I do the redneck calculation of heat sink size, use the biggest one that still fits, lol and if that fails i used a larger mosfet. or pop a TEC on it, joking of course
I do the redneck calculation of heat sink size, use the biggest one that still fits, lol and if that fails i used a larger mosfet. or pop a TEC on it, joking of course
Remember Remember The 8th of November, When No One Stood, but Kneel, In Surrender
In a popular government when the laws have ceased to be executed, as this can come only from the corruption of the republic, the state is already lost. Montesquieu
I'm interested in what measurements you are able to take as the formula only works on the junction temperature within the chip which I would imagine is very hard, if not impossible, to measure outside of a lab. In my example the case temperature is likely to be approximately 84ºC with a heat sink temperature of 82.5ºC while the junction temperature is 138ºC.
Whether the Øjc value has a practical application or not is a matter of opinion. I am only pointing out, from my research, the industry standard calculation for heat sink sizing.
If the formula is re-arranged to
Tj = Pd(Øjc + Øcs + Øsa) + Ta
This allows you to insert the heat sink value that you may want to use to see if it is possible to use it.
In my example above
Tj = 15*(3.6 + 0.1 + 3.5) + 30 = 15*(7.2) + 30 = 138ºC
Meaning my junction temperature should be within limits.
However if I were to use this MOSFET, http://www.farnell.com/datasheets/136333.pdf which has a Øjc of 0.75 and possibly a better candidate as it a lower RDSon and very similar characteristics, although it's not my design, then suddenly my junction temperature drops to 95ºC which is more acceptable with an approximate case temperature of 84ºC and heat sink temperature of 82.5ºC. Notice that the case and heat sink temperature remain the same but the junction temperature is significantly lower; this is why the Øjc value is important.
Of course all of this is only theory and actual measurements may be different.
- Neil