Buck switcher fails when potted

hondgm · 2014-03-07T17:02:58+00:00

We're using a buck switcher, LM2574HV-5.0 in SOIC package on a custom PCB. The load is less than 200mA and the input voltage can vary, but most testi ng as been at higher input voltages, around 55V. The IC has a max. input v oltage of 60V and absolute max of 63V so this should be fine. We were careful about component selection and when possible, went with the recommended parts in the datasheet. We started seeing failures where the i nternal switch shorts closed, which absolutely fries the load. The load is a microcontroller circuit, with the part causing the harshest load conditi on being a small relay. In an effort to track this down, we connected several boards to a power sup ply and a device that cycles power on and off. The only units that have ev er failed are potted. The un-potted units never fail when the input voltag e is kept within spec. This is bizarre to me. The potting epoxy we use is very thermally conducti ve, a very good dielectric, and has been used for years in other products. The best I can come up with is the chip overheats, as it does get quite wa rm with such a high input voltage. But, surely open air is worse than the potting when considering cooling effects. Even so, the chip supposedly has thermal and overload protection. I have dug out the potting and removed the faulty chip in a few units. The output pin isn't hard-shorted to Vin, but a diode check shows about 0.3V b oth ways which is much different than what a new chip measures. Any ideas? We also have power MOSFETs on that same board and when potted, are able to dissipate a good deal of power without failure. However, the F ETs are not carrying any current when the switcher fails.

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hondgm 12 years ago

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.

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Phil Hobbs 12 years ago

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

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John Larkin 12 years ago

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

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hondgm 12 years ago

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.

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John Larkin 12 years ago

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

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Joe Gwinn 12 years ago

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

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hondgm 12 years ago

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.

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hondgm 12 years ago

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.

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Jon Elson 12 years ago

Well, any further news on this problem?

Jon

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hondgm 12 years ago

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!

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Buck switcher fails when potted

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