Solution Stead-State Voltage The 117VAC, 110% high line condition is 129V. The closest voltage rating available is 130V.

Transcription

1 APPLICATION NOTES Lielfuse Varisor Design Examples This noe is mean o be a guide for he user in selecing a varisor by describing common applicaion examples, and illusraing he soluion process o deermine he appropriae varisor. The noe also describes series/parallel connecion rules. APPLICATIONS: POWER SUPPLY PROTECTION AGAINST LINE TRANSIENT DAMAGE I is desired o preven failure of he power supply shown in Figure b o be used on residenial 7VAC lines. A represenaive ransien generaor is o be used for esing as shown in Figure a. 5kV Figure a.transien Generaor V T = 5kV sin 5 X e --5 If he ransien is applied o he exising circui, he recifier will receive high negaive volages, ransmied hrough he filer capacior.the LC nework is here o preven RFI from being ransmied ino he power line (as in a TV se), bu also serves o reduce he ransien volage. An analysis shows ha he ransien will be reduced approximaely by half, resuling in abou.5kv insead of 5kV a he recifier. H D Figure b.typical Power Supply Circui This is sill o high for any pracical recifier, 5 +. F 5 F - so some suppression mus be added. I is desirable o use he buil-in impedance of he coil o drop he remaining volage, so he suppressor would bes be applied as shown. A selecion process for a Lielfuse varisor is as follows: Soluion Sead-Sae Volage The 7VAC, % high line condiion is 9V. The closes volage raing available is 3V. Energy and Curren The µh inducor will appear o be abou 3Ω is derived from he inducive reacance a he ransien generaor source frequency of 5 π rad.taking a firs esimae of peak varisor curren, 5V/Ω=3A. (This firs esimae is high, since i assumes varisor clamping volage is zero.) Wih a enaive selecion of a 3V Harris Varisor, we find ha a curren of 3A yields a volage of from 35V o 3V, depending on he model size, as shown in Figure a and Figure b. Revising he esimae, I=(5V- 35V)/Ω=7.A. For model V3LAA, 7.A coincides closely wih a 3V clamping level.there is no need o furher refine he esimae of peak curren if model A remains he final selecion. To arrive a an energy figure, assume a sawooh curren waveform of 7A peak, dropping o zero in wo ime consans, or µs. Energy is hen roughly equal o (7Ax3Vxµs)/, he area under he power waveform.the resul is.6j, well wihin he capabiliy of he varisor (7J). Peak curren is also wihin he 65A raing. Model Selecion The acual varisor selecion is a rade-off beween he clamping volage desired and MAXIMUM PEAK (V) UL449 CORD CONNECTED AND DIRECT PLUG-IN CATEGORY V3LA V3LA5 V3LAA V3LAA IMPULSE GENERATOR LOAD LINES (IMPLIED) UL449 PERMANENTLY CONNECTED CATEGORY, AND ANSI IEEE C6.4 (IEEE57) CATEGORY B PEAK AMPERES /µs WAVESHAPE Figure a. V3LA Varisor V-I Characerisics MAXIMUM PEAK (V) PEAK AMPERES /µs WAVESHAPE Figure b. V3LA Varisor V-I Characerisics VARISTOR CURRENT 7 I V Figure 3. Energy Approximaion µs µs he number of ransien curren pulses expeced in he life of he equipmen. A 7J Raed varisor will clamp a 35V and be capable of handling over 6 such pulses. An J uni will clamp o approximaely 35V and be capable of handling over 5 such pulses. Furhermore, he clamping volage deermines he cos of he recifier by deermining he volage raing required. A smaller, lower cos varisor may resul in a more expensive higher volage recifier diode.

