Re: power supply discharge

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.
 

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Often the only internal load is the resistive divider for the regulator
loop feedback.
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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.
 

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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.
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I'm designing some programmable multi-channel power suplies and that
is one of many tangled issues in the project.
 

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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