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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Wed, 09 Oct 2024 16:37:22 -0400
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On Wed, 09 Oct 2024 12:51:32 -0700, john larkin <jl@glen--canyon.com>
wrote:

>On Wed, 09 Oct 2024 13:40:49 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Tue, 08 Oct 2024 17:27:53 -0700, john larkin <jl@glen--canyon.com>
>>wrote:
>>
>>>On Tue, 01 Oct 2024 16:03:40 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>wrote:
>>>
>>>>On Tue, 01 Oct 2024 09:59:27 -0700, john larkin <jl@glen--canyon.com>
>>>>wrote:
>>>>
>>>>>On Tue, 01 Oct 2024 11:24:34 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>wrote:
>>>>>
>>>>>>On Mon, 30 Sep 2024 18:49:14 -0700, john larkin <JL@gct.com> wrote:
>>>>>>
>>>>>>>On Mon, 30 Sep 2024 11:49:54 -0700, Joerg <news@analogconsultants.com>
>>>>>>>wrote:
>>>>>>>
>>>>>>>>On 9/30/24 11:24 AM, john larkin wrote:
>>>>>>>>> On Mon, 30 Sep 2024 08:39:27 -0400, legg <legg@nospam.magma.ca> wrote:
>>>>>>>>> 
>>>>>>>>>> On Sun, 29 Sep 2024 08:23:01 -0700, john larkin <JL@gct.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> On Sat, 28 Sep 2024 22:28:07 -0700, Joerg <news@analogconsultants.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On 9/27/24 8:07 AM, john larkin wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Given a benchtop power supply, you can turn the voltage up and then
>>>>>>>>>>>>> down, and it goes down. Most have a substantial amount of output
>>>>>>>>>>>>> capacitance, and can be driving an external cap too. So something
>>>>>>>>>>>>> pulls the output down.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Often the only internal load is the resistive divider for the regulator
>>>>>>>>>>>> loop feedback.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> I guess that there are no standards for this, but I've never seen a
>>>>>>>>>>>>> supply that just hangs high when it's cranked down.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I have some. They drop very slowly when there isn't much load on the output.
>>>>>>>>>>>
>>>>>>>>>>> Customers might whine if they ask for 10 volts and see 30. Amd that
>>>>>>>>>>> may be mostly held up by their capacitive load.
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> I'm designing some programmable multi-channel power suplies and that
>>>>>>>>>>>>> is one of many tangled issues in the project.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> A synchronous buck architecture should work quite well if you need to
>>>>>>>>>>>> slew fast. I've used that on a driver that had to modulate a hard
>>>>>>>>>>>> capacitive load at several kHz and above 100V.
>>>>>>>>>>>
>>>>>>>>>>> I'm doing some multichannel non-isolated supplies that will be sync
>>>>>>>>>>> buck, using multiple TI DRV8962 chips.
>>>>>>>>>>>
>>>>>>>>>>> One problem is that a sync buck can become a boost in the wrong
>>>>>>>>>>> direction, and start charging my +48 supply. If it hits, say, 55
>>>>>>>>>>> volts, I'll disable the switcher chips, and the outputs can hang. I
>>>>>>>>>>> need to discharge the outputs. I'm thinking about 20 mA of depletion
>>>>>>>>>>> fet per channel.
>>>>>>>>>>
>>>>>>>>>> You might consider overvoltage protection or a (switched ?)
>>>>>>>>>> internal minimum load.There's usuaally some point in the
>>>>>>>>>> control loop that's a good indicator of a pull-down requirement.
>>>>>>>>>> A single ovp or autoload on the input looks likely to serve
>>>>>>>>>> all of your many sync-bucks.
>>>>>>>>>>
>>>>>>>>>> RL
>>>>>>>>> 
>>>>>>>>> An MOV on the bulk supply could limit the reverse-pump excursion until
>>>>>>>>> the software can notice and shut things down.
>>>>>>>>> 
>>>>>>>>> MOVs can gobble a lot of joules, but their clipping is very soggy.
>>>>>>>>> 
>>>>>>>>
>>>>>>>>MOVs are usually cumulative. They can take a certain amount of 
>>>>>>>>dissipation over their lifetime and then *PHUT* ... POOOF. Like a bank 
>>>>>>>>account that runs dry.
>>>>>>>
>>>>>>>What kills MOVs? Integrated joules? Time-temperature?
>>>>>>>
>>>>>>>I don't expect a lot of joules per event. Just enough energy to keep
>>>>>>>my supply voltage down until a slowish ADC and the software can shut
>>>>>>>the buck switchers down. 15 milliseconds max, maybe.
>>>>>>
>>>>>>I think it's integrated joules per cubic centimeter of the MOV
>>>>>>material.  This is discussed in the literature on MOVs for protecting
>>>>>>line-powered equipment from pulse overvoltages, such as from nearby
>>>>>>lightning strikes.  <https://www.deltala.com/>
>>>>>>
>>>>>>Joe Gwinn
>>>>>
>>>>>Makes sense. It looks like most MOV appnotes assume that it's across
>>>>>an AC line, with kilo-amps available. Or lightning bolts.
>>>>>
>>>>>I'll get a few and test them at much lower loads.
>>>>
>>>>For smaller MOVs, I think that the data sheet specifies capacity in
>>>>Joules.  I bet this is the max integrated dose, not the pre-event
>>>>limit.  Well, the one-event limit as well.
>>>>
>>>>Joe Gwinn
>>>
>>>I'm torturing an MOV, a 470KD14. It's rated for 47 volts and 0.1 watt
>>>and 10 joules.
>>>
>>>At a constant 15 mA, it's at 58.1 volts, which is 0.86 watts. It's
>>>pretty warm. The voltage seems very stable after 4 hours so far.
>>>That's about 12K joules.
>>>
>>>It's likely it could do that forever, but the data sheets suggest that
>>>high power shots can do cumulative damage. I might set up to try that
>>>somehow.
>>
>>I bet that the duty cycle affects the cumulative damage, with smaller
>>duty cycles (more powerful pulses, but more widely separated) doing
>>more damage than just the cumulative energy.
>>
>>I looked at the Yageo 470KD14 MOV datasheet.  It  does not seem to
>>mention any wearout effect.  Perhaps they figured the mechanism out
>>and remedied it, which would be a good thing.  
>>
>>But the "surge life" items under "Reliability" on page 9 only does ten
>>surges and notes no visible damage, so we have no idea what happens
>>beyond that simple surge test's parameters.
>>
>>Joe Gwinn
>
>On page 5, it doesn't say so but I think the curves are parametreized
>on the number of shots, 1 to 1e6.

Yes, one can certainly read it that way.  Probably have to ask Yageo
how to read those plots, and the underlying physical mechanism.


>I might have to cut over to using mosfets and resistors to dump my
>overshoot energy. MOVs may be too risky longterm. Pity... they are so
>simple.

 How large are the surges and how long will it be to get to 10^6
surges in total?

Joe Gwinn