I am upgrading a "power amplifier" that a fellow engineer has designed , my job is to add thermal protection against failure in prolonged use (or operator miss use).
The package of the transistors used is To-247-3, the metal area is 140mm2 according to :infineon the interface material (silicon pad , insulator and grease free) used has thermal impedance of 0.23°C-in2/W (@50 psi) , the thickness, conductivity and other parameters are provided in this table :
The tempreture sensor has a lug mount and will be screwed into the heatsink directly close to the transistor similar to this amplifier.
The power dissipated by each transistor is 5W , the maximum junction temperature is 175 degree Celsius and Rjunc/case is 0.29 degree / W.
I think we should keep the junction temperature below 150 to be a bit away from the absolute maximum.
Now for calculations :
- 140mm2 = 0.217 inch2 , so sil-pad resistance is 0.23/0.217= 1.05 degree C/ watt.
- total resistance from junction to sink is 1.05+0.29= 1.34 .
- delta T = 1.34*5w= 6.74 degree C . So the heat sink is allowed to reach 150-6.74= 143.25 degree Celsius ?!!
EDIT: due to comments and answers i would like to add the following , the transistos used are mosfets used switched 45 A in half bridge topology at 20khz from a decent gate driver to minimize switching losses , the conduction losses are : RxIxI = 0.0022 x 45 x45 = approximatly 5W .
QUESTION: I feel the numbers are not adding up (because other parts on the pcb would be toasted if the case is at such a high tempreture) please correct my calculations and tell me what i am missing ? and state what is usually the case temperature at which the thermal limit is reached ??