2 SCR MOTOR CONTROL The circui shown in Figure 4. experiences failures of he recifiers and SCR when he ransformer primary is swiched off.the manufacurer has ried V componens wih lile improvemen. 4V AC 6Hz 4: Soluion Add a varisor o he ransformer secondary o clamp he ransformer inducive ransien volage spike. Selec he lowes volage Lielfuse Varisor ha is equal o or greaer han he maximum high line secondary AC volage.the V3LA ypes fulfills his requiremen. Deermine he peak suppressed ransien volage produced by he ransien energy source.this is based on he peak ransien curren o he suppressor, assuming he wors-case condiion of zero load curren. Zero load curren is normally a valid assumpion. Since he dynamic ransien impedance of he Lielfuse Varisor is generally quie low, he parallel higher impedance load pah can be negleced. Since ransien curren is he resul of sored energy in he core of he ransformer, he ransformer equivalen circui shown in Figure 5 will be helpful for analysis.the sored inducive energy is: FIELD R 5k R 33k ARMATURE SPEED CONTROL R3 5k C I.µF FIGURE 4 SCR MOTOR CONTROL Figure 4. SCR Moor Conrol E LM = -- L M I M V PRIMARY Z P MUTUAL INDUCTANCE REPRESENTED BY IRON CORE I M L M N IDEAL TRANSFORMER Z S SUS Figure 5. Simplified Equivalen Circui of A Transformer SCR V SECONDARY The designer needs o know he oal energy sored and he peak curren ransformed in he secondary circui due o he muual inducance, L M. A no load, he magneizing curren, (I NL ), is essenially reacive and is equal o I M.This assumes ha he primary copper resisance, leakage reacance and equivalen core resisive loss componens are small compared o L M.This is a valid assumpion for all bu he smalles conrol ransformers. Since I NL is assumed purely reacive, hen: V pri X LM = I NL and i M = I NL I NL can be deermined from nameplae daa.where nameplae is no available, Figure 6 and Figure 7 can guide he designer. Assuming a 3.5% value of magneizing curren from Figure 7 for a kva ransformer wih 4V AC primary, and V AC secondary: i M = (.35 ) kva V =.46A i M = i M X LM = 4V/.46A = 39 L M = X LM /w =.7H.7(.6 ) E LM = =.5J Wih his informaion one can selec he needed semiconducor volage raings and required varisor energy raing. Peak varisor curren is equal o ransformed secondary magneizing curren, i.e., î M (N), or.4a. From Figure, he peak suppressed ransien volage is 3V wih he V3LAA selecion, 95V wih he V3LAB.This allows he use of 3V raed semiconducors. Safey margins exis in he above approach as a resul of he following assumpions:. All of he energy available in he muual inducance is ransferred o he varisor. Because of core hyseresis and secondary winding capaciance, only a fracion less han wo-hirds is available..the exciing curren is no purely reacive. There is a % o % safey margin in he peak curren assumpion. Afer deermining volage and peak curren, energy and power dissipaion requiremens mus be checked. For he given example, he single pulse energy is well below he V3LAB varisor raing of 7J a 5 C maximum ambien emperaure. Average power dissipaion requiremens over idling power are no needed because of he nonrepeiive naure of he expeced ransien. PERCENT MAGNETIZING CURRENT f = 5...6Hz 4 6 TRANSFORMER RATING (kva) Figure 6. Magneizing curren of ransformers wih low silicon seel core PERCENT MAGNETIZING CURRENT f = 5...6Hz 3 4 TRANSFORMER RATING (kva) Figure 7. Magneizing curren of ransformers wih high silicon seel core or square loop core Should he ransien be repeiive, hen he average power is calculaed from he produc of he repeiion rae imes he energy of he ransien. If his value exceeds he V3LAB capabiliy of.w, power varisors of he HA, DA, or DB Series may be required. Should he ambien emperaure exceed 5 C or he surface emperaure exceed 5 C, he single pulse energy raings and he average power raings mus be deraed by he appropriae deraing facors supplied on he daa shee. CONTACT ARCING DUE TO INDUCTIVE LOAD To exend he life of he relay conacs shown in Figure and reduce radiaed noise, i is desired o eliminae he conac arcing. + C C V DC Figure. Relay Circui L R C RELAY C C = STRAY CAPACITANCE L = RELAY COIL INDUCTANCE R C = RELAY COIL RESISTANCE FIGURE. RELAY CIRCUIT When relays or mechanical swiches are used o conrol inducive loads, i is neces-

