{"id":195,"date":"2012-06-01T20:50:12","date_gmt":"2012-06-02T01:50:12","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=195"},"modified":"2014-04-13T20:21:23","modified_gmt":"2014-04-14T01:21:23","slug":"everything-you-know-is-wrong-february-2001","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=195","title":{"rendered":"Everything You Know Is Wrong February 2001"},"content":{"rendered":"

Answers to those Doggone Thermal Design Questions<\/strong><\/p>\n

By Tony Kordyban<\/strong><\/p>\n

Copyright by Tony Kordyban 2001<\/p>\n

Dear Tony,<\/em><\/p>\n

My boss thinks I need to put in a time-delay in our fan-cooled system, so that the fan will stay on for about five minutes after the power is turned off to the electronics.\u00a0 He claims that if we turn off the fan at the same time as the electronics, the component temperatures will “overshoot\u201d because of \u201cthermal inertia.\u201d\u00a0 I can\u2019t find \u201cthermal inertial\u201d in any of my old textbooks.\u00a0 Is my boss nuts, or can my parts really overheat this way?<\/em><\/p>\n

Over-worked and Passed-over in Overton<\/em><\/p>\n

 <\/p>\n

Dear Over,<\/p>\n

Your question seemed to have a simple answer.\u00a0 Unfortunately, I had two simple answers in my head, and I couldn\u2019t decide which one was right.\u00a0 Besides that, not being trained in psychology, I am not qualified to say whether you boss is nuts.<\/p>\n

I never heard of \u201cthermal inertia\u201d either.\u00a0 I asked a representative sample (2 people) if they had ever experienced \u201covershoot\u201d of component temperatures when they shut off the power to their system.\u00a0 The vote was 50% yes, 50% no, with absentee ballots yet to be counted.<\/p>\n

That taught me that voting is a dumb way to decide anything important.\u00a0 But now I not only had to figure out an answer to your question, but I had to have a way to explain why two people would get opposite results.<\/p>\n

I knew, in the depths of my hypothalamus (the part of the brain that senses body temperature), that a heat source like the die of a component could not increase in temperature once its power was cut off.\u00a0 If the fan were shut off at the same time, the die would cool more slowly, but it could not increase in temperature without more power.\u00a0 No overshoot was possible.\u00a0 Case closed.<\/p>\n

As soon as I thought the word \u201ccase\u201d in \u201ccase closed,\u201d it occurred to me that the same might not be true for case temperature.\u00a0 The case or surface temperature of a component is NOT the hottest spot in the component.\u00a0 It is somewhere between the die temperature and the air temperature.\u00a0 Just because power is cut off to the die, does not mean the case temperature won\u2019t go up, at least temporarily.\u00a0 Heat stored in the die could conduct over to it.\u00a0 But heat also would continue to flow out of the case into the air, even if it were not moving, because of natural convection.\u00a0 Suddenly I was not so sure of my answer.\u00a0 Another disturbing fact is that it is much more likely you would measure case temperature during a test, instead of die temperature.<\/p>\n

Voting and gut feelings didn\u2019t provide an answer.\u00a0 I could have built a fixture and explore this problem experimentally.\u00a0 But that sounded too much like work, plus it would be hard to try lots of different power levels, air velocities and material properties, in case the answer depended on those things.\u00a0 Next I thought of making a CFD model.\u00a0 That could work, but I can never figure out the transient menus in FLOTHERM.<\/p>\n

Thank goodness I didn\u2019t need an exact answer for a real component.\u00a0 All I cared about was whether overshoot is possible or not.\u00a0 This problem can be adequately modeled with transient, one-dimensional heat transfer.\u00a0 That only needs a small spreadsheet.\u00a0 It\u2019s not accurate or detailed, but it is good enough to see if overshoot happens.<\/p>\n

I made the simplest, one-dimensional component possible that would still be able to tell the difference between die temperature and case temperature.\u00a0 It is shown in Figure 1.<\/p>\n

