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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Mon, 30 Sep 2024 18:49:14 -0700
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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.