Author Topic: Protecting against Voltage Regulator failure?  (Read 4959 times)

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Offline kizmit99Topic starter

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Protecting against Voltage Regulator failure?
« on: March 04, 2019, 10:33:05 pm »
Hi All,
I was tinkering today on an ongoing project that uses one of these Cyclone-II dev boards:


(They are cheap and widely available, so I have no idea who made it, or frankly even who I got it from at this point.)

I was testing a new PCB I had made which uses one of these as a 'daughter board'.  The board had been running for about a day without any issues, then while trying to debug a portion of the circuit which really doesn't involve the FPGA much at all I noticed everything had crashed.  After doing a post-mortem it appears that the 3.3V regulator on the dev board has failed with a short from Vin to Vout.
The regulator is an AMS1085CT-3.3 which is basically a 3 pin 3.3v regulator - Vin max should be 15V, max current 3A.  I was powering it with a 9VDC (1.5A) wall wart (basically something sold as an Arduino/RasPi wart).  I suspect the failure fed 9V to the FPGA (and the rest of my board) and fried all those 3.3v parts.
As to the cause - it is certainly possible that I shorted 3.3V to Gnd with a scope probe on one of the chips.  I was debugging an audio out amp, and these two pins are very close to one another.  I did not notice a spark, pop or magic smoke so I can't say for certain that I shorted them, but certainly a possibility.  However, since the regulator is rated to supply 3A and the 9V supply was rated at only 1.5A I'm a little suspect that a brief short should have killed the regulator - but again, always possible I suppose.  The only other idea I have is that the regulator just failed on it's own, so I'm willing to chalk the failure up to me shorting the output...

What I'm not happy with is the failure mode - shorting Vin to Vout seems like the worst possible failure mode.  And, from what I can tell, it does seem to have taken out all of the 3.3v chips on both boards.  So, given this kind of failure, is there a reasonable way to protect against your voltage regulator crapping out and over-volting the entire board?

Also, I'm curious if anyone else has seen something similar - cheap regulators failing in Vin shorted to Vout mode?

I've used a different copy of this board on the previous iteration of the main board for many months without any kind of similar mishap.  And I'm probably just as likely to have shorted out that board given I was using it for so long.  I had been powering that one from a 5VDC (2A) wart instead of the 9V one though.  My new design has another 5V regulator that needs to be powered from this Vin, that's why the change to 9V on this one.

Anyway, I'm open to any thoughts or suggestions on how to protect against this failure mode...  I don't relish the thought of toasting everything with a small slip of a scope probe...

 

Online Ian.M

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Re: Protecting against Voltage Regulator failure?
« Reply #1 on: March 04, 2019, 11:33:13 pm »
Ringing on the input (input capacitance resonating with the cable inductance) can transiently double the input voltage.  This normally only happens if you connect the PSU to the board with the power already on or if the PSU has a switch in series with its DC output, but briefly shorting the regulator output could also cause it.   

You didn't have any problems wth 5V in because the ringing never exceeded 10V

Therefore any time you use a LDO with an input voltage greater than half of its abs. max. rating, and there is a significant length of wiring between it and the PSU, you need to make sure that any ringing is damped or clamped at well under the regulator's abs. max input rating.

Typically one adds an aluminum electrolytic capacitor of ten times the total value of any ceramic input capacitors used so its ESR damps the ringing.   A beefy 12V Zener across the input to clamp the ringing would also work.

To protect against the consequences of regulator failure due to other causes you need a crowbar circuit that will short the output to ground if the rail voltage becomes excessive.    It must latch on so it holds the short till the supply fuse blows or over current protection trips, or if the supply is current limited, hold the short until power is removed.  This is easy for rails 5V and upwards - see the classic TL431 + TRIAC crowbar circuit in the full TL431 datasheet - but with only a 3.3V rail, a TL431 wont have enough headroom to trigger a TRIAC reliably.   MOSFET based crowbar circuits are possible, but you need a really beefy MOSFET to guarantee it wont fail before the fuse, and plenty of gate drive from a maintained supply so it continues to hold the MOSFET hard on.
« Last Edit: March 05, 2019, 12:42:42 am by Ian.M »
 
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Offline georges80

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Re: Protecting against Voltage Regulator failure?
« Reply #2 on: March 04, 2019, 11:56:16 pm »
Possibly overheated/died.

