Re: Best way for mounting high voltage transistors on heat sink

I use nylon shoulder washers a lot but as you pointed out the screw is not isolated and I don't know is it safe to leave a bolt at 100V inside the case non-isolated.
nylon screws as you said should stand high temperature. I think only Teflon can stand these temperatures safely.
Clips are also very good idea, thanks but I have to design them and order them because I don't think I can find anything ready that fits my heat sink. as I indicated before because of high density and low space I'm very limited at choosing heat sinks and I found a suitable small heat sink that a 8x8 fan can be mounted on it and has very good dissipation.

PS: my lost post was in response to Vladimir post that mistakenly posted in response of myself, sorry.
"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message news:w5WdnWzZrLbcfofXnZ2dnUVZ_vadnZ2d@xxxxxxxxxxxxxxx
On Mon, 25 May 2009 20:42:18 +0430, Saman wrote:

(top posting fixed)

regards,
<bill.sloman@xxxxxxxx> wrote in message
news:e6fa87a9-61c3-46ce-8a31-
acf72be38f35@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On May 25, 6:20 am, "Saman" <saman75sa...@xxxxxxxxx> wrote:
Hello, I have a board that has 8 transistors mounted on a heat sink.
each TO-220 transistor(IRF 630) dissipates about 10W. for lower
voltages I usually used rubber silicon pads for isolation. I have
tested them and they
can dissipate up to 15W without making transistor too hot. Now I have a
high
voltage board. the voltage of metal flange(drain) of each transistor is
100V. I haven't worked with such high voltage yet, so I'm worry about
safety. The whole heat sink assembly is inside a metal box that has
earth wire and also both live and null of input 220V line are fused.
Still I'm not
sure it is enough. your experience is appreciated. thanks

Check the voltage rating on your insulating silicone rubber mounting
pads - most of them seem to be good for a couple of kilovolts, so 100V
shouldn't be a problem.

Make sure that you've also got proper insulators on the mounting bolts
that hold your transistors onto the heatsink.

Thanks Bill,

How should I isolate mounting bolts? I have bought an epoxy that the
retailer says it can resist up to 300 degree centigrade but I'm not sure
if I spray an epoxy layer on top of transistor then it can exchange
heat, because though most of heat are exchanged through silicon pads but
some heat are also exchanged through top of TO-220 body. another problem
that I have encountered in past using these pads is that when I tighten
the screw, sometimes it causes sharp edges of TO-220 to cut the pad and
make a short circuit with heat sink. Thermal conductivity of these pads
are directly related to the pressure on them. I am worry that after a
while and because of different mechanical stresses even if I do not
tighten the screw too much it cuts the pad. One of the hardest part of
my job is power dissipation because I have a very limited space(a fixed
size box that is too small to accommodate big heat sinks. so I have to
use every cubic cm of box space. Are these silicon pads the favorite
choice? I mean isn't there better alternatives?

Nylon shoulder washers? Usually these are done by using an undersized
(i.e. #4) screw and a washer with a thin thin shoulder, but you could
come in from the back with a non-isolated screw and a shoulder washer on
the sink, as long as you didn't mind using a nut and having a HV point on
that side of the sink.

Or nylon screws, if you can stand nylon and temperature cycling.

Or clips, which vastly increases your opportunities for isolation.

--
www.wescottdesign.com

.

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