3 sary o use he conacs a only abou 5% of heir resisive load curren raing o reduce he wear caused by arcing of he conacs.the energy in he arcing is proporional o he inducance and o he square of he curren. Each ime he curren in he inducive load is inerruped by he mechanical conacs, he volage across he conacs builds up as - L di/d. When he conacs arc, he volage across he arc decreases and he curren in he coil can increase somewha.the exinguishing of he arc causes an addiional volage ransien which can again cause he conacs o arc. I is no unusual for he resriking o occur several imes wih he oal energy in he arc several imes ha which was originally sored in he inducive load. I is his repeiive arcing ha is so desrucive o he conacs. In he example, R C is 3 and he relay conacs are conducing nearly A.The conacs will draw an arc upon opening wih more han approximaely.4a or V.The arc coninues unil curren falls below.4a. Soluion To preven iniiaion of he arc, i is necessary o reduce he curren and volage of he conacs below he arc hreshold levels a he ime of opening, and hen keep hem below breakdown hreshold of he conacs as hey open.two obvious echniques come o mind o accomplish his: ) use of a large capacior across he conacs ) a volage clamp (such as a varisor).the clamp echnique can be effecive only when he minimum arc volage exceeds he supply volage. In his example a clamping device operaing above he supply volage will no preven arcing.this is shown in Figure 9. ARC VOLTAGE (V) 5 ARCING VOLTAGE CLAMP ABOVE ARC VOLTAGE VOLTAGE CLAMP BELOW ARC VOLTAGE BREAK TIME ( s) BREAKDOWN LEVEL Figure 9. Volage clamp used as arc suppressor The capacior echnique requires he capaciance o be sufficienly large o conduc he inducor curren wih a volage rae-of-rise racking he breakdown volage rae-of-rise of he conacs as hey mechanically move apar.this is shown in Figure a. ARC VOLTAGE (V) 5 SMALL C WITH R (ARCING) BREAK TIME ( s) Figure a. R-C arc suppression The limiaions in using he capacior approach are size and cos.this is paricularly rue for hose cases involving large amouns of inducive sored energy. Furhermore, he use of a large capacior alone creaes large discharge currens upon conac reclosure during conac bouncing. As a resul, he conac maerial may mel a he poin of conac wih subsequen welding.to avoid his inrush curren, i is cusomary o add a series resisor o limi he capaciive discharge curren. However, his addiional componen reduces he nework effeciveness and adds addiional cos o he soluion. A hird echnique, while no as obvious as he previous wo, is o use a combinaion approach.this echnique shown in Figure b parallels a volage clamp componen wih an R-C nework.this allows he R-C nework o preven he low volage iniial arcing and he clamp o preven he arcing ha would occur laer in ime as he capacior volage builds up.this approach is ofen more cos effecive and reliable hen using a large capacior. ARC VOLTAGE (V) 5 CONTACT BREAKDOWN LEVEL LARGE C WITH R (NO ARCING) SMALL C WITH R AND VOLTAGE CLAMP COMBINATION BREAK TIME ( s) Figure b. R-C and clamp arc suppression CONTACT BREAKDOWN LEVEL Also, wih AC power relays he impedance of a single large R-C suppressor migh be so low ha i would allow oo much curren o flow when he conacs are open.the combinaion echnique of a small R-C nework in conjuncion wih a varisor is of advanage here, oo. In his example a.µf capacior and Ω resisor will suppress arcing compleely, bu by reducing he capaciance o.47µf, arcing will sar a 7V. Thus, o use a varisor as a clamp in conjuncion wih he R-C nework, i mus suppress he volage o below 7V a A and be capable of operaing a a seadysae maximum DC volage of V + %, or 3.V (assumes a ±% regulaed V DC supply). The hree candidaes ha come closes o meeing he above requiremen are he MA series V39MAB model and he ZA series V39ZA and V39ZA5 models, all of which have maximum seady-sae DC volage raings of 3V.The V39MAB and V39ZA5 V-I characerisics a A shows a maximum volage of 73V, while he V39ZA characerisic a A shows a maximum volage of 67V. Thus, he laer varisor is seleced. Use of a.6µf capacior in place of he.47µf previously chosen would allow use of he V39MAB or V39ZA5. Placing only a Lielfuse Varisor raed for 3V DC across he conacs resuls in arcing up o he 66V level. By combining he wo, he capacior size and volage raing are reduced and suppression complee. Besides checking he varisor volage and arcing eliminaion, he designer should review energy and peak curren requiremens.varisor energy is deermined from a measuremen of he coil inducance and he calculaion E = / Li. Peak curren, of course, is under A. Power dissipaion is negligible unless he coil is swiched ofen (several imes per minue). In hose cases where muliple arcs occur, he varisor energy will be a muliple of he above / Li value.the peak curren is well wihin he raing of eiher he MA or ZA series of varisors, bu he number of conac operaions allowable for eiher varisor is a funcion of he impulse duraion.this can be esimaed by assuming a L/R C ime consan a he A or peak curren value. Since he volage across he varisor is 67V a A, he varisor saic resisance is 67.The coil R C value is V/A,

