, if he peak impulse curren applied o a V130LA2 is 10A, ha model will limi he ransien volage o no higher han 340V

4 Applicaion Noe 9771 MAXIMUM PEAK (V) MAXIMUM CLAMPING VOLTAGE COMPARED BY MODEL SIZE V M(AC) = 130V RATING T A = -55 TO 85 o C UL1449 CORD CONNECTED AND DIRECT PLUG-IN CATEGORY V130LA2 V130LA5 V130LA10A IMPULSE GENERATOR LOAD LINES (IMPLIED) UL1449 PERMANENTLY CONNECTED CATEGORY, AND ANSI/IEEE C61.41 (IEEE587) CATEGORY B PEAK AMPERES 8/20µs WAVESHAPE V130LA20A FIGURE 6. TRANSIENT V-I CHARACTERISTICS OF TYPICAL LA SERIES MODELS If he ransien curren is unknown, he graphical mehod of Figure 3 can be uilized. From a knowledge of he ransien volage and source impedance a load line is ploed on he V-I characerisic. The inersecion of he load line wih he varisor model curve gives he varisor ransien curren and he value of clamped peak ransien volage. The abiliy of he varisor o limi he ransien volage is someimes expressed in erms of a clamp raio. For example, consider a varisor applied o proec he power erminals of elecrical equipmen. If high line condiions will allow a rise o 130V AC, hen 184V peak would be applied. The device seleced would require a volage raing of 130V ACRMS or higher. Assume selecion of a V130LA2 model varisor. The V130LA2 will limi ransien volages o 340V a currens of 10A. The clamp raio is calculaed o be, Clamp Raio = = V C a 10A Peak Volage Applied 340V 184V = 1.85 The clamp raio can be found for oher currens, of course, by reference o he V-I characerisic. In general, clamping abiliy will be beer as he varisor physical size and energy level increases. This is illusraed in Figure 7 which compares he clamping performance of he differen Lielfuse Varisor families. I can be seen ha he lowes clamping volages are obained from he 20mm (LA series) and 60mm (BA series) producs. In addiion, many varisor models are available wih wo clamping selecions, designaed by an A, B, or C a he end of he model number. The A selecion is he sandard model, wih B and C selecions providing progressively igher clamping volage. For example, he V130LA20A volage clamping limi is 340V a A, while he V130LA20B clamps a no more han 325V. MAXIMUM CLAMP RATIO AND MAXIMUM INSTANTANEOUS VOLTAGE RATIO K 5K 10K INSTANTANEOUS CURRENT (A) MA4 LA10 PA, LA20 BA NOTE: CLAMP RATIO EQUALS VARISTOR VOLTAGE DIVIDED BY V NOM OR 184V FOR 130V AC RMS LA4 FIGURE 7. VARISTOR V-I CHARACTERISTICS FOR FOUR PRODUCT FAMILIES RATED AT 130V AC

5 Applicaion Noe 9771 MAXIMUM STEADY-STATE APPLIED VOLTAGE PEAK CURRENT (A) ENERGY (J) VOLTS AC RMS VOLTS DC ,000 1, 2,800 3, 6,000 7,000 DISC SIZES/ PACKAGES CP, SERIES 22, 20, 16 GAUGE AUML, ML, MLE, MLN, CH SERIES MA SERIES x 8mm 3mm ZA SERIES 5, 7, 10, 14, 20 (mm) RA SERIES 5 x 8, 10 x 16, 14 x 22 (mm) 1, - 10, C-III, LA, UlraMOV SERIES 7, 10, 14, 20 (mm) PA SERIES 20mm 25,000-40, ,050 HA, HB, DA/ DB SERIES 32, (mm) 50,000-70, ,000 BA/ BB SERIES 60mm 30,000-40, NA SERIES 34mm SQ. 20,000-70,000-10,000 CA SERIES 32, 40, 60 (mm) 65,000 -,000 2, - 12,000 AS SERIES 32, 42, 60 (mm) Lielfuse mulilayer suppression echnology. FIGURE 8. VARISTOR PACKAGE STYLES AND RATINGS RANGE

6 Varisor Ordering Informaion The varisor par number includes raings informaion. Some ypes include he working volage, ohers indicae he nominal volage. See he varisor ordering nomenclaure guides below. Applicaion Noe 9771 ULTRAMOV TYPES V XX E XXX LX X X DEVICE FAMILY: Varisor DISC DIAMETER: 07, 10, 14, or 20 (mm) ENCAPSULATION: E = Epoxy V M(AC)RMS : 130 o 625 (V) LEAD FORMATION: L1 = Sraigh L2 = Crimped L3 = In-Line L4 = Trim/Crimp (Bulk pack only) NONSTANDARD LEAD SPACING OPTIONS (DO NOT ADD IF STANDARD) (NOTE 2): 5 = 5mm Lead Spacing 7 = 7.5mm Lead Spacing 1 = 10mm Lead Spacing PACKAGING: B T A = Bulk Pack = Tape and Reel = Ammo Pack OTHER VARISTOR TYPES BA, BB, CA, CP, CS, DA, DB, HA, HB, LA, NA, PA, VARISTOR SERIES CH, MA, ZA, VARISTOR SERIES V 130 LA 20 A V 220 MA 4 A Selecion - Clamping Volage (A or B) Relaive Energy Indicaor or Disc Size Produc Series Max RMS Applied Volage V = Meal-Oxide Varisor (MOV) MOV Varisor Relaive Energy Indicaor Produc Series Selecion - Clamping Volage (A or B) V N (DC) Nominal Varisor Volage The five major consideraions for varisor selecion have been described. The final choice of a model is a balance of hese facors wih device packaging and cos rade-offs. In some applicaions a prioriy requiremen such as clamp volage or energy capabiliy may be so imporan as o force he selecion o a paricular model. Figure 8 illusraes he Lielfuse varisor package syles in a marix ha compares energy and curren raings o he working volage range