High Voltage Protection Techniques with TP3210 Subscriber Line Interfa
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High Voltage Protection Techniques with TP3210 Subscriber Line Interface Module
High Voltage Protection Techniques with TP3210 Subscriber Line Interface Module
INTRODUCTION objective this application note demonstrate solution which removes some traditional accuracy constraints protection components without impacting performance then goes develop protection schemes which completely resettable note begins with brief discussion protection problems associated with subscriber line interface circuits outlines basic requirements which these devices meet This followed discussion National Semiconductor Subscriber Line Interface Module (SLIM) device demonstrating unique advantages this part over more conventional solutions Finally protection systems analyzed detail measured performance these shown These results combined with information contained note will allow linecard designer completely specify protection components required meet desired performance level SLIC PROTECTION PROBLEMS understand problems protecting line circuits basic review traditional protection layout useful appreciating direction which protection technology moving line interface protection networks traditionally split into primary secondary tertiary protection components Figure shows layout conventional switch protection scheme both subscriber trunk lines exiting central office secondary tertiary levels normally combined linecard protection levels typically 1000V peak after primary protection around peak after secondary protection This value course dependent clamp voltage shunt protection element used secondary circuit Primary Protection primary components responsible handling large disturbances such lightning strike close central switch location They normally situated main distribution frame (MDF) where subscriber cables
National Semiconductor Application Note Duncan Bremner Telecom Products January 1990
enter office disturbing currents arrested primary protection components directed away from main body switch separate protection ground connection discharged harmlessly earth typical implementation primary protection would employ discharge tubes (GDT) which limit voltages exiting less than 1000V peak benefit positioning primary protection twofold Firstly avoids having large transient currents flowing office wiring This ensures that current rating wiring never exceeded also ensures that voltages present after relatively (less than 1000V peak) which avoids damaging secondary arcing between adjacent points inside office Secondly ensuring that energy shunted safely away earth operation majority switch remains unaffected This especially important when serving large rural areas with high incidence thunderstorms Secondary Protection secondary protection components reside either linecard itself immediately backplane adjacent card connector designed handle residual current which passes primary protection This consists power cross currents power induction products which sufficiently voltage pass through primary protectors secondary protection schemes employ separate elements protect sensitive line card components Firstly series element which limits current flowing onto linecard secondly shunt element which limits voltage This shunt element simple bridge rectifier connected between battery terminals which shunts current either battery ground more commonly active thyristor device which shunts current protection ground only This device when triggered returns transient energy either local protection ground connection linecard preferably returns remote protection ground usually same protec-
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FIGURE Conventional Switch Protection Arrangement
C1995 National Semiconductor Corporation 10553 RRD-B30M75 Printed
tion ground used When using remote grounds this give rise 1000V differences between Battery ground protection ground This configuration popular North America specialized knowledge design techniques must employed cope with this since voltage stress line card components high SLIM device devices which meet this requirement thus especially useful applications using separate ground systems While providing necessary protection protection components must degrade transmission characteristics during normal speech signaling modes this area that compromises between good protection meeting specifications frequently made these avoided when using SLIM device PROTECTION COMPROMISES There areas which affect selection protection components used These voltage headroom requirements electronic SLICs operate correctly Longitudinal Balance requirements imposed circuit transmission specifications Headroom Limitations headroom implications shown Figure order electronic SLIC function correctly certain voltage headroom required linear operation This means that there interaction between feeding line current maximum long loop requirement allowing sufficient headroom SLIC amplifer operation From Figure seen that line current flows from amplifier Rprotect series protection elements values these carefully chosen voltage headroom impacted hence maximum long loop requirements compromised
longitudinal balance tests probably most stringent requirements placed front line interface circuit normally directly affected selection protection resistors order meet these requirements matching resistance from Tip(A) Ring(B) legs circuit ground must typically better than most circuits burden this precise matching requirements placed directly series protection elements This results escalating costs these components with SLIM these precise requirements eliminated However problems headroom longitudinal balance eliminated resourceful design techniques employed desensitise series protection elements from impacting feeding These techniques employed National Semiconductor SLIM which optimal complementary technologies removes much restrictions accuracy requirements from protection components SLIM Protection National Semiconductor SLIM device completely concept subscriber connections central switch which uses mixture technologies attain optimal