Re: power supply discharge

On 29/09/2024 2:13 am, john larkin wrote:

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:
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On 27-Sept-24 11:07 pm, john larkin wrote:

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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.
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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.
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I'm designing some programmable multi-channel power suplies and that
is one of many tangled issues in the project.
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Be easy enough to sink current when the output voltage exceeds the set
point by more than, say, 0.1V.
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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.
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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.
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Or some yahoo could connect the battery backwards.
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This is actually a nice multidimensional dilemma. I'll be using the
DRV8962 quad half-bridge, which also constrains things.
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As usual with data sheets, it isn't entirely clear.
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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.

You usually end up with a better result if you work out what you are trying to do before you start designing it - or in your case, slinging it together. It's called system engineering.
When dim newbies post here, we often have to ask them what they are trying to do before we can work out how they might solve the problem they think they are seeing.

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.

 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.

By "worst" do you mean cheapest or smallest?

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

LTSpice simulations tend to include the kind of information that gets left out of your pencil sketches.
--
Bill Sloman, Sydney