thermal information for dvhf2805s/dvhf2800d/dvhf2800t the dvhf dc/dc converters utilize hybrid technology. all the power semiconductor chips such as linear regulator, mosfet, and schottky diode are attached to an alumina substrate and the substrate is then attached to the metal case which houses the dc/dc converter. the pow er transformer however is directly mounted to the metal case through a thermally conductive epoxy preform. table 2.1 shows the maximum junction temperature rise ( d t jc ) at full load operation of all the power components referenced to the outside of the bas e plate of the metal case. the case - ambient thermal resistance ( q ca )of the metal case between the base plate of the metal case and ambient is also listed in the table. a simplified and conservative way to calculate the worse case junction temperature of a ll the power components is as follows: 1) measure the worse case input power (pi) and output power (po) of the dc/dc converter according to the system requirement and calculate the total power dissipation (pd) equal to pi - po. 2) if no additional heatsink is att ached to the converter, the case temperature of the base plate of the metal case (t c ) is equal to pd* q ca + t a where t a is the ambient temperature and the value of q ca comes from the table. if any additional heatsink is attached to the converter, the new ef fective q ca needs to be used. 3) the worse case junction temperature (t j ) of all the power components is then equal to tc + d t jc where the value of d t jc is from table 2.1. the maximum junction temperature of all the power components are rated at 175 c. add itional heatsink is always recommended in order to maintain a low junction temperature of the power components so that a higher reliability number can be achieved. table 2.1 thermal data for dvhf dc/dc converters d t jc of linear regulator 12 c d t jc of mosfet 7 c d t jc of schottky diode 9 c d t jc of power transformer 12 c q ca , case to ambient thermal resistance 25 c/w
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