{"id":172,"date":"2012-05-31T12:54:00","date_gmt":"2012-05-31T12:54:00","guid":{"rendered":"http:\/\/localhost\/ascenten.net\/tech-notes\/?p=172"},"modified":"2018-10-26T12:51:23","modified_gmt":"2018-10-26T12:51:23","slug":"before-the-heat-kicks-in","status":"publish","type":"post","link":"https:\/\/ascenten.net\/tech-notes\/before-the-heat-kicks-in\/","title":{"rendered":"Before the Heat Kicks in"},"content":{"rendered":"

Heat Sink Calculations<\/strong><\/h2>\n

Today’s demanding applications require components and systems to work under intense environmental conditions. Survivability is critical, as features are only as good as their ability to operate without fail.<\/p>\n

 <\/p>\n

Innovation in thermal management has become essential to meeting the requirements for fault-free performance. The choice of an appropriate thermal management system depends on cost and development time of a product. The techniques for heat dissipation include heat sinks and fans for air cooling, liquid cooling and peltier cooling (Thermoelectric cooling) etc. Designing a cost competitive electronics system requires careful consideration of the thermal domain also. To find the most optimized solution requires a good understanding of thermal properties of devices and knowledge on how to remove unnecessary heat from the devices.<\/p>\n

 <\/p>\n

This article presents the basic properties and principles of the thermal system of a device and the method of selection of a heat sink to prevent overheating under normal operating conditions.<\/p>\n

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

Thermal Property \u2013 Definitions<\/strong><\/h3>\n

 <\/p>\n

    \n
  • TA<\/span> \u2013 Temperature at reference point \u201cA\u201d<\/li>\n
  • TJ<\/span> \u2013 Junction temperature, often assumed to be constant across the die surface<\/li>\n
  • TC<\/span> or TCase<\/span> \u2013 Package temperature at the interface between the package and its heat sink; should be the hottest spot on the package surface and is the dominant thermal path<\/li>\n
  • Theatsink<\/span> \u2013 Temperature at heat sink<\/li>\n
  • PD<\/span> \u2013 Power dissipation, source of heat flux (Watts)<\/li>\n
  • H \u2013 Heat flux, rate of heat flow across a unit area (J\u00b7m-2\u00b7s-1)<\/li>\n
  • R\u019fJMA<\/span> \u2013 Junction to moving air ambient thermal resistance<\/li>\n
  • R\u019fchs<\/span> \u2013 Case to heat sink thermal resistance<\/li>\n
  • R\u019fJC<\/span> \u2013 Junction to case thermal resistance of a packaged component from the surface of its silicon to its thermal tab, or R\u019fJC<\/span><\/li>\n
  • R\u019fhs-a<\/span> \u2013 Heat sink to ambient thermal resistance<\/li>\n
  • R\u019fJA<\/span> \u2013 Junction to ambient thermal resistance<\/li>\n
  • C\u019fAB<\/span> \u2013 Thermal capacitance between reference points \u201cA\u201d and \u201cB\u201d; A and B should be junction, case or heat sink<\/li>\n
  • \u00b0C or K \u2013 Degrees Celsius or degrees Kelvin<\/li>\n
  • C\u019fJC<\/span> \u2013 Junction to case thermal capacitance<\/li>\n<\/ul>\n

     <\/p>\n

    \u00a0Thermal Circuit \u2013 Basic Principle<\/strong><\/h3>\n

     <\/p>\n\n\n\n\n
    \"\"<\/td>\n<\/tr>\n
    Figure 1: Thermal circuit<\/em><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

     <\/p>\n

    \u00a0Determining the Necessity of Heat Sink and Heat Sink Calculation<\/strong><\/h3>\n

     <\/p>\n

    The following procedure provides a method one can use to determine whether a heat sink is required. This example uses a semiconductor device, with the condition listed below.<\/p>\n

    (1) Power \u2013 20W, (2) Maximum TA<\/span> – 50 \u00b0C, (3) Maximum TJ<\/span> – 85 \u00b0C, (4) Air flow rate \u2013 400 feet per minute, (5) \u019fJA<\/span> \u2013 4.7 \u00b0C\/W, (6) \u019fJC<\/span> \u2013 0.13 \u00b0C\/W.<\/p>\n

