Paralleling Switching Pwr Supplies...safe? How?

I have 3 Radio Shack 13.8V, 25A power supplies (model #22-510,
http://www.radioshack.com/product.a...y_name=CTLG_009_001_005_000&product_id=22-510
) which are working great.

But, I need to get up to 40A output though by hopefully paralleling
two or three of them. Can this be safely done using either or both of
the following methods?
- Series resisitance on the output of each unit to act as a ballast to
help balance the out puts (similar to what's done to help balance
current thru paralleled MOSFETs)?
- Series diode (probably paralleled 6A diodes) on the outut of each
supply to prevent the unit with a slightly higher voltage output from
causing unwanted current flow back into the slightly lower-voltage
unit? The voltage drop across these diodes wouldn't be too much of a
problem but I'd love to avoid it if I could.

I'm guessing that I'd need to adjust the output voltage of each power
supply to be as close to each other as possible and use short,
large-gauge, identical length connections from each supply.

I hesitate to start experimenting without knowing a bit more of what
the possible options are for paralleling these power supplies. IMHO,
I probably wouldn't have too many problems until I reached higher
current levels, but by then any problems would be pretty nasty and
probably happen too quickly for me to do anything about.

Thanks for any info you can offer!
John Muchow
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Cross-posted...

I have 3 Radio Shack 13.8V, 25A power supplies (model #22-510,
http://www.radioshack.com/product.a...y_name=CTLG_009_001_005_000&product_id=22-510
) which are working great.

But, I need to get up to 40A output though by hopefully paralleling
two or three of them. Can this be safely done using either or both of
the following methods?
- Series resisitance on the output of each unit to act as a ballast to
help balance the out puts (similar to what's done to help balance
current thru paralleled MOSFETs)?
- Series diode (probably paralleled 6A diodes) on the outut of each
supply to prevent the unit with a slightly higher voltage output from
causing unwanted current flow back into the slightly lower-voltage
unit? The voltage drop across these diodes wouldn't be too much of a
problem but I'd love to avoid it if I could.

I'm guessing that I'd need to adjust the output voltage of each power
supply to be as close to each other as possible and use short,
large-gauge, identical length connections from each supply.

I hesitate to start experimenting without knowing a bit more of what
the possible options are for paralleling these power supplies. IMHO,
I probably wouldn't have too many problems until I reached higher
current levels, but by then any problems would be pretty nasty and
probably happen too quickly for me to do anything about.

Thanks for any info you can offer!

John Muchow
-- remove SPAMMENOT for e-mail responses --

 

There are a bunch of issues.
Where is the current limit set? Is it set at a level that the PS can
tolerate at 100% duty cycle?
What's the shape of the current limit? If it's foldback, you've got
problems.
Is there overvoltage protection? If one crowbars it has to take all
three down. Probably in a puff of smoke.
Can they tolerate voltage at their output when turned off?
Even if you switch them with a power strip, they'll come up differently.

What's the cost of the thing you're powering? How does that compare
to the savings on the power supply? Is it worth the risk?

Once you get all that sorted out, you have three choices.
Resistors to force load sharing, give up on low output impedance and
hope the settings are stable enough to stay where you balanced it.

Redesign the supplies to put one regulation loop around the whole thing.

Sell the two and buy one good one.
mike

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There are a bunch of issues.

PS turns off at 25A, 100% duty cycle, after a few seconds. Runs
forever at 22A, 100% duty cycle. Seems to be thermally activated.
Not sure where the current limiting is set but it does reliably shut
down if output is shorted.

Don't know...no reference other than "electronic overload protection"
in the instructions, but's that's probably for current.

Hmm...good point. I can make sure that the load is off until I
manually turn it on after the PS's have settled. Wouldn't diodes in
series with the outputs prevent any problems with this though?

That's the big decision. I have three perfectly good PS that I still
need for other uses and would prefer not to buy a 4th, if possible.
Especially since I don't have much room on the bench.

The device(s) I'm powering are fairly inexpensive. There's a lot of
deciding to do on whether it's worth the time to try to parallel the
units I have or just bite the bullet and buy a hi-amp unit.

The load is stable at one current value for the entire time it's on
but it will be turned off while the PS's are running. I was *hoping*
that would be a better situation for them to deal with.

Good idea, IMHO, probably not worth doing since choice #3 is easy to
do.

Yea, might just have to do this one (but keep all the units). Just
hate to have to fit another unit in somewhere.

Thanks for the info/ideas Mike!

John

John Muchow
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