Buck switcher fails when potted

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.

Reply to
hondgm
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I wonder about the "very thermally conductive" part. You're supplying ~1W, and at the high input voltages the spec sheet says the part will be dissipating nearly that much. Maybe your inductor is getting warm too so the "ambient" temperature isn't as cool as you think. Might those power MOSFETs contribute to this as well? Can you measure the temperature, perhaps in an experimental unit?

Reply to
Frank Miles

On a sunny day (Fri, 7 Mar 2014 09:02:58 -0800 (PST)) it happened snipped-for-privacy@yahoo.com wrote in :

Any pressure due to potting solidifying on package?

Reply to
Jan Panteltje

Potting can be evil. Compressive forces can be huge, crushing parts.

Try a softer epoxy, or apply something softer over the parts before you pot.

Potting is a messy process, too.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Some answers and clarifications to everyone's replies:

-the power MOSFETs, as I mentioned in the original post, are inactive when this occurs. There is nothing else on the board generating heat worth ment ioning.

-the inductor is nearby, but we have potted temperature sensors on the IC, both on top and soldered to a pin, and it wasn't near overtemping. Additio nally, the IC is thermally protected according to the datasheet FWIW.

-I'm sure there's pressure on the package due to potting, and we discussed that internally. But how much does that affect the die? An SO package is fairly robust. But potting does flow around the part, even underneath.

-We considered putting something over the IC before potting, but then the t hermal conductive properties of the potting will be less.

-The epoxy we use is considered soft, but the kicker is, this company has b een potting electronics for decades and never run into an issue like this. The specific epoxy currently in use has been used for several years at lea st. However the product in question is a new design I should mention.

I am attempting some more testing and ideas, but in the meantime, I'm open to any other suggestions.

Reply to
hondgm

Potting can be evil. Compressive forces can be huge, crushing parts.

Try a softer epoxy, or apply something softer over the parts before you pot.

Potting is a messy process, too.

Potting is such sweet sorrow!

Just could not resist. Harry

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John Larkin                  Highland Technology Inc 
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Reply to
Harry D

Parking is such street sorrow.

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Reply to
John Larkin

Any chance that the potting is wrecking the inductor? If it shorted it, or open up a gap and reduced the inductance, that might fry the switcher. Many inductors really don't like being potted.

Or maybe there's something about the switcher lead frame that doesn't like mechanical stress.

We use that same switcher, but not potted. It seems fine.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Do you have complete control over the sourcing of the parts in both potted and unpotted units?

Any chance the potted parts have a substitute or counterfeit part?

--sp

Reply to
Spehro Pefhany

Yes, the product is being designed and built in-house. The test units are potted by yours truly.

Reply to
hondgm

Hmmmm, the comment about the potted inductor is interesting. I'll have to prevent the epoxy from encasing the inductor on some test units and give it a try.

Reply to
hondgm

Could the potting cause a change in the circuit operation due to the different dielectric properties compared to air? Can you compare some waveforms before and after potting? tm

Reply to
Tom Miller

Do you have an impedance bridge? If you have opened up a failed unit, you could just check that the inductance is still the right value.

Jon

Reply to
Jon Elson

This switcher is slow, so probably not.

That's another problem with potting; scope probes quit working.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I have considered using RTV which we do use on other products, but I worry about the lack of heat conduction. Still an option.

I haven't checked the inductor's value, however if I dig out the epoxy to replace just the chip, the power supply works fine once again.

I could certainly put wires on certain places in the circuit for scope probe attachment before potting. With a 52KHz switching frequency it should alter it much.

Reply to
hondgm

Okay, I only ask becase that particular part number seems suspicously cheap and available in China.

Then I have to echo the others about the evils of potting- I've had it cause issues. A non-acid cured RTV silicone glopped over the chip before potting may help. Do NOT NOT use the kind of consumer silicone that has acetic acid in it or you may have even worse problems.

Reply to
Spehro Pefhany

did you check your date code to see if a bad run?

Not potted all came from good lot, potted came from bad lot.

Reply to
RobertMacy

Not likely. All boards came from the same run, but I potted some for final testing since the end product will be potted.

Reply to
hondgm

Easy. Tack some leads into the circuit and bring the signals out of the potting.

A higher er will increase capacitances everywhere. maybe enough to slow down the switching speeds and get things out of the SOA.

Reply to
Tom Miller

Bwahahahaha! ROTFLMAO... Good One, John! ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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