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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th May 2006, 08:30 PM   #11
analogspiceman is offline analogspiceman  United States
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Originally posted by Eva
Forget abut simulating discrete switching circuits. Simulation models are intended for linear applications only and you will get weird surprises if you try to build what the simulator tells to be optimum.

What you can simulate with proper software is class D control strategies, but always in a theoretical basis with ideal components.
Can't speak for you or anyone else, but I've found a properly posed simulation can be a great tool for all aspects of class d design, both in the frequency and time domains.  The art is in knowing what to include and disinclude in the models so the results are a meaningful match to reality and are obtained quickly enough to make simulation a useful iterative design tool.  For example, nonlinear Cdg and lead and trace inductance must be properly modeled for useful gate drive design.

Regards -- analogspiceman
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Old 20th May 2006, 10:02 PM   #12
BWRX is offline BWRX  United States
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Originally posted by darkfenriz
Thanks Brian
Sounds reasonable.
Do you know any 'thumb' rule to design output filter, like, say: 120% nominal load resistance, 1/3 of switching freq., Butterworth (blind shot here)?
There's no real rule of thumb with class d output filter design. But a good place to start would be a second order Butterworth like you mentioned with a cutoff frequency in the neighborhood of 40-50kHz. You could try 30uH and 0.47uF as a starting point.

Have a look at this pdf from TI for some design tips: http://focus.ti.com/lit/an/sloa119/sloa119.pdf
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Old 21st May 2006, 04:38 AM   #13
Eva is offline Eva  Spain
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Concerning output filter:

Inductor value determines the amount of current ripple and how wide is the resulting class AD output power range in which soft resonant switching and little EMI is achieved. That has some similarity to class AB idle current in linear circuits, altough there is inherently no added distortion due to mode transition.

Capacitor value determines how much voltage ripple the inductor current ripple is going to be translated into.

Both capacitor and inductor together determine the resonant frequency and the resonant impedance of the filter. That's only critical for pre-filter feedback designs. Real loudspeaker impedance curve has to be considered, as most tweeters are inductive and their impedance starts to rise above 5Khz, reaching up to 20 ohms at 20Khz (and further rising), thus causing unwanted filter peaking.
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