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From: legg <legg@nospam.magma.ca>
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Subject: Re: power supply discharge
Date: Mon, 30 Sep 2024 08:39:27 -0400
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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