Heatsink Calculation

[regomodo]

Hi,
I'm calculating the heatsink required for a MOSFET (irf2204pbf) but i'm getting confused by the results.

To operate at 200W (constant rather than switched) at an ambient temp of 25C i need a heatsink of -0.215 C/W.

What does a negative thermal resistance imply? Refrigeration?

 

[dknguyen]

Are you sure it's negative? THat doesn't make sense. It sounds like you are doing your calculations incorrectly.

 

[regomodo]

that's what i thought. I'll attach the values and equation i'm using in an image. Hope you can read it as i can't scan in Linux.
[edit] Could i be using the wrong value for Rbh? I'm using the maximum value which i'd assume is metal-metal contact.

 

[Roff]

Apparently, you are only allowed a 15 deg temperature rise from ambient. Note that the sum of the two thermal resistances over which you have no control (with a passive heatsink), Rjb and Rbh, is 0.95deg/watt. Even if you had an infinite heatsink, you would get (204*0.95)=193.8 deg temp rise. So, I think you are correct in concluding that you have to change T2 by adding refrigeration.

 

[regomodo]

Rbh is at a maximum of 0.5 C/W. I assume this varies with how you have the heatsink and FET in contact as i've seen some examples where people have taken the figure to be zero.

I just ran the numbers and, with a 0.1 C/W HS (expensive) T2 would have to be -35C. Hmm. Well, i've ordered some heatsinks (a measly 3.3 C/W) and was going to put big fans across them anyway. I've got a few FETs spare so i'll run 'em into the ground.

 

[Roff]

Rbh is at a maximum of 0.5 C/W. I assume this varies with how you have the heatsink and FET in contact as i've seen some examples where people have taken the figure to be zero.

I just ran the numbers and, with a 0.1 C/W HS (expensive) T2 would have to be -35C. Hmm. Well, i've ordered some heatsinks (a measly 3.3 C/W) and was going to put big fans across them anyway. I've got a few FETs spare so i'll run 'em into the ground.

Just curious -What sort of environment and device do you have where ambient is 2C, and the max device temperature is only 17C?

 

[regomodo]

Just curious -What sort of environment and device do you have where ambient is 2C, and the max device temperature is only 17C?

Sorry if i'm being a touch dense but i don't follow. I haven't stated i'm using those temperatures.

 

[Roff]

Sorry if i'm being a touch dense but i don't follow. I haven't stated i'm using those temperatures.

In your equations, what are T1 and T2?

 

[regomodo]

In your equations, what are T1 and T2?

In all of the calculations i've use 175C as the max package temperature (T1) and 25C for the ambient temp' (T2). However, i've fudged T2 when calculating max power dissipated with certain heatsinks, etc, when messing with the numbers

 

[Roff]

In all of the calculations i've use 175C as the max package temperature (T1) and 25C for the ambient temp' (T2). However, i've fudged T2 when calculating max power dissipated with certain heatsinks, etc, when messing with the numbers

I thought your 5's were S's. I thought I was seeing 17S and 2S, when it was 175 and 25. Duhhh. I'm not sure what I thought the S's were. I think I forgot about them.

 

[regomodo]

ah. that would explain it. I can remember my primary school moaning about that to me many, many years ago. She also had a go at me about my "1"s looking like "2"s. Additionally i randomly uses "S"s instead of "2"s. No idea why.