technical note july 1996 thermal management for fc- and fw-series 250 w?00 w board-mounted power modules introduction board-mounted power modules (bmpms) enhance the capabilities of advanced computer and communi- cations systems by providing ?xible power architec- tures; however, proper cooling of the power modules is required for reliable and consistent operation. maintaining the operating case temperature (tc) within the speci?d range keeps internal component temperatures within their speci?ations. this, in turn, helps keep the expected mean time between failures (mtbf) from falling below the speci?d rating. tyco's fc- and fw- series 250 w to 300 w bmpms are designed with high ef?iency as a primary goal. the 5 v output units have typical full load ef?iencies of 83%, which result in less heat dissipation and lower operating temperatures. also, these modules use temperature resistant components, such as ceramic capacitors, that do not exhibit wearout behavior during prolonged exposure to high tempera- tures, as do aluminum electrolytic capacitors. this application note provides the necessary infor- mation to verify that adequate cooling is present in a given operating environment. this information is applicable to all tyco 250 w to 300 w bmpms in the 4.6 in. x 2.4 in. x 0.5 in. package. basic thermal management proper cooling can be veri?d by measuring the case temperature of the module (tc) at the location indi- cated in figure 1. note that the view in figure 1 is of the metal surface of the module (the pin locations shown are for reference). tc must not exceed 100 ? while operating in the ?al system con?uration. after the module has reached thermal equilibrium, the measurement can be made with a thermocouple or surface probe. if a heat sink is mounted to the case, make the measurement as close as possible to the indicated position, taking into account the contact resistance between the mounting surface and the heat sink (see heat sink section). 8-1303a figure 1. case temperature measurement (metal side) while this is a valid method of checking for proper thermal management, it it is only usable if the ?al system con?uration exists and can be used as a test environment. the graphs on the accompanying pages provide guidelines to predict the thermal per- formance of the module for typical con?urations that include heat sinks in natural or forced air?w environ- ments. however, due to differences between the test setup and the ?al system environment, the module case temperature must always be checked in the ?al system con?uration to verify proper operation. 1.20 (30.5) 3.25 (82.6) case sync in v i (? v i (+) v o (+) v o (? sync out measure case temperature here on/off
2 2 tyco electronics corp. technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series basic thermal management (continued) the goal of thermal management is to transfer the heat dissipated by the module to the surrounding environ- ment. the amount of power dissipated by the module as heat (p d ) is the difference between the input power (p i ) and the output power (po) as shown by the equa- tion below: p d = p i ?po also, module ef?iency ( h ) is de?ed as the ratio of output power to input power as shown by the equation below: h = po / p i the input power term can be eliminated by the combi- nation of these two equations to yield the equation below: p d = po (1 ? h ) / h this equation can be used to calculate the module power dissipation. however, ef?iency is a nonlinear function of the module input voltage (v i ) and output current (io). typically, a plot of power dissipation versus output current over three different line voltages is given in each module-speci? data sheet. this is because each module has a different power dissipation curve. a typical curve of this type is shown below in figure 2 for a fw300a1 power module (5 v output voltage). 8-1313 figure 2. fw300a1 power dissipation vs. output current module derating experimental setup the derating curves in the following ?ures were obtained from measurements obtained in an experi- mental apparatus shown in figure 3. note that the module and the printed-wiring board (pwb) onto which it was mounted were vertically oriented. the passage has a rectangular cross-section. the clearance between the top of the module and the facing pwb was kept constant at 0.5 in. 8-690a figure 3. experimental test setup v = 72 v v = 54 v v = 36 v 70 60 50 40 30 20 10 0 010203040 5060 output current, i o (a) power dissipation, p d (w)
