1/6 AN1542 application note ? the thermal runaway law in schottky used in or-ing application may 2002 by y.lausenaz nowadays, some critical applications require very high available power supplies. typically, these applications are servers or telecommunication base stations. in such systems, the power supplies are built with several power supplies connected in parallel in order to be fault tolerant. thanks to redundancy, the total failure rate stays very low and the avail- ability can exceed 99.99%. the connection of several power supplies needs the or function, commonly built with diodes, to tolerate faults in the smps. introduction smps 2 smps 1 or function load vout fig. 1: supplies connected in parallel the or-ing function is commonly built with diodes. the diode has to let the current pass through when the associated smps is working in normal opera- tion. when a smps fails in short circuit, the diode has to block reverse voltage in order to maintain output voltage on the load. the purpose of the or function is to prevent fault propagation between supplies connected in parallel. 1. or-ing function presentation in normal operation the diode is conducting in forward mode. so, the first requirement of the component, irrespective of the maximum repeti- tive reverse voltage (v rrm ) and the current rat- ing (i f(av) ), is the forward voltage drop (v f ). the lower the forward voltage drop, the lower the forward losses in the diode, and the better the smps efficiency. for this reason, power schottky diodes are com- monly used in or-ing application. the l series (for example stps60l30cw) are optimized to provide very low forward voltage drop: v f typ = 0.33v (30a @125c per diode). the following graph presents the typical schottky used in or-ing application on common voltage outputs: 2. typical preferred device output voltage 24v 48v 12v 5v 3.3v l15 l30 l25 l45 l60 h100 schottky voltage fig. 2: typical schottky used as or-ing function on common voltage outputs using schottky diodes provides very low forward losses. but the main important technology trade off for schottky is between forward voltage drop and leakage current: the optimization of forward voltage drop is inevita- bly made to the detriment of leakage current. high leakage current gives rise to the thermal runaway problem.
application note 2/6 the risk of thermal runaway comes from the fact that leakage current increases quickly with the junction temperature. 3. thermal runaway risk reverse current reverse losses junction temperature fig. 3: thermal runaway diagram using a schottky as or-ing function provides a very low forward voltage drop. but when the diode is blocking because its associated supply has a fault in short circuit mode, the diode has to operate in reverse mode with high junction temperature (due to preceding forward losses) and so with rela- tively high reverse current. this high reverse current can generate high re- verse losses, and so increase junction tempera- ture, and so reverse current as well this is the thermal runaway phenomenon. 3.1. problems in the classical simple case where both the following assumptions are made: n constant thermal resistance system n or-ing diode on its own heatsink the reverse losses in the schottky diode, due to associated smps short circuit failure, is a mo- notonous function of the time. consequently the thermal runaway diagram of fig. 3 is covered in only one rotation- sense. to determine if the power schottky will goes into thermal runaway mode consists of finding the ele- ments that will determine the rotation sense of fig. 3. during the forward mode, the forward current (i f ) defines the junction temperature (t j ) (linked to forward voltage (v f ), device thermal resistance r th(j-a) ) and ambient temperature (t amb ): tt r ixv jambthjaffi fwd =+ - () @ () during the fast mode change of the diode (from the forward mode to the reverse one, the change is fast in comparison to device thermal constant), the junction temperature due to the preceding for- ward mode stay continuous (c.f. fig. 5) and will de- termine the leakage current (i rev ) (linked to the reverse voltage v rev ): ()( ) ( ) itv i cv e rev j rev rev rev ct c j ;; = - 100 100 c ? 0.055c -1 (thermal constant) this reverse current will determine the new junc- tion temperature trend (linked to reverse voltage and device thermal resistance). this variation trend between the initial junction temperature (due to forward mode) and the new one (due to reverse mode) gives the t j variation and the rotation-sense in fig. 3. in a constant thermal resistance system, the ther- mal stability can be determined by comparing for- ward losses (p fwd ) in the power schottky just before the smps failure (t 0 - d t) and the reverse losses (p rev ) occurring just after (t 0 + d t) the even- tual smps short-circuited fault. 3.2. result in classical cases the stability can be guaranteed if p fwd >p rev @t 0 the problem is to quantify the risk of thermal runaway in order to prevent it.
