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Class D Switching Power Amplifiers and Power D/A conversion

Class D amp using TL494
Class D amp using TL494
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Old 7th November 2005, 11:02 AM   #11
Pierre is offline Pierre  France
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I have set-up a small circuit for sumulation of the offset, gain, etc, issues.
I have tried your feedback proposal (of course, with the error amplifier polarity corrected, that is, entering by the - input), and the problem is the one I suspected:
You can adjust the voltage divider resistors for zero offset at the output, but as soon as the rails move a little bit, the adjustment is ruined and it is not corrected by the feedback loop.
You need to relate all to a estable reference, like Vref. But that's not easy without level shifting.

If you see my proposed simulation schematics, I found a way to have no DC offset at the output, with a unipolar-fed input stage, such as a PWM controller. But it was sitting on GND , not -Vss, and hence required level shifting.
I doubt if it worths the pain to try to elliminate the level shifting stage at any cost: in fact it only adds a transistor and a couple of resistors and solves a lot of problems, so I would leave it and reference the input circuitry to GND (0V)!
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Old 7th November 2005, 11:27 AM   #12
ledmania is offline ledmania  Philippines
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Quote:
Originally posted by Pierre
About the basic setup of the circuit:



That's what I wanted to confirm: if you are referencing all to -50V, that's ok (I tried that once) , but you will have to be very careful with the design of feedback.

Now with the feedback setup:

As you have pointed out, your 50% duty cycle point will be _around_ 2V input to the error opamp, producing 0V at the output.
So, if you have 0V at the output, you will have 50V difference between output and -50V rail, ok so far? Then your voltage at junction between R14 and R15 will be 50*400/(4600+400)=4V, not 2V (referred to -50V, of course). So you need to modify the resistors, for example, R15=220 ohm and R14=4.7k aprox.
Anyway, I don't like this way of introducing the offset, as it will be dependant on supply voltage. Perhaps you should try something related to Vref, that is quite estable.
Pierre nooo!
Maybe you lost your eye glasses again! LOL!
Please verify your comment on your first paragraph....
Quote:
Originally posted by Pierre

So, if you have 0V at the output, you will have 50V difference between output and -50V rail, ok so far?
I absolutely disagree because at 50% duty cycle(no music) the o/p after the filter is 0 in referenced to center tap neutral....But!!! On the o/p referenced to global ground.....drum roll... you will get +25V...
look at the sch please. did you see the 25V???
This 25V is devided by R15 and R14 to seat at 2 volts..Please recalculate you ohms law here now.

Quote:
Originally posted by Pierre

Let's continue...

That signal is then lowpass-filtered by 22k+100pF (72KHz aprox), and enters to the positive input of TL494. Shouldn't it go to the negative input? (Yes, it should IMHO). So I think you have connected the inputs of the error amplifier backwards.

Now it rests to check if the polarity of the PWM signals is ok or backwards. Remember that the output stage is inverting as you have connected it.
Pierre,
Did you download the PDF that I mentioned on the very beginning of this post? if not then "GOTO" my reply to Kenwar regarding the input of the error amp relative to its o/p duty cycle...the o/p is reverse in response to its output....

Regards
ledmania
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Old 7th November 2005, 11:46 AM   #13
ledmania is offline ledmania  Philippines
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Pierre,
Here is my reply based on your comment on the non inverting input that I use....Please read it carefully...


Quote:
Originally posted by ledmania



On the next paragraph of you comment regarding midpoint bias of the error amp, 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...

Best regards,
ledmania
Regards
ledmania
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Old 7th November 2005, 12:36 PM   #14
Pierre is offline Pierre  France
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Let's see if we are looking at the same sch...
I am looking at the attached image in Post #1 of this same Thread. Is this correct?
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Old 7th November 2005, 06:04 PM   #15
powerbecker is offline powerbecker  Germany
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Hi ledmania,

nice idea, do you have a "working" simulation from it?
Can you put it here with symbols and subcircuits?
I think it will work BUT only if you have c o n s t a n t powersupplys.
If not, you need a 2. divider from ps-midpoint to get a new reference!
Iam too lazy to check all polaritys.
May be you need some C between pin 3 and 15 from TL494.
Regards
Heinz!
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Old 7th November 2005, 11:03 PM   #16
ledmania is offline ledmania  Philippines
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Quote:
Originally posted by Pierre
Let's see if we are looking at the same sch...
I am looking at the attached image in Post #1 of this same Thread. Is this correct?
Well, I hope that you are looking at the right sch because I dont see any other sch around here except mine. Unless you accidentally hit lumanauw's thread button. LOL!