performance cost ratio This ratio just component cost required implement SLIC function total manufacturing cost SLIM device designed minimize number cost external components resulting substantial cost saving complete line circuit basis This system cost reduction philosophy particularly prevalent protection scheme implemented tolerances protection components orders magnitude less than normally required attain performance levels which this part achieves with corresponding reduction cost these components working knowledge principles employed allow this will presented followed some results showing insensitive technique changes mismatches protection components Conventional protection schemes outlined Figure separate linecard protection into secondary protection tertiary protection Using SLIM approach module itself carries tertiary protection components series elements secondary protection included sensing loop which cancels errors which arise poor matching resistance values shown Figure benefit this approach that burden matching these components removed large degree from board manufacturer This approach only possible using careful technologies which enable voltage ratings primary secondary interface This achieved manufacturing SLIM thick film hybrid module which will stand excess 1000V peak without failing using these thick film techniques accurately trimming very high degree longitudinal rejection maintained Typical figures 75dB measured final test stage completed module using 100X protection resistors Since module employs control loop guarantee longitudinal balance system also synthesize longitudinal terminating resistance each subscriber line resistance from each ground
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FIGURE Headroom Limitations
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FIGURE SLIM Protection Arrangement
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FIGURE Synthesized Longitudinal Resistance response longitudinal signals advantage synthesizing this resistance that control longitudinal voltages appearing line terminals much tighter maintaining more consistent longitudinal balance figure Secondly more importantly carefully matching synthesized resistance with physical resistance consisting module resistance external protection resistance improvement signal handling capability presence high levels longitudinal current achieved Figure shows this graphically Referring conventional protection layout Figure important that module protection sufficient withstand currents which allowed pass through secondary shunt protection device Using SLIM module series protection elements incorporated design which capable surviving excess each applications advantage this that shunt protector directly connected across output SLIM device terminals Overall SLIM device been designed with application mind optimum technologies been used achieve most cost effective solution just component cost also external components manufacturing costs This philosophy seen which protection function partitioned enabling user achieve previously unattainable levels performance from wide tolerance components this systems approach problem which enables SLIM users obtain competitive advantage compared with conventional solutions protection problem remaining sections this applications note will deal with results laboratory tests followed examination various protection options available with this versatile device outlining strengths weakness different solutions
LABORATORY MEASUREMENTS previous sections have concentrated effectiveness SLIM device meeting longitudinal requirements while incorporating very loose tolerance protection components However there have been quantitive measure performance which attained with part results which presented here were measured under laboratory conditions using Wilcom T207E Longitudinal Balance Test measured accordance with IEEE 455-1976 Recommendations tests were measured typical devices from standard production runs representative results which expected from SLIM based line circuit tests were designed investigate behavior part under different line conditions while varying protection resistances Rprotect graphs show results different test frequencies allow appreciation sensitivity results devices were tested sensitivity absolute value resistors (both resistors matched within then mismatch sensitivity between protection resistors resistor being held constant other reduced value This second test carried with fixed resistor having values 120X 100X other having resistors different results presented below worst case results devices tested measurements were made line current These correspond onhook 1900X loop resistance 750X loop resistance
Absolute Value Sensitivity Figures show results Longitudinal Balance against Resistor Value Rprotect both resistors matched within These results measured 1000 3400 stipulated line conditions overall trend these results indicate that Longitudinal Balance better than values protection resistor between 120X Values substantially greater than 120X recommended since these will reduce operating voltage headroom output amplifiers explained earlier Matching Sensitivity tests matching sensitivity were carried similar fashion except resistor legs held constant results recorded Figures indicate worst case result between legs results shown Figure illustrate trade-off between resistor matching ratio longitudinal balance which achieved Iloop while Figures show Iloop respectively This shown fixed resistors 120X 100X while other varied graphs show results mismatch between legs From graphs seen that longitudinal balance achieved using very loose tolerance parts very high tighter specifications required slightly closer tolerance resistors specified these still cheaper than high tolerance devices required meet these specifications using conventional line circuits important note that during these tests
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FIGURE Longitudinal Balance Resistor Value
FIGURE Longitudinal Balance Resistor Value
FIGURE Longitudinal Balance Resistor Value
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FIGURE Longitudinal Balance Mismatch