tyco electronics corp. 3 technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series module derating (continued) convection without heat sinks increasing air?w over the module enhances heat transfer via convection. figures 4 and 5 show the maxi- mum power that can be dissipated by the module with- out exceeding the maximum case temperature versus local ambient temperature (t a ) for natural convection through 800 ft./min. a natural convection condition is produced when air is moved only through the buoyancy effects produced by a temperature gradient between the module and surrounding air. in the test setup used, natural convection air?w was measured at 10 ft./min. to 20 ft./min., whereas systems in which these power modules may be used typically generate natural con- vection air?w rates of 60 ft./min. due to other heat dis- sipating components in the system. the 100 ft./min. to 800 ft./min. curves are for air?w added externally to the test setup, usually through the use of fans. note that there is a thermal performance improvement when the long axis of the module is perpendicular to the air- ?w direction (transverse orientation). 8-1314 figure 4. convection power derating with no heat sink; air?w along length (lon- gitudinal) 8-1315 figure 5. convection power derating with no heat sink; air?w along width (trans- verse) figures 4 and 5 can be used to determine the appropri- ate air?w for a given set of operating conditions as shown in the following examples. example 1: air?w required to maintain tc what is the minimum air?w necessary for a fw300a1 in the transverse orientation, operating at 54 v input, an output current of 50 a, and a maximum ambient tem- perature of 35 ?? solution: given: v i = 54 v, io = 50 a, t a = 35 ? determine p d (figure 2): p d = 46 w determine air?w (figure 5): v = 800 ft./min. example 2: maximum power output what is the maximum power output for a fw300a1 in the longitudinal orientation, operating at 54 v input, in an environment that provides 600 ft./min. with a maxi- mum ambient temperature of 40 ?? solution: given: v i = 54 v, v = 600 ft./min., t a = 40 ? determine p d (figure 4): p d = 34 w determine io (figure 2): io = 40 a calculate po = (vo) * (io) = 5 x 40 = 200 w although the above two examples use 100 ? as the operating case temperature, for extremely high reliabil- ity applications, one may design to a lower case tem- perature as shown later in example 4. 0 10203040 100 0 40 60 70 local ambient temperature, t a ( c) power dissipation, p d (w) 30 20 10 90 80 70 60 50 800 ft./min. 700 ft./min. 600 ft./min. 500 ft./min. 400 ft./min. 300 ft./min. 200 ft./min. 100 ft./min. 50 20 ft./min. (nat. conv.) 0 10203040 100 0 40 60 70 local ambient temperature, t a ( c) power dissipation, p d (w) 30 20 10 90 80 70 60 50 800 ft./min. 700 ft./min. 600 ft./min. 500 ft./min. 400 ft./min. 300 ft./min. 200 ft./min. 100 ft./min. 50 20 ft./min. (nat. conv.)
4 tyco electronics corp. technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series module derating (continued) heat sink con?uration several standard heat sinks are available for the fc- and fw-series 250 w?00 w bmpms, as shown in figures 6 and 7. the heat sinks mount to the top surface of the module with m3 x 0.5 screws torqued to 5 in.-lb. (0.56 n-m). placing a thermally conductive dry pad or thermal grease between the case and the heat sink minimizes contact resistance (typically 0.1 ?/w to 0.3 ?/w) and temperature drop. all heat sink curve data taken had such a dry pad present. 8-1316 figure 6. heat sinks with longitudinal fins 1/4 in. (mhsl02555) 1/2 in. (mhsl05055) 1 in. (mhsl10055) 1 1/2 in. (mhsl15055) 4.56 2.36
tyco electronics corp. 5 technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series module derating (continued) heat sink con?uration (continued) 8-1317 figure 7. heat sinks with transverse fins nomenclature for this family of heat sinks is as follows: mhsxyyy55 where: x = ? orientation; longitudinal (l) or transverse (t) yyy = heat sink height (in 100ths of inch) for example, mhst10055 is a heat sink that is transverse mounted (see figure 7) for a 4.6 in. x 2.4 in. module with a heat sink height of 1 in. the ? pre? represents a heat sink kit with metric hardware. 1/4 in. (MHST02555) 1/2 in. (mhst05065) 1 in. (mhst10055) 1 1/2 in. (mhst15055) 2.36 4.56
6 6 tyco electronics corp. technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series module derating (continued) natural convection with heat sinks figures 8 and 9 show the power derating for a module in natural convection with the heat sinks shown in fig- ures 6 and 7. natural convection is the heat transfer produced when air in contact with a hot surface is heated, causing it to rise. an open environment is required with no external forces moving the air. figures 8 and 9 apply when the module is the only source of heat present in the system, generating air?w of approximately 10 ft./min. to 20 ft./min. again, a typical system with other heat dissipating components will usually generate higher air?ws in natural convection. 8-1318 figure 8. heat sink power derating curves natural convection, longitudinal ori- entation 8-1319 figure 9. heat sink power derating curves natural convection, transverse ori- entation figures 8 and 9 can be used to predict which heat sink a module will require in a natural convection environ- ment, as shown in the following example. example 3: sizing a heat sink what heat sink would be appropriate for a transverse mounted fw300a1 in a natural convection environ- ment at 54 v input and 2/3 load with a maximum ambi- ent temperature of 35 ?? solution: given: v i = 54 v, io = 2/3(60) = 40 a, t a = 35 ? determine p d (figure 2): p d = 35 w determine heat sink (figure 9): 1 1/2 in. heat sink allows up to t a = 35 ? 0 10203040 100 0 40 60 70 local ambient temperature, t a ( c) power dissipation, p d (w) 30 20 10 90 80 70 60 50 1 1/2 in. 1 in. 1/2 in. 1/4 in. none 50 0 10203040 100 0 40 60 70 local ambient temperature, t a ( c) power dissipation, p d (w) 30 20 10 90 80 70 60 50 1 1/2 in. 1 in. 1/2 in. 1/4 in. none 50