4 or.the coil inducance was found o be mh. Thus, he approximae ime consan is: From he pulse raing curves of he V39ZA mh τ = L/R C = = s 95 model, he number of allowable pulses exceeds million. NOISE SUPPRESSION Swiching of a small imer moor a V, 6Hz, was causing serious malfuncions of an elecronic device operaing from he same power line. Aemps were made o observe he ransien noise on he line wih an oscilloscope as he firs sep in curing he problem. Observed waveforms were hash, i.e., no readily idenifiable. Noise in an elecromechanical sysem is a commonly experienced resul of inerruping curren by mechanical conacs. When he swich conacs open, a ho cahode arc may occur if he curren is high enough. On he oher hand, low curren will permi swich opening wihou an arc, bu wih ringing of circui resonances. As a consequence, volages can exceed he conac gap breakdown resuling in a replica of he old spark gap ransmier. I is he low curren case ha produces he mos serious noise disurbances which can resul in malfuncions or damage o elecrical equipmen.these pulses cause noise problems on adjacen lines, rigger SCRs and riacs, and damage semiconducors. In addiion, hey can disrup microprocessor operaion causing memory o be los and vial insrucions o be missed. Soluion A es circui (Figure ) was se up wih lumped elemens replacing he measured circui values.the moor impedance was simulaed by R,L, and C, and he AC line impedance by L and C. A DC source allowed repeaable observaions over he V CC + L C 4 P F 5 H V " AWG # WIRE Figure.Tes Circui S V C P F R L 6.H 44 V RF full range of curren ha could flow hrough he swich in he normal AC operaion. A diode deecor was used o observe he RF volage developed across a lengh of wire (5nH of inducance). The supply is se a 5mA o represen he peak moor curren in normal V AC operaion. As swich S was opened, he waveform in Figure was recorded. Noe he showering arc effec.the highes breakdown volage recorded here is V, and he highes RF deecor oupu (shown in he lower race) is 3V. A B s/cm UPPER V : V/cm LOWER V RF : V/cm :.ms/cm Figure. Unproeced Conacs Obviously, some correcive acion should be aken and he mos effecive one is ha which prevens he repeaed breakdown of he gap. Figure 3 shows he waveform of V (upper race) and V RF (lower race) for he same es condiions wih a Lielfuse Varisor, ype V3LAA, conneced direcly across he swich erminals.the varisor compleely eliminaes he relaxaion oscillaions by holding he volage below he gap breakdown volage (abou 3V) while dissipaing he sored energy in he sysem. PROTECTION OF TRAN- SISTORS SWITCHING INDUCTIVE LOADS The ransisor in Figure 4 is o operae a solenoid. I may operae as frequenly as once per second.the circui (wihou any suppression) consisenly damages he V/cm V/cm V/cm V/cm s/cm UPPER V : V/cm LOWER V RF : V/cm :.ms/cm Figure 3. Varisor proeced conacs ransisor. The inducor drives he collecor volage up 6V V+ 5 mh 47 V C V C V+ 6V when he ransisor base is grounded (urning off ).The inducor forces curren o flow unil he energy sored in is field is dissipaed.this energy is dissipaed in he reverse bias condiion of he ransisor and is sufficien o cause breakdown (indicaed by a sudden collapse of collecor volage during he pulse). Soluion This condiion can be eliminaed eiher by shuning he ransisor wih a suppressor or by urning i on wih a varisor conneced collecor-o-base.the firs mehod will considerably reduce he demands upon he safe operaing area (SOA) of he ransisor. If he volage is kep below is breakdown level, all energy will be dissipaed in he suppressor.the laer mehod will cause he ransisor o once again dissipae he sored energy, bu in he forward-bias sae in which he ransisor can safely dissipae limied amouns of energy.the choice is deermined by economics and reliabiliy. A suppressor conneced collecor-emier (C-E) will be more expensive han one conneced C-B, since i is required o absorb more energy, bu will allow he use of a ransisor wih reduced SOA. If a collecor-emier varisor is used in he above example, i is required o wihsand.6v DC wors-case (6 + % regulaion).the sored energy is / Li or / I C Figure 4a. Basic solenoid circui I C 6V V+ I V V C V+ 6V I C I V PERIOD OF HIGH SOA REQUIREMENT V C = COLLECTOR EMITTER VOLTAGE Figure 4b. Solenoid circui wih varisor proecion

