Interesting failure of buck converter - Page 1
Forum section: electronics / projects

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 24, 2019, 03:11:37 pm
content: I have a prototype board using a ST ST1S14PHR buck converter, converting 48V to 13.5V at 1.25A max output current.I'm seeing an odd failure mode - after it's been running hot at high loads for a few 10s of secs, instead of the ST1S14 going into thermal shutdown, the diode fails to a near-short.At lower loads ( ~1A) the thermal protection works OK, the chip cycling on and off.I've confirmed that it is the effect of heat ( from the chip - the diode produces minimal heat itself), as putting a soldering iron on its ground pin induces the same failure quite quickly.The diode is an STPS5L60U, rated 60V 5A. (Datasheet recommends STPS3L60, the 3A version)I know Shottkys get quite leaky at higher temps, so my current theory is the 60v rating isn't enough when it gets hot. I've ordered some 100v diodes to try as I can't find any to hand.

author: Fraser
author_rank: Super Contributor
timestamp: May 24, 2019, 03:41:49 pm
content: Failure of the diode appears to be a challenge to other designers as well.See here....https://electronics.stackexchange.com/questions/86791/cause-of-diode-failure-in-step-down-switching-mode-power-supplyMay be nothing to do with your failure mode but some interesting comments. High voltage ringing looked to be one possibility and with high currents and any inductance high frequency, high voltage ringing could potentially occur ? An RC snubber does look like a good idea if this is suspected.Another suggestion mirrors yours. That of thermal runaway due to external heat causing increased leakage.Hope you solve the mystery.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 24, 2019, 03:50:58 pm
content: The last comment on that referenced thread looks pretty convincing......To quote it here .......“It will be over-voltage spikes that are causing the Schottky shorts (many years experience in SMPSU). If you look at the waveform with a higher bandwidth scope you will seen them. 100 MHz not really good enough, suggest 500 minimum to see all. Use snubber as suggested or higher voltage rating.I realise this is very late, but may help others, who find the thread as I did.”End quote.Hope this helpsFraser

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2019, 04:13:21 pm
content: Shouldn't be much overshoot on turn-on though, at least not unless the bypassing is terrible?That diode is rated for avalanche, so it's very suspicious that it would fail due to overvoltage without also heating up. It's probably not rated for much avalanche, so maybe it fails pretty suddenly when subjected to a large enough surge?The other way diodes can fail suddenly, is large peak currents.And dV/dt, but I don't really know what that does for diodes (and MOSFETs). It's hard to even generate that much rate, though. (Easier with GaN nowadays.)Tim

author: thinkfat
author_rank: Unknown
timestamp: May 24, 2019, 04:56:54 pm
content: I would expect the diode to generate some heat. At the given conversion ratio, there should be a significant ripple current which the diode has to bear.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 07:13:01 pm
content: This is just an idea but at high currents, the parasitic PN junction in parallel with the schottky junction can become forward biased enough to store charge and create reverse recovery. A short is created during the reverse recovery which can lead to reliability problems.Maybe make a measurement of the high speed diode current waveform to see what is going on?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 25, 2019, 09:52:12 am
content: Mike,I suspect that this is just over voltage on the diode.There will be transients on the switch node. This might be very short, but enough to damage the diode.While you are waiting for the higher voltage diodes, try testing the circuit with a lower input voltage, say 36VThe higher current diode, 5A versus 3A, may have higher junction capacitance which could increase the transient on the switch node.The transients on the switch node will change with temperature.60V diode in a 48V (+/- _ %) application isn't enough margin.Regards,Jay_Diddy_B

