I need a HV insulator for TO247AC FETs...

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batee

batee

Member
Joined 2004
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
 
A

AndrewT

R.I.P.
Joined 2004
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?
 
batee

batee

Member
Joined 2004
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
 
batee

batee

Member
Joined 2004
Which Flyback Diodes?

Does someone have a suggestion for what flyback diodes should use? I don't have load specs, but expect that it'll be a resistive or maybe small AC motor, probably <1HP.

I have confirmed no shoot-thru (deadtime over 1mSec for testing purposes). I have confirmed correct functionality at +12VDC using a resistive load. I have not operated with a load at high voltage yet.

I like the idea of the insulated package, however I'm not sure that IRFI4229PBF would meet my needs. Load current will be <= 15A. Depending on the operating temp, the FETs wouldn't be derated at all (they'd be operating at or near the full spec current). I would expect more/sooner failures from these.
 
batee

batee

Member
Joined 2004
Measured the resistance of the Q3 material:

Face-face: 240Ohm

Corner-corner on one side: 750Ohm.

The case wasn't grounded, so it went to +170V when the power was switched on through the top FET Drains. When the MCU started switching the bottom FETs, they started to conduct and destroyed the Q3 material, blew the FETs which always seem to fail in short condition, then arced to the heatsink and left a pitted surface.

I guess the moral of the story is to check the datasheet on everything.

Bryan
 
T

TechGuy

Member
Joined 2006
Eva said:
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.
Click to expand...


FWIW/Tip of the Day:

Mica and alum. Oxide insulators require thermal grease for good thermal conduction. Over time, Thermal Grease will evaporate reducing themal conductivity. Silicone pad work better and over time the thermal conductivity improves.
 
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