5 (.) (.57) =.37J.The energy conribued by he power supply is roughly equal o his (coil volage supply volage, since varisor clipping volage x supply volage). Ignoring coil resisance losses for a conservaive esimae, varisor energy dissipaion is.65j per pulse.the peak curren will be.57a, he same as he coil curren when he ransisor is swiched off. If he ransisor operaes once per second, he average power dissipaion in he varisor will be.65w.this is less han he.w raing of a small 3V DC varisor (V39ZA). From he daa shee i can be seen ha if he device emperaure exceeds 5 C, deraing is required.the non-recurren joule raing is.5j, well in excess of he recurren value.to deermine he repeiive joule capabiliy, he curren pulse raing curves for he ZA series mus be consuled.two are shown in Figure 5. To use Figure 7, he impulse duraion (o he 5% poin) is esimaed from he circui ime consans and is found o be 4µs. From Figure 7A, for his example, he RATED PEAK PULSE CURRENT (A) INDEFINITE 3 MODEL SIZE 7mm VZA - V6ZA.,, IMPULSE DURATION (µs) 7mm V39ZA would no be limied o a cumulaive number of pulses. 4 In cases where he peak curren is greaer and inersecs wih he recommended pulse life curves, he designer mus deermine he maximum number of operaions expeced over he life of he circui and confirm ha he pulse life curves are no 5 6 Figure 5a. ZA Series VZA-V6ZA (7mm) RATED PEAK PULSE CURRENT (A), INDEFINITE MODEL SIZE 4mm VZA3 - V6ZA,, IMPULSE DURATION (µs) Figure 5b. ZA Series VZA3 o V6ZA (4mm) exceeded. Figure 5B shows he curves for he larger, 4mm V39ZA6 device and, illusraes he resulan higher capabiliy in erms of number of ransiens for a given peak pulse curren and duraion. Also, i may be necessary o exrapolae he pulse raing curves.this has been done in Figure 6 where he daa from Figure 5B is ransposed. A low currens he exrapolaion is a sraigh line. Finally, he V-I characerisics curves mus be consuled o deermine he varisor maximum clamping volage in order o selec he minimum ransisor breakdown volage. In his example, a.57a he V39ZA6 (if chosen) provides a maximum of 6V requiring ha he ransisor have abou a 65V or 7V capabiliy. PEAK PULSE CURRENT (A) ZA SERIES VZA3 TO V6ZA NOTE: PULSE RATING CURVE FOR,4µs PULSE WIDTH NUMBER OF PULSES Figure 6. Exrapolaed pulse raing curves Moor Proecion Frequenly, he cause of moor failures can be raced o insulaion breakdown of he moor windings.the source of he ransiens causing he breakdown may be from eiher inernal magneic sored energy or from exernal sources.this secion deals wih he self-generaed moor ransiens due o moor saring and circui breaker operaion. In he case of DC moors he equivalen circui consiss of a single branch.the magneic sored energy can be easily calculaed in he armaure or field circuis using he nameplae moor consans. Wih AC inducion moors he equivalen magneic moor circui is more complex and he circui consans are no always given on he moor nameplae.to provide a guide for moor proecion, Figures 7,, 9 were drawn from ypical inducion moor daa. While he acual sored energy will vary according o moor frame size and consrucion echniques, hese curves provide guidance when specific moor daa is lacking.the daa is conservaive as i assumes maximum moor orque, a condiion ha is no he ypical running condiion. Sored energy decreases considerably as he moor loading is reduced. Experience wih he suppression of magneic energy sored in ransformers indicaes ha Lielfuse Varisors may be used a heir maximum energy raings, even when muliple operaions are required.this is because of he conservaism in he applicaion requiremens, as indicaed above, and in he varisor raings. Thus, no aemp is made o derae he varisor for muliple operaion because of he random naure of he ransien energy experienced. STORED ENERGY PER PHASE (J) V5PA V5HA3 46V RMS LINE - LINE POLE 3V RMS LINE V3PA4 V3HA3 Y CONNECTED POLE MOTOR (hp) Figure 7. Sored energy curves for ypical wye-conneced inducion moor NOTES:.Y conneced 6Hz.. Energy a Max orque slip speed. 3. See Figure for varisor circui placemen. STORED ENERGY PER PHASE (J) DELTA CONNECTED 3V RMS LINE - LINE POLE V7HA3/V75PA4 46V RMS LINE - LINE V5PA MOTOR (hp) POLE V5HA Figure. Sored energy curves for ypical dela-connecion inducion moor NOTES: 4. Dela conneced a 6Hz. 5. Energy a maximum orque slip speed. 6. See Figure for varisor circui placemen. As an aid in selecing he proper operaing volage for Lielfuse Varisors,Table gives guidelines for wye-conneced and delaconneced moor circuis a differen line-o-line applied volages. Figure provides guidance in proper placemen of he varisor.

