How do I accurately hand calc/make assumptions for active convection of a TO220 heatsink?
Hi, student here! I am working on a side project where I have a MOSFET dissipating a certain amount of power. How much power is not really possible to say at the moment because resistance Rds_on varies with temperature, so it's currently implicit and based on the thermal resistance of the dissipation. However, by using the maximum possible junction temperature Tj = 150, you can calculate that my dissipation solution (after Rjc and Rth of thermal paste interface), the Rth of my heatsink has to be less than 3 C/W.
I am looking at this heatsink shown below, AL 6061 with black anodization. A very rough CFD places it at about 8-10 C/W in passive convection, so I'm putting a fan blowing down on it. (it will be mounted on the PCB from this view looking straight down onto the board). As you can see from the last picture, the TO220 package is very small relative to the spreader/base surface and it is aluminum. So I'm struggling to hand calc a thermal resistance for this. I guess I could go to CFD, but for me to get results that I'm confident about I'd have to way deep dive new concepts and a new software, which I'm hesitant to do. 3C/W is not trivial and I'm like 500 hours in on this project already.
So, I'm looking for if anyone has advice on the proper hand calcs/assumptions I can make? Here's where I am and a couple of options so far:
The stackup
- Source
- Baseplate
- Fins
- Airflow over fins + base to convect out
Options:
- Yovanovich approximation for conductive spreading resistance: I must use a conical profile here. Doing 1D conduction through the baseplate is either going to be way too conservative or way too optimistic. 45 degree assumption will be better but with such a thin baseplate I am unsure how accurate it'll be.
- Fins + Baseplate surfaces: here's where I get lost. ideas? - Shah and London for a Uduct/1 adiabatic wall
- Don't treat as a Uduct and do just the fins separately, use Bar-Cohen Rosehnow to get a correction factor for narrow parallel plates. Either do the base strips of the spreader with the same correction factor or just as a infinite free plate and hope for the best
I'm a little overwhelmed here just bc I need to pick what to do and then the hand calcs itself are gonna be pretty intense- I assume i'm going to have to do all 7 parallel fin channels separately because the thermal resistance to get to them from the conical spreading is different. am I on the right track? Thanks!