FIGURE Longitudinal Balance Mismatch
FIGURE Longitudinal Balance Mismatch
module passed longitudinal capability tests easily handling mArms specified Datasheet Summarizing results presented this section absolute value resistors range 120X with matching tolerance However order meet more stringent requirements such Bellcore cope with long term effects Lightning power cross resistors should 100X This will ensure meeting requirements life These results enable protection options examined detail with understanding capabilities SLIM device prediction effectiveness these options made PROTECTION OPTIONS These options split into roughly areas which follow slightly different philosophies regarding purpose protection These Fusible manual resettable systems Auto-resetting systems Both these consist same elements difference between these type series protection element used Before discussing differences detail study individual protection components worthwhile Shunt Protection Devices shunt protection device this application three configurations Figure shows least expensive shunt protector available application consists bridge rectifier connected across subscriber wires returning fault current either battery ground battery supply dependent polarity fault current This system very effective protecting line circuits since voltage transitions restricted approximately forward diode voltage beyond supply rails however injection large fault transients onto battery supply desirable many administrations This particularly problem when negative battery potential generated using switch mode power supply which while capable sourcing large currents incapable sinking current fault condition which dumps substantial current into battery could increase battery potential causing damage this protection system does prove adequate application then required specification diode bridge surge capability continuous) 100V rating
device exceed zener voltage (VZ) rating device unit enters voltage clamp region fault voltage continues rise current into shunt protector will rise correspondingly through series protection element until breakover current threshold (Ibo) exceeded this point device fires causing voltage across device collapse returning current battery ground terminal device remains this state until current reduced below holding current device whereupon protector resets itself
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FIGURE Protection Using Shunt Suppressor Device understanding characteristics shown Figure highlighting important features characteristics This diagram also shows characteristics basic types suppressor namely symmetric asymmetric types Asymmetric (shown dotted) have advantage forward diode characteristic direction which reduces power dissipated SLIC during fault condition
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FIGURE Cost Shunt Protector second shunt protection configuration Figure designed avoid dumping fault transients into office battery supply thus ensuring that battery potential remains unaffected during fault condition this circuit positive going transients returned battery ground connection diode bridge negative transients passed transient surge protector This terminal device operates voltage current sensitive thyristor When voltage appearing terminals
FIGURE Shunt Suppressor Characteristic final shunt protection scheme Figure development circuit shown Figure this circuit diode bridge discarded shunt components replaced single device This terminal device protects against faults occurring between subscriber wires ground These devices slightly more expensive than previous option competitive when savings board area assembly costs taken into account
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FIGURE Terminal Shunt Protector
with SLIM module ideal type shunt protector unipolar type since this limits potential within forward diode voltage (VBE) battery ground potential advantage this limitation power dissipation module during fault condition breakover voltage should less breakover current important correct SLIM operation choice terminal made purely financial basis whether addition diode bridge circuit increases cost over terminal device Series Protection Elements complete protection function method limiting current into shunt device required this task series elements These elements each subscriber wires operate case fusible systems interrupting current flow after exceeds predefined value case auto-resetting systems reducing current level safe level when ensure that choices constrained strengths weaknesses both manual auto resetting systems forward thus allowing designer appraise both systems choose most suitable choice series protector must made with protection specifications mind Many specifications require that protection components must withstand particular level disturbance without damage Usually these which govern choice series element principle factors affecting choice power dissipation device during fault condition rupturing current case fusible systems However there time constraint placed test conditions thermal mass device must incorporated into calculations though this often done referring manufacturers datasheet which contains this information choice between fusible resettable system which often governed specifications usually because resettable systems cannot achieve long term balance requirements been shown results section previously this constraint does apply users SLIM device thus opening opportunities users desire auto-resetting protection without compromise balance performance Fusible (Manual Resettable) Systems benefit employing manual resettable system that fault occurs particular line after protection been fired line disconnected from switch until fault cleared system reset This means that remainder switch function completely without board heating problems which occur using other protection systems However disadvantage manual system requirement human intervention replace reset line protection This especially important distributed switch system where protection circuits same location
