This paper proposes the key design guidelines for a fully digitally controlled 400-Hz active power filter (APF), in which the circuit parameter selections and control design remarkably affect the performance of the APF. Detailed design considerations, including APF grid-connected inductance, line inductance, dc bus capacitance, and digital signal processor control implementation, were presented to meet a specific goal. Simulation was given to verify correctness of the design analysis. A 20-kVA prototype was set-up to verify the validity of the APF design considerations. The experimental results show that the APF can compensate the harmonic order up to 13th and reduces total harmonic distortion from 28.9% to 5.7% at full load and achieves good dynamic performance as well.