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Old 10th May 2005, 12:39 PM   #11
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Here's the same one with current sharing disabled.

Things are starting to wander about a bit.

DNA
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Old 10th May 2005, 01:20 PM   #12
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Quote:
Originally posted by Jaka Racman
Hi,

may I suggest that you take a serious look at Bruno's US pat no. 6,803,816. His method of using autotransformers for automatic current sharing between phases of multiphase system is a very good idea. I will try to implement it in multiphase nonsynchronous buck converer where I expect it will alow me to reduce volume of magnetics components by more than half.
I suggest you do some simulations. Coupling coefficient can be as high as 0.99997 (that is what I have measured on my prototypes). Maybe you will find that you do not need output filter at all.

Best regards,

Jaka Racman
I went and had a look, perhaps I should go and have another look.

Patents make hard reading. Didn't see the autotransformer thing.

Just for the fun of it here's what happens if I take out the filter capacitors.

I was surprised and I'm not so sure I want to think about it....

DNA
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Old 10th May 2005, 01:56 PM   #13
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Hi,

attached is picture from patent showing how phase summation circuit looks like. I added dots to indicate winding polarity. On the right side of the picture you see combined pwm signal from four phases. As you can see there is hardly any filtering necessary.

Maybe you can ask Bruno to mail you his AES article. It is much more descriptive than patent.

BTW, such autotransformer combining is nothing new in other fields. In my lab I have 7kW single phase variac which consists of two physical variacs on the same axis and whose outputs are combined with such autotransformer. I also have 1978 Motorola RF handbook where autotransformers are used to combine RF power of several smaller RF amplifiers.

Best regards,

Jaka Racman
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Old 10th May 2005, 02:27 PM   #14
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Oh, sort of. Well, not really

One of the things you get is ripple cancellation.

Take two phases 180 degrees out of phase and the zero voltage sum has zero ripple.

Take four phases 90 degrees out of phase and you get zero ripple at 0V out and zero ripple at +/- half the supply voltage out and so on.

Perhaps when you are talking about 'autotransformers' you are referring to what I'd call coupled inductors.

If you look at a full bridge circuit with a differential output filter then it should be possible to wind the two filter inductors on the same core.

Ideally they experience the same volt-seconds so it works.


DNA
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Old 10th May 2005, 02:44 PM   #15
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Hi,

you might also call them coupled inductors, but I prefer term autotransformer (and if I recall correctly Bruno also) because they carry no net DC current so they use high permeability core. You also dimension them like transformers ( Volt second product). If you have time and will, do a simulation, you will find that phase currents do not look like you expect them to be (classic ripple cancelation thing). Use small LC filter at the output for first simulation.

Best regards,

Jaka Racman
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Old 10th May 2005, 05:08 PM   #16
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I'd post the paper here but I'm not sure how this goes with the AES copyright (they charge for downloading papers from their site). So if those who are interested send me a mail I'll send the paper.

Looking forward to see how an analogue multiphase amp works out.
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