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Old 26th June 2012, 06:15 AM   #1
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Default Quick (stupid?) question on paralleling PSs

Hi all,

A general question but not saw answers/solution after searching a bit...

Take a 2-secondary transformer to make a single regulated DC power supply, it is wiser (read better as far as 'performances' go) to parallel this...

1- To start with, AC level (tx secondaries)
2- At DC level (independent secondaries, rect and filtering, then parallel)
3- At regulated DC level (as 2 but paralleling after regulation)

I have a feel that maybe with "3" you may have the same noise as "1" but feel stupid as you have worked (and spent) double on electronics?

Thanks!
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Old 26th June 2012, 06:53 AM   #2
marce is offline marce  United Kingdom
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I have done paralled multi phase SMPS designs, but not linear, as you get less noise and EMC problems, though what noise there is is spread over a slightly wider bandwith, but with good layout this shouldn't be a problem. Also had spread spectrum clock to further reduce the noise.
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Old 26th June 2012, 07:39 AM   #3
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Thanks Marce. As one can see I am thinking on simple 'linear' PS off course... small clarification!
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Old 26th June 2012, 09:11 AM   #4
DF96 is offline DF96  England
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You can only parallel supplies if you can guarantee exactly equal voltages, or you are happy to drop some voltage in equalising resistors. In your case (1) is best, then (2). (3) would need equalising resistors unless you can somehow guarantee that two separate regulators have exactly the same output voltage.
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Old 26th June 2012, 09:13 AM   #5
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If it is a single transformer with two IDENTICAL secondaries it is normal practice to parallel the secondaries before the rectifiers, i.e. immediately at the transformer.
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Old 26th June 2012, 12:52 PM   #6
macboy is offline macboy  Canada
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If the AC voltage at the two paralleled transformers differs even slightly, then a high current can develop between them. It will be roughly equal to the voltage difference / winding resistance. Since winding resistance is low, the current will be high.

A smarter idea is to have a separate rectifier for each transformer, then have a single set of filter caps. This way, no current can flow from one transformer to the other.
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Old 26th June 2012, 01:02 PM   #7
DF96 is offline DF96  England
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Two nominally identical secondaries on the same transformer should be close enough to parallel without any problems. Paralleling after rectification avoids back-feeding current, but may force one winding to do all the work as rectifier diodes will not be identical.
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Old 26th June 2012, 01:25 PM   #8
macboy is offline macboy  Canada
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Quote:
Originally Posted by DF96 View Post
Two nominally identical secondaries on the same transformer should be close enough to parallel without any problems. Paralleling after rectification avoids back-feeding current, but may force one winding to do all the work as rectifier diodes will not be identical.
Yes, two nominally identical secondaries on the same transformer are OK to parallel, because they are usually (always?) wound together. If there are two such secondaries, then two stands of wire are wound together. This reduces variances due to manufacturing tolerances and helps ensure that they are essentially identical.

I would not parallel secondaries from two separate but "identical" transformers.
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Old 26th June 2012, 06:07 PM   #9
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Maybe I was not clear but I wrote "a 2-secondary transformer" so yes, there will bo no two separate transformers (!).
Ok, so the easiest (and cheapest) way seems to be too the 'wiser' one too.
This was my idea too, but one never knows!
Thanks guys
Regards,
Josep
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