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Old 1st September 2004, 08:04 AM   #1
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Default UCD Based Fully Differential Full Bridge 450W RMS Amp

Hi,

This thread is intended to be a take off of the "Development of a "reference" class D starting point" thread, for which all the background information of this project can be found at:

Development of a "reference" class D starting point

It is as complete as I care to make it, though I've included no by-passing on the driver power supplies, along with a few other minor changes which would be required for a real implementation of it.

In this first post I will include extensive simulation results, to be followed with a FFT done with 100mV input at 1Khz, then I'll provide the circuit schematic, both in the form of a bitmap or jpg, and the pspice *.DSN file.

I do not intend to produce a pcb design for this, so if anyone decides to make one, and gets it working well, please contribute.

All the simulations for which the results you are about to see, have been done with default pspice options, and being spice, obviously, need to be taken with a grain of salt, or two.

However, I'm very pleased with the results it shows. That's not to say it can't be improved on, and anyone who cares to is invited to contribute there as well.

Regards,
Chris
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Old 1st September 2004, 08:12 AM   #2
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FFT done at 100mV 1Khz input ~8 watts peak output.
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Old 1st September 2004, 08:38 AM   #3
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Schematic in .bmp format.

A few notes here..

Note the 5K resistors used to bias the op amps. It was more of a proof of concept thing than anything else, it shows a worthy improvement in simulation, so I left them in. This schematic is exactly what produced the test results given in post #1.

The OPA627 ties to the positive rail, as the datasheet has a diagram of its internals I think it was the proper choice, simulation backs that up. As the OPA134 did not have such a schematic of its internals in the datasheet, it was biased to the negative rail through an educated guess, and what simulated best.

Being spice, since they're both tied to a perfect voltage source, it simulates a perfect current source. For a real implementation I'd change that to a cascode Jfet + bias resistor, which I think is the most robust way of doing it. There's alot of info out there on how to do it.

Also the IRF540 mosfets won't do. I'd recommend trying something like Fairchild's FDB3632 for it, as a starting point. It's a 100V 80A device with 0.009 Ohms Ron with Qg total of 84nC, should have a good charge ratio as well but I didn't check. They do provide a model of it, but from what I've experienced, they've tried to make the models so advanced that I've found it impossible to force them to converge.
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Old 1st September 2004, 08:43 AM   #4
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Here's the Orcad Capture *.DSN file.

Cheers,
Chris
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Old 1st September 2004, 08:57 AM   #5
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Hi,

Saving your schematic as 4-bit gif files is more practical than using zip.

How on earth are you going to make a cascode work with the bases tied together? If it passes any current at all, the bottom transistor is in complete saturation.

B.
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Old 1st September 2004, 09:13 AM   #6
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Hi,

If I saved it as a gif there'd be a loss of quality and the ability to zoom right in would be lost, which means no values would be seen.

Wasn't happy with the biasing schemes I had tried for the cascode, they weren't very elegant, and that seemed to work great, I hadn't realized it would cause the other transistor to saturate. Might look into a clamp circuit . Hmmm... two diodes might do it... but I need sleep. I'll give it more thought later.

Thanks for pointing that out,
Regards,
Chris
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Old 1st September 2004, 09:16 AM   #7
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Quote:
Originally posted by classd4sure
If I saved it as a gif there'd be a loss of quality and the ability to zoom right in would be lost, which means no values would be seen.

Wasn't happy with the biasing schemes I had tried for the cascode, they weren't very elegant, and that seemed to work great, I hadn't realized it would cause the other transistor to saturate. Might look into a clamp circuit .
Gifs are lossless. I wasn't talking about jpeg.

Simulation models generally fall apart in saturation. Circuits that simulate well may not even work at all in real life.
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Old 1st September 2004, 10:27 AM   #8
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Quote:
Originally posted by Bruno Putzeys

Gifs are lossless. I wasn't talking about jpeg.

Simulation models generally fall apart in saturation. Circuits that simulate well may not even work at all in real life.
I wasn't talking about jpg either.... it is 5AM though.

You're right of course, no quality is lost. I'm also wrong in that you can't zoom in with them, you can.

A problem with Gifs (and jpeg) I've encountered in the past is, while compressed very well, they sometimes wind up being too big, and further zipping them does nothing, or makes them bigger, so I just tried zipping the bmp first and it was a decent size.. out it went.

Anyway, back to business, I tried the two diode idea, elegant as I can think of..

DC COMPONENT = 7.427910E-04
TOTAL HARMONIC DISTORTION = 5.485285E-03 PERCENT

That's 500mV input at 20Khz, looks great but shoot through is through the roof now, those results are not comparable.


It needs to be re optimised, but not today.. kind of pointless doing it anyway since spice isn't very exact, I kind of view it as a good ballpark figure to start with in a real circuit, meaningless as it is.

Ah well, here's the change I made, should do the trick.

Regards,
Chris

PS:

The image that you have attached is too big. Please make it no bigger than 800 x 1200.

Siiiiiiiiiiiigh
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Old 1st September 2004, 11:19 AM   #9
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Quote:
Originally posted by Bruno Putzeys
Hi,

Saving your schematic as 4-bit gif files is more practical than using zip...
4-bit PNG would be even better (smaller).


Quote:
Originally posted by classd4sure
...A problem with Gifs (and jpeg) I've encountered in the past is, while compressed very well, they sometimes wind up being too big, and further zipping them does nothing, or makes them bigger, so I just tried zipping the bmp first and it was a decent size.. out it went...
Zipping GIFs and JPEGs doesn't do much because they are already compressed. In that case you don't have to zip them at all.
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Old 1st September 2004, 11:29 AM   #10
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Hi there,

Quote:
Zipping GIFs and JPEGs doesn't do much because they are already compressed. In that case you don't have to zip them at all.
Unless you get this "The image that you have attached is too big. Please make it no bigger than 800 x 1200." Though.. rotating it might have been good for a laugh.
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