9V in and 3.3V out, so 9-3.3 = 5.7V (could be worse if the wallwart is outputting higher voltage) that has to be dropped (linear regulator). No idea what the 3.3V side is drawing, but say it was 300mA. Then you have 5.7 x 0.3 = 1.7W dissipated in the regulator...

Your 5V operating one is only having to drop 5-3.3 = 1.7V...

cheers,
george.
 
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Offline kizmit99Topic starter

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Re: Protecting against Voltage Regulator failure?
« Reply #3 on: March 05, 2019, 12:32:10 am »
Therefore any time you use a LDO with an input voltage greater than half of its abs. max. rating, and there is a significant length of wiring between it and the PSU, you need to make sure that any ringing is damped or clamped at well under the regulator's abs. max input rating.

Very interesting - that's the first I've heard of this effect.  (To be fair, my background was in 1990 era digital, and we had analog guys for the power supplies  :P)
The variant of this regulator that Digikey stocks shows a max Vin of 30V, so perhaps replacing these parts on my remaining dev boards might be prudent...  (I'm quite certain the folks making these boards are using the cheapest, sketchiest parts available)

Typically one adds an aluminum electrolytic capacitor of ten times the total value of any ceramic input capacitors used so its ESR damps the ringing.

That very well may be why I've not experienced it on other projects in the past.  I usually drop a bulky cap on both the inputs and outputs of regulators, just for good measure.  I usually use tantalum caps (because I have a supply) - you specifically mentioned aluminum, do you think there would there be a difference in this application?

Possibly overheated/died.

Certainly another possibility.  I'm not sure what the board was pulling, but I suspect it is in the sub 100mA range.  But as you point out that would still be a half-Watt the little package would need to dissipate (with no heat sink of course), compared to just around 100mW at 5V...  I didn't notice the regulator getting hot prior to the meltdown, but I may just not have noticed.  Although, now that I look, this part is supposed to have an internal thermal shutdown, so I would have thought that a slow-burn would probably just shut it off, not melt it down.  In fact they don't even spec the max power-dissipation, just temperature related numbers.  My gut now says this isn't the culprit.

I should also probably also point out that the specs on the board itself do specify 5VDC input.  But looking at the schematic, verified with inspection of the board itself, the input power only goes to this Vreg (and its input cap) nowhere else.  So I was thinking, since the part was spec'd for 15V input I should be ok with the 9V input (really only need about 7V in for the other regulator, but 9V is what I had lying around).  I did figure the additional power waste would be less than it probably is, and I definitely didn't consider input voltage ringing...
 

Offline Doctorandus_P

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Re: Protecting against Voltage Regulator failure?
« Reply #4 on: March 05, 2019, 12:33:08 am »
It is a part of the price you pay for cheaping out.

Putting too much voltage on circuits often does not destroy them immediately, but causes too much current flow, which heat up the components too much and they release the magic smoke.

This is why Lab power supplies with adjustable current limit are invented.

If you set it to a voltage and current slightly above what your board needs then if it fails, on over current, the output voltage collapses and your circuit has a chance to survive. 3V3 circuits tend to be more sensitive to abuse than the older higher voltage stuff.

Alternatively you can add a "Crowbar" circuit.
This is most often a TL431, Thyristor, (poly)Fuse and some passives.
The Idea is that if the TL431 detets an over voltage, it fires the Thyristor, and the thyristor shorts Vcc to GND, which protects the circuit.
Deliberaty shorting the power will of course cause too much current to flow, and the fuse wil blow. (Or use a decent Lab Power supply).
 
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Offline kizmit99Topic starter

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Re: Protecting against Voltage Regulator failure?
« Reply #5 on: March 05, 2019, 12:43:59 am »
It is a part of the price you pay for cheaping out.
Hey! I resemble that remark!   ;)

This is why Lab power supplies with adjustable current limit are invented.