application note 3/6 losses p fwd forward mode smps break down reverse mode thermal runaway time monotonous variation t 0 p rev fig. 4: typical loss variation in the or-ing before and after the smps failure in these more complicated cases, device thermal behavior can be simulated with tools like pspice. the following analogies have to be used: t j forward mode continuous variation smps break down reverse mode monotonous variation time thermal runaway t amb fig. 5: typical junction temperature variation in the or-ing before and after the smps failure more complicated cases, where the assumptions of 3.2 do not exist, can be considered. for example: n or-ing diode not on its own heatsink. the or-ing diode can be mounted on common heatsink with other dissipative devices. in this case, the junction temperature of the or-ing di- ode can be influenced by the other devices, thanks to coupling thermal resistance. n non constant thermal resistance system: the convection can be forced by a fan con- nected to the anode side of the or-ing diode. in case of smps failure, the fan will stop and the r th(j-a) will increase. in this case, the junction temperature variation will not be monotonous. 3.3. results in more complicated cases t j forward mode continuous variation smps break down fan switch off reverse mode thermal runaway t amb time non-monotonous variation t 0 fig. 6: example of junction temperature variation in non-constant thermal resistance system rth (c-a) p (losses) rth (j-c) t amb t case t junction junction case ambiant fig. 7: thermal / electric analogy for simulation this analogy can be use to analyze any complex thermal problem. thermal: electrical: temperature voltage power current resistance resistance
application note 4/6 stmicroelectronics has developed a schottky family dedicated to the or-ing function. this l family demonstrates very low forward voltage in order to reduce conduction losses. consequently, the leakage current is relatively high. for example, the l15" family (v rrm =15v) is op- timized for 3.3v, 5v and eventually 12v output as or-ing diode. due to the specific thermal runaway law of the schottky in or-ing application, we can optimize the device choice in order to improve the smps efficiency, while keeping the risk of thermal run- away under control. for example, lets take a 3.3v 35a output, with a stps40l15c as or-ing diode. the two diodes have to be considered like connected in parallel: 4. from thermal runaway to product optimization stps40l15c i = 35a (=2i ) out fwd v = 3.3v out p = 115.5w out fig. 8: smps output synopsis in the forward mode, the forward losses can be calculated as: () () pviri iid fwd t fwd d fwd fwd fwd = + = + - 2 2 0 18 8 010 0 2 32 .. ( atasheet w ) . = 112 theses losses decrease the global smps effi- ciency about 9.7%. the risk of thermal runaway can be evaluated by calculating the maximum junction temperature that must not be reached in forward mode to avoid reverse losses being higher than forward losses, thus avoiding thermal runaway. () p v i t v diodes in parallel v rev out rev j out = = 2332 2 ;. ( ) () ( ) icve c c thermal cons rev ctj c 100 3 3 0 055 100 1 ? - - ;. . tan () () t i c v per diode datasheet rev 100 3 3 ;. ( , ) note that it is very important to use maximum re- verse current values to evaluate reverse losses. actually, the worst case must be considered to evaluate junction temperature in order to be sure to avoid thermal runaway. the limit of the thermal runaway criteria being de- fined by p fwd =p rev , the maximal junction tempera- ture t j max corresponds to p rev max =p fwd : () ( ) pvicve t rev out rev ct c j j max max ;. max = = - 2 100 3 3 1 100 () 00 1 2 100 3 3 137 + ? ? ? ? ? = c c in p vi c v c fwd out rev ;. the maximum junction temperature reachable in forward mode before the risk of thermal runaway occurs is so high, that we can consider a well adapted device. this one will have a lower forward voltage so a highest reverse current. the same process can be applied to different devices stps80l15c forward losses p fwd = 9.0w the efficiency loss about 7.8%. maximal junction temperature before thermal run- away: t jmax = 127c stps120l15 forward losses: p fwd = 7.6w the efficiency loss is about 6.6% maximal junction temperature before thermal run- away: t jmax = 100.3c stps20l15 (the 20a average current specified is only indicative value) forward losses: p fwd = 16.1w the efficiency loss is about 13.9% maximal junction temperature before thermal run- away: t jmax = 155.9c the comparison between the 4 parts considered on the 3.3v 35a output can be summarized on the following graph:
