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-   -   New to PWM. (http://www.diyaudio.com/forums/class-d/79787-new-pwm.html)

darkfenriz 18th May 2006 10:53 AM

New to PWM.
 
2 Attachment(s)
Hi
I would love to learn something about class D amplifiers, but it still seems to be kind of esoteric knowledge. There is still not much "know how" or "how to build" information, at least I didn't find much.
I think I know some basical basics of basic class D amplifier design, probably still not enough to design one myself. I played a little bit with a simulator but I don't trust stupid digital models, life showed that sims cannot predict switching of even small signal bjt's.
The outcome schematic is in the attachment, I hope you all class D gurus will share a bit of knowledge to help a newbie.

I have several questions:
1. Would it be potentially workable at all?
2. If so, would it be worth building?
3. What are general ways to avoid cross-conduction keeping dead time short and slew rate high? Any suggestions how to improve the schematic below?
4. How to know that class D amp using bootstrap is going to explode on start-up or not?

... and much more...
I am really looking forward to suggestions and discussion.

best regards
Adam

darkfenriz 18th May 2006 01:27 PM

oooops!
zeners should be of much higher voltage.

regards
Adam

kartino 19th May 2006 12:58 AM

Hi, finally......

Adam, please to try your schematic with simulation. LTSpice (Switchercad) will be good to know your questions.

Because the speed of switch that very high, a discrete class-d needed to be simulated and fine tuned until you have good output result, fastest switching, less cross conduction, etc.

darkfenriz 19th May 2006 06:14 AM

Quote:

Because the speed of switch that very high, a discrete class-d needed to be simulated and fine tuned until you have good output result, fastest switching, less cross conduction, etc.
And what if I get satisfactory simulation results? Should I be assured to be able to put schematic to really working circuit? With similar parameters?

regards

kartino 19th May 2006 06:29 AM

Fist, you will know that your circuit works, then you push to the limit during fine tuning.

After that then you make a real test. During real test it may you will find problem, such as heat, cross conduction, so you also make fine tuning on real.

classd4sure 19th May 2006 06:52 AM

Yeah, by analyzing every limitation of the first simulation, you stand a good chance of learning ways of circumventing them one at a time in order to achieve optimal theoretical performance.. which a few exceptions to the real world.

Then compare the first real world prototype performance via test and measurement, to the simulation ideal. Use your powers of obsvervation combined with what you know in theory through research, and possibly collaboration via collegues aand this place, to work out the understanding that will allow the final optimal solution before posting it to us all :)

darkfenriz 19th May 2006 09:00 PM

Thank you for responses
I conclude that you cannot tell any possible pitfalls or blatant mistakes from schematic, right?

BWRX 19th May 2006 09:53 PM

You might want to decrease the value of the inductor a bit. 80uH is pretty high and will have a lot of copper loss unless you want to use heavier gauge wire and end up with a huge inductor.

darkfenriz 20th May 2006 03:39 PM

Thanks Brian
Sounds reasonable.
Do you know any 'thumb' rule to design output filter, like, say: 120% nominal load resistance, 1/3 of switching freq., Butterworth (blind shot here)?

Eva 20th May 2006 05:17 PM

Forget abut simulating discrete switching circuits. Simulation models are intended for linear applications only and you will get weird surprises if you try to build what the simulator tells to be optimum.

What you can simulate with proper software is class D control strategies, but always in a theoretical basis with ideal components.


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