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From: Jan Panteltje <alien@comet.invalid>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Wed, 02 Oct 2024 06:17:31 GMT
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On a sunny day (Tue, 01 Oct 2024 11:24:34 -0400) it happened Joe Gwinn
<joegwinn@comcast.net> wrote in <kj4ofj56re4nl4h6qses2erq6fdgl6gbj7@4ax.com>:

>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

Protects my washer:
 https://panteltje.nl/pub/mains_protector_IXIMG_0501.JPG
 https://panteltje.nl/pub/main_protector_IXIMG_0503.JPG
  VDRs with spark gaps.