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Old 11th October 2007, 06:20 PM   #1
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Default Switch Mode PS question (Chip Amp)

I am a complete Noob when it comes to Switch Mode Power supplies, so forgive me if this is a dumb question.

I have access to several small SWPS's, and I am hoping to use them on an upcoming project (Chip Amp)...But I need to know if I can use them in a pair to achieve the voltage I need.

They output 20vdc each, and I would like to use them connected like the picture below to get 40vdc +/-.

Click the image to open in full size.

Is this possible, or is my ignorance going to be my downfall?

Thank You for the help!

Tall Shadow
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Old 11th October 2007, 07:24 PM   #2
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Unfortunately, there is no easy answer to your question without a few more details. The main issue is if the outputs of the supplies in question are completely isolated or not. If they are isolated, then this should be possible. However, I have also seen cases where the output 0V line is tied to the plug ground. If this were the case, the schematic you have shown would effectively short the bottom supply. If you can either post the schematic (if you have it), or trace it yourself and verify that the output is isolated, then you can probably safely try this. No guarantees, though. Also, other more subtle problems may arise, depending on a bunch of other factors (beat frequencies, for example).
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Old 13th October 2007, 12:17 AM   #3
Eva is offline Eva  Spain
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The arrangement that you propose is definitely valid, but you will have to modify internal earthing of three of the units if their negative/ground output is earther. You may also have to electrically isolate the metal cases of the units because they tend to be connected to output ground tool.
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Old 27th October 2007, 04:39 PM   #4
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Default Laptop P.S.?

Won't it be nice to use laptop SMPS for this. A lot of them don't have an earth pin, so the earth problem won't arise.

Besides this they pack a lot of power in a very small foot print. I have a few units lying around and waiting for a good destination. Output voltage 19.5 to 19.8 volts at 3.3 to 3.6 Amps. Use two of these for feeding a gainclone might be nice.

Which leads me to another question: these Laptop supplies might also be a good starter for a power supply for a pre-amp or a DAC by following them with a regulator like LM317 for example. But then, do we have to get rid of all sorts of switching noise, and how do we do that?

anyone comments?
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Old 29th October 2007, 02:19 PM   #5
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Interestingly, I have tried the laptop supply idea before! I agree that they are an ideal solution in many cases for moderate power levels. In particular, I built a 4x20W system with them in the past for a friend. They are also cheap and easy to obtain (the university once threw out a stack of them with some very old laptops once, for example).

In terms of switching noise, it will vary based on the quality of the design. Generally, moderate filtering and a decent amp design will mean very little gets through to the output. For example, I saw more 60Hz on the output than I did of the switching fundamental. That being said, I was able to modify the supplies to synchronize the oscillators - if you can't do this, you may get audible beat frequencies. Also, the switching noise itself is usually out of the audio range, anyway.

One thing though... most newer laptop supplies have the third prong added, so you will have to look out for those that don't, as deduikertjes suggested. If you are looking to use existing supplies that you already have, then modification (as per Eva's suggestion) may be the only way. Always be careful, when dealing with earthing, to not only avoid short circuits, but also not to put something touchable at a high potential by accident (I know this is common sense, but it is worth repeating, I have seen it happen).
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Old 29th October 2007, 08:20 PM   #6
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Quote:
Generally, moderate filtering and a decent amp design will mean very little gets through to the output.
How does moderate filtering look like in this case?

Quote:
That being said, I was able to modify the supplies to synchronize the oscillators - if you can't do this, you may get audible beat frequencies.
For me modifying the supplies is not an option. So, can you elaborate a bit on this audible beat frequencies? What's that? Can we get rid of them?

thanks, MArco
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Old 29th October 2007, 09:03 PM   #7
gootee is offline gootee  United States
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Often, a simple LC lowpass filter, followed by a three-terminal adjustable regulator, will get rid of the switching noise, very well. For the LC filter, you could probably use something like a series J.W.Miller high-current toroidal choke, in the 10uH to 100 uH range (usable values might depend on max current), followed by a large-ish electrolytic capacitor to ground (probably 2200uF or less, depending on choke's inductance value, and the switching frequency). A regulator after the filter should help quite a bit, too.

Beat Frequencies are frequencies that are the arith-metic difference between two mixed frequencies. If the two frequencies are very close to each other, but not exactly the same, then there could be a low-frequency "beat frequency" developed. That might be a pain. But maybe the three-terminal regulators (mentioned above) would take care of it.

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Old 30th October 2007, 04:17 AM   #8
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Concerning radiated stuff, you will only get beat frequencies if the amplifier picks up massive EMI from the power supplies. This is something that should never happen if you use two decent quality PSUs and you don't place them too close to small signal circuits. Sometimes you can even place them too close and nothing happens.

Concerning conducted stuff, if output ripple voltage is too high it can also disturb amplifiers, but with a good LC output filter you can achieve as much ripple attenuation as you want.
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