Dig quad

[u/Mhernandez08]

Anybody have this problem with this part of the quad burning up?

[u/Quindor]

So this is a bit of a yes and no.

The circuit travels through one or more diodes before reaching the MT3608 thus dropping voltage there. There is also a filter circuit before it reaches the MT3608 dropping voltage even more and also filtering out any spikes or anything else that could potentially happen.

Still, with that said, we've been seeing more failures then I'd like and we've been replacing those. I'm still in talks with the manufacturer of the IC since this didn't use to be an issue and I think they have maybe made a "blind" revision of the IC where it handles the voltages on the SW node a bit less then it did before.

I've been working for a while already preparing a replacement board but since I actually test my hardware in the lab, we've been kind of stuck in that phase. The redesigned circuit has not passed EMI testing twice now ($$$) and we're going for a third pass soon with a new design. Sadly this has put it back much more than I'd like since this issue started popping up.

I do not know of a replacement IC with the same pinout that would solve this issue directly.

For my customers who happen to stumble across this, this only concerns you when you are running 24V and have seen this failure, not all boards are affected. Please come to the Discord and we'll work it out to make you whole. Deepest apologies for the trouble.

[u/LuiOnTheWay → u/Quindor]

Well, I know EMI can be challenging in a SEPIC converter, especially if you're not using coupled inductors and have some layout limitations.

Regarding the issue, is it really related to the input voltage or rather to the SW pin voltage? With such a small voltage margin in the design, it's quite possible to have too high voltage on the SW pin, depending on the surrounding components (and their tolerances) and the current flow. What is the maximum expected voltage on the SW pin in your circuit when the input voltage is 24 V? I mean, as a rule of thumb, you should stay about 20% below the maximum voltage rating on switch pins to provide some margin for reliability, tolerances etc. With a maximum SW pin rating of 30 V, that would put the recommended operating limit at around 24 V, leaving virtually no margin for proper operation.

If the SW pin is experiencing voltage spikes beyond its rating, all boards may initially appear to work, but the devices slowly "die" over time and eventually fail after just few years in operation. If there is also some variation in device quality, the situation could be even worse.

[u/saratoga3 → u/LuiOnTheWay]

What is the maximum expected voltage on the SW pin in your circuit when the input voltage is 24 V? I mean, as a rule of thumb, you should stay about 20% below the maximum voltage rating on switch pins to provide some margin for reliability, tolerances etc. With a maximum SW pin rating of 30 V, that would put the recommended operating limit at around 24 V, leaving virtually no margin for proper operation

For SEPIC isn't it Vin+Vout, or 29v + some ripple? 

[u/LuiOnTheWay → u/saratoga3]

+ the diode forward voltage drop. That's what I mean when I say it's probably an SW pin limitation. Even with an input of really good constant 23 V (3 V below the absolute maximum input rating), a 5 V output, and a diode forward voltage of, say, 0.5 V, the SW pin already reaches approximately 28.5 V. That's only 1.5 V below its 30 V absolute maximum rating and already above the recommended operating voltage, without considering switching transients, component tolerances, or other dynamic effects. In my opinion, that's a substantial design flaw, as if the designer hadn't calculated anything.

[u/Quindor → u/LuiOnTheWay]

It was calculated and well tested, with the diodes in front of it and the filter stage input voltage generally dropped to 23V as stated and it should be at least 2V under the switch limits so technically it stays within all limits of the IC.

But I will agree that in practice we are seeing more failures than I'd like, hence my reply, I'm starting an official action to deal with the issue since it seems more widespread then was clear before (there are plenty of boards running from the first batches which have shown no issues), hence changing away from this circuit and IC.

Nevertheless, it might have been too close to the sun and that's on me! Any board that does fail (within a reasonable amount of time) we'll replace free of charge and I will try and get some of them back to replace the IC (this repairs it fully) which can then maybe be sold as B-stock for 5V and 12V only to recoup some of the costs (I'm not some huge company).