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FIGURE Fusible Resistor Implementation example Vrms from generator impedance 150X period seconds reason this ensure that protection systems trip short term faults caused transient power cross situation survive long enough electricity supply authority circuit breakers respond fault thus avoiding false unnecessary failure Until manual resettable fusible systems have been most popular industry since matching accuracy series elements carefully controlled ensure good balance problem future using this type protection expansion distributed switching systems employing local street furniture costs replacing protection these locations much greater argued that incidence faults will also reduce shorter line lengths order reduce this cost minimum improve time taken reinstate service client auto resetting systems more effective these described following section Auto-Resetting Systems Auto-Resettable protection system always been attractive line card designers inherent advantage system guards against fault which occur line after occurence resets automatically without requirement human intervention Unfortunately compromises achieve this were traditionally great warrant change auto systems With advent SLIM these compromises necessary implementation automatic protection becomes realistic possibility irrespective severity longitudinal balance specification basic principle behind these protection schemes device series element secondary protection circuit During fault condition this device dissipates power which causes self heating temperature increase device causes resistance increase thus regulating current flow Eventually thermal equilibrium reached this state held until fault condition removed device then cools normal service re-established choice device relatively painless once critical parameters decided understand impact these parameters appreciation operation construction device useful CONSTRUCTION OPERATION Positive temperature coefficient devices come variety forms depending application required line card protection application devices required designed switching elements with carefully defined
Figure shows schematic fusible protection circuit series elements fuses resistors circuit breakers which triggered rupturing current device exceeded SLIM device maximum current which element must pass correct operation maximum line feed current plus worst case longitudinal current which appear line However many authorities require that protection elements withstand short term power cross finite period time typical
abrupt switching characteristic most common method implementing switching function thermistors operation these discussed Thermistors Switching type thermistors made from semiconducting barium titanate ceramic material This material exhibits temperature-resistance characteristic shown Figure Over lower portion their characteristic thermistor resistance relatively constant slight negative coefficient present temperatures intrinsic negative temperature coefficient from semiconducting material) temperature raised above Curie point magnetic domains material realign themselves material becomes more resistive until eventually material approaches insulator switching type device this action designed take place abruptly over 10-15 temperature change Over this range resistance thermistor changes five orders magnitude from
current value less than value trip current flowing through device temperature rise thermistor insufficient trip device however current increased above trip level temperature rise sufficient resistance increase ensure correct operation application thermistor chosen must rated that Trip Point never exceeded during normal operation maximum ambient temperature Maximum Ratings When using thermistors imperative never exceed manufacturers ratings device These ratings give maximum voltage current which device capable switching voltage rating principally defined thickness resistivity device switching current rating very important since exceeding this value will cause device fracture This caused physical properties material which have intrinsic energy-time product capacity attempts made dissipate excessive amount energy differential expansion inside device local heating large stresses brittle ceramic material which consequently shears fractures general larger cross sectional area device greater switching current handled this parameter most crucial selecting correct device these limits exceeded resistance value thermistor very predictable will return initial starting value repeatedly independent number switching cycles device undergone tests thermistor repeatedly with second burst Vrms mains voltage sufficient stress maximum ratings then allowed cool minutes after which resistance measurement made This cycle repeated over times results logged resistance device vary more than over this test Hopefully this sufficient dispel myth that thermistors unstable devices repeatably resettable after undergoing numerous switching operations that basic operating principles thermistors have been explained limitations from applications point view appreciated into context Specifying Thermistors outlined above section thermistors have five basic parameters which must specified ordering These Operating temperature range Normally telecom equipment this This does need take temperature rise self heating into account Maximum operating current (non switch) This normally range most telecom applications important rate this parameter maximum operating ambient temperature Maximum voltage rating This parameter voltage which device must withstand when source resistance generator zero This important when resistance device increases high value (much greater than source resistance generator) when rating should equal exceed test voltage
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FIGURE Thermistor Temperature-Resistance instead using external temperature heating source current flowing through device used traditional characteristic this type device measured Figure This curve distinct regions which important application
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FIGURE Thermistor Characteristics First linear region This area normal operation where device does exhibit switching behavior
