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Old 14th December 2012, 02:31 PM   #1
xrk971 is online now xrk971  United States
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Default Laptop SMPS Bricks in Parallel?

I have asked this question in several forums now with no response, and have searched but not found an answer. So I am posting this as a new thread.

Can one connect multiple switch mode power supply (SMPS) bricks from laptops for example, to boost the amps rating? I have lots of 19 volt 4.6 amp (90W) bricks lying around, and if need more, they cost $9 ea to buy. Maybe use blocking diodes and a fat capacitor at common supply point to smooth out? What are the problems or issues with this?

I want to tie maybe 4 of them together to power a class D amp.

Thanks in advance.

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Old 14th December 2012, 04:55 PM   #2
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It isn't recommended to parallel two or more of them if they hasn't a "share" or similar circuit to make both (Or more) PS give the same voltage AND current at the same time.

For example, let one of them to have, say, 12V and another 12.1V, making gross number to see the difference. When paralleling, the 12.1V will be idle. You can use diodes or ballast resistors, but it will degrade the supply performance as voltage sense will be done before this resistors.
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Old 14th December 2012, 06:13 PM   #3
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Ok, does not recommended mean it won't work? I know you can't do this with linear supplies without having a master supply driving slave pass transistors. For switchers, their internal sense tries to maintain the constant voltage. If the voltage sags on one power supply and it can't keep up, would not the common tie point voltage drop and hence force all power supplies to do their thing to keep up? Maybe I am being too simple minded but I am thinking about as a reservoir with the big capacitor and each supply does its job to refill this cap to maintain voltage. I agree that the voltage needs to be close, but maybe what you are saying that is even if the voltage is off by tiny amount, the power supply will not switch on to refill The capacitor?
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Old 14th December 2012, 06:23 PM   #4
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Returning to my previous example, if you wire them in parallel directly, and if both of them has (as must have) a big open loop gain in their error amplifier, then the higher voltage supply will be driven by the lower to a so small duty cycle that it may be full shut down. Or if not, the higher voltage SMPS will carry the other behind the duty cycle limit (For example in a forward converter in which max duty is <50% by definition), and to destroy or go overvoltage shutdown. The cap you suggest, I believe is useless, because for DC it is open circuit. It can help in transients only.

I believe you have certain technical knowledge, why not to try to make a kind of master/slave configuration in similar way as linear counterparts?

I use it in a 180 deg out of phase solar cells switching regulator and performs fine.
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Last edited by Osvaldo de Banfield; 14th December 2012 at 06:25 PM.
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Old 14th December 2012, 06:40 PM   #5
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I would rather build speakers and buy power supplies. I used To build my own linear supplies all time in my younger days, but have no experience with switchers. Maybe hack into a laptop brick and intercept the voltage sense and connect it to common point? Use switching drive voltage from one master to all slave mosfets. It might be fun, but alas, I barely have enough time to make speakers.... thanks for your input though. Does anyone have any ideas on how to make cheap higher current supplies for Class D amps?
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Old 14th December 2012, 09:31 PM   #6
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I have done the parallel trick with 2 SMPS when desperate but only for short testing purposes. What you ideally want are outputs that sag a little on load, say 50mv, but have voltages at the same load within say 10mv. So, you might have to hand select supplies or find a way to adjust the feedback. One way if there are sense wires is to add a little resistance or miniscule ratio potential divider in the sense lead. The sagging could be realised with a series output resistor in each output, as I say 50mv is enough to get reasonable sharing. You don't want supplies that latch off in overcurrent, then at the limit they'll share anyway because as one shuts down its voltage falls and the other one becomes the main source and wants to deliver all the current but then that will shutdown etc etc.
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Old 14th December 2012, 10:01 PM   #7
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Diode isolation will help distribute load sharing without having any one supply pump current into another.

Equal (same V & I output) supplies recommended.

Use schottkey diodes at 5 to 10 times rated current load for less drop.
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Old 14th December 2012, 10:10 PM   #8
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Quote:
Originally Posted by DUG View Post
Diode isolation will help distribute load sharing without having any one supply pump current into another.

Equal (same V & I output) supplies recommended.

Use schottkey diodes at 5 to 10 times rated current load for less drop.
Dug,
Thank you! I suspected that it might work, and I do have identical bricks to try this with. I hope those Schottky diodes aren't too pricey
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Old 15th December 2012, 08:07 AM   #9
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Quote:
Originally Posted by xrk971 View Post

Can one connect multiple switch mode power supply (SMPS) bricks from laptops for example, to boost the amps rating? I have lots of 19 volt 4.6 amp (90W) bricks lying around, and if need more, they cost $9 ea to buy. Maybe use blocking diodes and a fat capacitor at common supply point to smooth out? What are the problems or issues with this?

I want to tie maybe 4 of them together to power a class D amp.

Xrk971
I suspect that you need more than 19 volts for your amp.
So, are we talking about series connecting 4 bricks for +/-38 volts center tapped ?
If so then no paralleling, no problem ...
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Old 15th December 2012, 05:06 PM   #10
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I wasn't going to series them with center tap but that is not an option? Why?
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