Heat Sink Calculation

Hello All,

I have a question regarding calculations on heat sink. It is pretty straight foward just like explainned everywhere (i.e **broken link removed** )

First you need to get the Maximum thermal resistance, which it is gotten by diving the max temperature increase by the power.

Rja < (Tmax - Tamb ) / P(W) (1)

And in case of MOSFET we get that:

Rsa = Rja - Rjc - Rcs (2)

where R´s are the thermal resistance for sa: Sink to ambient; ja: junction to ambient; jc: junction to case ; and cs: case to sink.
let´s assume that equation 1 gave us Rja ≦ 10 K/W and we know that a mosfet gives us Rja≈60K/W without any sink which is greater. Inoder to get a new Rja, it is possble to add a sink by using equation 2. Asusming Rjc = 0.5 K/W and Rcs = 1 K/W, we have:

Rsa = 10 - 1 - 0.5 = 8.5K/W (3)
An that is the thermal resistance of the sink.

Alright, with all that said. What happens if my Rja max was 0.2K/W which is avalue lower that Rjc which I cannot do anything about? The temperature will keep increasing until it reaches the final temperature which is higher than 150C. Thta does happen and predicted.


To overcome this, I see 3 options:
1. New part with lower Rjc
2. Allow part to grow hotter.
3. Dissipate less power.

Here is the other catch, by attaching metal blocks onto the tab of the mosfet ( therfore, reducing thermal resistivity ), it only reaches 115C. How can a heat sink, that cannot be calculate, exist?

Any insight guys?

" it only reaches 115C.", this is probabley too low a measurement, also with such a small heat sink your measuring probe could introduce significant extra heat dissipation area.
Frank

chuckey said:

" it only reaches 115C.", this is probabley too low a measurement, also with such a small heat sink your measuring probe could introduce significant extra heat dissipation area.
Frank

Click to expand...

The point is how can the temperature stabelize? being Rjc higher than the needed Rja for a safer final temerature.

we are using a thermal camera, btw.