If you set it to a voltage and current slightly above what your board needs then if it fails, on over current, the output voltage collapses and your circuit has a chance to survive. 3V3 circuits tend to be more sensitive to abuse than the older higher voltage stuff.
Yeah, just happens that my bench supplies are on a different work area (and too bulky to use where I was doing this testing).  If it's of any consequence, that's what I used *after* it melted down to figure out what went bad...  ::)

I guess most of what bugs me here is the regulator failing with Vin tied to Vout.  I'm not too worried that it blew up ('specially if I shorted it), I just think it's a pretty damn inconsiderate way to fail...  :-//

 

Offline helius

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Re: Protecting against Voltage Regulator failure?
« Reply #6 on: March 05, 2019, 01:21:04 am »
Very interesting - that's the first I've heard of this effect....I usually use tantalum caps (because I have a supply) - you specifically mentioned aluminum, do you think there would there be a difference in this application?

See https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf
Tantalum or aluminum electrolytics can both be used as snubbers, as long as they have sufficient ESR to damp this parasitic oscillation.
In your case, if there is no ceramic bypass capacitor, I don't think that this is your problem.

I guess most of what bugs me here is the regulator failing with Vin tied to Vout.  I'm not too worried that it blew up ('specially if I shorted it), I just think it's a pretty damn inconsiderate way to fail...
Linear regulators have a series pass element (one or more transistors) between Vin and Vout. Short circuit is a common failure mode.
« Last Edit: March 05, 2019, 01:32:48 am by helius »
 

Offline David Hess

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Re: Protecting against Voltage Regulator failure?
« Reply #7 on: March 05, 2019, 01:39:30 am »
I would use a fast power precision shunt regulator combined with a thyristor crowbar circuit.  Low voltage supplies do not have much margin for error.
 

Offline T3sl4co1l

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Re: Protecting against Voltage Regulator failure?
« Reply #8 on: March 05, 2019, 07:46:53 am »
Very possible that input overvoltage or short circuit caused the failure.

The AMS1085 does claim reasonably accurate current limiting (3 to 4A) and thermal protection, but neither should be relied upon, at least without careful consideration.  And, for the combination of 9V wall wart (which might be ~12V open circuit!) with this particular circuit, who knows if it was considered?

You might not see or hear a spark, because a spark requires considerable power to be perceptible.  The satisfying little "pop" you get from discharging a modest size electrolytic cap, might entail a peak current of a hundred amperes, for tens of microseconds!  The board probably only has ceramic caps on the low voltage side, which don't store much energy, and so don't spark much when shorted (though they can discharge even faster still).  3A from the reg may still be enough to see some sparking though.

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Online magic

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Re: Protecting against Voltage Regulator failure?
« Reply #9 on: March 05, 2019, 09:26:13 am »
This is easy for rails 5V and upwards - see the classic TL431 + TRIAC crowbar circuit in the full TL431 datasheet - but with only a 3.3V rail, a TL431 wont have enough headroom to trigger a TRIAC reliably.
There are similar chips with 1.25 reference which may replace the 431 in low voltage applications. Something like TLV431, TLC431, TL431LV, I don't remember. Multiple types exist from many vendors. Some tolerate only 6V, others 12V, maybe even 18V.
 

Offline 2N3055

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Re: Protecting against Voltage Regulator failure?
« Reply #10 on: March 05, 2019, 10:04:14 am »
Board was speced at 5V for dispation not maximum input voltage. If you put in 9 V unreg you prbably fed it 12V. So 5-3.3V=1.7 V. 12-3.3=8.7V. . That is 5 times more dissipation than speced. I wonder why it did not smoke within 60 seconds.
This board should be powered by 5V stabilized PSU. Like the USB ones or one of bricks. Even high quality ones by Meanwel are cheap. And you can use it for all other uses too.
 

