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Old 30th October 2008, 02:21 PM   #1
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Default My TOY MP-7720 Class-D Amplifier

I have been working in Power Electronics field since 1989 and I always stay in RD department. So, it is very easy for me to get free samples. I made a 20W/4ohm single-ended Class-D amplifier with MP-7720. Please refer to the following for detail:

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Old 31st October 2008, 03:42 PM   #2
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Join Date: Oct 2008
Default How to clarify how much current an inductor can support?

Basically, we can use core specification to calculate how much current to make a core saturate. But we donˇ¦t know the Ae (Effective Cross Section Area) of a molding core!

Again, Ampereˇ¦s Law is a very useful formula:
di / dt = V / L
It means when we add a fixed voltage across an inductor, the inductor current goes up in proportion to the voltage across it. The di / dt speed changes when the core saturates.

So, we can set a R-S F-F to begin the voltage across the inductor.
Use a current sensor to monitor the inductor current.
Use a comparator to compare sensor voltage and adjustable reference voltage.
Reset the R-S F-F to stop the inductor voltage and turn on the magnetic reset circuit when inductor current goes up from zero to reference threshold.
Use current probe to capture the inductor current.
The waveform is exactly the half B-H Loop.

The following waveform exhibits an example of above description:
Click the image to open in full size.

It can support about 6.5A, however, care should be taken:
Ferrite has 4000 Gauss Bsat at 25 degreeC, but it drops to 3000 Gauss at 100 degreeC.
So, this inductor can support max. 6.5 x 3000 / 4000 = 4.875A, include half ripple current, with enough design margin.
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Old 1st November 2008, 01:44 AM   #3
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Location: Canada

Where the wave form taken from this board...

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Old 1st November 2008, 05:45 AM   #4
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Hi, e_fortier

Thanks for your information.

I did the PCB layout myself, please refer to the following pictures:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

I also put them on the original topic of my blog.

I put four channel MP7720 on the PCB because I would like to connect it and speakers bi-amp.
The extra circuit on the PCB is UVLO, Under-Voltage-Lock-Out, enable signal.
It means the MP7720 only can be turn-on when the supply voltage is higher than 16V.
Once MP7720 is turned-on, it only can be turn-of when the supply voltage is lower than 10V.

I used Panasonic ECQE(F) Metalized Polyester Film Capacitor for the L-C Filter because it is for Active PFC input capacitor which can withstand both high voltage and high ripple current stress.

The PCB on opposite side of MP7720 related circuit is GND for noise immunity issue.
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