Active PWM ripple reduction in class-D amplifiers using digital loop filters

This thesis is focused on reducing the electromagnetic interference (EMI) due to the residual power after the output filter at the pulse-width modulation (PWM) frequency, the so called ripple. To reduce the ripple current through the speaker leads, it is possible to use a higher order filter, use a multi-phase or multi-level output stage or to inject a cancellation signal after the output filter. First the disturbance is sensed and processed and afterwards a cancellation signal is synthesized and injected into the circuit. A development board utilizing the AX5689 digital amplifier controller IC was used as a design vehicle to realize a prototype bridge-tied-load (BTL) Class-D amplifier with ripple reduction. The built-in analog-to-digital converters (ADCs) were used to measure the output signals of both bridge halves. A selective digital band-pass loop filter was designed in the digital-signal-processing (DSP) core of the AX5689 to only provide high loop-gain around the PWM frequency without affecting the audio band. A ripple reduction of 28 dB for the common-mode (CM) and 18 dB for the differential-mode (DM) was achieved across all signal frequencies and output powers up to 10 W. The Class-A drivers used to inject the anti-ripple have a negative impact on the total system efficiency, which drops from 84% without ripple reduction to 79% with ripple reduction enabled. The technique has virtually no impact on THD+N performance and could be used as an add-on to any Class-D amplifier with a fixed (PWM) frequency. In summary, this thesis shows a feedback technique to reduce the ripple current after the output filter. Thereby the amount of EMI that can be radiated off the speaker leads decreases. The effect of unit delays in the feedback loop has been mitigated by applying a special filter design method to obtain a more stable system. A prototype has been made around an existing amplifier to show the effectiveness of the proposed feedback ripple reduction solution.