3 FETS 0.5 In X 2.0 In dissipating 12 watts each are bonded on a plate 2 in X 2

Question

3 FETS 0.5 In X 2.0 In dissipating 12 watts each are bonded on a plate 2 in X 2 in using thermal epoxy of 1.75 W/M/K conductivity with bond thickness of 10 mil. It is immersed in a pool of FC-72. Will the junction temperature exceed 110 C assuming a theta –JC of 1.5 C/W. Check if critical heat flux is exceeded. If junction temperature is exceeded suggest a safe operating power for the FETS.

Hint: This is pool boiling. The heat of FETs are at the base which is mounted to a 2 X 2 In plate. Calculate the pool boiling heat transfer coefficient and the critical heat flux of FC-72 using the thermo-physical properties of FC-72 (from 3-M Co.). Search internet to get this. Total area of the plate is 2 * 2 and convert to proper units. h*A*DT (plate surface - Boiling point) = 3*12 watt converted to proper units. Note DT is the plate surface temperature - Boiling point of FC-72. This will give the plate temperature. Bond thermal resistance = Bond thickness / (k*A) where k is the thermal conductivity of epoxy. Bond resistance * 3*12 = DT (case temperature - Plate temperature) = FET case temperature - Plate temperature. Theta j-c * 3*12 = DT = junction temperature - case temperature. Check if critical heat flux is exceeded

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