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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Thu, 10 Oct 2024 19:06:03 -0700
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On Thu, 10 Oct 2024 20:30:50 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:

>On Wed, 09 Oct 2024 14:35:47 -0700, john larkin <jl@glen--canyon.com>
>wrote:
>
>>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:
>>>
>>>>On Wed, 09 Oct 2024 13:40:49 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Tue, 08 Oct 2024 17:27:53 -0700, john larkin <jl@glen--canyon.com>
>>>>>wrote:
>>>>>
>>>>>>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:
>>>>>>>
>>>>>>>>On Tue, 01 Oct 2024 11:24:34 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>>>>>wrote:
>>>>>>>>
>>>>>>>>>On Mon, 30 Sep 2024 18:49:14 -0700, john larkin <JL@gct.com> wrote:
>>>>>>>>>
>>>>>>>>>>On Mon, 30 Sep 2024 11:49:54 -0700, Joerg <news@analogconsultants.com>
>>>>>>>>>>wrote:
>>>>>>>>>>
>>>>>>>>>>>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:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> 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
>>>>>>>>>>>> 
>>>>>>>>>>>> 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.
>>>>>>>>>>>> 
>>>>>>>>>>>
>>>>>>>>>>>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.
>>>>>>>>>>
>>>>>>>>>>What kills MOVs? Integrated joules? Time-temperature?
>>>>>>>>>>
>>>>>>>>>>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.
>>>>>>>>>
>>>>>>>>>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/>
>>>>>>>>>
>>>>>>>>>Joe Gwinn
>>>>>>>>
>>>>>>>>Makes sense. It looks like most MOV appnotes assume that it's across
>>>>>>>>an AC line, with kilo-amps available. Or lightning bolts.
>>>>>>>>
>>>>>>>>I'll get a few and test them at much lower loads.
>>>>>>>
>>>>>>>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
>>>>>>
>>>>>>I'm torturing an MOV, a 470KD14. It's rated for 47 volts and 0.1 watt
>>>>>>and 10 joules.
>>>>>>
>>>>>>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.
>>>>>>
>>>>>>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.
>>>>>
>>>>>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.
>>>>>
>>>>>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.  
>>>>>
>>>>>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.
>>>>>
>>>>>Joe Gwinn
>>>>
>>>>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
>>
>>That's tricky. Some user might slam a capacitive load or a motor a lot
>>of times.
>
>Unh.
>
>
>>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>
>
>Cute, even in pairs.  If two, would stepped response (two MOSFETs) be
>worthwhile?
>
>For the record, VFD (Variable Frequency Drive) for three-phase motors
>also have the backdrive problem when stopping.  The 2 HP motor and
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