Re: power supply discharge

On Wed, 09 Oct 2024 12:51:32 -0700, john larkin <jl@glen--canyon.com>
wrote:

On Wed, 09 Oct 2024 13:40:49 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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On Tue, 08 Oct 2024 17:27:53 -0700, john larkin <jl@glen--canyon.com>
wrote:
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On Tue, 01 Oct 2024 16:03:40 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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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.
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Joe Gwinn

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I'm torturing an MOV, a 470KD14. It's rated for 47 volts and 0.1 watt
and 10 joules.
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At a constant 15 mA, it's at 58.1 volts, which is 0.86 watts. It's
pretty warm. The voltage seems very stable after 4 hours so far.
That's about 12K joules.
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It's likely it could do that forever, but the data sheets suggest that
high power shots can do cumulative damage. I might set up to try that
somehow.

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I bet that the duty cycle affects the cumulative damage, with smaller
duty cycles (more powerful pulses, but more widely separated) doing
more damage than just the cumulative energy.
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I looked at the Yageo 470KD14 MOV datasheet.  It  does not seem to
mention any wearout effect.  Perhaps they figured the mechanism out
and remedied it, which would be a good thing. 
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But the "surge life" items under "Reliability" on page 9 only does ten
surges and notes no visible damage, so we have no idea what happens
beyond that simple surge test's parameters.
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Joe Gwinn

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On page 5, it doesn't say so but I think the curves are parametreized
on the number of shots, 1 to 1e6.

Yes, one can certainly read it that way.  Probably have to ask Yageo
how to read those plots, and the underlying physical mechanism.

I might have to cut over to using mosfets and resistors to dump my
overshoot energy. MOVs may be too risky longterm. Pity... they are so
simple.

 How large are the surges and how long will it be to get to 10^6
surges in total?

Joe Gwinn