6 S O O G GS O CO C O O S RMS Line Volage (Line-Line) Dela Conneced Applied V. Varisor Raings Y Conneced Applied V. Varisor Raings Table.. Lielfuse varisor selecion guideline for 7VAC applicaions STORED ENERGY PER PHASE (J) DELTA CONNECTED 3V RMS LINE - LINE POLE V7HA3/V75PA4 46V RMS LINE - LINE V5PA MOTOR (hp) POLE V5HA Figure 9. Sored energy curves for a ypical moor wih salled roor NOTES: 7. 6Hz, see Figure for varisor circui placemen.. Energy a sar, i.e., SLIP =. 9. Inducion moor.., and 4 pole moors. Inerrupion of moor saring currens presens special problems o he user as shown in Figure 9. Since he sored magneic energy values are approximaely imes he running values, proecion is difficul a he higher horsepower levels. Ofen he moor is sared by use of a reduced volage which will subsanially reduce he sored energy. A reducion in saring curren of a facor of wo resuls in a fourfold reducion in sored energy. If a reduced volage sarer is no used, hen a decision mus be made beween proecion for he run condiion only, and he condiion of locked roor moor curren. For mos applicaions, he saring condiion can be ignored in favor of selecing he varisor for he wors-case run condiion. To proec a wo-pole, 75hp, 3φ, 46V RMS line-o-line wye-conneced moor from inerrupion of running ransiens. Specific Moor Daa Is No Available Soluion Consul Figure 7 along wih Table. Sandard varisors having he required volage raings are he 3V RMS raed models. This allows a % high-line volage condiion on he nominal 46V line-o-line volage, or 66V line-neural volage. Figure 7 shows a wo-pole 75hp, wye-conneced inducion moor, a he running condiion, / /4 5/75 has 5J of sored magneic energy per phase. Eiher a V3PA4 series or a V3HA3 series varisor will mee his requiremen.the HA series Lielfuse Varisor provides a greaer margin of safey, alhough he PA series Lielfuse Varisor fully mees he applicaion requiremens. Three varisors are required, conneced direcly across he moor erminals as shown in Figure. V L-L V VARISTOR = V 3 L L Figure a. Wye conneced FIGURE A. WYE CONNECTED V L-L Figure b. Dela conneced 46 5/ Power Supply Crowbar Occasionally i is possible for a power supply o generae excessively high volage. An accidenal removal of load can cause damage o he res of he circui. A simple safeguard is o crowbar or shor circui he supply wih an SCR.To provide he riggering o he SCR, a high-volage deecor is needed. High volage avalanche diodes are effecive bu expensive. An axial leaded Lielfuse Varisor provides an effecive, inexpensive subsiue. In he circui of Figure, he volage, wihou proecion, can exceed wice he normal 4V peaks, damaging componens downsream. A simple arrangemen o crowbar he supply is shown. M V VARISTOR = V L-L / V + - V A C.B. FULL WAVE (RECTIFIED) Figure. Crowbar circui C6D NORMAL VOLTAGE < 4V PEAK ABNORMAL VOLTAGE > 4V PEAK The supply shown can provide A RMS of shor-circui curren and has a A circui breaker. A C6D SCR having a 4A RMS capabiliy is chosen.triggering will require a leas.4v gae-o-cahode, and no more han.v a A a 5 C ambien. Soluion Check he MA series Lielfuse Varisor specificaions for a device capable of supporing 4V peak.the V7MA4B can handle (7V RMS ) = 4V. According o is specificaion of 7V ±%, he V7MA4B will conduc ma DC a no less han 43V.The gae-cahode resisor can be chosen o provide.4v (he minimum rigger volage) a ma, and he SCR will no rigger below 43V.Therefore, R GK should be less han 4.The highes value 5% olerance resisor falling below 4 is a 36 resisor, which is seleced.thus, R GK is 37 maximum and 34 minimum. Minimum SCR rigger volage of.4v requires a varisor of.4v/37, or.6ma for a minimum varisor volage of 45V. The maximum volage o rigger he circui is dependen upon he maximum curren he varisor is required o pass o rigger he SCR. For he C6 a 5 C, his is deermined by calculaing he maximum curren required o provide.v across a parallel resisor comprised of he 36 R GK seleced and he equivalen gae-cahode SCR resisor of.v/a, since he C6 requires a maximum of A rigger curren.the SCR gae inpu resisance is 4k and he minimum equivalen gae-cahode resisance is he parallel combinaion of 4k and R GK (MIN), or 36Ω -5%, 34.The parallel combinaion is 35Ω.Thus, I VARIS- TOR for maximum volage-o-rigger he C6 is.v/35, or.54ma. According o he specificaion shee for he V7MA4B, he varisor will no exceed 33V wih his curren.the circui will, herefore, rigger a beween 45 and 33V peak, and a 4V raed C6 can be used.the reader is cauioned ha SCR gae characerisics are

