Best way for mounting high voltage transistors on heat sink

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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.

-- Bill Sloman, Nijmegen

100 is not really high voltage. The silicone pads are rotten thermally, but will not be bothered by 100 volts.

John

Nothing to worry about. The 100V and 15W is not much for TO-220 with the rubber silicon pad.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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?

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.

-- Bill Sloman, Nijmegen

I like the mica sheets, not the plastics.

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Do not put any screws through the mounting hole of TO-220. Use the spring loaded mounting clamp over the TO-220 body.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

(top posting fixed)

snipped-for-privacy@g37g2000yqn.googlegroups.com...

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

The combination of TO-220 and sil-pad is terrible. Sil-pads need high contact pressure to be even mediocre thermally, and the TO-220 machanics is all wrong... the force is where it does the least good.

Mica is at least thin, flat, and imcompressible.

Sil-pads work a little better with TO-247 type packages: more heat transfer area and better centered force.

Incidentally, the convention here is bottom-posting. Top posting is confusing and arguably impolite.

John

Saman TOP-POSTED:

[...]

Use of standard techniques

*-character+outlook.express+*-=*-*-before-*-quoted-text+default+Trim+*-a-test-post+move-*-cursor-to-*-bott= om-*-*-*-*+unable#ss3.1 would make your Usenet posts look less noobish.

::[Blockquoting is a complete mess] :: ...and if you're going to use MicroCrap tools

--especilly for tasks for which they are poorly suited--

*-*-*-*-inferior+published+proper+do.not.properly.mark.quo=ted.text+top.post+Micro.oft.programs+QUILA+Outlook.Express+official+do-not-=*-use-Micro.oft-programs+average+Netiquette+Richard.Torrens+dry+top.posting= &strip=3D1#quila

it would be a good idea to apply ALL the available patches:

't.exactly.feature.the.=most.intelligent.quoting.algorithm

There's two issues: with higher voltage, may come higher power (and you might want more power dissipated). Also, this is above shock- hazard threshold (but if it's inside a grounded box, that's already handled).

Easiest would be to use thermal compound and direct bolt-to-heatsink (so the heatsink is electrically live, but it's inside a grounded box, so you're OK there). The only insulation problem is how to connect the heatsink to the box (lots of solutions, at real HV we used a lot of ceramic standoff posts).

There's IRF630 variants that have factory insulation in the package, the IRFI630G costs circa $1.53 in a fully isolated 'fullpak' package, as opposed to IRF630N at $1.05, and the cost of mounting washers and insulators makes the apply-your-own-insulation route more expensive.

It's also convenient, sometimes, to put a bunch of transistors onto a small heatsink (like an aluminum L section) that fits onto the circuit board, then bolt the small L section to a big heatsink when the board is mounted in the box. The L section can be electrically live, it's relatively easy to put a layer of Kapton tape on the flat mating surface and fasten the big sink with nylon screws or other nonconducting fasteners.

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If you want to connect to the tab using the screw, the shoulder washer keeps that screw from the sides of the heatsink hole. If you don't want the screw live, the shoulder washer keeps the screw from the sides of the tab hole.

If it isn't the screw that's at 100V, it's other items inside the box. So, close the box before powering it up. Apply a bit of lock adhesive (Loctite 2240 or similar) to the nut so it doesn't loosen.

yes because of higher power I used 8 parallel MOSFETs. So you are suggesting having a live conductor inside a grounded box is Ok?

I'm not sure about saftey but if I could do such a thing it would save me from a lot of troubles when trying to accomodate big heat sinks inside a small box. Because as I said earlier if I could directly mount transistors on heatsinks then each transistor could dissipate considerably more and I could use fewer transistors. Another problem with this scheme is when I have 8 parallel transistors I either have to use a separate heatsink for each one or isolate them.

Thanks I will investigate about it.

good idea, I have seen some L shape heat sinks on PCBs but I do not know how much they can dissipate. I remember once (about a year ago) I made some measurements about power dissipation of different heat sinks and I came to this conclusion that for anything above 10W I need either a huge heat sink(and for 20W even that's not enough) or a fan cooled heat-sink the shape of heat sink is very important in its efficiency and honestly I don't think these L shaped aluminum sections can dissipate beyond a few Watt.

the sholder washers that I've seen have a small cylindrical body and usually their tube-shape section is long enough to only cover transistor hole not the heatsink.

Good advice about adhesive thanks

It was my thought that multiple transistors could be bolted to an intermediate heat spreader, that the L section was really just another kind of heatsink tab; the REAL heatsink bolts to the L section, possibly with a layer of insulation between.

You could use your rubber washers and peek screws. $$

Or you could use the mica insulators with thermal compound on either side.

I'd be leary about the shoulder wahsers unless you counterbore the screw hole in the heatsink and get one that passes all the way through it. I've had the short shouldered ones get knicked then POOF!!!.

I would upwith Sil-pad1500 and a mylon screw. You know need about 16 oz-in of torque to generate the 100-200PSI required for the sink to work.

Leave your heatsink a ground potential!!! If you have high dv/dt on the drains fo the fets that big heatsink will radiate noise everywhere.

BUT also be aware that the closer you get the drain tap to the heatsink the more capacitance you have from drain to ground. That might cause some problems so you'll have to check that out.

Can you get this fet in to-220fullpack? a screw and some grease and your good to go. Junction would be about 42C (thea Jc-4.17C/w) above the heat sink temperature but the to-220 isn't much better when you consider the theta Jc = 1.4 + at least 1C/w for the interface. the difference is ~18C. As long as the heatsink temp is below about 50C you should be OK.

hard anodise is good for a about 200V insulation