Heatsink Dissipation Problem

[bacworth]

I have finished building an Aleph 2, therefore I have used 12 fets each side. I have attached two heatsinks, for each side, together. I need to keep the fets in the middle of the two heatsinks due to space constraints in the case. The fins run verticle to the case.

The major problem is the excessive heat in the center ends of the two heatsinks, too hot too touch and the lack thereof at the ends no heat at all.

What should i do so i can avoid major structural modifications? I see two options a. use a fan on the hotest parts of the heatsink or b. use a copper plate running horzontally along the heatsinks and the fets due to copper's better thermal conductivity that aluminum.

thanks guys,
Brent

[viktor1986]

I will use fan on 220V!

[Samuel Jayaraj]

Use a thick copper flat bar. If you allow a temperature build up in the centre of the heatsink, it means that the heat is getting produced faster than it spreads thru' the heatsink and you will not be able to avoid hot-spots on the Mosfets in the middle. Either life-span will get affected or you are going to burn them up in normal use.

A fan will help only if the heat is being sunk into the metal at about the same rate it is being produced.

[AndrewT]

Hi,
I don't think it is the speed of the heat production or dissipation.
I think the problem is thermal resistance.
How big are the sinks, height(H), width(W), overall depth(D), backplate thickness(T), fins thickness(Ft), fin depth(Df) (out from the backplate, D=T+Df), Fin spacing(Fs), number of fins,
Where are the semis located?
This data should let us trouble shoot.

[EWorkshop1708]

Are your mosfets real close together in the middle of the heatsink?

If so and your space constraints make it hard to spread them out, is you could run thick wires from the PCB to the transistor legs instead of putting them directly in the PCB.

On my amp, what I did to get good even heating of the sinks, (amp has two 3"x 8" heatsinks, one for NPN one for PNP) is to use 5 pairs of outputs, and spread them out among the sinks. Makes it that the transistors do not feel any hotter than the heatsink when touching them. Also the base is 1/4 inch thick, and draws the heat well. But the amp has forced fan cooling and only output ~300W for the amount of cooling I'm using. 2 80mm fans per sink.

Just for testing, I biased the amp real high with no fans just to get the heatsinks hot, and the heat is even. Nice and spread out.

If a high amount of heat in a single area, the middle of the sink will be the hottest. That's also a problem with computer CPU's, esp Athlon, so they use a thick heatsink base 1/4 inch or more, and sometimes with copper to help more because of this problem.

If yours is thinner than 1/4, you will need to spread them out more. You mentioned you did have 12 pairs of mosfets.

[AKSA]

Brent,

What is the dissipation of each device, and what heatsink are you using?

If you have say 10W in each device, and there are 12 devices, this is 120W, and with a 0.35C/watt sink you are looking at a 42C rise above ambient IF the heat is injected evenly along the sink.

You might need to use forced air cooling; it doesn't have to be a high speed fan, 1500rpm on a 120x120 fan could do it easily, but this will increase thermal capacity around 2.5 times.

Cheers,

Hugh

[EWorkshop1708]

Originally posted by AKSA
Brent,

What is the dissipation of each device, and what heatsink are you using?

If you have say 10W in each device, and there are 12 devices, this is 120W, and with a 0.35C/watt sink you are looking at a 42C rise above ambient IF the heat is injected evenly along the sink.

You might need to use forced air cooling; it doesn't have to be a high speed fan, 1500rpm on a 120x120 fan could do it easily, but this will increase thermal capacity around 2.5 times.

Cheers,

Hugh

I agree 100% fan cooling helps a ton, and if he's dissipating that kind of power, he will need good heatsinks AND fan cooling.

[amplifierguru]

You really have to ask - For what all this waste power? What is the power output vis a vis the consumption? Great if everyone did it!

We only have one climate/planet perhaps GAIAN principles should be imposed.

Cheers,
greg

[mastertech]

well , waste of energy could come from a lot of things, it is
actually happening, so whats your point guru, dont you like
class-a as power output?

i think i should go listen to my state of art class-a amp now


cheers guys

[bacworth]

All - The base is of the heatsinks are 8mm thick base, 250mm long, 151.5mm wide, 48mm deep. They are made by Conrad (www.conradheatsinks.com), MF25-151.5, 0.29 C/Watt.

ASKA - The amp is rated 250watt/channel therefore 20.83w per fet or 125w per heatsink.

I have attached a pic of the layout.