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Old 6th March 2012, 04:51 PM   #1
tomchr is offline tomchr  United States
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Default TO220 isolation kits breakdown

Folks,

I like to have the heatsinks in my project grounded for safety reasons. But I've noticed that when mounting a TO-220 like the IXYS 10M45 CCS to a grounded heat sink, I often get a high-voltage breakdown (sparks, carbonization, short circuit) between the tab of the device and the mounting screw. The breakdown seems to happen right where the shoulder washer ends on the back side of the TO-220.

I measure the resistance from the TO-220 tab to the heat sink with an ohmmeter before turning the power on. I have had cases that would short out even though the meter showed no connection from tab to heatsink.

Are some TO-220 mounting kits rated for higher voltages than others? How can I avoid this breakdown issue?

~Tom
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Old 6th March 2012, 05:01 PM   #2
kevinkr is offline kevinkr  United States
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Hi Tom,
What sort of voltages are we talking here? I've not had problems at 200V and below in recent times and built some Maidas back in the '80's that ran at much higher voltages (330V out and 350 - 360V in). I use nylon screws and silpads where the power to be dissipated is low since nylon is not particularly good for clamping.. I always drill the mounting holes in heat sinks roughly a drill size large than the fastener and debur carefully before installing the insulator. (I usually use mica/grease for higher dissipation and silpads with nylon or stainless steel screws for modest dissipations.)

In some instances I also use small ungrounded heat sinks on vector board inside a chassis where they cannot be touched in normal operation.
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Old 6th March 2012, 06:17 PM   #3
tomchr is offline tomchr  United States
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Quote:
Originally Posted by kevinkr View Post
What sort of voltages are we talking here?
I've had issues from 150~200 V and up. In recent incarnations of my current amp design, I've run B+ of 400 and 470 V and experienced fireworks at both voltages.

I'm using the silicone pads as they're so much more convenient than the mica+goop. But I've also noticed that after they get covered in carbon from the breakdown arc, they don't insulate very well anymore...

Quote:
Originally Posted by kevinkr View Post
I use nylon screws and silpads where the power to be dissipated is low since nylon is not particularly good for clamping..
Yeah... That's my concern as well. Nylon tends to stretch so you don't get good clamping pressure.

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Originally Posted by kevinkr View Post
In some instances I also use small ungrounded heat sinks on vector board inside a chassis where they cannot be touched in normal operation.
That may be where I end up in the final design, but I'd still like to figure out a reliable way for isolating TO-220's when mounted on a grounded heat sink. That's handy for prototyping and such.

~Tom
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Old 6th March 2012, 06:36 PM   #4
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Tom,

The payback for the SILpad convenience is the high sensitivity to punctures. They're good for kV when everything is polished flat and cleaned down - but totally intolerant to one speck of swarf.

I'm usually back with Mica for high voltage!
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Old 6th March 2012, 06:54 PM   #5
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What I use for high voltage TO-220 mounting:

Aavid 7721-8PPSG PPS material shoulder washer .040 x .245 oversized shoulder

Aavid 4170G Al oxide ceramic insulating pad

the ceramic pads are awesome, about .050 thick and reusable

All heat sink mounting is super sensitive to nice flat clean surfaces

PS I don't know if perhaps some of the heatsink compound (arctic silver, etc.) is electrically conductive but I stick to the white stuff.

Last edited by Michael Koster; 6th March 2012 at 06:56 PM.
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Old 6th March 2012, 08:52 PM   #6
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Rod Coleman View Post
The payback for the SILpad convenience is the high sensitivity to punctures. They're good for kV when everything is polished flat and cleaned down - but totally intolerant to one speck of swarf.
I've noticed that as well. Also, I've noticed that the hole for the screw is pretty large in the SILpads I have. I think it's made for a 4 mm (#6) screw. I use 3 mm screws...

I'm using a pretty small heat sink. They're made for TO-220 packages but have a slot where the screw goes rather than a hole. I'm wondering if the thermal pad ends up getting pinched on the slot edges. I'm using something else for the final product, but for prototyping these small heat sinks are pretty handy. Well, aside from the arcing that is...

~Tom
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Old 6th March 2012, 11:20 PM   #7
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I have blasted through the sil pads too, but I was running about 500 volts on some big Fuji TO247 fets. Made a big bang and fried the plastic on the fet. The fet was still alive but wasn't flat anymore. The bang took out a 2 amp line fuse.

I got some new pads and will try coating them in white goo first. There is no plastic washer on a TO247. The hole is plastic and the metal contact area is spaced away from the hole. I might have torqued the screw a bit too much, so I will try a 6 inch pound calibrated screwdriver. This time there will be 4 fets per big heatsink with only 350 volts. This is for the mother of all driver boards for screen drive and other extreme tube amp experiments.
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Old 6th March 2012, 11:31 PM   #8
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Quote:
Originally Posted by tubelab.com View Post
I have blasted through the sil pads too, but I was running about 500 volts on some big Fuji TO247 fets. Made a big bang and fried the plastic on the fet. The fet was still alive but wasn't flat anymore. The bang took out a 2 amp line fuse.

I got some new pads and will try coating them in white goo first. There is no plastic washer on a TO247. The hole is plastic and the metal contact area is spaced away from the hole. I might have torqued the screw a bit too much, so I will try a 6 inch pound calibrated screwdriver. This time there will be 4 fets per big heatsink with only 350 volts. This is for the mother of all driver boards for screen drive and other extreme tube amp experiments.
The ceramic pad for TO-247, TO-3P, etc is Aavid p/n 4180G

I was able to use these at ~500V swinging up to >800V with no problems

Most linear triode-strapped pentode

Last edited by Michael Koster; 6th March 2012 at 11:49 PM.
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Old 7th March 2012, 12:10 AM   #9
tomchr is offline tomchr  United States
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I gotta believe we're burning through those SILpads because the mating surfaces aren't completely clean. It wouldn't take much of a burr to create a connection through one of those pads. At least all of Aavid's products are rated at breakdown voltages in excess of 4 kV. Pretty impressive...

~Tom
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Old 7th March 2012, 12:32 AM   #10
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Quote:
The ceramic pad for TO-247, TO-3P, etc is Aavid p/n 4180G
Digikey wants a minimum order of 6000 units, but Mouser has stock at 92 cents each. I will get some on my next order just in case I decide to turn up the volts. I am using the multi tapped Edcor "tinker box" power transformer with the output stacked on top of a 120 volt isolation transformer. I get output voltages from +/- 40 volts to +/- 550 volts.

Quote:
I gotta believe we're burning through those SILpads because the mating surfaces aren't completely clean.
I know that the heat sink, pad and mosfet were wiped down with alcohol and were clean. The heat sink I used on the last experiment was tapped before it was anodized and burr free. It was brand new and still in the box. It wasn't big enough for my purpose and got quite warm. Thermal cycling probably weakened the pad in the area that failed. Overtorquing was probably a factor too.

I got bigger heat sinks, but I must drill and tap them myself this time. I will be careful to clean things up before another big bang. No torque specs in the Aavid lit though.
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