GaN-based High-Frequency Isolated Single-Stage AC-DC Converters for More Electric Aircrafts
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There has been an increased focus on the electrification of aircrafts, with the objective of improving overall system efficiency, weight and reliability. Power electronics is a key enabling technology for this transition, and there is a greater emphasis on the design of lightweight and efficient power electronic interfaces. Traditionally, the generation of the low-voltage 28 V DC bus from the high voltage variable frequency output of the turbine generators has been performed using passive diode-bridge rectifier systems, known as transformer rectifier units (TRU). Compared to active power electronic interfaces, TRUs have higher weight, lower efficiency, and inferior voltage regulation. This work proposes an active power electronic converter architecture to replace the TRU, referred to as the Regulated Transformer Rectifier Unit (RTRU). The proposed RTRU converter topology and control are specifically formulated to harness the advantages of wide-bandgap Gallium Nitride (GaN) power transistors. The system comprises three modular single-stage high step-down isolated AC-DC converters based on the Dual Active Bridge (DAB) circuit. The modular design allows for improved failure-tolerant operation, resulting in increased overall reliability which is critical for aircraft applications. The proposed DAB AC-DC converter achieves the functions of power factor correction and isolated voltage step-down with soft switching in a single power stage, thus eliminating the bulky intermediate DC-link capacitor typically associated with two-stage converter topologies. Furthermore, the three-phase converter architecture allows for automatic pulsating power cancellation at the output DC port. In the first part of this work, the suitability of the single-stage converter topology for a modular RTRU architecture is established through a comprehensive analytical comparison framework that considers the volume and efficiency tradeoffs for all passive components, including heat sinks. On the modeling aspect, the steady-state operation of the DAB AC-DC converter can show a high dependence on the circuit non-idealities and on the transient nature of the consistently changing phase shifts necessary to achieve AC-DC operation. These aspects are not fully captured using traditional modeling approaches derived for the DC-DC DAB converter. To address these issues, an improved unified modeling approach is presented – comprising of hybrid frequency and time-domain analyses that encompass the transient nature of the AC-DC converter while providing the advantages of highly generalized steady-state frequency-domain analysis. The proposed modeling approach demonstrates a significant reduction in modeling inaccuracies, which in turn lead to more accurate tracking of optimal operating points (i.e. higher efficiency) and improved power quality. In the second part of this work, the low passive component requirement of the single-stage topology is harnessed to develop a power-dense converter design featuring a compact, high-efficiency planar integrated magnetic structure with adjustable leakage inductance. The detailed modeling, design, and optimization of the planar magnetics are presented, with a special focus on unique PCB winding layouts to achieve low AC resistance in high step-down high-current applications. Moreover, the use of paralleled GaN transistors in high current applications presents several challenges with regards to current sharing and conduction loss optimization, which are addressed by a new design approach presented in this work, that leads to optimized layouts with low parasitics. Lastly, a holistic design process is formulated to analytically estimate the differential-mode (DM) conducted emissions in a DAB AC-DC converter, which is then coupled with a multi-objective EMI filter optimization algorithm to minimize the DM EMI filter weight and converter losses. Through these improvements, the developed hardware prototype achieves a 40% higher power density than the existing state-of-the-art. In the third part of this work, the proposed modeling approach is combined with a numerical optimization routine is proposed to find the optimal-conduction-loss modulation trajectories. A hybrid closed-loop control method with offline-generated feedforward lookup tables is subsequently realized for optimal loss tracking over the entire operation range, while satisfying the stringent transient operating requirements for airborne equipment. The implementation of the closed-loop control for the multi-phase modular RTRU with variable input frequency and variable switching frequency is carried out on a single microcontroller with parallelized execution. Finally, to verify the modeling, design, optimizations, and control methods, a 5 kW 230 V – 28 V fully-GaN based RTRU is developed as a hardware proof-of-concept, which achieves a peak efficiency of 96.8% and a power density of 1.2 kW/L, and satisfies the power quality and transient requirements for airborne equipment.