Btl class d filter design

bowman1710 said:

Is the signal seen by the left hand side on the inductor a 300Khz switched audio signal, as in isn't it basically the 300Khz signal being switched to produce the audio signal??? Below is the signal on the inductor

View attachment 123103


Isnt this why the LP filter is used to remove this 300KHz?

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Yes the PWM of the triangle yields an AC component from the modulator which shifts the threshold of the comparator.
Your waveform looks at the differential input with the slicer being at 0V so in this case it swings full scale with no DC offfset.

It's not really accurate to model the frequency reponse of a reactive filter unless you consider the reactive impedance of a dynamic speaker.
But I suppose accurate enough for government work.

bowman1710 said:

Isnt this why the LP filter is used to remove this 300KHz?

Click to expand...

Yes. The LP filter can be an inductor, or capacitor, or both.

Your scope trace is the voltage across the inductor. It it is the sum of (a) applied voltage and (b) internal counter-emf. (I look forward to the day when I can get an intuitive grasp on how these interact.)

As you can see the voltage waveform does not necessarily resemble the Ampere waveform. Inductors are strange that way.

The appearance of the sine-shaped envelope indicates that the inductor value is in the correct range to do the filtering job. If you were to test different Henry values, you will see the shape change. You'll get an idea how it behaves when the rolloff frequency is too low (choke effect), or when the rolloff frequency is too high (no attenuation of 300kHz).