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Class-D Amplifiers in an Undergraduate Power Electronics Course Daniel W. Hart Valparaiso University

Abstract The class-D amplifier is a recent development in audio electronic engineering. Class-D amplification utilizes power transistors as switches, producing a pulse-width modulated signal that is filtered and delivered to a speaker. Compared with other designs such as class AB amplifiers, the class-D scheme is much more energy efficient and compact. Because the circuitry is similar to others in power electronics, the class-D amplifier was a natural addition to our undergraduate power-electronics course.

This paper describes the student-designed experiences and the hardware that students selected to implement class-D amplifiers. Included are results from the student projects based on Texas Instruments equipment, National Semiconductor components, and an original amplifier design.

Introduction Power electronics has been a popular elective course in the Department of Electrical and Computer Engineering at Valparaiso University for several years. In power electronics, electronic devices are used as switches in circuits that convert one form of voltage or current to another. One example of a power electronics circuit is an inverter for converting DC to AC. Class-D amplifiers use the same switching theory as in pulse- width modulated (PWM) power inverters. One power-inverter switching scheme uses PWM, wherein a reference waveform (usually a sinusoidal) is compared to a triangular "carrier" waveform operating at a frequency much higher than the reference [1, 2]. In class-D amplifiers, the sinusoidal reference waveform is replaced with a low-power audio signal, and the filtered PWM output of the inverter becomes the amplified audio signal.

The energy-savings aspect of class-D amplification has long been known and has been well documented [3]. In class-D amplification, an audio signal is converted to a pulse- width modulated voltage that changes between two levels. Power losses are mainly due to imperfect switching in the transistors during voltage transitions. Most audio amplifiers use the well-known class AB amplification method. The maximum theoretical efficiency of class AB amplification is 78%, compared to 100% for class D. In practice, with real audio signals, class-AB efficiency is much lower. In one Texas Instruments test, a class- AB amplifier was 20% efficient, compared to 75% efficient for class D [3]. Class-D amplifiers are becoming more prevalent in consumer electronics applications were greater efficiency results in reduced size and increased battery life.

In the fall of 2003, a guest speaker from Texas Instruments explained and demonstrated class-D audio equipment to the power-electronics class. Seeing that students were

Proceedings of the 2005 American Society For Engineering Education Annual Conference & Exposition Copyright  2005, American Society for Engineering Education

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Hart, D. (2005, June), Class D Amplifiers In An Undergraduate Power Electronics Course Paper presented at 2005 Annual Conference, Portland, Oregon. 10.18260/1-2--14735