Re: power supply discharge

On Wed, 09 Oct 2024 16:37:22 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

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

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

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

That's tricky. Some user might slam a capacitive load or a motor a lot
of times.

Here's a Riedon ceramic DPAK 50 ohm resistor. It could absorb at least
50j, 100 with two in parallel. That would work. They will need a
mosfet to switch them on when the 48v supply gets over-driven to 58
maybe.

https://www.dropbox.com/scl/fi/octctz94vdi4ac4aageit/Dpak-50r-joules.jpg?rlkey=y21a3x8xmkno82ezrb4vefxrr&raw=1

The Caddock TO-220 resistors have a big metal tab like a mosfet and
would absorb more joules, but are more expensive. They would be an
option.