Author Topic: Power supply part 6  (Read 13179 times)

0 Members and 1 Guest are viewing this topic.

Offline EEVblog

  • Administrator
  • *****
  • Posts: 29422
  • Country: au
  • Country: au
    • EEVblog
Re: Power supply part 6
« Reply #25 on: January 12, 2012, 08:24:58 am »
I feel that the primary hazard lies in end-user ignorance, and if the almost 4 hours (thus far) of edited video content enumerating just about every facet of the current prototype doesn't satisfy safety concerns, then Tim Tebow praising baby Jesus in a manger wouldn't be able to save that chap's life. To be sure, I have yet to encounter a design process comparable in documentation scope and made freely available outside academia.

And I ain't done yet, just kinda video documenting as I go...

I really can't possibly cater for everyone's requirements here in terms input and output voltage and other protection. At some point you have to just say that this design is meant for an intended purpose, and if you want to use it outside of that scope then you'll likely have to modify it, and that entails knowing what you are doing. That is the same for almost every design, there is no escaping it.

Dave.
 

Online Mechatrommer

  • Super Contributor
  • ***
  • Posts: 8993
  • Country: my
  • Country: my
  • reassessing directives...
Re: Power supply part 6
« Reply #26 on: January 14, 2012, 07:41:35 pm »
just watched psu#6. i'm concerned about the lt3080 burnt out. i had a situation of burnt npn power transistor that just simply short out the Vin to Vout, ie max voltage input=output. say we are powering 3.3V device and then poof the lt3080, if Vin something like 20V, then poof the device as well. so Dave, is there any protection/voltage sense on the output side to cut off supply in case the psu fails to regulate? or is it not necessary?
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline metalphreak

  • Frequent Contributor
  • **
  • Posts: 815
  • Country: au
  • Country: au
  • http://d.av.id.au
    • D.av.id.AU
Re: Power supply part 6
« Reply #27 on: January 14, 2012, 09:37:31 pm »
It's not hard to put an in-line fuse when connecting the external input voltage. It doesn't necessarily have to be a part of the PCB design.

Bourns make PTCs that go up to 3A holding current: http://www.bourns.com/data/global/pdfs/mfsm.pdf

They're not hard at all to solder onto pads as they're pretty big.

Heck, there's heaps of through hole ones up to 13A! http://www.bourns.com/ProductLine.aspx?name=polymer_ptc_resettable_fuses_multifuse
« Last Edit: January 14, 2012, 09:39:25 pm by metalphreak »
 

Offline EEVblog

  • Administrator
  • *****
  • Posts: 29422
  • Country: au
  • Country: au
    • EEVblog
Re: Power supply part 6
« Reply #28 on: January 16, 2012, 12:45:40 pm »
just watched psu#6. i'm concerned about the lt3080 burnt out. i had a situation of burnt npn power transistor that just simply short out the Vin to Vout, ie max voltage input=output. say we are powering 3.3V device and then poof the lt3080, if Vin something like 20V, then poof the device as well. so Dave, is there any protection/voltage sense on the output side to cut off supply in case the psu fails to regulate? or is it not necessary?

You have to ask yourself in what way can the circuit fail?
In essence, you can't really fully protect the output of a variable supply from circuit failure, because you don't know what the required output voltage the user has set, nor what maximum voltage the external circuit can tolerate.
You simply have to rely upon the voltage regulator working correctly and not failing in order to protect your circuit, there is little choice in that. That's the same with every supply, unless you designing some elaborate super redundant power supply for a 20 year life span on a space craft or something.
If the LT3080 suddenly shorts out for some reason then there isn't much you can do about it, the circuit you are powering won't get the regulated voltage any more. Not that the LT3080 is going to suddently "short out", as there usually has to be a deliberate external factor in order for that to happen.
If the supply was fixed then it would be easier of course, you could have a big zener on the output as additional fail-safe protection for example.

It is simply unrealistic to expect anything like this level of redundancy or protection from a regular variable bench power supply.

Dave.
 

Offline Rufus

  • Super Contributor
  • ***
  • Posts: 2094
Re: Power supply part 6
« Reply #29 on: January 16, 2012, 01:08:15 pm »
It is simply unrealistic to expect anything like this level of redundancy or protection from a regular variable bench power supply.

You are vastly more likely to blow up something with operator failure than circuit failure.

You could provide a secondary max voltage/current limit in firmware for free, giving the operator a second chance not to screw up. That or point users at a range of BIG zener diodes :)
 

Online Mechatrommer

  • Super Contributor
  • ***
  • Posts: 8993
  • Country: my
  • Country: my
  • reassessing directives...
Re: Power supply part 6
« Reply #30 on: January 16, 2012, 01:50:30 pm »
yes i hope the lt3080 is not as simple as a single power transistor, which it should be (internal protection or cut out power to output during failure? etc). the same case with other regulator such as lm317 in case other ppl want to do minor modification. i was just asking my concern. i believe redundant protection will increase the cost and complexity. and the "already" current limiting feature is good enough to avoid the regulator from blowing up.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 1904
  • Country: us
  • Country: us
Re: Power supply part 6
« Reply #31 on: January 16, 2012, 02:06:21 pm »
The LT3080 has built in over-current and thermal protection.  It shouldn't be damaged under anything like normal operating conditions as long as the input voltage stays in range.  Like most regulators, they are fairly bullet proof.  The only thing that really kills these guys is reverse bias.  If you short out the input and apply external voltage to the output (or have way oversized output capacitors) as dave did, you dump a lot of current through the substrate diode, which can destroy the chip.  If that is an expected failure mode for your input, you just add a reverse diode from the output to the input that will take the fault current and protect the regulator.

