Load exceeding supply current

[trillium]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?
Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.

 

[Don Foreman]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

You'll need more like 32,000 uF to hold droop to 1 volt in 1
millisecond at 32 amps.

 

[John Popelish]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?
Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.

The average current for the above pattern is 10 amperes, so you need a
supply with greater capacity than that, or it is going to be in
current limit all the time.

A 30mF cap will sag 1 volt in 1 millisecond while delivering the whole
32 amperes. but if the supply can deliver 10 of those amperes, then
22 mF will deliver the remaining 22 amps during that millisecond while
the voltage sags by a volt. It will recharge back to full voltage
with the excess current capacity during the rest of the cycle.
Current limit is not a problem for the regulator as long as it is
designed to be in current limit, continuously. Current limiting is
just regulation with a different condition controlling the pass
device, except that the current is high. But any supplies are not
designed to handle the temperature rise of continuous current limit
operation.

Coupling the big capacitor to the supply through a big inductor will
let the cap carry more of the peak current (requiring a larger
capacitor) but will smooth out the current drawn from the supply (and
also waste some voltage with its resistance). A bigger supply is
probably cheaper than this solution.

 

[Roger Lascelles]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?
Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.

If your power supplies are switchmode types, you have to consider :

- will they like starting into a large capacitance ?

- "current limit" can be a vague term for switchmodes. Most do not have the
classic laboratory power supply current limit where voltage drops to keep
current at the limit. Often a protective cycle by cycle limit cuts in, but
way above the rated current, and the effect on output current depends on
mains voltage. Some supplies shut down when excessive current is drawn.
Most switchmodes are intended to be wired up to a load which does not have
massive peaks above the average value.

- A little bit of resistance works wonders - say 20 or 50 milliohms between
power supply and capacitor will help lower peak current and assist supply
control loop stability. An inductor is good too.

You can get switchmodes with an external control terminal or feedback
terminals, and it is possible to control the switchmode for your own
purposes. We incorporated this in a 34A output supply to give current limit
and precise voltage control. We had to consider control loop stability.
Might be too complex for your needs.

With series power supplies, I have seen a reversed diode wired across each
supply output, so one can never drive current thru the other.

Roger Lascelles

 

[trillium]

will they like starting into a large capacitance ?

I'm thinking about one that let's you slow down the start-up.

"current limit" can be a vague term for switchmodes. Most do not have the
classic laboratory power supply current limit where voltage drops to keep
current at the limit

Thank you so much for that! Exactly what I was worried about...

little bit of resistance works wonders

Yes.

 

[mike]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?
Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.

This is a valid concern. It's generally bad to put supplies in series.
If you expect them to behave in current limit, you're dreaming.
They will never be exactly the same. You may be in the situation that
only one ever current limits. You won't be able to use foldback
limiting. There's probably a glitch filter in the limit that can put
both supplies in a frenzy. If you're only building one system, you only
have one customer to worry about. If you build a lot of them, you're
asking for trouble. It's unlikely that the current limit behavior you
need is in the spec. Using any component outside it's specification is
a recipe for disaster.

Some bean counter in your organization is gonna save a buck by changing
vendors. Or some bean counter at the supplier
is gonna change one of his vendors. And all of your careful evaluation
goes out the window and your replacement is on a plane to do an
emergency retrofit.

Put a filter between the supply and the load that guarantees you never
put the supplies into current limit. Make sure your caps are rated for
the repetitive peak current you are gonna put thru them. Once you've
met the specs on the supply, you can remote sense the output of the
filter. Yes, there will be phase issues and potential for oscillation.

Pay attention to your resistance budget. 1V 32A gives you 1/32 ohms
total resistance for all the caps, wire, connectors, etc. And that's
before you get to any capacitive droop.

Buy a supply that's rated for the peak and average currents you need.
The design problem is the same, it just goes inside the regulation loop
and is guaranteed to work.
mike


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[trillium]

Pay attention to your resistance budget.

First thank you so much for comments! Resistance budget: yes, very
important, especially after I noticed (duh!) how the load will take
more current from the low-series-resistance component, and that may be
the power supply, not the caps as in some rosy dreams... (The caps
won't provide the bulk of current.)

you can remote sense the output of the filter. Yes, there will be phase issues and potential for oscillation.

Yep. It's tempting to use remote sense to compensate droop. I'm
thinking of... filtering the filter output... Something like 1s delay.
(The 32 amps -- and droop -- happen every 8ms.) Won't help
fast-changing firing patterns (I'm firing solenoids), but would catch
up to a constant one.

 

[Jasen Betts]

The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.

A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.

Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?

probably what's the surge rating on those supplies?
If it worries you use a bigger cap

Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.

I'd say to use unregulated power supplies, but you've got that 1V limit
maybe use two unregulated power supplies and then regulate their series
output. (or just get a 24V powersupply.)

What sort of device has nice sharp current fluctuaions like that?

Bye.
Jasen

 

[Rich Grise]

probably what's the surge rating on those supplies?
If it worries you use a bigger cap


I'd say to use unregulated power supplies, but you've got that 1V limit
maybe use two unregulated power supplies and then regulate their series
output. (or just get a 24V powersupply.)

What sort of device has nice sharp current fluctuaions like that?

The imaginary device in the homework assignment.

Cheers!
Rich

 

[trillium]

What sort of device has nice sharp current fluctuaions like that?

Solenoid drivers for fuel injectors. The box must be able to drive 8
simultaneously. One runs up to 4A, then settles at 1A. Max freq 125Hz.

just get a 24V powersupply

This needs to work at 12V as well.

use unregulated power supplies

I was thinking of using one unreg in series with one reg with sense
divided by two. But I found a small enough variable supply (yes, size
matters...) and I'm going with that.

what's the surge rating on those supplies?

Many go up to something like maybe 200%, but surge and periodical seem
to be disjunctive concepts...

If it worries you use a bigger cap

That's basically were I am heading too.

 

[Jasen Betts]

Solenoid drivers for fuel injectors. The box must be able to drive 8
simultaneously. One runs up to 4A, then settles at 1A. Max freq 125Hz.

And you're planning of firing all 8 of them at once?

125Hz on a 4 cylinder engine would be 3750 Rpm - kind of slow for an upper
limit. and on an 8 half that.

It sounds more like you want do so sequential multipoint injection (or
possibly direct injection) with one injector feeding each cylinder.

this way only one injector is fired at a time, and 125Hz (on each injector,
with the injectors phased by 45 degree) would translate to 15000 RPM,
the sort of RPM race cars, (and motorcycles) run at. and basically you
have an 8 amp load with a 3A ripple. not so scary at all.

Bye.
Jasen