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

Abstract

The research field around sound reproduction is driven by a desire to continuously pack more powerful amplifiers in a small fan-less enclosure while providing state-of-the-art noise and distortion performance. Class-D amplifiers are a perfect candidate for these requirements, specifically because of the inherently low dissipation in their switching power stage with respect to more conventional linear amplifiers. There is, however, always a caveat: switching high currents at high frequencies causes electromagnetic emissions that could interfere with the correct operation of other electronic devices. Regulations limit the amount of electromagnetic interference (EMI) that devices are allowed to emit to maintain interoperability between the many electronic devices in the Internet of things (IoT) era.Speakers are commonly connected to an amplifier using leads of several meters. These leads double as an antenna for high frequency signals above the audio band, such as the switching frequency and its harmonics or parasitic ringing in the output stage, thereby causing unwanted radiated emissions. This thesis is focused on reducing the EMI due to the residual power after the output filter at the pulse-width modulation (PWM) frequency, the so called ripple. Examples in literature reduce the power at the PWM frequency and its harmonics by applying spread spectrum modulation to smear power out over a larger bandwidth. However, in this way they still radiate the same amount of power, just not concentrated at distinct frequencies. 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. In the research field of power electronics, cancellation schemes have been presented. First the disturbance is sensed and processed and afterwards a cancellation signal is synthesized and injected into the circuit. The processing can be done using a feedback or feed-forward topology, each having its advantages and disadvantages.An extensive case study on multi-phase systems is done to evaluate their merits in reducing the ripple current after the output filter. In a multiphase system, multiple half-bridges are combined in a parallel fashion to drive one or both sides of the load. Interleaving of the PWM carriers in the modulator corresponding to each half-bridge provides a reduction in the observed ripple current after combining the half bridge outputs in the output filter. The effect of adding more phases on the amount of ripple is vii ix ABSTRACT high-frequency poles.In summary, this thesis has shown a 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 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 so...