\"\"<\/a><\/p>\n

Each layer of the component body has only a single temperature.\u00a0 I assumed all the heat from the die has to pass through the case to get to the air.\u00a0 I also simplified things by assuming the air temperature was constant.\u00a0 Power is applied only to the die, and heat is carried away by convection only from the case.<\/p>\n

All these assumptions make it fairly simple to write a set of energy balance equations for the two layers of the component body, both for the steady state when the power and fan is on, and for the transient situation when the fan is suddenly turned off and so is the power.\u00a0 I won\u2019t develop them for you here, mainly because you\u2019ll skip to the end anyway.\u00a0 But I\u2019ve buried them is a spreadsheet that you can play with yourself.<\/p>\n

overshoot<\/a><\/p>\n

It allows you to plug in dimensions and material properties for the die and case, power dissipation (when the power is on), and values for heat transfer coefficient for when the fan is on and the fan is off (please choose larger values to represent the fan being ON, OK?)\u00a0 It gives as a result a table of die and case temperatures vs. time after the fan and power are cut off, and a graph plotting the same thing.\u00a0 Have fun with it.<\/p>\n

In the meantime, Figure 2 shows one typical result.<\/p>\n

\"\"<\/a>It\u2019s based on fairly realistic values for a component, I think.\u00a0 Figure 2 shows a pattern that stayed the same no matter what values I tried.\u00a0 The die temperature NEVER overshoots.\u00a0 (As my hypothalamic reaction told me.)\u00a0 But the case temperature ALWAYS overshoots.\u00a0 (Like my second guess.)\u00a0 How much it overshoots varies a lot with the parameters you choose.\u00a0 But note also that the case temperature never goes higher than the die temperature.<\/p>\n

That explains why two experimenters could get opposite results.\u00a0 If one was measuring die temperature and the other case temperature, one would see overshoot and the other wouldn\u2019t.<\/p>\n

Now, as to Over-worked\u2019s question about his fruitcake of a boss:\u00a0 Do you care if case temperature overshoots when you turn off the fan?\u00a0 I generally don\u2019t, but you might, depending on the kind of product you are designing.\u00a0 If you turn it on and off a lot during its life, and the overshoot is large and fast, that could create lots of repetitive mechanical stress on the components, which could shorten their life.\u00a0 My telecom customers never shut off power to my products once they are installed, so overshoot is not important.\u00a0 I worry about maximum junction temperature, and junction temperature (die temperature) doesn\u2019t overshoot.<\/p>\n

So your boss has a valid point, even if there is no such a thing as \u201cthermal inertia.\u201d<\/p>\n

\n

\u00a0<\/strong><\/p>\n

Isn\u2019t Everything He Knows Wrong, Too?<\/strong><\/p>\n

\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014<\/strong><\/p>\n

\"\"<\/a>The straight dope on Tony Kordyban<\/strong><\/p>\n

 <\/p>\n

Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years.\u00a0 Maybe that doesn\u2019t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to cool electronics.\u00a0 He does have some book-learnin\u2019, with a BS in Mechanical Engineering from theUniversityofDetroit(motto:Detroit\u2014 no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn\u2019t Nobels count more than Rose Bowls?)<\/p>\n

In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney.\u00a0 He has run into so much nonsense in the field that he has found it easier to just assume \u201ceverything you know is wrong\u201d (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.<\/p>\n

Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down.\u00a0 These have been published recently by the ASME Press in a book called, \u201cHot Air Rises and Heat Sinks:\u00a0 Everything You Know About Cooling Electronics Is Wrong.\u201d\u00a0 It is available direct from ASME Press at 1-800-843-2763 or at their web site at http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0 <\/em><\/strong>Order Number 800741.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2001 Dear Tony, My boss thinks I need to put in a time-delay in our fan-cooled system, so that the fan will stay on for about five minutes after the power is turned off to the electronics.\u00a0 He claims that if […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-195","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/195","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=195"}],"version-history":[{"count":5,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/195\/revisions"}],"predecessor-version":[{"id":227,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/195\/revisions\/227"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=195"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}