Path: ...!news.nobody.at!2.eu.feeder.erje.net!3.eu.feeder.erje.net!feeder.erje.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Joerg <news@analogconsultants.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Mon, 30 Sep 2024 11:49:54 -0700
Lines: 70
Message-ID: <lm0a6iFg65cU2@mid.individual.net>
References: <c5idfjp9miqru154ei6tnmg8m14qd30m6d@4ax.com>
 <lls6r9Frm70U1@mid.individual.net>
 <4nrifjdkjuhai9dujuhir4eu91alovqjf6@4ax.com>
 <7i6lfjh7m3bt17jn2ponboi0a2refvpuob@4ax.com>
 <qvqlfjt4ttk1qeae20tje6mblci4h4d2ku@4ax.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-Trace: individual.net xHgP9v1+GRp9mT0ItY1t9Q8f5Nbh/RazfxLiC2PShrsj7m9CR+
Cancel-Lock: sha1:R94LBY9oHSPYFPt6xE9K8ydfVSM= sha256:29iQWJz3waAAxGkhGU2SnHdUwRg4UEza4MFIYBsChBk=
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.8.1
In-Reply-To: <qvqlfjt4ttk1qeae20tje6mblci4h4d2ku@4ax.com>
Content-Language: en-US
Bytes: 3696

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.

-- 
Regards, Joerg

http://www.analogconsultants.com/