Maximum current capability This parameter must chosen dependant peak cold current device will required handle instant fault occurs Finally cold resistance device This will defined large extent previous parameters choice exists should chosen large possible order increase switching time reduce currents injected into line card during fault WORKED EXAMPLE Take case linecard which must survive mains cross fault Vrms minutes from source resistance after which part must reset normal operation Assume that longitudinal specification must exceed across frequency band 3400 Operating temperature range First referring performance graphs measurements section seen that absolute value matching requirements meet this spec very loose have relatively free choice value device eventually choose Temperature Operating Range Operating current SLIM device maximum feed current Modulating this worst case longitudinal current mArms gives maximum operating current mArms Therefore require thermistor which must able handle mArms maximum operating temperature Note currents since interested heating effect current Voltage spec device Vrms (min) (Since resistance series element will increase become much greater than source resistance Peak current handling requirements Given that test condition source impedance then short circuit current capability source Apeak Arms) This suggests fairly large thermistor withstand peak current high this suitable device which current rating close this YS960 which peak current rating This device cold resistance which when added source resistance gives total resistance across Vrms source This gives peak current alternative method choose thermistor with higher cold resistance This increases resistance series with source resistance thus reduces peak current handling requirements thermistor cold resistance only requires handle maximum current example given added advantage that current flow into protection ground also reduced This tradeoff dependent availability suitable thermistors calculation this parameter must always done using this somewhat iterative method arrive suitable device choice suitable device often restricted limitations test requirements experience shows that this often peak current requirements After device been selected predicted performance level read graphs plotted results section checking validity selection WORKED EXAMPLE longitudinal requirements much more strict such those imposed Bell specifications value
lected becomes more critical Referring results graphs suggests value 120X series elements case thermistor selected first example this made very easily Since shunt protector device connected across junction series elements module pins anything behind this node protected secondary protection order improve longitudinal balance performance circuit necessary place additional resistance series with thermistors increase combined value resistance closer 100X This done with resistors which would raise resistance around overall Care should taken power rating these resistors since during fault conditions these must cope with difference shunt protector trigger voltage battery supply This cause substantial amount power dissipated this condition schematic showing implementation this technique given Figure
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FIGURE Protection Schematic Stringent Longitudinal Requirements CONCLUSIONS outset this applications note objective show ease designing protection networks using SLIM module philosophy behind SLIM reducing cost overall application protection constraints much less stringent than conventional line interface circuits which allows previously unavailable protection schemes employed Below summary advantages disadvantages fusible autoresetting systems Fusible Systems Advantages Installation Cost Power dissipation board during fault conditions Total disconnection subscriber wires when fault occurs Disadvantages Relatively high replacement costs Labor intensive especially distributed switching systems Non-resettable thus causing unnecessary service time users Possibility operating transient fault
Auto Resetting Systems Advantages Good remote locations unnecessary down time users Completely automatic maintenance instances transient operation Disadvantages Increased Installation costs Small amount on-card heating thermistor dissipation -3W) Line totally disconnected during fault condition Given these choices bearing mind industry moving toward decentralized switches auto resetting systems technique more applicable requirements market where maintenance costs premium There companies taking advantage this opportunity manufacture complete modules containing thermistors shunt protectors designed direct mounting onto line card These modules ideal applications attempting minimize board footprint compact systems employing lines card incur slight cost penalty Finally included appendix this note list suppliers shunt protectors series fuse elements fusible resistors fusible links supplier list thermistors This list intended exhaustive more starting point those interested pursuing subject further hoped that writing this note that author highlighted major problems trying design protection schemes line card applications helped dispelling misconceptions application thermistors this area
APPENDIX Below listed suppliers protection components Suppliers Fusible series protection components Welwyn Electronics Bedlington Northumberland NE22 England (0670) 822181 International Resistive Company Post Office 1860 Boone North Carolina 28607-1860 (704) 264-8861 Suppliers Series Elements Raychem Corporation Polyswitch Products Constitution Drive Menlo Park Calif 94025-1164 (415) 361-6900 Mullard Mullard House Torrington Place London WC1E Components Thermistor Division Crown Industrial Estate Priorwood Road Taunton Somerset England (0823) 335200 Suppliers Shunt Protection Devices Texas Instruments Power Products Division Teccor Electronics 1801 Hurd Drive Irving Texas 75038-4385 (214) 580-1515 Lucas Semiconductors Garetts Green Lane Birmingham England (021) 784-6855
High Voltage Protection Techniques with TP3210 Subscriber Line Interface Module
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