Thermal runaway during blocking

Transcription

1 Thermal runaway uring blocking CES_stable CES ICES_stable ICES k 6.5 ma s

2 Thermal runaway uring blocking Application Note 5SYA Raffael Schnell Nano Kaminski ABB Switzerlan Lt, Semiconuctors April 25 Table of Contents: 1 INTRODUCTION THE ROBLEM CALCULATING THE STABILITY CRITERION CONCLUSIONS REFERENCE...5 Doc. No. 5SYA Apr. 5 age 2 of 6

3 1 Introuction Since e beginning of semiconuctor technology, ermal runaway has been a well-known effect. Thermal runaway occurs when e power issipation of a evice increases rapily wi temperature. A classic example is e ermal runaway uring blocking, when e applie voltage causes a leakage current an eir prouct s up e evice. As e evice gets hotter, leakage current increases exponentially an so, erefore, oes e ing. If e ing of e evice is not aequate, e evice will get progressively hotter an will ultimately fail. The recent evelopment of very high-voltage IGBT moules has lea to increase values of issipate power uring off-state, ue to eir higher blocking voltages, even if e leakage currents remain at similar levels as evices wi lower blocking voltage. This can cause problems when such evices are characterise at high temperature (e.g. 125 C). In is case, e whole measurement system (ing plate, moule, chip) is e up to a constant temperature an no temperature graient exists to sink e generate. This is in strong contrast to real-worl applications where e unction temperature may inee reach a maximum value of 125 C but e case temperature never excees, say, 11 C allowing leakage current losses to be e away across e temperature graient between unction an case. This Application Note is intene to escribe e limits of ermal stability, which have to be taken into account when testing an operating semiconuctor evices. In orer to keep e analysis straightforwar, a number of simplifying assumptions are mae. It shoul be note in particular at ing an ing is generally not homogeneous as assume here, so a reasonable safety margin is always aitionally require. Nevereless, e finings provie a goo guieline for esigning ing requirements. 2 The problem The power issipate by e evice is simply e applie voltage times e leakage current I(, T ) at e sai voltage an e respective unction temperature T. Usually, a power law can conveniently fit e temperature epenence of e leakage current. Following a well-known rule of umb, which says at e leakage current oubles every ~11 K, a power law wi e base 2 an e characteristic constant T 11 K (temperature increase to ouble e leakage) is chosen. The current I is en e leakage current at e applie voltage an e reference temperature T : I (, T ) I 2 T T (1) The power, which is e away from e evice, is given by e ifference between e unction temperature T an e stable reference temperature T (e.g. e controlle er or ambient temperature) an e ermal resistance R : T NB: Transient effects can be ignore because R is alreay e worst case. T R (2) In Fig. 1, e generate power ( ) an e power which is e away ( ) are plotte against e unction temperature. The reference temperature in is example is T 125 C. Assuming a 65 Hiak moule wi a leakage current of 6mA at 36 an 125 C, e re curve representing is obtaine. The e (sunk) power is shown for ifferent ermal resistance values. The blue curve represents a moule wi sufficient ing. The black curve shows a moule mounte wi an inaequate mounting technique an e green curve shows e limit of ermal stability. Reference to e blue curve of Fig. 1 shows at up to e first crossing point (stable operating point), e chips will up until power balance is reache (e.g. at C). Above is point e unction temperature rops back to e stable operation point. However, if e unction temperature excees C (unstable point of operation) e balance of e power is isturbe an e evice will continue to itself up until it fails. The black curve shows e power e away in case of an inaequate mounting technique (e.g. moule not bolte to -sink). In is case, is always higher an which means at no stable operating point can be reache an ermal runaway will result. The necessary ing power to ust reach e limit of ermal stability is shown by e green curve. In is case, e stable point an e unstable point of operation are merge when an are tangential. In such a case, a small ermal imbalance coul provoke ermal runaway. Doc. No. 5SYA Apr. 5 age 3 of 6

