Project Analog - February 2009 - (Page 19) open constructions with a throughflow of cooling air. Additionally, the measured case temperature can be used to decide on a suitably sized heat sink so the maximum case temperature is not exceeded at the maximum ambient temperature. Nevertheless, the internal losses and thermal resistances can also be derived mathematically. figurE 1 EfficiENcy vs. loaD currENT aT miNimum, NomiNal aND maximum iNPuT volTagE. Tambient is environment temperature Pdissipation is internal loss Pin is input power Pout is output power η(oper) is efficiency under given operating conditions With the help of the above formulas, the maximum allowable ambient temperature for a given set of operating conditions can be calculated, but it is important to remember that efficiency depends on both the output load and the input voltage (see Figures 1 and 2). The formulas also demonstrate that case temperature is not the same as operating temperature. If the thermal dissipation calculations reveal that the dc/dc module will overheat at the desired ambient operating temperature, then there are still a number of options available to reach a solution. One option is to derate the converter, that is, use a higher- Contents Viewpoint Selecting the Right dc/dc Converter Selecting the Right Battery System Understanding the Buck Switchmode Power Converter Analog news For the calculations, Ohm’s law (R=V/I) can be modified so R is thermal resistance, V is temperature and I is power dissipation. The following equations can thus be derived: RTHcase-ambient = (Tcase – Tambient)/ Pdissipation figurE 2 EfficiENcy vs. iNPuT volTagE aT miNimum, 50% aND 100% loaD. Pdissipation = Pin Pout = [Pout/η(oper) Pout where: RTHcase-ambient is thermal impedance from the case to the ambient surroundings Tcase is case temperature Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 19 · PRojeCt ANAlog · VolUme 3 / NUmBeR 1 http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423
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