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Class D Switching Power Amplifiers and Power D/A conversion

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Old 29th March 2009, 04:08 PM   #11
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Hey guys sorry i havn't got back to yous. I am tied up at the moment working on other projects, i will be working on the amp again tomorrow for a few hours. Thanks for the suggestions, i will keep yous posted regarding it.
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Old 1st April 2009, 11:52 PM   #12
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sorry i have taken so long to reply, i am just so busy with other projects. ok so i sat down with the amp today with the little time i had and took the following measurements:

Conditions:

Positive rail voltage: 20V
Negative rail voltage: -20V
No load
MOSFETS used: IRFB4019PbF


Measurements:

VAA(pin1) with respect to GND(pin2) = 7.3V
VAA(pin1) with respect to VSS(pin6) = 13.1V
VSS(pin6) with respect to GND(pin2) = -5.7V

VAA(pin1) with respect to COM(pin10) = 26.2V
VSS(pin6) with respect to COM(pin10) = 13.2V
VB(pin15) with respect to COM(pin10) = 38.9V
VS(pin13) with respect to COM(pin10) = 38.6V

CSD(pin5) with respect to GND(pin2) = 7.3V
Vref(pin7) with respect to GND(pin2) = -19.0V
Ocset(pin8) with respect to GND(pin2) = -19.0V
DT(pin9) with respect to GND(pin2) = -19.0V
Vcc(pin12) with respect to GND(pin2) = -19.0V
VS(pin13) with respect to GND(pin2) = 20V
VB(pin15) with respect to GND(pin2) = 20V
CSH(pin16) with respect to GND(pin2) = 20V


I have also attached some pictures of the breadboard layout and also a copy of the typical application circuit that i am basing my amp on. I am confused regarding the grounding and the different grounding potentials that exist in this design.
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File Type: zip breadboard1.zip (96.1 KB, 371 views)
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Old 1st April 2009, 11:55 PM   #13
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sorry about each image being zipped, i tried to downsize the pictures as much as possible.
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File Type: zip breadboard2.zip (72.0 KB, 255 views)
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Old 1st April 2009, 11:57 PM   #14
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labeled breadboard
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File Type: zip breadboard3.zip (93.5 KB, 217 views)
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Old 1st April 2009, 11:58 PM   #15
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application circuit
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File Type: zip application_circuit.zip (75.9 KB, 342 views)
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Old 2nd April 2009, 05:57 AM   #16
ssanmor is offline ssanmor  Spain
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Location: Madrid
Hello,
You are not likely going to get decent results with that layout, but, anyway, at least the DC measurements should be ok, and they are not.
For a start, your VB is not ok, you are getting 38V with respect to COM, when the IRS2092 has an internal 20V zener, so something is not connected properly around this point. It is not that the amplifier has not started or something and you don't get the proper Vb, it is that it doesn't seem to be connected to the proper IC pin.

Also, your output is stick to the positive rail. Please try with some load connected just in case it hasn't started oscillating, but to be sincere, I would not spend much more time until you fix the VB issue, because it is clearly indicating that you have some major errors.

Once this is ok, you WILL need an oscilloscope. Don't even think on debugging and putting it to work with only the measurements of a voltmeter.

You also say you don't understand the different references, and that's the most important point to have clear in a Class-D design, so please spend some time reading the datasheets throughly or ask here. Basically, you have a GND reference at the input of the chip, and the output is referenced to the negative rail (COM pin). Also, the VB is referenced to the switching pin (VS) that should swing from -Vbus to +Vbus.


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Old 2nd April 2009, 10:36 AM   #17
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Thanks very much for the comments. going to investigate the situtaion with VB now. regarding the layout is there any recconmendations you would make. I know it is not really ideal to build any type of high frequency circuit on a breadboard. I would be looking to implement this design/layout on to a PCB.
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Old 2nd April 2009, 04:57 PM   #18
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Is their an advantage to using external regulators to power VAA and VSS?
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Old 2nd April 2009, 05:21 PM   #19
ssanmor is offline ssanmor  Spain
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Yes, the PSRR will increase using external regulators. But, anyway, this is something you should take care off if your PSU is not regulated and anyway after you have fixed your main problems. It should work reasonably well with the internal zeners by the moment.
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Old 3rd April 2009, 02:42 PM   #20
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i have done a slight re-design. i am using external Positive and Negative regulators now for VAA and VSS. I also tried to shorten the length of all wires, legs of caps & resistors as short as possible while still retaining the appropriate spacing between the various components. i would like to to keep the power output stage (MOSFET's) disconnected from the IRS2092 chip completely (for the time being) and try to get some decent initial conditions with the IRS2092. Is this plausable? Once i am happy with the measurements and waveforms at various points of the IRS2092 i will then connect the power output stage and build upon that. what waveforms should i expect to see on pins HO (pin 14) & LO (pin 11)? square waveforms? of what amplitude ?
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