Understanding output filters for Class-D amplifiers

The heart of a Class-D amplifier filter is a L-C low-pass filter. The corner frequency of the filter is chosen so that the filter will have minimal effect on the desired output frequency range while attenuating the switching noise as much as possible.

Low-Pass Filter
The optimum value for the filter inductor is

where f_C is the desired corner frequency of the filter and R_L is the load (speaker) resistance. Note that the inductor value is dependent on both the desired corner frequency and the speaker impedance so the inductor value will changes if the speaker impedance changes.

Practical designs require the use of standard component values so small adjustments usually have to be made to the ideal inductor and capacitor values. Rather than calculating the inductor and capacitor values independently and then adjusting their values, it's better to calculate the inductor value, select the closest standard inductance value, and then calculate the required capacitance using the selected inductor.

The quality factor (Q) of a filter is the ratio of the center frequency to the filter bandwidth.

A high Q produces an underdamped curve and a low Q produces an overdamped curve. The Q of the filter should be in the range 0.6 > Q > 0.8 to avoid underdamped or overdamped behavior. If you use the equations above the filter should have a Q of about 0.7, which provides good performance and allows for impedance variation in the speakers. Note that the Q of the filter will change if the speaker impedance is changed without adjusting the filter component values, which can result in an underdamped or overdamped response.

Component Selection
Not only is it important to choose the correct L-C filter values, it is also important to choose the correct types of components for the class-D amplifier in order to avoid losses and minimize harmonic distortion.

The DC current rating of filter inductors must be greater than or equal to the maximum current that it will see. The change in inductance versus load current should not be more than 10%. The core material can affect the amplifier's harmonic distortion and should have very low hysteresis losses.

The capacitor should be a multilayer polyester, polypropylene or polycarbonate film capacitor. Avoid using ceramic capacitors in the low-pass filter. Ceramic capacitors experience large changes in capacitance as the voltage across them changes, which can result in distortion.