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Old 8th November 2004, 03:11 PM   #1
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Lightbulb Thinking of starting a 'new' Class D design.

I don't really know what to say but I need a prod up the behind to actually get down and do this..... please give me one.

I'll be posting the design info at...

http://groups.yahoo.com/group/classdamplifier/

The rest is open to discussion.

I'm going to be doing a clocked thing using PWM with multiple stages operated in phase shifted parallel, if that makes sense.

I've got LTspice and some other bits as well as schematic capture and PCB design software (I can release Gerbers). I'll try to present the documentation in PDF format



Ask some questions and we'll see how it goes. All and any input appreciated.


Thanks

DNA
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Old 13th November 2004, 05:04 AM   #2
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Default phase shifted parallel = poly phase?

Do you mean it will be set up like a multi phase switcher, similar to the latest CPU power supplies?
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Old 13th November 2004, 09:15 PM   #3
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Default Re: phase shifted parallel = poly phase?

Quote:
Originally posted by thespeakerguy
Do you mean it will be set up like a multi phase switcher, similar to the latest CPU power supplies?

Yes.

I'll be using a number of 'modules' to share the output.

Each will be operated using current mode control and provide its share of the output current according to an overall demand signal.

DNA
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Old 14th November 2004, 03:49 AM   #4
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Awesome, I'd like to see this in action
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Old 17th November 2004, 03:36 PM   #5
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I played around with a multi phase design for a while but always found that the changing output impedance resulted in distortion folding down into the audio band. To see this effect in simulation I had to add non-linear inductors and model speaker back EMF effects.

I'm sure there are solutions for these problems but I gave up on it because there did not seem to be any advantage and there are a lot of added costs.

ClassDunce
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Old 17th November 2004, 04:50 PM   #6
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Quote:
Originally posted by ClassDunce
I played around with a multi phase design for a while but always found that the changing output impedance resulted in distortion folding down into the audio band. To see this effect in simulation I had to add non-linear inductors and model speaker back EMF effects.

I'm sure there are solutions for these problems but I gave up on it because there did not seem to be any advantage and there are a lot of added costs.

ClassDunce
I'm not sure I understand what you are saying.

However, if you want to do a 'multi phase' design you have to bear in mind that the filter inductors in each stage integrate the volt seconds applied to them to give a final output current.

If you simply apply the same, but phase shifted, pwm signal to each stage then it is likely that one of the stages will 'hog' the output current because off differential delays and switching times.

If you add a current mode control loop to each stage then you can force them to share the current.

DNA
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Old 10th May 2005, 11:04 AM   #7
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Default Long time, haven't done much

Got back to it and started writing some of the theory behind what I'm doing.

Done a bit of design work to try and get things together and then modelled things in spice using 'proper' bits and pieces.

The piccy is an FFT of four stages operated in parallel with current sharing. The base modulator frequency is 200KHz for an effective 800KHz.

You can see a bit of a lump at 200KHz with the rest of the switching hash up at 800KHz. These components are 40dB down or about 10mV rms(?). That's one of the reasons for doing it this way.

This is 1.5Vpp in for 60Vpp out at 10KHz off 80V rails. Mosfets are IRF640/9640.

If the theory, and small signal model, agree with the switching model then the -3dB bandwidth is about 200KHz. I'd be bothered about current slew rate limiting.

As is always the case with spice.... don't believe everything you see. I'm fairly confident that things are not too wide of the mark.

DNA
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Old 10th May 2005, 11:36 AM   #8
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This is the output voltage at start up along with the stage currents.

Each stage here is modelled the same as all the others but D gets caught first and is then brought back into line with the others by the control loops.

The spikey bits are body source diode reverse recovery.

DNA
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Old 10th May 2005, 12:04 PM   #9
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Here's the 10KHz square wave response.

The peaking is present in the linear model as well, I've had to clamp the voltage error amplifier to tame the result in the switching model.

Oh, it's a 6R resistive load.

DNA
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Old 10th May 2005, 12:05 PM   #10
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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
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