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Class D Switching Power Amplifiers and Power D/A conversion

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Old 22nd September 2007, 03:09 PM   #1
Cyguy84 is offline Cyguy84  United States
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Default 25 W Amplifier for Senior Project, due Friday

Short Version:

I am using a full bridge design, and I have PWM signals coming from my output NMOS to the inductors. Before the inductor, it is a 0-20V PWM signal... after the inductors, it is a 5-20V mostly-sine wave.

The resulting differential signal across the 8 ohm resistor has Vpp of 27V-- resulting in 11.7W.

I need 25W to get a B. That means I need the Vpp to be closer to 40. How can I get less voltage drop across this inductor?

(Inductor value: 220 uH. Capacitor value: 4uF.
Set up in typical
L-R-L
-C-
arrangement with capacitors to ground as well.)



Long version:
Hello all--

First I would like to apologize for the rushed nature of this post. I was trying to design the entire thing on my own, but I have run into a problem that I can't solve, so I would like to get a B instead of a D.

Background: I started this project with a partner, we were to build the entire amplifier over the summer. It was my partners idea, so I was basically letting him set the pace. Bad idea-- he got kicked out of my major with a GPA below 2.0, and I was left to finish his half of the project.

I have built a working Class D amplifier, full bridge, using a triangle wave and a comparator, but the output is nothing to be proud of. That was his half of the project, so I am learning it on the fly.

In order to get a B, I need 25W. I was able to get 11.7W today, but I would like any sort of expert advice on how to improve it.


Output:
For now, I think it is safe to ignore all of the circuitry before the Gate drivers. It is a basic Preamp design, with a triangle wave and a comparator, and it works within the range I need it to.

The gate drivers are Fairchild 73832 parts. I have a .22 uF capacitor acting as the boostrap capacitor to drive the high side NMOS. The NMOS are Fairchild FDB8447 parts. If you haven't noticed, I get free Fairchild stuff

I have the Gate Driver running on a 15V supply, while the NMOS power supply goes from 20V to 0V. I can increase or decrease this within any reasonable limit.


Output Filter:
I had to scurry about and make the full bridge design work, so I didn't plan things out as well as I would typically like. I took the basic design that I have seen for this sort of load-- inductors in series with the load, with a capacitor in parallel, and two capacitors to ground.

I only need the low pass filter to pass 10Hz-2kHz input, so I think my values are appropriate. Doubling the 4uF and halfing the 8 ohm load gives a basic LC filter with 220uH, 8 uF and 4 ohm load. The cutoff frequency should be somewhere around 4kHz.


If anyone wants more information, I can gladly fill in the rest of the circuit-- I am a bit ashamed of it, as it is very crude and basic, but as I said-- I had to just throw things together a bit.


Any advice on any part of this output stage would be awesome. Thank you very much!
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Old 22nd September 2007, 05:46 PM   #2
Cyguy84 is offline Cyguy84  United States
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Old 23rd September 2007, 08:46 PM   #3
Tim__x is offline Tim__x  Canada
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What's your switching frequency and your signal frequency?
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Old 24th September 2007, 02:09 AM   #4
Cyguy84 is offline Cyguy84  United States
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Switching at only 20kHz, as my input has to be between 10Hz and 2 kHz.

I can change the switching frequency up to 1 MHz with the chip I am using, but I would only do that to get full audio, which I don't need for B grade.
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Old 24th September 2007, 02:43 AM   #5
BWRX is offline BWRX  United States
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Try increasing the switching frequency to say 100kHz and raising the cutoff frequency of the output filter (so you can use smaller inductors and caps).

A good rule of thumb is to a use a switching frequency that is at least 10x the highest frequency you want to reproduce.
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Old 24th September 2007, 03:23 AM   #6
Cyguy84 is offline Cyguy84  United States
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At what point will the increase in the switching frequency start causing trouble?

I can change the switching frequency basically at will-- just slap a new cap in there and it works. But the cutoff filter is the hard part, I have to order new parts if I want to change that. And I only have a week.

I think I will try pumping the switching frequency to about 40kHz, so 20X... and leaving the filter. (FYI, I put a .47 uF cap in the filter, instead of the 4, and it changed the output but did not increase the power... so I can't increase the power by moving the cutoff frequency unless I order a new inductor, I guess)

Any more advice? Thanks for the feedback, I really appreciate it-- I forgot completely about the switching frequency, and there are probably more things you guys can point out.
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Old 24th September 2007, 03:24 AM   #7
Cyguy84 is offline Cyguy84  United States
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And I phrased it wrong, but I meant that as in I will test it tomorrow, at 40kHz and with the same components, because I would have to order new ones.


If that doesn't help, I'll order some new inductors and maybe go for full audio band.
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Old 24th September 2007, 03:34 AM   #8
BWRX is offline BWRX  United States
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If you are using toroidal inductors you can just unwind some turns to lower the inductance.

Higher switching frequencies require better PCB layout and closer consideration of parasitics. The 300-400kHz is a good range to aim for as far as class d audio amps are concerned (that's just my opinion). Higher is better, of course, because it shifts the switching frequency further away from the audio band and allows the use of a higher cutoff frequency (smaller inductors, capacitors).
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Old 24th September 2007, 12:18 PM   #9
Cyguy84 is offline Cyguy84  United States
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Yeah, this project is completely not up to my standards of work. I was told that we didn't have to do the PCB design until fall semester, so the project that I have now is just on circuit board.

There are inches of wires connecting the inductor to the resistor, and unfortunately, there is little I can do about it. Thats one reason I am trying to lessen the switching frequency for now.

Turns out the entire project needs to be built by now, but I think I will design it in PCB just for prides sake, even though it won't help my grade.
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Old 24th September 2007, 03:22 PM   #10
Cyguy84 is offline Cyguy84  United States
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Here is the current output. I'm getting 75% efficiency, I finally got 25W but I would like to clean it up a little.

Here is just the output waveform:
Click the image to open in full size.


Here is the + and - of the resistive load... what is causing all the craziness?
Click the image to open in full size.
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