5.2 An 8Ω 10W 91%-power-efficiency 0.0023%-THD+N multi-level Class-D audio amplifier with folded PWM

As the portable device market tries to enhance user experience, high-power audio systems with boosted supply voltage have been the main design focus recently. Several past works have addressed issues related to boosted supply voltages [1,2]. Nevertheless, the power stage retained the classical H-bridge structure in the previous works, which resulted in aggravated electromagnetic interference (EMI) from high switching amplitude and poor efficiency due to voltage boosting. The use of multi-level pulse-width modulation (PWM) shown in Fig. 5.2.1 can naturally eliminate the complications caused by high supply voltages. Since the audio signal has a high crest factor, a multi-level Class-D amplifier draws most power directly from a low-voltage battery source, which in turn improves the power efficiency significantly [3]. Spread spectrum techniques prevent energy localization in the power spectral density [2]. Nevertheless, the diffusion of switching harmonics into the nearby frequencies complicates EMI management. However, the multi-level switching scheme suppresses EMI by reducing the switching amplitude without spreading the energy spectrum [4]. In this work, a new folded-PWM (FPWM) architecture implementing a multi-level H-bridge topology is presented.