insulating TO-247 fets on heatsink

[Jamie Morken]

Hi all,

What is the best material to use for insulating a fet on an aluminum
heatsink? Anyone have a digikey part# or another source for this
material? I was thinking of using 1mil kapton film or tape and cutting
out the holes for the attachment screws. Thanks,

cheers,
Jamie Morken

 

[John Larkin]

On Fri, 05 Nov 2004 05:30:36 GMT, Jamie Morken <jmorken@shaw.ca>
wrote:

Hi all,

What is the best material to use for insulating a fet on an aluminum
heatsink? Anyone have a digikey part# or another source for this
material? I was thinking of using 1mil kapton film or tape and cutting
out the holes for the attachment screws. Thanks,

cheers,
Jamie Morken

Kapton doesn't conduct heat very well, but would be OK if your power
density isn't too high. Hard aluminum anodize is pretty good, with a
dab of silicone grease.

How much power?

John

 

[Jamie Morken]

Hi John,

John Larkin wrote:

Kapton doesn't conduct heat very well, but would be OK if your power
density isn't too high. Hard aluminum anodize is pretty good, with a
dab of silicone grease.

How much power?

Aluminum anodized heatsinks I guess? Interesting idea but I would
rather use a removeable insulator to avoid having to do the anodizing.
This is for a 3phase hbridge brushless DC motorcontroller and the peak
power would be ~160Amps, but for the motors I am using right now it will
be running at 20Amps or so. Also I would like the insulator to be able
to insulate up to 240V. It is a pretty general purpose motor controller

cheers,
Jamie

John

 

[Pooh Bear]

Jamie Morken wrote:

Hi all,

What is the best material to use for insulating a fet on an aluminum
heatsink? Anyone have a digikey part# or another source for this
material? I was thinking of using 1mil kapton film or tape and cutting
out the holes for the attachment screws. Thanks,

What's wrong with the readily available commercial insulators ? Pre-cut
with holes.

If you need it to conduct heat well, ceramic aluminium oxide washers work
well. Don't forget to use thermal transfer compound ( silicone grease or
aluminium loaded same ).

Try Googling motorola AN1040 ? IIRC. A classic application note on mounting
power semiconductors. Very good source of info. It's on On Semi's website
now btw.


Graham

 

[Terry Given]

Pooh Bear wrote:

Jamie Morken wrote:


Hi all,

What is the best material to use for insulating a fet on an aluminum
heatsink? Anyone have a digikey part# or another source for this
material? I was thinking of using 1mil kapton film or tape and cutting
out the holes for the attachment screws. Thanks,


What's wrong with the readily available commercial insulators ? Pre-cut
with holes.

If you need it to conduct heat well, ceramic aluminium oxide washers work
well. Don't forget to use thermal transfer compound ( silicone grease or
aluminium loaded same ).

but they are both expensive and fragile. Torque control is imperative,
as are well-machined heatsink surfaces.

Try Googling motorola AN1040 ? IIRC. A classic application note on mounting
power semiconductors. Very good source of info. It's on On Semi's website
now btw.

One of the essential app notes. I refer people to it all the time, its
horrifying how many so-called engineers I have met who think its OK to
pop-rivet TO-220 devices to heatsinks - aaargh!

Graham

My personal perference is to use spring clips to mount piddly wee
devices to heatsinks. For a decent semiconductor, use DCB substrates and
bolt them to an 0.01K/W heatsink.....

If you can live with the fairly high thermal impedance the various
"boots" are great for production (requiring clips or clamping bars) and
can be very convenient when isolation is required

The funniest thing I have ever seen was the use of a nylok nut on a smps
FET that ran quite hot - a little over 100C at max Tambient. The nylon
melted and oozed out the back of the nut, which duly loosened, thereby
ensuring the FET self-destructed from thermal runaway. When I looked at
the RMA, the FET rattled around but the nut wouldnt undo without some
effort. oops.

Cheers
Terry

 

[John Larkin]

On 5 Nov 2004 07:41:06 -0800, optoeng@pioneernet.net (Paul Mathews)
wrote:

Jamie Morken <jmorken@shaw.ca> wrote in message news:<CzEid.59278$E93.56131@clgrps12>...
Hi John,

John Larkin wrote:

Kapton doesn't conduct heat very well, but would be OK if your power
density isn't too high. Hard aluminum anodize is pretty good, with a
dab of silicone grease.

How much power?

Aluminum anodized heatsinks I guess? Interesting idea but I would
rather use a removeable insulator to avoid having to do the anodizing.
This is for a 3phase hbridge brushless DC motorcontroller and the peak
power would be ~160Amps, but for the motors I am using right now it will
be running at 20Amps or so. Also I would like the insulator to be able
to insulate up to 240V. It is a pretty general purpose motor controller

cheers,
Jamie



John



Search on SilPad Hiflow, e.g.

http://www.bergquistcompany.com/tm_hi_flow_detail.cfm?oid=104290&tab=config&prod_type=hiflow

anodizing is not considered an acceptable insulator by UL/CSA

Paul Mathews

The Hiflow stuff isn't an insulator at all. Silicone grease would be a
lot better thermal conductor.

I've used hard anodize+grease up to 200 volts; theta is low and it's
very reliable. UN doesn't care what goes on inside a box if it's not a
shock or fire hazard.

John

 

[John Larkin]

On Fri, 05 Nov 2004 09:08:43 +0000, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

If you need it to conduct heat well, ceramic aluminium oxide washers work
well. Don't forget to use thermal transfer compound ( silicone grease or
aluminium loaded same ).

Since BeO is toxic, it's falling out of favor. AlN is almost as good,
very nice stuff.

John