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Path: news.eternal-september.org!eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin <JL@gct.com> Newsgroups: sci.electronics.design Subject: Re: power supply discharge Date: Thu, 10 Oct 2024 19:01:19 -0700 Organization: A noiseless patient Spider Lines: 139 Message-ID: <ml1hgjpq9ak1q72euir8t2vtsvtm5ejhmr@4ax.com> References: <qvqlfjt4ttk1qeae20tje6mblci4h4d2ku@4ax.com> <lm0a6iFg65cU2@mid.individual.net> <01lmfj52p9aurg23v6oq29j3shutja5tnb@4ax.com> <kj4ofj56re4nl4h6qses2erq6fdgl6gbj7@4ax.com> <6baofjtquh870j7ccl49q9lc1lalcgmedv@4ax.com> <p8lofj9162t9ianni8fepgbv31drc7et26@4ax.com> <bnibgj9532fp554uek2l2rnn8ls12khl89@4ax.com> <ve4q38$2grjl$1@news.eternal-september.org> <rmndgjh7sp1s187g4fphd97216ah7c4enc@4ax.com> <ve98mk$38rlc$1@news.eternal-september.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Fri, 11 Oct 2024 04:01:20 +0200 (CEST) Injection-Info: dont-email.me; posting-host="d9b8c3293df3a30fd5566a2f75982ea2"; logging-data="3672095"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+9nASTcUwLBgdD2scGeRUz" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:744U6e8Ycc4gRRI0SW7I5xTyHOo= On Thu, 10 Oct 2024 15:08:03 -0400, ehsjr <ehsjr@verizon.net> wrote: >On 10/9/2024 3:53 PM, john larkin wrote: >> On Tue, 8 Oct 2024 22:34:16 -0400, ehsjr <ehsjr@verizon.net> wrote: >> >>> On 10/8/2024 8:27 PM, john larkin 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. >>>> >>> >>> Now lower the voltage. At what voltage does the current drop >>> to 0? >>> >>> Ed >> >> >> 0 is a fuzzy concept. >> >> I drops 48.2 v at 1 mA, about the same as always, after 62 K joules. >> > >Ok, thanks. Looks like it proves your idea. (-: >Ed I think there is damage at higher pulse energies, as few as a single pulse.