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Class D Switching Power Amplifiers and Power D/A conversion

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Old 12th December 2007, 06:13 AM   #21
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Ionutgaga,
Did you put output inductor at the power supply?
Recently, I also face the same problem. I add output inductor at the power supply, it helps the cross conduction problem.

I don't know is it just lucky or something else.
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Old 12th December 2007, 06:17 AM   #22
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darw82, probably i miss something. Do you want to be a more explicit?
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Old 12th December 2007, 06:25 AM   #23
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Ionutgaga,
As you said before, you need to sync this class-D amp with power supply. So I assume you use SMPS to power this amp.
In my opinion, cross conduction can also happen because of bad designed power supply.

My question is:
Do you use output inductor, at the SMPS output?
Some people design SMPS for amplifier without output filter.

If you haven't try this. You should. It helps me.
But I don't know for sure, whether will this help you or not.
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Old 12th December 2007, 06:35 AM   #24
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Ok, darw82...
I'm not using a smps for this prototype. Thanks anyway...

Floating voltages is from push-pull sync smps
(SG3525+2 x IRFZ44N+EE19@62.5khz).
I used a ground plane pcb (see my photo).

I'm unnhappy about a small white noise at output
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Old 12th December 2007, 10:40 AM   #25
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Quote:
Originally posted by ionutgaga
I'm unnhappy about a small white noise at output
Is it at idle no signal conditions or with some signal......

Which type of supply is used for preamp
seperate or tapped from main rails??
If tapped from main rails then try to insert small bead inductors in the main rails......
another possibility could be the modulator in close proximity with output inductor or feedback node is interferencing some disturbance from stray fields.....
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Old 12th December 2007, 11:06 AM   #26
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Thanks WorkHorse for your suggestions.
I have a separate power supply for the +/-15v, and another for the +12v for smps floating voltages.
I tryiet to add a small bead.. no improving...
My modulator is another pcb, 10cm distance...
I think that feedback node is remaining to try

At output i have a sinusoidal 4Vvv@250khz before the notch. Perfect sinusoidal, viewed with my old 20mhz oscilloscope


LE ....white noise at idle... it is small, but not corresponding with my standard...
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Old 12th December 2007, 11:17 AM   #27
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Quote:
Originally posted by ionutgaga
LE ....white noise at idle... it is small, but not corresponding with my standard...
Then another factor could be a bad pcb layout also........
what is the average amplitude of white noise at the output.....
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Old 12th December 2007, 11:30 AM   #28
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I don't know the average... very small, but perceptible...
I think it is not from my modulator, because the same white noise appears at output without modulator, only power stage (explained in first post and the photos), 250khz generator with deadtime control (4mhz quartz, CD4060, 74HC02, 1k+82p+1n4148, driving 6n137 in the power stage with 300 ohm at 5v)
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Old 12th December 2007, 11:45 AM   #29
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Only option is to try to post the exact schematic, the way you have done it on the pcb.....
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Old 12th December 2007, 12:12 PM   #30
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Hi, Ionutgaga,

Sometime ago, I make classD with IRF240 output mosfets. It has problem like you describe. It turns out these TO-247 devices are not so good for classD, because of (1) slow body dioda, (2) huge gate charge/discharge, (3) parasitic capacitance of the flange of the mosfets (drain) to the heatsink (if you put those 2 mosfets on a grounded heatsink).
The best is to use TO-220 device, like EVA said. if you still want to use them, (1+2) can be remedied by bypassing body dioda (using schottky in series and 30ns external diode in parrarel). (2) you must have A (mA will not enough) gate charge/discharge. Can be done by cascading totem-poles. (3) is confusing. You have to buy ceramic/aluminum oxide insulator (1-1.5mm thick) or use independent heatsinks for each mosfets that is not grounded, blown by cooling fan.
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