Reviewing Class D Output Inductor Current Beyond Audio RMS

The current in a class D output filter inductor includes the wanted audio current and switching ripple. Selecting the part only from speaker RMS power can miss peak stress, magnetic nonlinearity, and high frequency loss. Review the waveform that passes through each winding rather than treating the inductor as an ideal filter coefficient.

Identify the relevant component evidence

The Coilcraft class D inductor bulletin distinguishes inductance behavior under current from temperature rise ratings and discusses the importance of linearity for audio distortion. These quantities describe different limitations.

Record inductance test conditions, current dependent curves, winding resistance, temperature limits, and any loss information relevant to the switching frequency. For a dual part, check whether thermal ratings assume both windings carry current simultaneously.

Estimate the complete current waveform

Calculate the audio current from the intended load voltage and impedance, then superimpose the expected switching ripple using the actual amplifier modulation and filter topology. Do not import a ripple expression from a different bridge configuration without checking its assumptions.

A hypothetical resistive load receiving 2 A RMS of sine current has an audio peak near 2.83 A before switching ripple is included. The magnetic peak can therefore exceed the RMS current substantially even when average speaker power is within its target.

Examine inductance through the signal cycle

Compare the current excursion with the inductance versus current curve. A gradual loss of inductance can alter ripple and distortion before an abrupt looking saturation threshold is reached. The printed saturation current normally corresponds to a specified percentage change, so retain that definition.

Check positive and negative peaks and any DC imbalance. An asymmetry in the amplifier or load can shift the waveform and use more magnetic range on one side.

Measure electrical and thermal results together

Use an appropriate current probe to inspect idle ripple and representative high output conditions. Measure audio distortion at several output levels and frequencies, keeping the analyzer’s class D filtering consistent.

Observe inductor temperature after the required operating duration and in the intended enclosure conditions. A low DC winding resistance does not account for every loss caused by switching ripple. Conversely, a cool part does not prove sufficiently linear inductance for a demanding distortion target.

Save the waveform, peak and RMS estimates, component curves, distortion sweep, and thermal conditions. Compare candidate parts at matched output and load conditions. An acceptable output inductor satisfies the filter response, magnetic behavior, and temperature requirements together; a current rating larger than the calculated speaker RMS current is only the beginning of that assessment.