application note heatsink calculation and examples AN646/0594 1/3 in many cases, gs-rx and gsxty-z modules dont require any additional cooling methods because the dimensions and the shape of the metal boxes were studied to offer the minimum possible thermal resistance case to ambient for a given module. it should be remembered, that gs-r and gs-t modules are power devices i.e. products that de- liver power and dissipate power and, depending on ambient temperature, an additional heat-sink or forced ventilation or both may be required to keep the unit within safe temperature range. we would like here to eliminate a wrong parameter that has been plaguing the technical literature of power devices for 30 years: the operating ambient temperature specified among absolute maximum rating. the concept of operating ambient temperature is totally meaningless when we deal with power com- ponents, because the operating ambient tempera- ture depends on how a power device is used. what can be unambiguously defined is the maxi- mum junction temperature of a power semiconduc- tor device or the case temperature of a module. to prove this, lets consider the following example: gs-r1005 at : v in = 24v i o = 10 a p o = 50 w t casemax = 75 c gs100t300-12 at : v in = 300 v i o = 8 a p o = 96 w t casemax = 70 c from data sheets we can get the respective effi- ciencies h and power dissipations p d = p o ( 1 h - 1 ) gs-r1005 h = 0.83 p d = 10.2 w gs100t300-12 h = 0.84 p d = 18.3 w in case of natural convection (no heat-sink or forced ventilation) the thermal resistance case to ambient and the maximum ambient temperature (t ambmax = t cmax - r th p d ) will be: gs-r1005 r th = 7.5 c/w t ambmax = 75-7.5 10.2=-1.5 c gs100t300-12 r th = 7.5 c/w t ambmax = 70-7.5 18.3=-62.25 c as data show, the maximum operating ambient temperatures are a "non-sense" in the two cases, due to the fact that both devices are for use with an external heatsink. in practice a designer must fix four preliminary values such as: v in = input voltage v out = output voltage i out = output current t amb = maximum ambient temperature at which the system must operate. from these data, it is easy to determine whether an additional heat-sink is required or not and the rele- vant size i.e. the required thermal resistance. the step by step calculation is as follows: 1. calculate output power: p o = v o i o 2. on data sheet, from v o , v in , i o , the efficiency is obtained directly or by calculation: h = p o p in 3. the actual power dissipation is given by: p d = p o ( 1 h - 1 ) 4. the case temperature is calculated: t case = t ambmax + r th p d (rth is shown on data sheet) 5. if t case < t casemax no external heat-sink is re- quired if tcase > t casemax then proceed as follows. 6. lets calculate what thermal resistance case to ambient is needed: t th ( tot ) = t casemax - t ambmax p d
2/3 this is the total thermal resistance i.e. the parallel of the module and external heat- sink thermal re- sistances. 7. the thermal resistance of the additional heat- sink is calculated: r th ( hs ) = r thmodule r tot r thmodule - r tot as an example, lets consider two cases. conditions: gs-r1005 gs100t300-12 v in1 = 24 v v in2 = 300 v v o1 = 5v v o2 = 12 v i o1 = 2a i o2 = 5a r th1 = 7.5 c/w r th2 = 7.5 c/w t casemax =75 ct casemax =70 c t ambmax = 55 c 1. output powers: p o1 = 5 2 = 10 w p o2 = 12 5 = 60 w 2. from data sheet: h 1 = 0.83 h 2 = 0.84 3. power dissipations: p d1 = 10 ( 1 0.83 - 1 ) = 2.0 w p d2 = 60 ( 1 0.84 - 1 ) = 11.4 4. case temperatures: t c1 = 55 + 2.0 7.5 = 70 c t c2 = 55 + 11.4 7.5 = 140.5 c 5. the gs-r1005 does not require heat-sink that is, on the contrary, required for gs100t300-12. 6. total thermal resistance for gs100t300-12 r th ( tot ) = 70 - 55 11.4 = 1.31 c / w 7. required thermal resistance of heat-sink: r th ( hs ) = 7.5 1.31 7.5 - 1.31 = 1.58 c / w heatsink application notes
the following table gives the thermal resistance of commercially available heat-sinks. manufacturer part number rth c/w mounting fastening sgs-thomson hs01 2.8 vert. screw thermalloy 6177 3 horiz. screw thermalloy 6152 4 vert. screw thermalloy 6111 10 vert. adeshive thermalloy 6155 4.5 vert. screw thermalloy 6601 5 vert. screw thermalloy 6176 4.5 vert. screw thermalloy 6320 1.5 horiz. screw alutronic pr139 3 vert. screw alutronic pr140 2 horiz. screw alutronic pr159 2.5 vert. screw aavid 60885 4.5 vert. screw aavid 60660 1.5 horiz. screw aavid 62355 3 vert. screw austerlitz ks50 3 vert. screw austerlitz ks100.3 2.5 horiz. screw fischer sk18 3 vert. screw fischer sk48 3 vert. screw fischer sk16 1.5 horiz. screw fischer sk52 2 horiz. screw fischer sk07 4 vert. adeshive sge bosari l30 3 horiz. screw sge bosari lz50 3 vert. screw sge bosari sr50 6 vert. adeshive assman v5280 2 horiz. screw assman v5805 2 vert. screw assman v5440 4 vert. adeshive assman v5382 4 horiz. screw assman v5460 3 vert. screw assman v5510 3 vert. screw 3/3 information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics ass umes 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 f rom its use. no license is granted by implication or otherwise under any patent or patent rights of sgs -tho mson microelectronics. specification ment ioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical com ponents in life support dev ices or systems without express written approval of sgs-thomson microelectronics. ? 1994 sgs-thomson microelectronics C all rights reserved sgs-thomson microelectronics group of companies aust ralia - brazil - china - france - germany - hong kong - it aly - japan - ko rea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. heatsink application note
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