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Old 30th July 2009, 02:32 AM   #1
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Default Doing a class D Amp project using TL494

I'm building a class D amp using the following components
TL494, IR2110 Gate driver IRF540N mosfets.

I've never worked with this chip before and need some help. I've looked up on the "designing switching voltage regulators with the Tl494" guide and understand how to implement the oscillator components and also how to implement Dead-Time. But i do not understand how to actually implement the musical signal itself.

Do i just implement the musical signal into one of the error amplifiers (+'ve input of the error amplifier) and ground the other input (-'ve) ?

I couldn't find a application note on using this chip to create a class d amp, hence my confusion with implementing the music. I've also tried to follow how ledmania implemented the signal but don't understand why he has a reference voltage signal in the the negative input of the error amp with the music signal in the positive input.

http://www.diyaudio.com/forums/attac...amp=1131481319

Here's a quick spice model of what i've done, though it doesn't run. Gives me errors.

http://img338.imageshack.us/img338/7121/attempt1w.png

In the schematic diagram above, I implemented the Fail-safe operation for the oscillator as specified in the design guide.

I wanted to simulate a 1Khz music freq and i'm running it on a single 20V power supply.

I will worry about Dead-time circuit and feedback and the other goodies later once i get the fundamental's corrected.


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Old 30th July 2009, 10:09 AM   #2
Eva is offline Eva  Spain
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Far too many mistakes and misconceptions.

Have you ever heard about control loops, error amplifiers and frequency compensation? You seem to be overlooking everything related to these subjects.

Learn elementary electronics first

Then do some discrete linear amplifiers, then some switching regulators and SMPS, then you would be ready to start with class D
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Old 30th July 2009, 02:24 PM   #3
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In further reading off ledmania's thread

Quote:
"the data sheet says that "when the o/p of the error amp modulates from 0.5V to 3.5V, the o/p duty cycle will vary from 97% to 0%"in reverse order! So the proper bias point on this is to get the average voltage of 0.5V+3.5Vdevide by 2,you will get 2V. this will correspond to an o/p of 48.5%.bingo!.
So, 2V should be the quiescent bias voltage at the i/p..."

So now i understand why he's using voltage divider into the inputs of the error amplifier. But he's incorporating it as a feedback system. While i would like to initially test it as a open-loop system. So if i use Vref ( 5V ) and use voltage divider rule to get 2V from that, into one of the inputs of the error amplifier (Negative input - Pin 15). And in the other input (Pin 16) the music is fed through, do i have to make sure that the voltage is also 2V in this input (i.e Pin 16)?

If so i have to make sure the source music signal is 2V?

Also what's with the 3.3uF cap and 2.2K resistor which the music signal is put through?

What's the purpose of the other error amplifier (Pin 1 and 2)?
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Old 30th July 2009, 02:24 PM   #4
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I have limited knowledge of control loop and error amplifier, but not for freq compensation.
The trouble is it's a actual school project that i have to complete by the end of the semester.

Any tips then on taking it little bits at a time with the TL494?

For example i was thinking i would just get the oscillator started and running and put a music signal in and use earphones like Ledmania used to check out initially to see if i've implemented it correctly?

Would this be a good starting point?
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Old 30th July 2009, 02:24 PM   #5
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sorry should have added this as well to my last post.

I understand the basics of a class D amplifier..

The musical signal is compared with a high freq carrier (sawtooth or triangular) signal. This generates pulse width modulation, which is then fed into Gate drivers to operate the output MOSFETS. Harmonic distortion is introduced because of the switching action. The LC filter at the output filters the switched waveform from the MOSFETS to a sine wave (in a ideal world), before it's sent to a speaker.

A feedback is generally used to minimize errors at the output. Feedback is achieved usually by taking either the output right before it's filtered or after it's filtered, and filtering that by means of a passive RC filter and integrating that with the musical signal.