7 sensiive o juncion emperaures, and a value of 5 C for he SCR emperaure was merely chosen as a convenien value for demonsraing design procedures. The maximum energy per pulse wih his waveform is deermined as approximaely / x K x I PK x V PK x (duraion of / wave pulse), or.5mj for his example. Since he volage does no drop o zero in his case, he SCR remains on, and he varisor sees only one pulse; hus, no seady-sae power consideraion exiss. General Proecion of Solid Sae Circuiry Agains Transiens On 7VAC Lines Modern elecronic equipmen and home appliances conain solid sae circuiry ha is suscepible o malfuncion or damage caused by ransien volage spikes.the equipmen is used in residenial, commercial, and indusrial buildings. Some es sandards have been adoped by various agencies and furher definiion of he environmen is underway by he IEEE and oher organizaions. The ransiens which may occur on residenial and commercial AC lines are of many waveshapes and of varying severiy in erms of peak volage, curren, or energy. For suppressor applicaion purposes, hese may be reduced o hree caegories. Firs, he mos frequen ransien migh be he one represened by a 3kHz or khz ring wave.this es surge is defined by an oscillaory exponenially decaying volage wave wih a peak open circui volage of 6kV.This wave is considered represenaive of ransiens observed and repored by sudies in Europe and Norh America.These ransiens can be caused by disan lighning srikes or disribuion line swiching. Due o he relaively high impedance and shor duraion of hese ransiens, peak curren and surge energy are lower han he second and hird caegories. The second caegory is ha of surges produced by nearby lighning srokes.the severiy of a lighning sroke is characerized in erms of is peak curren.the probabiliy of a direc sroke of a given severiy can be deermined. However, since he lighning curren divides in many pahs, he peak curren available a an AC oule wihin a building is much less han he oal curren of he sroke.the sandard impulse used o represen lighning and o es surge proecive devices is an /µs curren waveshape as defined by ANSI Sandard C6., and also described in ANSI/IEEE Sandard C and IEC A hird caegory of surges are hose produced by he discharge of energy sored in inducive elemens such as moors and ransformers. A es curren of /s waveshape is an acceped indusry es impulse and can be considered represenaive of hese surges. Alhough no hard-and-fas rules can be drawn as o he caegory and severiy of surges which will occur, a helpful guideline can be given o sugges varisors suiable in ypical applicaions. The guideline of Table recognizes consideraions such as equipmen cos, equipmen duy cycle, effec equipmen downime, and balances he economics of equipmen damage risk agains surge proecion cos. Failure Modes and Varisor Proecion Varisors are inherenly rugged and are conservaively raed and exhibi a low failure rae.the designer may wish o plan for poenial failure modes and he resulan effecs should he varisor be subjeced o surge currens or energy levels above is raing. Failure Modes Varisors iniially fail in a shor-circui mode when subjeced o surges beyond heir peak curren/energy raings.they also shor-circui when operaed a seady-sae volages well beyond heir volage raings. APPLICATION TYPE DUTY CYCLE LOCATION EXAMPLE SUGGESTED MODEL Ligh Consumer Very Low A Mixer/Blender V7E3 or VE3 Consumer Low A Porable TV/Elecronics V4E3 Consumer Medium A Home Theaer, PC V4E3, VE3 Ligh Indusrial/Office Medium B Copier, Server VE3, VE4 Indusrial Medium B Moors, Solenoid, Relay VE4, V3HA3 Indusrial High B Large Compuer Moor Conrol V3DA4 or DB4 Indusrial High B Elevaor Conrol Heavy Moors V5DA4 or DB4 Table. Lielfuse varisor selecion guideline for 7V AC applicaions This laer mode of sress may resul in he evenual open-circuiing of he device due o meling of he lead solder join. When he device fails in he shored mode he curren hrough he varisor becomes limied mainly by he source impedance. Consequenly, a large amoun of energy can be inroduced, causing mechanical rupure of he package accompanied by expulsion of package maerial in boh solid and gaseous forms. Seps may be aken o minimize his poenial hazard by he following echniques: ) fusing he varisor o limi high faul currens, and, ) proecing he surrounding circuiry by physical shielding, or by locaing he varisor away from oher componens. Series and Parallel Operaion of Varisors In mos cases he designer can selec a varisor ha mees he desired volage raings from sandard caalog models. Occasionally he sandard caalog models do no fi he requiremens eiher due o volage raings or energy/curren raings. When his happens, wo opions are available: varisors can be arranged in series or parallel o make up he desired raings, or he facory can be asked o produce a special o mee he unique applicaion requiremen. Series Operaion of Varisors Varisors are applied in series for one of wo reasons: o provide volage raings in excess of hose available, or o provide a volage raing beween he sandard model volages. As a side benefi, higher energy raings can be achieved wih series conneced varisors over an equivalen single device. For insance, assume he applicaion calls for a lead mouned varisor wih an V RMS raing of 375VAC and having a I TM peak curren capabiliy of A.The I TM requiremen fixes he varisor size. Examining he LA series volage raings near 375VAC, only 3V and 4V unis are available.the 3V is oo low and he 4V uni (V4LA4B) resuls in oo high a clamp volage (V C of 6V a A). For a V3LAB and a V5LA4B in series, he maximum raed volage is now he sum of he volages, or 3V.The clamping volage,v C, is now he sum of he individual varisor clamping volages, or 945V a A.The peak curren capabiliy is sill 65A bu he energy