application note 5/6 efficiency loss 9.7% forward losses 16.1w 11.2w 13.9% 9.0w 7.6w stps l15 20 stps l15c 40 stps l15c 80 stps l15 120 t = 156c jmax t = 137c jmax t = 127c jmax t = 100.3c jmax 7.8% 6.6% v = 3.3v i = 35a out out fig. 9: comparison between 4 parts, forward losses, efficiency loss and t jmax . using the specific thermal runaway law, the smps designer can optimize the or-ing diode choice in order to improve the global efficiency. the risk of thermal runaway is controlled by limiting the junction temperature during the forward mode below the maximum value evaluated. stmicroelectronics is developing l family diodes dedicated to the or-ing application. this family shows very low forward voltage in order to reduce conduction losses and to improve efficiency: stpsxxl15, stpsxxl25, stpsxxl30, stpsxxl45, and stpsxxl60. with the very simple law presented, it becomes straightforward to optimize devices choice by evaluating the risk of thermal runway in schottky used in or-ing function in smps. this reliable and accurate law allows the optimi- zation of the devices used in order to improve converter efficiency while controlling the risk of thermal runaway risk. conclusion to evaluate the limit before thermal runaway, the maximum value of the reverse current has to be considered. actually, this parameter is critical for thermal runaway and the worst case must be con- sidered. to evaluate the maximum reverse current of a power schottky, take the typical value given in figure. apply the ratio between typical and maxi- mal value given in the table (in the adapted v r and t j field). finally, use the adapted formula to get the expected junction temperature. example: stps80l15c (twin diode in parallel) under 3.3v @125c figure 5 of the stps80l15c datasheet gives the typical value of the reverse current @100c for 3.3v (per diode): annexe: evaluation of maximum re- verse current from datasheet 3.3v 220ma 0123456789101112131415 1e-1 1e+0 1e+1 1e+2 1e+3 ir(ma) tj=25c tj=75c vr(v) tj=100c fig. 5: reverse leakage current versus reverse voltage applied (typical values , per diode). i rev typ (100c ; 3.3v) = 220ma the static electrical characteristics table gives the ratio between typical and maximum values (per diode): symbol parameter tests conditions min. typ. max. unit i r * reverse leakage current tj=25 cv r =5v 4 ma tj = 100 c 280 400 tj=25 cv r = 12v 11 tj = 100 c 0.44 1.1 a tj=25 cv r = 15v 16 ma tj = 100 c 0.53 1.3 a pulse test :* tp = 380 m s, d <2% static electrical characteristics (per diode).
application note 6/6 () icv ma rev max ;. 100 3 3 220 400 280 314 == the following formula allows the calculation of the reverse current in a power schottky for every junction temperature from a reference value: information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written ap- proval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 2002 stmicroelectronics - printed in italy - all rights reserved. stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany hong kong - india - israel - italy - japan - malaysia - malta - morocco - singapore spain - sweden - switzerland - united kingdom - united states. http://www.st.com ()( ) ( ) it v i c v e ccth rev j rev ctj c ;. ;. . 33 100 33 0 055 100 1 = ? - - () ermal cons t tan ()() ( ) icvicve a rev rev c 125 3 3 100 3 3 12 125 100 = = - ;. ;. . so, the global maximum reverse current value for the two diodes of the stps80l15c connected in parallel under 3.3v @125c is: () icva a rev max ;. . . 125 3 3 2 12 24 = ?
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