Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Tue, 01 Oct 2024 09:59:27 -0700
Organization: A noiseless patient Spider
Lines: 92
Message-ID: <6baofjtquh870j7ccl49q9lc1lalcgmedv@4ax.com>
References: <c5idfjp9miqru154ei6tnmg8m14qd30m6d@4ax.com> <lls6r9Frm70U1@mid.individual.net> <4nrifjdkjuhai9dujuhir4eu91alovqjf6@4ax.com> <7i6lfjh7m3bt17jn2ponboi0a2refvpuob@4ax.com> <qvqlfjt4ttk1qeae20tje6mblci4h4d2ku@4ax.com> <lm0a6iFg65cU2@mid.individual.net> <01lmfj52p9aurg23v6oq29j3shutja5tnb@4ax.com> <kj4ofj56re4nl4h6qses2erq6fdgl6gbj7@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 01 Oct 2024 18:58:15 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="2f9614aefd6af5edcc6ddd79fbba9342";
	logging-data="2966453"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+JVhA2a5WS/YIIpI/KXuO9"
User-Agent: ForteAgent/8.00.32.1272
Cancel-Lock: sha1:5t+Lqw/sxxWjfIWGFB2oM09wet4=
Bytes: 4940

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.