Re: power supply discharge

On Tue, 01 Oct 2024 16:03:40 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

On Tue, 01 Oct 2024 09:59:27 -0700, john larkin <jl@glen--canyon.com>
wrote:
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On Tue, 01 Oct 2024 11:24:34 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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On Mon, 30 Sep 2024 18:49:14 -0700, john larkin <JL@gct.com> wrote:
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On Mon, 30 Sep 2024 11:49:54 -0700, Joerg <news@analogconsultants.com>
wrote:
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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:
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On Sat, 28 Sep 2024 22:28:07 -0700, Joerg <news@analogconsultants.com>
wrote:
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On 9/27/24 8:07 AM, john larkin wrote:

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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.

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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.

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I'm doing some multichannel non-isolated supplies that will be sync
buck, using multiple TI DRV8962 chips.
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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.

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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.
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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.
 

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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.

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What kills MOVs? Integrated joules? Time-temperature?
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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.

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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/>
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Joe Gwinn

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Makes sense. It looks like most MOV appnotes assume that it's across
an AC line, with kilo-amps available. Or lightning bolts.
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I'll get a few and test them at much lower loads.

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For smaller MOVs, I think that the data sheet specifies capacity in
Joules.  I bet this is the max integrated dose, not the pre-event
limit.  Well, the one-event limit as well.
>
Joe Gwinn

That's probably a single-shot rating, such as to limit the peak
temperature. So that can be done many times, if it cools off between
shots.