Path: ...!feeds.phibee-telecom.net!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: power supply discharge Date: Mon, 30 Sep 2024 18:49:14 -0700 Organization: A noiseless patient Spider Lines: 77 Message-ID: <01lmfj52p9aurg23v6oq29j3shutja5tnb@4ax.com> References: <4nrifjdkjuhai9dujuhir4eu91alovqjf6@4ax.com> <7i6lfjh7m3bt17jn2ponboi0a2refvpuob@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Tue, 01 Oct 2024 03:49:15 +0200 (CEST) Injection-Info: dont-email.me; posting-host="760545f9a75e21855bf8aba83822a38d"; logging-data="2709263"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19deVUTqM9eo9hcmiVe8fuC" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:sLbPISig3MSxv4k8k8+DDrRd3Bw= Bytes: 4111 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.