Control Loop Design for Closed-Loop Class-D Amplifiers with 4th Order Output Filter

The paper presents a control scheme for a high bandwidth class-D power amplifier with two-stage LC output filter. The amplifier's output voltage is controlled using a simple PI-type controller with added single or double capacitor filter current feedback. With the proposed design principle a well-damped dynamic response of the total system (e.g., Butterworth or Bessel) can be achieved without any additional passive damping, whereas the large-signal frequency response is still defined by the intrinsic cut-off frequencies of the LC filter. The paper reports the control loop design analytically analyzing also the design space of the LC filter, the PI-control and the current feedback parameters. System verification is performed by digital simulation including also load variations. Finally, the principle is tested by implementation of a 1 kW / +-200 V laboratory amplifier prototype based on a GaN-MOSFET halfbridge operated at a rather high switching frequency of 200kHz.