Innoswitch-CH Temperature rating

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I require some assistance in explaining how to calculate the junction temperature of the Innoswitch-CH package. I am performing some temperature measurements on a USB Power Supply that has an Innoswitch-CH IC and using a thermal couple to measure the temperature on the surface of the component. From the datasheet I see a maximum junction temperature of 150 C, while my surface measurement from package was 95 C. When taking into account the thermal resistance from junction to case, I am unsure how to calculate the actual junction temperature.

Thank you,





If you refer to page 15 of the Innoswtich - CH datasheet, the junction resistance is 12° C / W.  So once you know the heat dissipation inside the IC (which will be the switching losses, condution losses etc), you can calculate the junction temperature from the case temperature. Also the Source pin temprerature is very close to the junction temperature.