A lot of the HP/Agilent programmable supplies do have programmable over-voltage and over-current protection that disconnect the output if they threshold is exceeded.  This is nice for powering very sensitive devices, but is an expensive feature, and rarely used for things as mundane as micro controllers.
 

Online jahonen

  • Super Contributor
  • ***
  • Posts: 1045
  • Country: fi
  • Country: fi
Re: Power supply part 6
« Reply #32 on: January 16, 2012, 06:31:09 pm »
Many Agilent power supplies (E3630-series at least) feature an adjustable thyristor crowbar circuit at the output (called OVP), maybe that is what you are looking for? Of course, it requires that one sets the crowbar limit accordingly.

Having that is not always a good thing, we had one supply going into smoke at work because of it, when it was used for charging a battery and then somebody accidentally turned it off, the crowbar triggered and tried to short out the battery (without any success :P).

Regards,
Janne
 

Online Mechatrommer

  • Super Contributor
  • ***
  • Posts: 8993
  • Country: my
  • Country: my
  • reassessing directives...
Re: Power supply part 6
« Reply #33 on: January 16, 2012, 06:56:58 pm »
ok, we learnt from agilent mistake :D
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

alm

  • Guest
Re: Power supply part 6
« Reply #34 on: January 16, 2012, 07:07:12 pm »
Having that is not always a good thing, we had one supply going into smoke at work because of it, when it was used for charging a battery and then somebody accidentally turned it off, the crowbar triggered and tried to short out the battery (without any success :P).
That's obviously a dumb thing to do. Few power supplies are happy when you force current into them, especially when turned off. Which is you put a diode in series when using one for charging.

OVP can help preventing a shorted pass transistor from killing your circuit. Depending on the costs and lead time of your circuit, a dead power supply may be preferable to a dead DUT.
 

Online jahonen

  • Super Contributor
  • ***
  • Posts: 1045
  • Country: fi
  • Country: fi
Re: Power supply part 6
« Reply #35 on: January 16, 2012, 07:17:29 pm »
Having that is not always a good thing, we had one supply going into smoke at work because of it, when it was used for charging a battery and then somebody accidentally turned it off, the crowbar triggered and tried to short out the battery (without any success :P).
That's obviously a dumb thing to do. Few power supplies are happy when you force current into them, especially when turned off. Which is you put a diode in series when using one for charging.

OVP can help preventing a shorted pass transistor from killing your circuit. Depending on the costs and lead time of your circuit, a dead power supply may be preferable to a dead DUT.

That is of course true, but one must always assume that people don't always read manuals. So I think that things should be as robust as possible against most common errors. I think the OVP should not spuriously trip on power up/down situations.

That thing is indeed mentioned in the power supply manual, use a series diode when charging a battery.  Power supply was ok after replacing the thyristor and cleaning up the burned PCB. That wonderful "radio" smell.



Regards,
Janne
« Last Edit: January 16, 2012, 07:44:07 pm by jahonen »
 

Offline jerry507

  • Regular Contributor
  • *
  • Posts: 246
Re: Power supply part 6
« Reply #36 on: January 19, 2012, 09:32:37 am »
Dave,

I was wondering if you looked at any other high side current sensing solutions other than the max4080. While it is convenient and does reduce your parts count not having gain resistors, it is also surface mount. There are many different rail to rail opamps with similar or better input offset voltages out there, and you could improve upon the full scale accuracy by using the 0.1% resistors you're already using elsewhere.

All spec reasons aside, I'd think the main thing would be being able to get it in a dip package.

edit - http://search.digikey.com/us/en/products/LT1636CN8%23PBF/LT1636CN8%23PBF-ND/891916

Something like that. It seems to outmatch the 4080 on Vos.
« Last Edit: January 19, 2012, 10:25:50 am by jerry507 »
 

Offline EEVblog

  • Administrator
  • *****
  • Posts: 29422
  • Country: au
  • Country: au
    • EEVblog
Re: Power supply part 6
« Reply #37 on: January 19, 2012, 10:33:09 am »
Dave,

I was wondering if you looked at any other high side current sensing solutions other than the max4080. While it is convenient and does reduce your parts count not having gain resistors, it is also surface mount. There are many different rail to rail opamps with similar or better input offset voltages out there, and you could improve upon the full scale accuracy by using the 0.1% resistors you're already using elsewhere.

All spec reasons aside, I'd think the main thing would be being able to get it in a dip package.

edit - http://search.digikey.com/us/en/products/LT1636CN8%23PBF/LT1636CN8%23PBF-ND/891916

Something like that. It seems to outmatch the 4080 on Vos.

That could possibly be a better solution.
I recall briefly considering a precision opamp, but decided a proper high side current shunt was neater.
With hindsight, the extra resistors and a DIP8 package would have been a real squeeze on the given PCB.
There would still be the So8 package on the uCurrent side though.
Too late now, the board is sent to manufacture.
Not out of bounds that I could eventually find an all though hole solution for another revision.

Dave.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf