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Old 5th November 2005, 11:49 PM   #1
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Lightbulb Class D amp using TL494

Quote:
Originally posted by Pierre
Conclusion: PWM chips CAN be used for Class-D half bridge designs, but several questions must be had in mind, such as WHERE to implement slow-start and overcurrent, as well as feedback. We will try to build a high-power one based on SG3525 (or UC3825 for better fidelity)

Best regards,
Pierre

Hi Guys,

Perhaps some of you have read the post of Lumanauw with a title "How to fix this sch?" then it was followed by several sch that was shown by Pierre's PDF sch, my UC3525, then Workhorse SG2524.

This is the the reason why I open up a new post and "Quoted" Pierres conclusion attached, because it simply sparks an encouraging challenge and interest on my part and perhaps some of you also regarding the implementation of soft start, over current protector, and the use of negative feedback in using the ubiquitous and cheap PWM controller chips.

The sch that I have just designed took almost two days to finish(seven cups of coffee) due to some challenging task like the elusive soft start, but fortunately, the over current and the negative fb were implemented.

I just want to emphasize that the circuit is not yet tried and tested "on the amplification side" but on the "bare TL494" alone(some caps and resistors) mounted on a breadboard + scope+ headphone, the sound is "WOW!". I preffer to use TL494 as one of my Pet b'coz it sounds more presentable compared to UC3525.

I am obliging you to download the PDF file of Texas Instrument with title "Designing switching voltage regulators with TL494" via Google b'coz most of your questions can be answered easily by reffering on that PDF file..

Also note that my schematic is my original design but you can use it freely for educational purposes or thesis alike. Of coarse you can modify or improvised it as you like b'coz it is an open source design and no copyright issue. However, I am not liable to any consequencial circumstances that may arise in using it or to accept a blame for damaging your property

Ok!!! Do you have the PDF now? If not yet then get it now and
lets talk about the sch....
There you go.............

Best regards,
Choy
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Old 6th November 2005, 12:56 AM   #2
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Hi guys,

I've noticed that when the over current starts, opamp NE5532 will shutdown the SD pin of IR2011 to logic 1 but the TL494 will still inject signal to the i/p of IR2011, so I add extra bias to the unused opamp of Tl494 so that this will forced down the o/p duty cycle of TL494 to minimum....

Do you have the PDF now? Then lets rock!!
Here is the added correction for it..
Click the image to open in full size.

Cheers
Choy
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Old 6th November 2005, 09:36 AM   #3
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Hi Ledmania,

Have you seen "A diode is connected between the output of error amp and input of comparator" in the datasheet of TL494....

I think this diode might restrict analogue signal funtioning properly.....


Your Schematic:
The Feedback isn't referenced to exact midpoint "as seen by TL494", its taken from the output ....whose level of voltage isn't in co-relation to GND....Therefore a Level Shifter is must required at the output of TL494 to reference its output to -VRail of mosfets.....

Instead of TL494 you should try UC3825B from TEXAS INSTRUMENTS for much better results in HI-FI domain .....We are also working on this Chip based kilowatt level power outputs.....

regards,
K a n w a r
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Old 6th November 2005, 11:21 AM   #4
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Hi Kenwar,
thanks for looking in the schematic. I am really looking for a sharp guys like you to comment on the drawing because more comments means more corrections will be made.

Yes I am fully aware of the diode connected at the o/p of the two error amps but as I have mentioned earlier that the real PDF that we must use should be the one that shows the detailed "transistor" connections of the entire circuit which I have a copy of it. According to my PDF, the diode that you are reffering to is a transistor buffer connected as "emiter follower" to block each of their two positive going signal in case the user use them both.
So there will be no restriction on any analog signal that will pass there.

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...

Kenwar, this is a nice topic but I am "obliging" you to download the PDF file mentioned. You will see that there are more than 100 transistors that were use inside the chip and you can easily grasp any application you want on that chip if you have that PDF.

On your last paragraph, YES, That UC3825 is my most favorite choice of chip but they are not availble here and you have to order online .
The TL494 is very cheap and available here thats why I made it as my model circuit

Best regards,
ledmania
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Old 6th November 2005, 09:58 PM   #5
Pierre is offline Pierre  France
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I will look at your schematics in more detail tomorrow, but at first sight, it definitley needs level shifting. You are mixing two stages with different references: the input/modulator (around TL494), that is referenced to GND, and the output driver/mosfets, that are always referenced to the most negative point in the circuit, in your case, the negative rail. (it would simply bang! as you are shortcircuiting GND and -Vrail!)

I suppose you have got confused by Kanwar schematics, that don't use level-shifting: that's because it uses unipolar supply (that is, only one rail) referenced to GND, so this is the most negative potential and there is no problem in having the IR2xxx driver referenced to it, and then no level shifting is needed. This circuit has application only if there are two equal halves and speaker is connected differentially between both of them, with inputs shifted 180, so the speaker sees no DC.

But for simplicity, it is better to have a single half-bridge stage, so bipolar rails are needed and hence level shifting.

I will post a more advanced schematics for you to have things a little bit more clear.

Best regards,
Pierre
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Old 6th November 2005, 11:34 PM   #6
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Quote:
Originally posted by Pierre
I will look at your schematics in more detail tomorrow, but at first sight, it definitley needs level shifting. You are mixing two stages with different references: the input/modulator (around TL494), that is referenced to GND, and the output driver/mosfets, that are always referenced to the most negative point in the circuit, in your case, the negative rail. (it would simply bang! as you are shortcircuiting GND and -Vrail!)

Hi Pierre,

Please take a look at the sch once more and notice that there are two reference points of signal involved... The "arrow" symbol at the end terminal of the speaker... And the "inverted triangle" as the global ground...
Now, the signal on the o/p is passed onto the speaker referenced to neutral "arrow" symbol which is the center point of your supply.
But, the small signal is commutated at the i/p of the error amp with reference to global groung (inverted triangle).

Naturally, all of the biasing and i/p schemes must be accomplished in reference to global ground(inverted triangle).
Now, take a look at the sch again that the feedback was tapped at the o/p which is 1/2 of VCC referenced to global ground, then voltage divide by R14 and R15 to get that 2v required to bias the error amp i/p with referenced to ground again. It is simple isnt it?

Now, from the o/p side of the TL494 that triggers the two scmitt inverters then triggers the hi side and low side of IR2011, every thing was referenced to digital global ground and levelshifting is not needed.
All hi/lo side fet drivers used levelshifting to interface their o/p to a much higher voltage while preserving their digital input capability. this means that both inputs doesnt need level shifting and must be triggered with referenced to global ground.

best regards
ledmania
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Old 7th November 2005, 12:51 AM   #7
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Default another correction on my sch

Hi,

I've noticed that when the over current occurs, the o/p of NE5532 may source to its own Vcc (12V) that may overdrive the pin 1 i/p of TL494, so I change it to 5V (to be connected to pin 14 of TL494).

So, there you go....
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Cheers
ledmania
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Old 7th November 2005, 05:38 AM   #8
Pierre is offline Pierre  France
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Let's see.
Please correct me if I am misunderstanding your sch, but I see this:
- You have bipolar rails, -50VDC and +50VDC.
- Pins 2 and 13 of the IR2010 (COM and VSS, respectively), are joined together, which is OK as VSS must be the same as COM (most negative point). So your "12VDC" must be referrenced to the negative rail, or you will be feeding the input stage of the IR2010 with 62V !!!
- Your "inverted triangle" symbol is at the same potential as -50V, or at the same potential as supply center tap (GND) ?

Please clarify this and if all is true so far, I can keep trying to debug the sch.
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Old 7th November 2005, 08:28 AM   #9
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Quote:
Originally posted by Pierre
Let's see.
Please correct me if I am misunderstanding your sch, but I see this:
- You have bipolar rails, -50VDC and +50VDC.
- Pins 2 and 13 of the IR2010 (COM and VSS, respectively), are joined together, which is OK as VSS must be the same as COM (most negative point). So your "12VDC" must be referrenced to the negative rail, or you will be feeding the input stage of the IR2010 with 62V !!!
- Your "inverted triangle" symbol is at the same potential as -50V, or at the same potential as supply center tap (GND) ?



Pierre,
-pins 2 and 13 of IR2010 are joined together and is common to global ground which is -50V.
-all terminal nodes with a "inverted triangle" are all connected to global ground which is -50V.
Now, The VCC and VDD of both TL494 and IR2010 are all tied to this +12V node..please take note that the -12V is also tied to -50V as the digital global ground.
now,when TL494 triggers IR2010's i/p to logic "1", this means Tl494 will source +12V(set aside Vdrop) on its o/p relative to global ground. There is no way you could get 62V.
Same is true when it triggers to logic "0", It mean sink or Vsat.. there is also no way you could get 62V here either.

Pierre, please dont be confused with "neutral" arrow symbol of the center tap of the supply because this is "exclussively" for speaker neutral only. Nothing more, nothing less and should not be included in analyzing any signal flow with global ground. Just leave it alone and forget it


Quote:
Originally posted by Pierre

Please clarify this and if all is true so far, I can keep trying to debug the sch.
Thats rights Pierre, I want someone will scrutinize it, debug it, or comment on it so that more comments means more corrections will be made, then more beter amp will come forth...

Regards,
ledmania
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Old 7th November 2005, 09:17 AM   #10
Pierre is offline Pierre  France
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About the basic setup of the circuit:

Quote:
all terminal nodes with a "inverted triangle" are all connected to global ground which is -50V.
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.

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.
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