     <\/p>\n\n\n\n\n\n\n\n\n
    <\/td>\n1.<\/td>\nUsing the junction temperature equation, calculate the junction temperature under the listed operational conditions:<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\nTJ<\/span> = TA<\/span> + P \u00d7 \u019fJA<\/span>= 50 + 20 \u00d7 4.7 = 144 \u00b0CThe junction temperature of 144 \u00b0C is higher than the specified maximum junction temperature of 85 \u00b0C. So a heat sink is absolutely required to guarantee proper operation.<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n<\/td>\n<\/tr>\n
    <\/td>\n2.<\/td>\nUsing the heat-sink-to-ambient equation (and a \u019fCS of 0.1 \u00b0C\/W for typical thermal interface material), calculate the required heat-sink-to-ambient thermal resistance:<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n\n\n\n\n\n\n\n\n\n\n
    \u019fSA<\/span><\/td>\n= (TJmax<\/span> -TA<\/span>) \/ P -\u019fJC<\/span> – \u019fCS<\/span><\/td>\n<\/tr>\n
    <\/td>\n= (85 -50) \/ 20 – 0.13 – 0.1<\/td>\n<\/tr>\n
    <\/td>\n= 1.52 \u00b0C\/W<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n<\/td>\n<\/tr>\n
    <\/td>\n3.<\/td>\nSelect a heat sink that meets the thermal resistance requirement of 1.52 \u00b0C\/W. The heat sink must also physically fit onto the device. The thermal resistance of Z40-12.7B at an air flow of 400 feet per minute is 1.35 \u00b0C\/W. Therefore, this heat sink will work since the published thermal resistance \u019fSA<\/span> is less than the required 1.52 \u00b0C\/W. Using this heat sink, and re-verifying:<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n\n\n\n\n\n\n\n\n\n
    TJ<\/span><\/td>\n= TA<\/span> + P \u00d7 \u019fJA<\/span><\/td>\n<\/tr>\n
    <\/td>\n= TA<\/span> + P \u00d7 (\u019fJC<\/span> + \u019fCS<\/span> + \u019fSA<\/span>)<\/td>\n<\/tr>\n
    <\/td>\n= 50 + 20 \u00d7 (0.13 + 0.1 + 1.35)<\/td>\n<\/tr>\n
    <\/td>\n= 81.6 \u00b0C<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n<\/td>\n<\/tr>\n
    <\/td>\n<\/td>\n81.6 \u00b0C is under the specified maximum junction temperature of 85 \u00b0C. This justifies that the Z40-12.7B heat sink solution will work.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

     <\/p>\n

    Factors Affecting Cost Effective Thermal Management<\/strong><\/h3>\n

     <\/p>\n

      \n
    • Selection of a good heat sink with higher heat sink to ambient thermal conductivity. Heat sink design and proper heat flow is the first and most critical factor in any thermal management\u00a0design.<\/li>\n
    • Selection of a highly thermally conductive and void free interface material.<\/li>\n
    • Use of a very thin thermal interface material.<\/li>\n
    • Use of void free interface between the power device, interface material and the heat sink (or heat pipe).<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

      Heat Sink Calculations Today’s demanding applications require components and systems to work under intense environmental conditions. Survivability is critical, as features are only as good as their ability to operate without fail.   Innovation in thermal management has become essential to meeting the requirements for fault-free performance. The choice of…<\/p>\n","protected":false},"author":1,"featured_media":909,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[],"class_list":["post-172","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-mansi"],"_links":{"self":[{"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/posts\/172","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/comments?post=172"}],"version-history":[{"count":1,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/posts\/172\/revisions"}],"predecessor-version":[{"id":940,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/posts\/172\/revisions\/940"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/media\/909"}],"wp:attachment":[{"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/media?parent=172"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/categories?post=172"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ascenten.net\/tech-notes\/wp-json\/wp\/v2\/tags?post=172"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}