I need a HV insulator for TO247AC FETs... - diyAudio
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Old 29th January 2007, 06:33 AM   #1
batee is offline batee  United States
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Default I need a HV insulator for TO247AC FETs...

I bought some of the Bernquist insulators recommended by the Class A guys, and it turns out that these didn't provide sufficient electrical isolation for my IRFP264N FETs when switching ~170V. The Drains arced to the chassis and destroyed several FETs.

Does anyone have a recommendation for an insulator that would provide 2-300V electrical isolation? I'm dissipating about 15W in each FET.

Thank you,

Bryan A. Thompson
bryan@batee.com
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Old 29th January 2007, 08:55 AM   #2
Eva is offline Eva  Spain
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Can't you just use the traditional mica sheets, silicone pads or aluminium oxide spacers? Mica and silpads are standard inexpensive stuff and will usually provide over 2KV of insulation with standard thicknesses.
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Old 29th January 2007, 11:35 AM   #3
IVX is offline IVX  Russian Federation
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batee, if your TO-247 dissipate just 15W, you has wrong choice obviously, best way is using TO-220FP packed mosfets (2500VAC RMS isolation).
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Old 29th January 2007, 12:17 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I think there was something wrong with your assembly/installation.

There was nothing wrong with using Bernquist insulators at 170Vdc and even @200 to 300Vdc.

Was there a metal fragment trapped under the insulator? or a burr on the edge of the device? or a solder whisker? or a clipping of wire? that could short from Drain to ground?
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Old 29th January 2007, 01:02 PM   #5
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some of the Bergquist insulators are, by design, conductive -- check the Digikey catalog page -- I haven't had any problems with the SP600 variety.

Jack
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Old 29th January 2007, 01:20 PM   #6
IVX is offline IVX  Russian Federation
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Guys, i guess, he'll happy with the mosfet like IRFI4229PBF (Rds <2 times, Qg<3 times) or so, without any additional insulators yet.
I hope it's a SMPS task generally?
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Old 29th January 2007, 01:32 PM   #7
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Have you implemented appropriate/adequate flyback/catch scheme? Assuming some kind of SMPS.
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Old 29th January 2007, 02:15 PM   #8
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the bergquist work very well for instance,
VO >6kv @ 10^11 vohms
SP400 >3500V at 10^11 ohms

if you ask nice they will send samples

http://www.bergquistcompany.com/tm_imsg.cfm is the index for their stuff
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Old 29th January 2007, 04:21 PM   #9
batee is offline batee  United States
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This is a full-bridge config. The FETs failed instantly with no load attached. Not all FETs failed during any test. Different FETs failed a during subsequent test. There were clear blast holes through the .005 Kapton washers, even when double-stacked. No FETs failed when I ran it with no load and the FETs *not* attached to the heatsink. The FETs are mounted to the back of a piece of heatsink that's 3/8" aluminum. It wasn't warped. All surfaces were cleaned before assembly. I tried it with and without non-silicon heatsink grease. I suppose it's possible that I overtightened the pads and caused the failure.

Why that transistor? Like Nelson says: That's what I had on the shelf.

Thanks everyone.

Bryan
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Old 29th January 2007, 04:31 PM   #10
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what part number are the fets?
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