I think I'm doing this the right way but I wanted to make sure.
I have 10 FETs being mounted on a single heat sink. To calculate the maximum theta-sa (thermal resistance, sink to air) for the heat sink can I merely add together the theta-jc (junction to case) and theta-cs (case to sink) for each FET and then divide by 10 to arrive at the "effective" theta-js to use for calculating the max theta-sa that the sink can have?
Example: IRF1405 theta-jc = 0.45 degrees C/W theta-cs = 0.50 degrees C/W (flat greased surface)***
Added together, the theta-js for one device is 0.95. Dividing this by
10, we get 0.095 degrees C/W.Assuming that the devices are spaced out well on the heat sink, can I use that 0.095 degrees C/W value for my theta-js for the other equations?
Thanks!
***Why is the spec 0.50 degrees C/W when the thermal resistance of a good compound is about 0.05? If the theta-jc takes care of the resistance all the way to the case, shouldn't the resistance from the case to the sink be just the resistance of the compound itself and not this 0.50 number? Where is the extra resistance coming from?-- remove SPAMMENOT for e-mail responses --