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Optimal Control of a High Frequency Class-D Amplifier

During the last decade, switch-mode audio amplifiers have become a common choice for audio applications because of efficiencies approaching 90% and distortions as low as 0.001%. Such amplifiers modulate the input audio into a high-frequency discrete signal that drives a Class-D power stage. The control loop is the key element in achieving high-quality performance. Modern control theory methods were used to design and simulate a full-state feedback integrating controller for use with a high-frequency bridge class-D amplifier. An optimal linear full-state integral controller based on the state-space model was designed using the Linear Quadratic Regulator (LQR) method, and verified on a linear and switching model. Measurements on a Class-D amplifier with the implemented controller showed that the step responses and THD+N measurements were aligned with theoretic predictions. A 30-fold reduction in THD+N was observed compared to open-loop. The results prove that the principals of modern control achieve good performance in Class-D amplifiers, even when the output filter has a large resonance.


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