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Old 11th February 2008, 11:24 AM   #1
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Default Slobodan Cuk classD

Cuk converter has low ripple. Maybe it can make a better classD. While searching, I found these :
http://www.ti.bfh.ch/fileadmin/img/H..._06-07_074.pdf
Mr. Cuk himself make classD :
http://www.freepatentsonline.com/4186437.html

What is the first paper about? I cannot read that.
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Old 11th February 2008, 12:13 PM   #2
ntropy is offline ntropy  Switzerland
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The first paper describes the use of a cuk converter for a class D amp. Looking at the title, it was a final year project by a Swiss polytechnic student.
They state that with this topology it is possible to have high dynamics, low noise combined with virtually no current ripple at the output of the amp. The lack of ripple renders additional filters needless and reduces EMI as well.
They write the hard part was to build a controller for this cuk converter since it shows a non-linear 5th order transfer function. The solution they found was a "one-cycle controller" with a simple 1st order PI transfer function.

My german is better than my english so I hope this translation was still usefull.
Btw, it's my first post after reading the forums for more than a year now :-)
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Old 11th February 2008, 12:16 PM   #3
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Hi, ntropy,

Thanks . With search engine, I cannot find the english version of that paper or block diagram or global/detailed schematic about that.
Can you read if there's any other information related to that first paper?
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Old 11th February 2008, 01:36 PM   #4
ntropy is offline ntropy  Switzerland
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hi lumanauw

I can't see any references in the first article, but there's an email adress of this former student. Maybe when you ask him nicely he will send you a copy of his diploma work?
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Old 11th February 2008, 02:02 PM   #5
iand is offline iand  United Kingdom
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Cuk's converters for SMPS were first proposed in (I think) the 1980's as a way to get a variable ratio step-up/step down DC-DC converter with continuous input and output currents, thus avoiding large ripple currents at either input or output, using a single switch at the input side and a single diode at the output side.

The input and output inductors can be realised as a single coupled inductor (depending on isolation voltage requirements), the circuit can provide DC isolation using a transformer, and in theory the coupled inductors and the transformer can be combined into one integrated magnetic structure.

Difficulties with this design include the fact that the non-isolated version is inverting, there's an inband resonance between the input/output inductor(s) and the coupling capacitor(s), the stress on the switching device is large, and the magnetic structure doesn't lend itself to conventional manufacturing techniques.

For one or all of these reasons the design never caught on. I looked at this architecture many years ago for class-D amplifiers and found the following:

Using two Cuk converters driven in antiphase (one with widening pulses, the other with narrowing ones) can produce a self-oscillating class-D amplifier with constant frequency (because each output voltage is (D/(1-D)) where D is the duty cycle -- for example, if each output is +100V at 50% duty cycle, at 33% one output has fallen to 50V and the other has risen to 200V. at 20% duty cycle the outputs are 25V and 400V and so on. This does mean that the combined voltage and current stress on the switching devices becomes *very* severe...

This works fine at DC and doesn't seem to need any output filter, but it all goes horribly wrong when the output voltage changes and the LC resonance inside the converter shows up, typically inside the audio bandwidth.

Maybe more advanced control techniques can fix this issue, but I doubt it, it seems to be an inherent problem with the architecture. I think this (together with the other issues mentioned above) is why this was never used for class-D.

Ian
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Old 11th February 2008, 02:31 PM   #6
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Hi, Iand,

Maybe that's why we doesn't see much of this topology, although it offers small ripple in and out.
Meanwhile, this paper is about Cuk's amp described in the patent.

http://ece-www.colorado.edu/~rwe/papers/Powercon5.pdf
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Old 11th February 2008, 03:23 PM   #7
iand is offline iand  United Kingdom
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Quote:
Originally posted by lumanauw
Hi, Iand,

Maybe that's why we doesn't see much of this topology, although it offers small ripple in and out.
Meanwhile, this paper is about Cuk's amp described in the patent.

http://ece-www.colorado.edu/~rwe/papers/Powercon5.pdf
He published lots of other papers too, especially about isolated versions of the converter which don't have the problem of needed both N and P switching devices and provide both regulation (step-up/step-down) and mains isolation.

This can be extended to the bridged class-D amp by putting a transformer in the middle and making the DC input rectified mains voltage (or PFC output). Then the mains side has 2 switching devices and so does the output side, with all bulk capacitor charge storage on the mains side -- however this then needs 2 coupled inductor/transformers (or "integrated magnetics" structures) per amplifier.

When I looked at it the biggest problem (assuming the LC resonance issue is solved) was the increased voltage/current stress in the switching devices, this is much less of a problem nowadays with the advances in power MOSFETs, IGBTs and high-speed diodes -- maybe it's time for another look at the circuit...

Ian
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Old 11th February 2008, 04:53 PM   #8
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Isolated Cuk, do you mean something like this?

Quote:
-- maybe it's time for another look at the circuit...
National made 1.4Mhz Cuk driver, LM2611
Attached Images
File Type: jpg cuk.jpg (63.5 KB, 733 views)
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Old 11th February 2008, 08:04 PM   #9
iand is offline iand  United Kingdom
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Quote:
Originally posted by lumanauw
Isolated Cuk, do you mean something like this?

National made 1.4Mhz Cuk driver, LM2611
That's the one -- for bidirectional power (needed for class D) you need a switch (MOSFET or IGBT) and diode on both sides, then for linearity you need to duplicate the whole thing with clock inverted.
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Old 11th February 2008, 08:07 PM   #10
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Hi,

you can flunk the first linked paper right from the start. It says (my translation to English, last sentence in first paragraph):

Quote:
The high phase shift in the output filter leads to a low loop gain, which results in higher non-linear distortion.
Low loop gain? No way. Look at UcD. Or any other topology. You can have loop gain as much as you want or as much you can handle.

Just another case of "he didn't looked far enough".

Bye
Baseballbat
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