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 25, 2019, 10:13:31 am
content: Schottky diodes have significant junction capacitance - much higher than the equivalently rated PN diode - and it's typically high-Q, so like the commenter reposted by Fraser, my vote is on ringing as the most likely cause of death here, too.That said, a high junction capacitance will also allow a big pulse of current to flow at the instant of reverse biasing which looks an awful lot like reverse recovery in a PN diode. This capacitvely coupled current shouldn't hurt the diode, though, since it isn't using charge carriers to travel across the barrier junction, but I am not a semiconductor device physicist so take that with the proverbial grain of salt.The idea floated by David Hess - the guard ring diode becoming forward biased at a high enough current - is another possible cause, but that one doesn't seem as likely to me unless the buck inductor is saturating and peak currents are way higher than the 5A diode rating.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 25, 2019, 10:50:03 am
content: Quote from: thinkfat on May 24, 2019, 04:56:54 pmThe diode is only there to bridge the dead time between top and bottom fet switching..OP: I assume you checked the diode out-circuit? If not, you may be measuring the bottom FET tossing it.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2019, 02:32:27 pm
content: I replaced the diode with a V3FM12-M3/H , which is rated at 120v, and it will now sit with its cathode lead in a blob of molten sodler and carry on working fine!

author: Fraser
author_rank: Super Contributor
timestamp: May 28, 2019, 04:14:10 pm
content: Excellent newsSo you think thermal runaway is the issue ?Fraser

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2019, 05:18:16 pm
content: Quote from: Fraser on May 28, 2019, 04:14:10 pmNot runaway - I'm pretty sure that it was the heat from the converter that was pushing the diode over the edge.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 28, 2019, 10:34:49 pm
content: Hi Mike,I had a look at the datasheet:That is not a lot of margin when operating from 48V supply.I also think it is telling that the do not present a data when operating at 48V on the datasheet. They show scope shots operating into a short with 43V input and lots of data at 12, 24 and 32V.They also show 50V input capacitors. I would want to see 20 or 30% margin.I know that they do say 'Operating input voltage from 5.5 V to 48 V'You might want to take a page out photonicinduction's playbook and turn one up till it pops.Regards,Jay_Diddy_B

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 29, 2019, 09:10:47 am
content: Unfortunately there aren't many small 48V converters like this at sensible prices - I'd seen that chip used in another big installation with several hundred nodes, so was fairly happy using it. Something I was concerned about, and did test for, was whether overvoltage transients caused by connecting & disconnecting the supply caused issues, but it seems fine.

author: thinkfat
author_rank: Unknown
timestamp: May 29, 2019, 01:23:45 pm
content: I think I'm going to try this on one of my LMR14030 based buck converters. I'm guessing it'd show a similar failure if I drive the input voltage high enough. That's going to be interesting to watch.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 29, 2019, 01:24:54 pm
content: I agree that that ain't much of a margin...MPS has some:Digi Link

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: May 30, 2019, 01:24:57 am
content: What did you mean by 'prototype board'? Is it a demo board for the regulator or some other prototype where it does supply something? When you compare the actual layout with their recommendation regarding heat removal, how far apart is it? I have made the experience that higher power SMD parts can't be used if not provided with the correct thermal vias etc. So if that heat is coming from the regulator, which you say is running correctly, it has to go somewhere else instead of towards that diode.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 30, 2019, 08:39:53 am
content: Quote from: Neomys Sapiens on May 30, 2019, 01:24:57 amIt's the pre-production protoype PCB, so as good a thermal layout as possible with the mechanical constraints I have.

author: ogden
author_rank: Super Contributor
timestamp: May 30, 2019, 09:43:46 am
content: Quote from: mikeselectricstuff on May 29, 2019, 09:10:47 amYou get what you pay forI agree that 50V is way too slim margin for 48V supply, especially when transients can be there. Better add some 1$ to BOM for better regulator and sleep wellIn your case I would usehttp://www.ti.com/product/LMR16030

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 30, 2019, 10:09:58 am
content: Quote from: ogden on May 30, 2019, 09:43:46 amEfficiency of the TI part looks way lower - about 82% vs. 91% for the ST, based on nearest graphs in the datasheet - ST has a sync. rectifier fet across the diode.For this application I'm very constrained for space to get rid of heat.

