Ultra-High Bandwidth GaN-Based Class-D Power Amplifier for Testing of Three-Phase Mains Interfaces of Renewable Energy Systems

AC power amplifiers are e.g. used to emulate the power grid for testing three-phase power electronics mains interfaces of renewable energy systems or measuring the inner impedance of power distribution grids. Typically, the output stages of the test systems are realized as analog amplifiers and thus achieve very high dynamics and a very high signal quality, but also have significant drawbacks, especially high losses and/or poor efficiencies, which leads to a large cooling volume and weight that dominates the full system power density. In addition, analog amplifiers cannot easily handle bidirectional power flow, i.e., power fed back from active loads such as renewable energy inverter systems is dissipated internally. Furthermore, future power distribution systems, e.g., of More Electric Aircraft, will have fundamental frequencies exceeding 1 kHz, thus ultra-high bandwidth (> 100 kHz) power amplifiers with multiple kW output power are required to emulate harmonic distortions and variations of voltage and/or frequency in such power grids. Currently available analog power amplifiers reach bandwidths of up to 30 kHz, which is too low for all desired applications. Switched-mode, i.e., class-D power amplifiers achieve a high efficiency, however, bandwidths of no more than 5 kHz are reached today due to the high required switching frequency (around a factor of 50 higher than the output bandwidth). However, as shown in this paper, novel wide bandgap semiconductor technology and suitable circuit techniques such as series interleaving (multi-level converter topologies) and parallel interleaving render a switched-mode realization with an effective switching frequency of 4.8 MHz and/or 100 kHz large signal bandwidth possible, while still keeping the switching losses at a moderate level. An analysis of designs with different numbers of voltage levels and interleaved branches is performed and shows that with a triple-interleaved three-level flying capacitor converter, where each switching cell operates at 800 kHz, the targeted effective switching frequency is reached and an efficiency above 95 % is feasible for both directions of power flow. Furthermore, a virtual prototype of the selected design is presented, showing that thanks to the high effective switching frequency, an extremely power-dense overall realization is possible (50 kW/dm
³
). Finally, simulations of the control behavior verify excellent control dynamics of the presented concept.