4 , [W] k, 6mA (125 C) Rh 2 K/kW Rh 27.2 K/kW Rh 35 K/kW stable operation point unstable operation point 5 fig.1: T [ C] ower balance of a moule for ifferent ing situations. Blue: proper ing, black: inaequate ing, green: limit of ermal stability accoring to E qn (1) 3 Calculating e stability criterion As alreay iscusse, e limit of ermal stability is e point at which e an curves are tangential. At is point, e ing power an e ing power have to be equal an e erivatives (wi respect to e unction temperature) of bo curves must also be equal: (3) The erivative of e ing function is given in E qn (4). Note at e erivative is given by e original function times a constant. T T T ln2 ln2 I 2 (4) The erivative of e ing function is simply e reciprocal of e ermal resistance: 1 R J (5) Starting wi e right-han sie of E qn (3), one obtains a formula for at epens on T an R only. utting is result into e left-han sie of E qn. (3), one etermines e critical temperature at which e two curves are tangential. ln2 T 1 R R ln2 T T T T T T (7) R ln 2 ln 2, crit, crit R (6) Doc. No. 5SYA Apr. 5 age 4 of 6

5 Interestingly, e ifference between e critical unction temperature T,crit an e constant reference temperature T is inepenent of e applie voltage, e leakage current I, an e ermal resistance R! The only influencing factors are e parameters ( T an 2) escribing e temperature epenence of e leakage current. Using is result e corresponing leakage current at T,crit can be calculate: I crit 1 T ln2, T, crit ) I 2 I( I e I crit (8) Furermore, if e result of E qn (7) is put into e right-han sie of E qn (3), e relation of e oer parameters in is critical situation can be calculate: J J 1 T ln2 ln 2 1 ln 2 1 I 2 I e R R (9) Finally, e criterion for stability reas: I R < e ln2 (1) Wi T 11K: T 5. 8 K e ln 2 (11) 4 Conclusions Equation (1) gives a simple criterion for stability. The left-han sie inclues e main influencing factors: applie voltage, leakage current I at reference temperature an an ermal resistance, while e righan sie can be even trace back to basic physics an has a value of 5.8K for silicon evices ( T 11K). In any case, is rule of umb value has to be verifie because of unusual contributions to e leakage current or particular evice features at can change e temperature epenence significantly. In aition, a safety margin is recommene since homogenous ing an leakage current over e whole evice are assume which is not realistic in real-worl applications. As shown in e example of Fig. 1, it woul be possible to operate e evice on a ing plate wi 36 an 125 C if e interface resistance between case an -sink were at an acceptably low level. However, it has to be consiere, at alreay in is case, e unction temperature stabilises significantly above 125 C, influencing e switching characteristics an even e SOA. In orer to characterise e evice at Tv125 C, one has two options: apply right before e test (e.g. some 1µs) an switch off irectly after e tests have been performe (e.g. some 1µs). aust e ing plate to a lower temperature in orer to compensate for e temperature rop across e ermal resistance cause by e expecte leakage losses In all cases, it is crucial to have e lowest possible ermal resistance. Therefore, it is manatory to mount e moule on e ing plate wi all screws properly tightene. In aition, ermally conuctive grease or at least ermally conuctive foil shoul be use to minimise e ermal resistance of e interface between case an -sink an to bring it to e specifie (ata-sheet) value. For more etaile information regaring e mounting of Hiak moules please refer to e Technical Note Mounting Instructions for Hiak Moules 5SYA Reference Nano Kaminski, TN TS 5-13, Thermal runaway uring blocking Doc. No. 5SYA Apr. 5 age 5 of 6

6 For furer information please contact: rouct Marketing Eric Carroll hone Fax eric.carroll@ch.abb.com Sales Engineering Raffael Schnell hone Fax raffael.schnell@ch.abb.com ABB Switzerlan Lt Semiconuctors Fabrikstrasse 3 CH-56 Lenzburg, Switzerlan hone Fax abbsem@ch.abb.com Internet Data sheets of e evices an your nearest sales office can be foun at e ABB Switzerlan Lt, Semiconuctors internet web site: ABB Switzerlan Lt, Semiconuctors reserves e right to change specifications wiout notice. Doc. No. 5SYA Apr. 5 age 6 of 6