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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Wed, 09 Oct 2024 13:40:49 -0400
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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