author: ogden
author_rank: Super Contributor
timestamp: May 30, 2019, 10:21:28 am
content: Quote from: mikeselectricstuff on May 30, 2019, 10:09:58 amSynchronous switcher then, definitely.http://www.ti.com/product/LM76002May blow your BOM, but efficiency is way better than ST part.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 30, 2019, 10:23:01 am
content: Well, honestly you ought to know better than to try to run a 50-55V rated part on 48V, and all just to shave less than a pound off the BOM... Given that the most common failure mode of a buck converter from overvoltage is a shorted switch, you are definitely playing with fire here, and perhaps not entirely figuratively, either.Personally, I'd consider the MPS MP9846 or the Microchip (nee Micrel) MIC28514 just from a quick sort of buck regulator parts on Mouser that can handle at least 60V input and 3A output current.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 30, 2019, 11:10:12 am
content: BOM cost isn't a huge deal, it's more that I'd already seen this part used in a reliable 48v application

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 30, 2019, 11:45:56 am
content: Quote from: mikeselectricstuff on May 30, 2019, 10:09:58 amMP4559 might still be worth a look.

author: Fraser
author_rank: Super Contributor
timestamp: May 30, 2019, 12:23:42 pm
content: This thread is really interesting as it discusses component ratings. I come from a background where components used in mission critical applications were heavily over engineered. Even when a component had a rating of X Volts, it was a pessimistic value and the components would often have better capability, it was just not guaranteed. We often tested components to destruction and were impressed with the true headroom on a component rather than that indicated by the data sheet. Whilst it is true that it would be unwise to deliberately run a component beyond its maximum ratings in an important role, it might not be such an issue to run it at or near that rating if the manufacturer has already calculated a decent margin of safety to cover issues like transients. If transients are considered a risk them maybe a TVS can be used in the supply to the chip ? In important applications, some decent supply filtering and transient suppression is wise anyway.If I were in Mike’s position I would sacrifice a few of the chips in a torture test to see at what voltage they fail and the failure mode. Such is useful information anyway as a non safe failure mode would either steer me away from a component or cause me to employ additional safety measures that might otherwise have not been required.When there is little choice of components for a task, it is sometimes worth a little experimentation to see just what can be used safely without breaking the bank or having to completely change a design. This is, after all, what OEM’s do when prototyping, often to test reliability, but also to see how cheaply a design can be built and still meet the needs of the customer. Those who get it wrong do cause issues with premature component failures though.Fraser

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 30, 2019, 12:37:17 pm
content: Quote from: mikeselectricstuff on May 30, 2019, 10:09:58 amHi,I am not convinced that the ST1S14 has a sync. rectifier FET. The block diagram for the part shows a FET in the sync rectifier position:But there is no mention of the characteristics of this FET on the datasheet.The eDesignSuite tool gives diode dissipation that is consistent with no sync FET.Rectifier dissipation = Vfwd x I x (1 - Vout/Vin)AssumeVfwd = 450mVI = 1.3AVout = 13.2Vin= 48Rectifier Dissipation = 424mWNo evidence of a sync rectifier.Regards,Jay_Diddy_B

author: ogden
author_rank: Super Contributor
timestamp: May 30, 2019, 12:38:17 pm
content: Quote from: Fraser on May 30, 2019, 12:23:42 pmOh, I know this "but we tested" very well. Problem with such "in-house" tests - they often are incomplete. When you test all voltage *and* temperature range extremes and their combinations, it appears that margins are not that big and they are left for component manufacturing tolerances. If you do not test your every finished product in environmental chamber, in whole spec range, it's good idea is to *not* overrate components.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2019, 03:54:14 pm
content: Quote from: ogden on May 30, 2019, 12:38:17 pmOr your test run used golden parts. Or the manufacturer changes the part in some way. Or purchasing changes the part to one which is cheaper but has the same specifications.