Heatsink Position, Temp, and Vgs

[MattM]

I am working an Aleph-5 Monoblock that will have 2 large heatsinks with 3 IRFP244's per heatsink.

However, due to the design of these heatsinks, the devices will be mounted vertically; i.e., one on the bottom third of the heatsink, one in the middle, and one near the top.

I am a little concerned that the upper device might be a little warmer than the bottom device due to natural convection. I know that Vgs is temperature dependent, but I'm not sure if being warmer will make the device run harder or easier.

So... Which of the following am I looking at?
[list=1][*]The top device, being warmer, runs harder, making it even warmer - a runaway effect. Bad.[*]The top device, being warmer, runs easier, making it cooler - a self-correcting effect. Good.[*]Because they are all on the same heatsink, the convection effect is minimal, so don't worry about it. Good[/list=1]

Any ideas?

[MattM]

Wow, 80+ people have read this and still no guesses? I thought it was an easy one and I just didn't understand the MOSFET performance well enough.

Maybe not.

[MEGA-amp]

A picture might be good, or some dimensions, maybe a drawing. That way we could help you better.


john


edit: If you're using these, using 2 per monoblock, it may run a little too hot for comfort. If your using those sinks, I would use 6 per monoblock and mount 1 transistor per sink, that way you could probably raise the bias as high as you want, but then again, you havent specified the size of your sinks.

[Alain Dupont]

MattM,

>> 1)- The top device, being warmer, runs harder, making it even warmer -
>> a runaway effect. Bad.
>> 2)- The top device, being warmer, runs easier, making it cooler -
>> a self-correcting effect. Good.
>> 3)- Because they are all on the same heatsink, the convection effect is minimal,
>> so don't worry about it. Good

First, I assume your heatsinks are mounted vertically, and the fins are also vertical...
Also there is space under the bottom of the heatsink ...
Plus these heatsinks were wisely calculated, so :

Q1: On my A75 no more than 1or 2 degree celcius between the bottom and the top
on 8" inches heatsinks ... after 20 minutes
Q2: Wrong, the Mosfets must share the same temp if possible otherwise
the sound will suffer....
Q3: The convection effect is the big principle of a vertical heatsink ; air when flowing
on the surface of the fins charges with heat, and so removes heat from the heatsink.

The 24 * TO3 Fets reach a case temperature of 70 degrees on each side ;
the angle that support the fets reach about 62 degrees and the heatsinks 52 degrees.

If too much difference between lower and upper mosfet case is > 3 to 5 degrees
it could be wise to install a support sharing the 3 mosfets attached to the heatsink
copper or brass to help share the same temp.

Without a complete schematic, models and dimensions of your heatsinks, fixation used
for the MosFets ; Bias etc, can't help you further...

PS:
Not so easy without any technical specifications...

Alain.

[yup]

Alain,
Certainly the air would be hotter as it travels up the heatsink, but I am not sure about the inside surface of the heatsink itself...

You could run a simple test to see if there is much difference in the heat sink temp from top to bottom. The thermal conduction of the aluminum may be great enough so that the transistor mounting surface temp is fairly even.

If you do conduct a test, let us know what you find.

Jeff

[MattM]

Thanks for the replies!

Yes, I am using these.

I plan to mount 3 devices per heatsink (two heatsinks per monoblock). The heatsinks will be mounted with the fins running vertically. In that orientation, they are 12" tall, 5.8" front to back, and the fins are 3" wide. Each heatsink weighs 5-1/2 pounds and has 16 fins. They are Wakefield Engineering part number MD387A, but I can't find them on their web site so they may have been a custom item. They are closest to extrusion profile 8415 - here is a link with some specs.

They will have air space under them, so I am expecting natural convection to create quite a healthy air flow across the 12" height.

Maybe the answer is simply to mount the devices at the same level. I had originally planned to spread them out across the heatsink (as depicted in the "wrong" half of the diagram below). While mounting them close together may or may not make them run a little hotter, at least they will all be at the same temp.

[Nelson Pass]

For an Aleph 5, it hardly matters. The bias is regulated and has
only a marginal temperature dependency.

[MattM]

Thank You Nelson!

I plan to experiment a bit with the heatsinks, but it's good to know that it shouldn't make much difference.

[GRollins]

My take is that you shouldn't worry so much about bias problems as reliability. The top-most device will run hotter, which will stress it more than the lower devices. The smaller the heatsink (hence hotter, all other factors assumed equal), the more pronounced the effect.

Grey

[djucki]

Question for Mr Pass:

what is IRFP 240 predictable life time in ZV9 at that mosfet case temperature?

Can you suggest maximum mosfet case temperature for lasting many years?

Miroslav