8 raing is now he sum of he individual energy raings, or J. In summary, varisors can be conneced in series providing hey have idenical peak curren raings (I TM ), i.e., same disc diameer.the composie V-I characerisic, energy raing, and maximum clamp volages are all deermined by summing he respecive characerisics and/or raings of he individual varisors. Parallel Operaion of Varisors Applicaion requiremens may necessiae higher peak currens and energy dissipaion han he high energy series of varisors can supply individually. When his occurs, he logical alernaive is o examine he possibiliy of paralleling varisors. Forunaely, all Lielfuse Varisors have a propery a high curren levels ha makes paralleling feasible.this propery is he varisor's series-resisance ha is prominen during he up-urn region of he V-I characerisic. This up-urn is due o he inheren linear resisance componen of he varisor characerisic. I acs as a series balancing, orballasing, impedance o force a degree of sharing ha is no possible a lower curren levels.this is depiced in Figure. A a clamp volage of V, he difference in curren beween a maximum specified sample uni and a hypoheical % lower bound sample would be more han o.thus, here is almos no curren sharing and only a single varisor carries he curren. Of course, a low curren levels in he range of A -A, his may well be accepable. Objecive SERIES Higher volage capabiliy. Higher energy capabiliy. Non-Sandard volage capabiliy. A high curren levels exceeding A, he up-urn region is reached and curren sharing improves markedly. For insance, a a clamp volage of 9V, he respecive varisor currens (Figure ) are 5A and A, respecively. While far from ideal sharing, his illusraion shows he feasibiliy of paralleling o achieve higher currens and energy han achievable wih a single model varisor. Pracically, varisors mus be mached by means of high curren pulse ess o make parallel operaion feasible. Pulse esing should be in he range of over ka, using an /µs, or similar pulse. Peak volages mus he read and recorded. High curren characerisics could hen be exrapolaed in he range of A -,A.This is done by using he measured daa poins o plo curves parallel o he daa shee curves. Wih his echnique curren sharing can be considerable improved from he near wors-case condiions of he hypoheical example given in figure. In summary, varisors can be paralleled, bu good curren sharing is only possible if he devices are mached over he oal range of he volage-curren characerisic. In applicaions requiring paralleling, Lielfuse should be consuled. Higher Curren Capabiliy Higher Energy Capabiliy PARALLEL Selecion Required No Yes Models Applicable All, mus have same I TM raing. All models Applicaion Range All volages and currens. All volages - only high currens, i.e., >A. Precauions I TM raings mus be equal. Mus be idenical volage raed models. Mus es and selec unis for similar V-I characerisics. Effec on Raings Clamp volages addiive. Volage raings addiive. Curren raings ha of single device. Energy W TM, raings addiive. Curren raings funcion of curren sharing as deermined graphically. Energy raings as above in proporion o curren sharing. Clamp volages deermined by composie V-I characerisic of mached unis. Volage raings ha of single uni. Table 3. Checklis for series and parallel operaion of varisors Reference For more informaion concerning Lielfuse Indusrial applicaion soluions visi he Lielfuse web sie-hp:// [] Kaufman, R., The Magic of I, IEEE Trans. IGA-, No. 5, Sep.-Oc PEAK VOLTAGE (V) LIMIT SAMPLE LOWER BOUND (%) SAMPLE UNIT T A = -4 o C TO 5 o MODEL V5BA6 C PEAK CURRENT (A) Figure. Parallel operaion of varisors by graphical echnique Some guidelines for series and parallel operaion of varisors are given in Table 3. Lielfuse, Inc. E. Norhwes Highway Des Plaines, IL 6 Specificaions, descripions and illusraive maerial in his lieraure are as accurae as known a ime of publicaion, bu are subjec o change wihou noice. Lielfuse is a regisered rademark of Lielfuse Incorporaed. EC63 Copyrigh Lielfuse, Inc., All Righs Reserved. Prined in U.S.A. JANUARY