Online Ian.M

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Re: Protecting against Voltage Regulator failure?
« Reply #11 on: March 05, 2019, 10:08:33 am »
This is easy for rails 5V and upwards - see the classic TL431 + TRIAC crowbar circuit in the full TL431 datasheet - but with only a 3.3V rail, a TL431 wont have enough headroom to trigger a TRIAC reliably.
There are similar chips with 1.25 reference which may replace the 431 in low voltage applications. Something like TLV431, TLC431, TL431LV, I don't remember. Multiple types exist from many vendors. Some tolerate only 6V, others 12V, maybe even 18V.
Yes, but a TLV431 is only rated for 10mA Ik max vs 100mA for a TL431, so its nowhere near as good at firing large TRIACs.  One can work around that by adding a PNP medium power transistor as a saturated switch between the TLV431 and the TRIAC to increase the available gate drive, which also lets you use ordinary SCRs as the gate drive is then positive going.
 

Online magic

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Re: Protecting against Voltage Regulator failure?
« Reply #12 on: March 05, 2019, 11:14:12 am »
TLV431 is only rated for 10mA Ik max vs 100mA for a TL431, so its nowhere near as good at firing large TRIACs. One can
... spend a minute in TME parametric search to come up with vendors better than TI, like SC431L and AZ431L :P
 

Offline perieanuo

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Re: Protecting against Voltage Regulator failure?
« Reply #13 on: March 05, 2019, 12:15:23 pm »
Hi, a good way to protect is an voltage supervisor who cuts power (5V from usbport in your case) when limits are exceeded.quick and efficient,there are some implementations like this on some projects, but is overkill for your board.
You can protect cheapies like this one with archaic methods like puting diodes all over the place to cut some induction,reverse polarity covered sometimes partially in the chips but with limited power absorbing diodes
Regards,pierre


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Offline Zero999

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Re: Protecting against Voltage Regulator failure?
« Reply #14 on: March 05, 2019, 01:25:59 pm »
This is easy for rails 5V and upwards - see the classic TL431 + TRIAC crowbar circuit in the full TL431 datasheet - but with only a 3.3V rail, a TL431 wont have enough headroom to trigger a TRIAC reliably.
There are similar chips with 1.25 reference which may replace the 431 in low voltage applications. Something like TLV431, TLC431, TL431LV, I don't remember. Multiple types exist from many vendors. Some tolerate only 6V, others 12V, maybe even 18V.
Yes, but a TLV431 is only rated for 10mA Ik max vs 100mA for a TL431, so its nowhere near as good at firing large TRIACs.  One can work around that by adding a PNP medium power transistor as a saturated switch between the TLV431 and the TRIAC to increase the available gate drive, which also lets you use ordinary SCRs as the gate drive is then positive going.
A couple of points:

Only a short pulse is required, so there's no harm in exceeding the current rating of the TLV431 by a fair margin. A surge of 100mA or so for the short length of time it takes to trigger the TRIAC will not damage it.

It depends on the TRIAC. There are sensitive gate TRIACs which can adequately trigger with under 10mA.
https://www.onsemi.com/pub/Collateral/MAC15S-D.PDF
 

Offline OM222O

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Re: Protecting against Voltage Regulator failure?
« Reply #15 on: March 05, 2019, 03:01:13 pm »
It is a part of the price you pay for cheaping out.
Hey! I resemble that remark!   ;)

This is why Lab power supplies with adjustable current limit are invented.

If you set it to a voltage and current slightly above what your board needs then if it fails, on over current, the output voltage collapses and your circuit has a chance to survive. 3V3 circuits tend to be more sensitive to abuse than the older higher voltage stuff.
Yeah, just happens that my bench supplies are on a different work area (and too bulky to use where I was doing this testing).  If it's of any consequence, that's what I used *after* it melted down to figure out what went bad...  ::)

I guess most of what bugs me here is the regulator failing with Vin tied to Vout.  I'm not too worried that it blew up ('specially if I shorted it), I just think it's a pretty damn inconsiderate way to fail...  :-//

to be fair the "inconsiderate" way to fail is how any LDO will fail ... they have an output element (be it a BJT,darlington pair or a fet) which is controlled by an op amp and is connected to input on the other end. If something happens and the device fails, it usually occurs on the output element. BJTs and FETs both tend to fail short ... so there's you're answer :D you can engineer the output element to fail open but that would drastically increase the cost of the device, so they're not really used in LDOs.
 

