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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Wed, 02 Oct 2024 13:36:39 -0700
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On Thu, 3 Oct 2024 00:02:31 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 30/09/2024 1:23 am, john larkin 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.
>> 
>
>There is no need to concede defeat and disable your switcher chips, just 
>turn on a big load instead.


The customer load could be a giant capacitor bank, or a battery. I
don't want to short either. And I do want a supply to recover
gracefully.


>
>In variable speed drives for induction motors, the voltage of the DC 
>rail and bulk capacitance can also rise when the motor is slowing down 
>with a lot of inertia attached to the shaft. They have a switch built 
>in, which you are supposed to attach a big load resistor to. When the DC 
>rail rises above some threshold, it turns on your external load 
>resistor. It cycles on and off to keep the bulk capacitor voltage in an 
>acceptable range.
>

Same problem. My fix is to protect our cap with the MOV, and shut off
the switchers when that voltage gets too high.