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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Sat, 28 Sep 2024 09:13:42 -0700
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On Sat, 28 Sep 2024 15:50:44 +0800, Sylvia Else <sylvia@email.invalid>
wrote:

>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.

Yes, a channel-channel short is possible, especially when a pair of
half-bridges drive a bidirectional motor coil.

>
>So it's down to requirements and specifications.

I am making those up as we go along, but I'd like to make the product
as good as I reasonably can.


>
>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.

The TI quad half-bridge has substrate diodes to ground, so a series
polyfuse may handle the reverse yahoo connection. I'll try that. I'll
need a gigantic power supply.

I suppose I should buy the worst series inductor that will work, to
limit the surge current.

I'll post some schematic scribbles as it goes along.