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Buck switcher fails when potted

Here's an update on what I've found. I took unpotted boards that have been power cycled hundreds, if not thousands, of times successfully and tried v arious tests. (I should note that unpotted boards have never failed when th e input voltage is kept within spec.)

The failures always occur immediately after power is applied, so it doesn't appear to be a heating issue. Here's what I did:

-put RTV (the stuff with acetic acid) around the inductor, and it failed wi thin a few power cycles. I don't believe the stuff was even close to fully cured.

-blobbed thickened potting epoxy around the inductor on 2 boards, but I got impatient and applied power before it cured. Power was left applied while it was curing. Amazingly, these two have not failed. A few days later I potted the entire board in the enclosure and they've been reliable.

-blobbed thickened potting epoxy around the inductor on 2 more boards, but this time I allowed the epoxy to fully cure. Both failed within a few powe r cycles.

-blobbed "electronics grade" (and no acetic acid) RTV around the inductor, let it fully cure, and failed within a few power cycles.

So, WTF? My only guess with the two that didn't fail is since they were po wered during curing, the inductor was allowed to magnetostrict and create a tiny cavity inside the epoxy. But, RTV causing it to fail? I don't under stand.

Reply to
hondgm
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Interesting--dunno what's going on. From a practical POV, perhaps you could just put a cap over the inductor before potting.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

OK, sounds like some change in the inductor is blowing the switcher.

Try applying the chemicals to just an inductor in the PCB, and then measuring it. Either L is going down, or it's shorting some turns.

What's the part number of the inductor?

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

It's Bourns SDR0805-102KL.

Something else I'm trying right now that is initially promising is putting a diode from the Vin to switch output of switcher IC. Someone here suggested this. I won't know for sure if this fixes it until possibly tomorrow though.

Reply to
hondgm

It's a full millihenry, 4.5 ohms, so the wire must be skinny.

We use a Coilcraft part like that. The bottom of the ferrite core is metalized in two places to form the PCB pads, and the wires are folded over that, maybe welded or something. The construction is a little scary.

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If the potting affects the inductor, I can't see why the diode would help.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation
Reply to
John Larkin

I recall from my days as a summer-hire plastics tech that we "buffered" strain-sensitive parts by first coating them in a soft urethane rubber (a two-part material), and only then potting everything in hard epoxy.

We used urethane, not RTV, for buffering. It never occurred to me to ask why, but there will have been a reason. This was for the US space program, and the hardware was literally going to the Moon.

Joe Gwinn

Reply to
Joe Gwinn

I already posted this, but apparently it didn't work:

I think we found the problem, or maybe more accurately the fix. I put a 1N

400x diode from the output of the IC to Vin and have had no more failures d espite potting the board as it is.

However, I did replace one more part; it was discovered that the free wheel ing Schottky diode's current rating was severely under-rated. The voltage rating was ok. None of those diodes ever failed despite the switcher IC fr ying, but could that have been the cause? I'd assume no, but it's difficul t to say. What I will try later is potting a board with only the correct S chottky and not the other diode, but right now we're running short on board s to blow up.

Reply to
hondgm

eeling Schottky diode's current rating was severely under-rated. The >volt age rating was ok. None of those diodes ever failed despite the switcher > IC frying, but could that have been the cause? I'd assume no, but it's >di fficult to say. What I will try later is potting a board with only the >co rrect Schottky and not the other diode, but right now we're running short o n >boards to blow up.

Oh yeah, I forgot to say that I think we should be coating the inductor in something before potting because of the whole possibility of the ferrite-ge tting-cracked thing.

Reply to
hondgm

Well, any further news on this problem?

Jon

Reply to
Jon Elson

Yeah. I thought I had it fixed by putting a diode from the IC output to Vin. These survived thousands of power cycles even when potted, which is a record.

But then I began setting up for surge and burst testing for CE on the power supply input. The regulator fried again on two UNPOTTED units before even applying the test waveforms.

We scrapped the buck regulator and replaced it with a ready-made 3-terminal buck regulator replacement. It costs more but volumes are too low to be of concern. Problem solved!

Reply to
hondgm

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