series input parallel output SMPS

I have a bunch of small 12 volt input ATX SMPS power modules each rated for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are harder to come by and significantly more expensive.

I'd like to slave two together to get ~200 watts out, with the inputs in series. 24 volt 8 amp power bricks are pretty common.

I found this whitepaper on voltage balancing SMPS inputs in series but it seems more applicable to from-scratch designs.

Got any suggestions?

Reply to
bitrex
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Are they even isolated ground?

SMPS certainly can't be stacked in that way without an extra controller.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

Good question, if the ground is tied on both sides that's gonna be a problem. They look to be strictly buck converters on the board, not flyback.

Reply to
bitrex

Yup the continuity tester indicating that the input jack negative and ATX output common are both joined to the same ground plane.

Reply to
bitrex

For "historical interest" the main semiconductor components on the board are two RT8105 synchronous buck controllers, one MC64063, one AP1538

18V/3A buck controller, one mystery 8 pin SOIC the manufacturer attempted to conceal with paint but that magnified/palette invert cell phone camera picked up fine, labeled what looks like "WT751002 S 429 ZSAX G", one 4825P P-channel MOSFET in an 8 pin package and four IRFS3006 N-channels in DPAK
Reply to
bitrex

I've done that with bench supplies and Meanwells. How about just buying a bench power supply. 30V @10A is about $150 from volteq.

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What are you doing with all that current?

George H.

Reply to
George Herold

Switchers generally have a negative input impedance, so a series connection will be unstable.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Oh the "mystery IC" is probably this"

Reply to
bitrex

Yeah they would need the input voltages actively balanced, but even so stacking these in series and taking the output in parallel is gonna be a problem due to the input and output sides sharing a ground.

Reply to
bitrex

If the outputs are in parallel and the switchers are identical isolated converters, the inputs could be force-balanced without much power dissipation, but with some complexity. There would be some midpoint input voltage that would be the perfect balance point, so would require zero balance current. That point would be unstable but could be forced.

Might not be too hard, actually. Sort of interesting, dynamically.

If these are not isolated converters, the ground sharing problem wrecks things.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Nope, won't work. Paralleled outputs will likely not current share, one will put out limit current, the other will pick up the remaining current demand. So, with delivered power not being shared equally, the voltage drop across the supplies will be different, but in series the input current MUST be the same. So, the one carrying less output current will have less voltage drop, and it will increase until the other unit is starved for voltage. It will be totally unstable, and the supplies will be pulsing on and off.

Jon

Reply to
Jon Elson

I've done it with bench power supplies. (individual adjustments for voltage and current.) I never thought much about it but it's worked fine the few times I've needed more current. I see bitrex is asking about DC-DC converters, which may behave differently.

George h.

Reply to
George Herold

it is the input side that is the problem

Reply to
Lasse Langwadt Christensen

Ahh, sorry. You know it's good to have a village idiot, we help all the other village idiots out there.

So what about if he has a real stiff source. :^) (apologies again, I got home early and had a beer.)

George H.

Reply to
George Herold

I thought I was cheap.

First of all what is wrong with the inputs being in parallel ? Is voltage too high, or the available current too low ?

Just what voltage and current (compared to the ratings) do you want out of this thingamajig ?

Reply to
jurb6006

Basically I need something like this:

Compact fanless 200 watt-level ATX power supplies are hard to come by domestically they're always either out of stock like above or stupid expensive.

I could probably parallel the 120 watt units but then I need a bigger external power brick, 24 volt 8 amp power bricks are cheap ($10-15) and commonly available, 12 volt 18-20 amp power bricks are like 70 bucks.

I'll probably have to end up ordering what I need from China but it takes a month, sucks.

Reply to
bitrex

I'm also skeptical as to whether these "250 watt" units from China can actually deliver the rated power.

Reply to
bitrex

Sure, paralleling the OUTPUTS of well-behaved supplies should work fine. Paralleling the inputs to DC-DC supplies should also work. The OP wants to parallel the outputs and SERIES the inputs! As far as I can see, that can NEVER work in the real world.

Jon

Reply to
Jon Elson

As TW pointed out not if the switchers input and output share a ground. If they're an isolated topology don't see why you couldn't "in theory", but you'd need some kind of active feedback circuit to keep the input rail split evenly.

Reply to
bitrex

Thanks Jon, sorry to be an idiot sometimes. As is often the case I didn't read the post that well... Well if it's going to oscillate maybe it can be used as an inverter. :^)

George H.

Reply to
George Herold

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