Path: ...!Xl.tags.giganews.com!local-3.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Wed, 09 Oct 2024 17:40:50 +0000 From: Joe Gwinn Newsgroups: sci.electronics.design Subject: Re: power supply discharge Date: Wed, 09 Oct 2024 13:40:49 -0400 Message-ID: <8nedgjld2g6dt2oif5q4ia483vfortc72n@4ax.com> References: <4nrifjdkjuhai9dujuhir4eu91alovqjf6@4ax.com> <7i6lfjh7m3bt17jn2ponboi0a2refvpuob@4ax.com> <01lmfj52p9aurg23v6oq29j3shutja5tnb@4ax.com> <6baofjtquh870j7ccl49q9lc1lalcgmedv@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 131 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-ZkT/fdALAwV08qBB1rCrIvR3AjRz7YJNuAH96Vp6KEcJTehfzM3hoxqEGza3eh7hzBvV5lYM+ygCpEX!n+I9g/PC48zL03MFcG7OhWxlOFLL1QezZfGdKqzY4DmnQUfofA6q5iXgRZDj8JoQjwldzSQ= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 6890 On Tue, 08 Oct 2024 17:27:53 -0700, john larkin wrote: >On Tue, 01 Oct 2024 16:03:40 -0400, Joe Gwinn >wrote: > >>On Tue, 01 Oct 2024 09:59:27 -0700, john larkin >>wrote: >> >>>On Tue, 01 Oct 2024 11:24:34 -0400, Joe Gwinn >>>wrote: >>> >>>>On Mon, 30 Sep 2024 18:49:14 -0700, john larkin wrote: >>>> >>>>>On Mon, 30 Sep 2024 11:49:54 -0700, Joerg >>>>>wrote: >>>>> >>>>>>On 9/30/24 11:24 AM, john larkin wrote: >>>>>>> On Mon, 30 Sep 2024 08:39:27 -0400, legg wrote: >>>>>>> >>>>>>>> On Sun, 29 Sep 2024 08:23:01 -0700, john larkin wrote: >>>>>>>> >>>>>>>>> On Sat, 28 Sep 2024 22:28:07 -0700, Joerg >>>>>>>>> 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. >>>> >>>>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