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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Mon, 30 Sep 2024 11:24:16 -0700
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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.