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Old 14th November 2014, 10:42 PM   #1
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Default How much smaller can heatsink be if fan used ?

I have designed a 1600 watt amplifier module.
I worked out I need a 0.1 degree/Watt heat sink.

How much smaller could this heat sink be if I kept it cool using a fan ?
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Old 14th November 2014, 11:00 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by nigelwright7557 View Post
I have designed a 1600 watt amplifier module.
I worked out I need a 0.1 degree/Watt heat sink.
How much smaller could this heat sink be if I kept it cool using a fan ?
Bear in mind that the rating is only for a uniform loading over the entire heat sink surface.
Here are some sinks that I've used with fans. They're big.

Standard Bonded Fin - heat sinks

Last edited by rayma; 14th November 2014 at 11:14 PM.
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Old 15th November 2014, 07:36 AM   #3
djk is offline djk
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The sink must be designed for a fan, then you can get about a 5:1 improvement.
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Old 15th November 2014, 08:22 AM   #4
sajti is offline sajti  Hungary
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Quote:
Originally Posted by djk View Post
The sink must be designed for a fan, then you can get about a 5:1 improvement.
Agreed. The maximum improvement is to reduce the thermal resistance with 80%. You need more than 5m/s air speed for it. So powerful fan is necessary.
This picture comes from the Fischer catalogue.

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Old 15th November 2014, 11:20 AM   #5
AndrewT is online now AndrewT  Scotland
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I would guess at almost 10:1 improvement using a fan.

A fanned heatsink is far better than 10:1 might imply.
i.e. take the fan off a fanned sink and the heatsink capability mat fall to only 5% of the fanned rating.
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Old 15th November 2014, 12:48 PM   #6
dmills is offline dmills  United Kingdom
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It also depends on the design of the heatsink, some of the better ones require very high pressures to get the airflow rate which can argue for blowers rather then fans (Blowers are generally quieter anyway).

Very careful optimisation of the interface between the heatsink and the device also pays huge dividends at this level, most extruded heatsink you buy has a mounting surface that is anything but flat and you can make a huge difference by running a fly cutter over the mounting area to mill away the height variations.

The RF guys like to fit a copper heat spreader under the power devices (Sometimes soldered to the device), this serves to increase the contact area and thus lower the interface thermal resistance.

Don't forget that lower interface thermal resistance directly equates to being able to run the heatsink hotter for a given junction temperature, so allowing a smaller heatsink or quieter fan.

For a project with similar power dissipation requirement I used one of these http://www.farnell.com/datasheets/1692180.pdf which are compact for what they do and come with the mating face allready milled flat.

Regards, Dan.
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Old 18th November 2014, 07:48 AM   #7
djk is offline djk
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"I would guess at almost 10:1 improvement using a fan."

So the manufacturer's data in post #4 is wrong?

"i.e. take the fan off a fanned sink and the heatsink capability mat fall to only 5% of the fanned rating. "

Not valid.

Forced air on a natural convection design will not yield anywhere near 10:1, and a forced convection design (with no fan) will suffer in comparison with a similar sized natural convection design.

A pure copper tunnel (like a Crown MA5000VZ) with direct metal mounted devices will give your 10:1 figure (easily), a natural convecion design with insulators under each device will not.
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Old 18th November 2014, 08:13 AM   #8
sajti is offline sajti  Hungary
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I agree with djk. The only thing is that any heatsink acting like in the diagram.
The best solution would be to avoid any solution which worse the heat transfer, such the insulator between the device and the heatsink.
See my solution on the picture. The heatsink will be insulated from the chassis, and the output devices will be mounted without insulation. The heatsink will act as power supply rail too.

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Old 18th November 2014, 02:08 PM   #9
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Quote:
Originally Posted by sajti View Post
The best solution would be to avoid any solution which worse the heat transfer, such the insulator between the device and the heatsink.
Depends on the type of insulator used.

Keratherm red does 0.07C/W for a TO-264, 0.06C/W for a TO-3, 0.05C/W for an MT-200 package.

Your suggestion does not yield zero Rcs, but lots of compromises and constraints instead (been there, done that)
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Old 18th November 2014, 02:58 PM   #10
JMFahey is offline JMFahey  Argentina
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To put some numbers into it [tm] , the excellent Aavid site posted above (thanks Rayma ) includes a design tool which factors in air speed:

- heatsink design tools

Do your Math

Suggesting numbers pulled out of thin air without justifying them won't carry you very far

By the way, did you answer my questions about this amp in your other post? (number and type of transistors, schematic, etc.).
Thanks.
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