tyco electronics corp. 7 technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series basic thermal model another approach for analyzing thermal performance is to model the overall thermal resistance of the module. this presentation method is especially useful when considering heat sinks, since their performance is also typically given as a resistance. total module thermal resistance ( q ) is de?ed as the maximum case temper- ature rise ( d tc,max) divided by the module power dis- sipation (p d ): q = d tc,max / p d this can be represented as an equivalent circuit as shown in figure 10. in this model p d , d tc,max, and q are analogous to current ?w, voltage drop, and electri- cal resistance, respectively, in ohm's law. also, d tc,max is de?ed as the difference between the inlet ambient temperature (t a ) and the module case tem- perature (tc) as de?ed in figures 3 and 1 respec- tively. d tc,max = tc ?t a 8-695 figure 10. basic thermal resistance module for fc- and fw-series 250 w to 300 w bmpms, the module's thermal resistance values versus air velocity have been determined experimentally and are plotted in figures 11 and 12 for a unit without a heat sink and for the various heat sink con?urations (see figures 6 and 7). note that the highest values on the curves rep- resent natural convection. in a system with free-?wing air and other heat sources, there may be additional air- ?w. it is important to point out that the thermal resistances shown in figures 11 and 12 are for heat transfer from the sides and bottom of the module as well as the top side with the attached heat sink; therefore, the case-to- ambient thermal resistances shown will generally be lower than the resistance of the heat sink by itself. the data in figures 11 and 12 were taken with a thermally conductive dry pad between the case and the heat sink to minimize contact resistance (typically 0.1 ?/w to 0.3 ?/w). 8-1320 figure 11. case-to-ambient thermal resistance curves, longitudinal orientation 8-1321 figure 12. case-to-ambient thermal resistance curves; transverse orientation bmpm p d
technical note july 1996 250 w?00 w board-mounted power modules thermal management for fc- and fw-series tyco electronics power systems, inc. 3000 skyline drive, mesquite, tx 75149, usa +1-800-526-7819 fax: +1-888-315-5182 (outside u.s.a.: +1-972-284-2626 , fax: +1-972-284-2900) http://power.tycoelectronics.com tyco electronics corporation reserves the right to make changes to the product(s) or information contained herein without notic e. no liability is assumed as a result of their use or application. no rights under any patent accompany the sale of any such product(s) or information. ?2001 tyco electronics corporation, harrisburg, pa. all international rights reserved. printed in u.s.a. july 1996 tn96-009eps printed on recycled paper basic thermal model (continued) figures 11 and 12 can be used to determine thermal performance under various air?w and heat sink con- ?urations as shown in the following examples. example 4: air?w required to maintain tc although the maximum case temperature for the fc- and fw-series 250 w?00 w bmpms is 100 ?, one may want to limit the maximum case temperature to a lower value for extremely high reliability. if an 85 ? case temperature is desired, what are the allowable minimum air?w/heat sink combinations necessary for a transverse mounted fw300a1 operating at 54 v input line and an output current of 50 a with a maxi- mum ambient of 40 ?? solution: given: v i = 54 v, io = 50 a, t a = 40 ? determine p d (figure 2): p d = 46 w q = (tc ?t a ) / p d = (85 ?40) / 46 = 1.0 ?/w use figure 12 to determine air velocity: no heat sink: v >> 600 ft./min. 1/4 in. heat sink: v >> 600 ft./min. 1/2 in. heat sink: v = 450 ft./min. 1 in. heat sink: v = 260 ft./min. 1 1/2 in. heat sink: v = 205 ft./min. example 5: determining tc suppose that there is an air velocity of 600 ft./min. available for the con?uration stated in example 4. what is the case temperature for the various heat sink con?urations? solution: given: v i = 54 v, io = 50 a, t a = 40 ?, v = 600 ft./min. determine p d (figure 2): p d = 46 w tc = ( q x p d ) + t a using thermal resistances ( q ) from figure 12: no heat sink: q = 1.5 ?/w tc = (1.5 x 46) + 40 = 109 ? 1/4 in. heat sink: q = 1.2 ?/w tc = (1.2 x 46) + 40 = 95 ? 1/2 in. heat sink: q = 0.9 ?/w tc = (0.9 x 46) + 40 = 81 ? 1 in. heat sink: q = 0.6 ?/w tc = (0.6 x 46) + 40 = 68 ? 1 1/2 in. heat sink: q = 0.5 ?/w tc = (0.5 x 46) + 40 = 63 ? in this con?uration, the module would not operate within the maximum case temperature of 100 ? unless a heat sink was attached. thermal shutdown the fc- and fw-series 250 w?00 w bmpms has a latching thermal shutdown circuit designed to turn off the module if it is operated in excess of the maximum case temperature. recovery from thermal shutdown is accomplished by cycling the dc input power off for at least 1.0 s, or toggling the primary referenced on/off signal for at least 1.0 s.
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