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From: Sylvia Else <sylvia@email.invalid>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Sat, 28 Sep 2024 15:50:44 +0800
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On 28-Sept-24 1:00 am, john larkin wrote:
> On Fri, 27 Sep 2024 23:50:21 +0800, Sylvia Else <sylvia@email.invalid>
> wrote:
> 
>> On 27-Sept-24 11:07 pm, 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.
>>>
>>> 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'm designing some programmable multi-channel power suplies and that
>>> is one of many tangled issues in the project.
>>>
>>
>> Be easy enough to sink current when the output voltage exceeds the set
>> point by more than, say, 0.1V.
>>
>> But there has to be a limit - connect the PS to your fully charged car
>> battery, and set the PS to 10V, and you're not going to see a 10V output
>> any time soon.
>>
>> Sylvia.
> 
> Right, the load could be a battery. The user could set the output
> voltage high with some current limit to charge the battery (or some
> giant capacitor), and then set the voltage low.
> 
> What's complicating my life is that the regulator is a half-bridge
> switcher that, in that case, becomes a boost converter, pumping
> backwards into my bulk power supply, which could then blow up. Or if
> the control loop cranks the PWM duty cycle down to zero in a futile
> attempt to reduce the output voltage, it soon shorts the battery.
> 
> Or some yahoo could connect the battery backwards.
> 
> This is actually a nice multidimensional dilemma. I'll be using the
> DRV8962 quad half-bridge, which also constrains things.
> 
> As usual with data sheets, it isn't entirely clear.
> 
> 
> 
> 
> 
> 

An even more extreme example of two PS connected together with different 
set points shows that no general solution exists, even in theory.

So it's down to requirements and specifications.

The reversed polarity battery case is I think usually handled with a 
diode and fuse. The controller can then email a manager pointing out 
that someone needs to be fired.

Sylvia.