At the moment i'm not worried about introducing gate drivers, mosfets and such. I would like to just get the TL494 itself started and running.
I'm not too sure if my interpretation of Ledmania first post is correct, but it sounds like he just used earphone to listen to the output from the collector C1 from the Tl494, without introducing gate drivers and such into the circuit.
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Old 30th July 2009, 02:47 PM   #6
wg_ski is offline wg_ski  United States
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One of the biggest problems with using one of these switching regulator chips for an audio amp is the limited duty cycle. Ones designed for push-pull (or half bridge) have maximum duty cycle of just under 50%. You really want 50% quiescent (minus dead time), and to be able to swing both higher and lower, approaching 0% and 100%.

I once demonstarted to myself that it works, but what I did to start with was "regulate" 20 volts from an 80 volt supply. Inject audio at the reference voltage (2V reference, gain of 10X) and get a shade under 40V p-p. LC filter, cap coupled output. It actually sounded cleaner than I expected considering that it was thrown together for the heck of it.
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Old 30th July 2009, 04:58 PM   #7
Eva is offline Eva  Spain
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TL494 is a bad choice, for example it uses sawtooth instead of a triangle wave. A simple class D amplifier with quite good performance can be built around just a fast comparator. Do some research on the UcD concept. It's a phase shift oscillator that amplifies audio and damps output filter resonance to flat response regardless of speaker impedance in the process...

Open loop amplifiers are a source of trouble, even getting no DC at the output is a challenge.
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Old 30th July 2009, 07:51 PM   #8
Pafi is offline Pafi  Hungary
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circuitcity!

Quote:
Here's a quick spice model of what i've done, though it doesn't run. Gives me errors.

http://img338.imageshack.us/img338/7121/attempt1w.png
Yes, because it contains many errors! For example: what is R2 for?!? Why did you attach + and - inputs of OPAs together? You should never do this! Have you learned about OPAs? Etc...

Ledmania's circuit is faulty also.

try this one!

Quote:
In the schematic diagram above, I implemented the Fail-safe operation for the oscillator as specified in the design guide.
Which is that design guide?

wg_ski!

TL494 is limited to 0...90 % if you connect pin13 to GND.

It's usable, but with care.
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Old 30th July 2009, 10:19 PM   #9
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Quote:
Originally posted by Eva
TL494 is a bad choice, for example it uses sawtooth instead of a triangle wave. A simple class D amplifier with quite good performance can be built around just a fast comparator. Do some research on the UcD concept. It's a phase shift oscillator that amplifies audio and damps output filter resonance to flat response regardless of speaker impedance in the process...

Open loop amplifiers are a source of trouble, even getting no DC at the output is a challenge.

Yes, from the theory i read in books and IEEE, triangular wave carrier is the better option. But i'm just trying to implement a amplifier for a 25W speaker, purposely for academic reasons, not a high powered amplifier. Also i'm aiming for a oscillator freq of 200Khz.

Ok i'll look into the UCD concept as well.

Quote:
Originally posted by Pafi

Yes, because it contains many errors! For example: what is R2 for?!? Why did you attach + and - inputs of OPAs together? You should never do this! Have you learned about OPAs? Etc...

try this one!
Ok i'll give the corrections you made, as a start to this project. I
ll try this configuration out.
Also your referring to Op Amp when you say OPA? We've covered op-amp theory in class.

Quote:
Which is that design guide?
I used the "Designing switching voltage regulators" guide off ti.com

Also i'm thinking of using a function generator as my music signal for the moment. Is this ok?
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Old 30th July 2009, 10:19 PM   #10
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Quote:
Originally posted by Eva
A simple class D amplifier with quite good performance can be built around just a fast comparator. Do some research on the UcD concept. It's a phase shift oscillator that amplifies audio and damps output filter resonance to flat response regardless of speaker impedance in the process...
I'm sorry, but a quick google search shows me UCD is a pre-built amp.
Have i looked it up wrong?
Also can you give me example models of fast comparators. I'll look into them and some schematics perhaps as guide.

All I'm attempting to get out is a simple structured class D amplifier. I'm definitely not ready for complex structured class d amp.

A very simple class D amp that'll do the job at the end of day is enough for me.
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