Towards an Ideal Switching (Class-D) Power Amplifier: How to Control the Flow of Power in a Switching Power Circuit

The design of a switching (class-D) audio power amplifier suitable for high-end audio applications is still a very challenging task for circuit design and signal processing engineers. Classical power stage topologies using Pulse-Width Modulation (PWM) in combination with voltage-controlled MOSFET H-bridges are already available on the market, but their performance in terms of signal bandwidth and linearity is still far below the one of traditional class-A and A/B power stages. Moreover, EMC is an issue that is very hard to control. Class-D output stages are considered from a totally different point of view in this paper: The flow of power in the output stage, containing the switching power stage as an "power control element", the output filter as an "energy store" and the load as both an "power sink" and an "power source" in case the load is not a resistor but a real world loudspeaker device. It is shown, where in a typical power stage the power loss occurs, which is dissipated as heat. To improve the quality and efficiency of high-frequency switched power stages, investigation has to be taken into the way, how to control the flow of power into the storage elements and how to charge them most precisely and most efficiently. Some fundamental approaches for this will be shown in this paper.