IC design of Switching Power Stages fro Audio Power Amplification

Research in Class D audio amplifiers stems back to the sixties, and practical use has increased mainly since the nineties, facilitated by advances in transistor technology. Most publications from this era focus on implementations with analog audio input and discrete output transistors. Since then, monolithic implementations have arrived, and these are the main focus of this thesis. Following the introduction in chapter 0, three chapters discuss each of three performance metrics by which Class D amplifiers are commonly assessed. Power losses in the output stage are analyzed in chapter 1. This topic is well covered in existing literature on switching power converters, but mostly under assumptions that can not be made for audio amplifiers. This analysis specifically addresses power losses in a switching output stage for audio signal reproduction. The analysis initially assumes an ideal power supply for the output stage, and subsequently includes the effects of parasitic inductances around the output stage. It is shown that the inclusion of parasitic inductance in the analysis causes fundamental changes in circuit behavior, and that the achieved results do not converge towards the results for an ideal power supply when the values of parasitic inductances go towards zero. Further, it is shown that conduction overlap between the output switches, which is typically prevented by use of switching dead time, is unavoidable when parasitic inductance is considered. Chapter 2 is an analysis of parameters influencing the maximum output power of a Class D amplifier. It includes a comparison of the die area of equivalent solutions in different topologies, and an analysis of the maximum output currents needed to drive loudspeakers. Distortion in a Class D amplifier system is mainly caused by the switching output stage, and is covered in chapter 3. This topic is especially relevant because most audio signal sources are digital (CD, DVD, digital media players, etc.), causing an increasing demand for low cost Class D amplifiers accepting a digital audio input. Feedback can not easily be implemented in such amplifiers, so low open-loop distortion is essential. The primary source of distortion is nonlinearities related to the switching transitions, and it is shown that when the influence of output current on switching transition waveforms is considered, the optimum amount of dead time for minimum distortion is not zero, but finite. The overall performance of the amplifier is shown to depend heavily on properties of the gate driver circuits, and a summary of relations between gate driver properties and performance is given in chapter 4, along with a presentation of a design example; a monolithic power stage from the Texas Instruments portfolio. Finally, modeling and simulation techniques specifically suited for Class D amplifiers are presented in chapter 5. 1 see www.erhvervsphd.dk 2 e.g. Norman H. Crowhurst “Two-State Power Amplifier with Transitional Feedback”, US patent #3,336,538