Pierre,
Yes you are looking at my post #1..
Please pay attention to the detail of the sch and contemplate on it carefully because there are some flaw on it but I'm not going to exposed it yet because I'm waiting for more adept guys that will notice it.
I think that my kirchhoffs voltage law know how is just working fine so the voltage drops on each identified nodes are just ok.

Regards
ledmania
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Old 7th November 2005, 11:29 PM   #17
ledmania is offline ledmania  Philippines
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Quote:
Originally posted by powerbecker
Hi ledmania,

nice idea, do you have a "working" simulation from it?
Can you put it here with symbols and subcircuits?
I think it will work BUT only if you have c o n s t a n t powersupplys.
If not, you need a 2. divider from ps-midpoint to get a new reference!
Iam too lazy to check all polaritys.
May be you need some C between pin 3 and 15 from TL494.
Regards
Heinz!
Hi Hienz!

I have no working simulations on it because I dont have the software yet but I hope soon. I just used my old technic of using breadboard+scope+headphone.

BTW, Thank you for your suggestion in putting C on pin 3 and 15 of TL494 but I think it will suppress the hi frequency response of the opamp that may negate the concept of full spectrum hi fi in which this sch was intended to.

Regards
ledmania
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Old 7th November 2005, 11:31 PM   #18
Perry Babin is offline Perry Babin  United States
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I don't know if this has been discussed but...

What about the fact that the 494 will only go to about 96% duty cycle?

There is a voltage source in the DT input. To overcome it, you can run pin 4 slightly below ground. You have to be careful not to go too low because the IC could be damaged. Simply go low enough to overcome the voltage source (~0.15 volts).
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Old 8th November 2005, 01:00 AM   #19
ledmania is offline ledmania  Philippines
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Quote:
Originally posted by Perry Babin
I don't know if this has been discussed but...

What about the fact that the 494 will only go to about 96% duty cycle?

There is a voltage source in the DT input. To overcome it, you can run pin 4 slightly below ground. You have to be careful not to go too low because the IC could be damaged. Simply go low enough to overcome the voltage source (~0.15 volts).
Hi Perry,

Your first question..er..concern I mean, about the chips duty cycle has a lot of sense indeed.
This is based on the fact that the o/p cant go less than 3%duty at 0 volt and 100%duty at 3V on pin 4(b'coz of that 110mv offset voltage).
Based on my scope, the chip cant go higher than 95% at 200khz on my proto using pin 4 as the modulating source.

But hey Perry, even you disable the two error amps and use pin 4 dead time pin as your sound i/p, with simple potentiometer bias on Vref, it still produce WOW! sound. LOL! And yes its true.

Humm, I'm going to give a try on your suggestion on u'r second paragraph..

Well!!, Nice shot Perry..

Regards
ledmania
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Old 8th November 2005, 04:38 AM   #20
Workhorse is offline Workhorse
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Hi LEDMANIA,

I am Kanwar ...not Kenwar ...Mind it....

Yes you were right about the Diode was an emmiter follower....
The design topology you are using isn't worth because it has many basic Flaws.... which aren't simply to ignore,

Pierre has showed it you, i think....



Hi Pierre,

Even if you referenced the TL494 to GND ...Its input NODE and the Feedback Node From Speaker Aren' t at perfect Zero ref with each other , because the GND of error amp is actually its -Vrail and Vref is +Vrail...You have to ascertain that and it requires carefull understanding for that too

The output of Half Bridge is cocentric with GND...not VREF/2..and for proper operation inverting node of error amp must be at Vref/2 cocentric with output node of HB...

regards,
K a n w a r
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