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Class D PWM switching power amplifiers use an LC filter at the output to filter out the switching harmonics:

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Other than the inductance value, what properties are important for this inductor? What types, specifically, are best?

Concerns would include distortion (from hysteresis/saturation of the core?), efficiency, EMI (toroidal shape, shielding), etc.

I think some of the important qualities would also be shared by inductors in switch-mode power supplies.

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  • \$\begingroup\$ Q factor plays a major role. concentrate on it \$\endgroup\$ – naveen Jan 11 '11 at 10:08
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Taking high enough current without saturation is important. Check peak and average current, and derate it some.

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In a switching power supply, the output inductor is designed to meet several criteria, including:

  • ripple voltage due to inductor current imposed on the ESR of the output capacitors
  • transient response, since larger inductance = slower response
  • non-saturation
  • zero or controlled roll-off of inductance due to DC biasing (permeability roll-off)
  • core loss / copper loss targets

Some of these are applicable to amplifier output filters. Permability roll-off due to DC biasing wouldn't be a concern since an audio amp isn't providing DC to the loudspeaker, whereas a buck converter output inductor is technically a 'choke' since there is a DC component which has to be taken into account.

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I did some experiments in 1995 .100 watt class D N chan mosfet full bridge Audio amp running on lab supply set to 28.8 VDC .The powdered iron torodial cores gave easily visible distortion when running my audio sig gen, my approx 2 ohm dummy load made from lots of 5 watt 22ohm wirewound resistors in parallel .The powdered iron toroidal cores were rated for the peak currents .I then wound 2 air cored coils of the same approx inductance on bobbins that were used for air cored crossover networks .The visible distortion vanished .So if you have the space the air core is best . This is reasonable because Audiophiles prefer air cored coils in their crossover networks . These days many people build the amp into the speaker box so there should be lots of space .Now there are more options for coils . I think that a planar with a gap would be worth a try .

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