Offline kizmit99Topic starter

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Re: Protecting against Voltage Regulator failure?
« Reply #16 on: March 05, 2019, 10:06:19 pm »
See https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf
Tantalum or aluminum electrolytics can both be used as snubbers, as long as they have sufficient ESR to damp this parasitic oscillation.
In your case, if there is no ceramic bypass capacitor, I don't think that this is your problem.

Very useful app-note  :-+ thanks very much for that link!

The dev board does in fact have a 22uF ceramic bypass cap on the input (although it also has a 47uF aluminum electrolytic, fed through a diode though so that might effectively isolate it), so I would say it's looking like this effect could be responsible for the failure of the ASM1085.  And I guess everyone (except me  :o) sees Vin to Vout shorts as an expected failure mode.  I can accept that, but I have to say I've used a fair number of 7805 type regulators over the years and haven't experienced this type of failure before... Live an learn though...

Just for completeness sake I took a look at the unloaded output of both the 5V and 9V "wallwarts".  Both are providing nicely regulated DC power (5.2V and 9.2V respectively) with less than 10mV ripple.  The 5V one actually is labeled as a "switching power supply", so perhaps referring to these as "wallwarts" was misleading (although they definitely fit my definition).  I don't think the suggestions that the actual Vin could have been significantly higher than the spec'd values apply.  [Just to be clear, I'm only referring to comments like "the 9V power was probably something like 12V", not the ringing effect.]

Thanks to all for the suggestions about adding a crowbar protection circuit.  I've never bothered with something like this in the past, but I'm finding that with surface mounted parts the debug and repair process is significantly more of a pain the arse than it was back in the day (sockets!).  I will look into what that would take to add to my design.

Since there seems to be a decent amount of helpful suggestions being offered - I figured I'd actually draw up a schema for the complete power plan for these boards (attached below because the imgur pic probably isn't readable). 


The design is actually two separate boards (Main and Mega) that connect together, plus the Cyclone dev board rides on the Main board as a daughter board.  (So three boards in total, two I control the design of, the dev board I use as is).  The Main board (plus the cyclone board) can be used standalone without the Mega board.  The Main board runs on 3.3V and basically just provides I/O for the FPGA.  The only exception being it needs to provide 5V to a PS/2 keyboard.  The Mega board is a re-spun Arduino Mega2560 with some peripherals - so it runs mostly on 5V, with some 3.3V level translators.  The power circuit on the Mega board is mostly lifted directly from the Arduino reference design (to support running on either external power or from a USB cable).

The idea was that I could run just the Main board by applying power to the Cyclone board.  In version 1 this was just 5VDC and I used that to power the keyboard.  In the next version I added the Mega board and wanted it to run on "normal" Arduino style power adapters.  I also was concerned that I would accidentally plug a >5V plug into the dev board, so I added the 5V regulator to the Main board.  The intent was if the Mega board was present everything would be powered from the Mega supply, but I didn't want anything burning up if both where accidentally present.

Having drawn this up on one sheet for this post, I now see several glaring problems with the overall design:
* With VDC2 being provided by Mega (and no VDC1) there is no 3.3V being generated for either the cyclone or the Main board - Doh!
* If running on USB power there is (potentially) neither VDC1 or VDC2, so again no power to Main or dev boards.

I think both of those can be worked-around by dropping D4 (diode from VDC2 to VDC_IN on Main board, and adding a diode from 5V (supplied by Mega, on Main board) to VDC1 on Main - which would feed 5V to the regulator on the dev board.  Of course plugging in power on J1 would be a no-no in that mode...

Oh well, I'm just babbling now -- thanks for the suggestions  :-+
« Last Edit: March 06, 2019, 12:06:29 am by kizmit99 »
 
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