Modelling and control of a reduced-stage isolated AC-DC converter for more electric aircrafts

This paper presents a reduced-stage three-phase isolated AC-DC converter topology applied to regulated transformer rectifier unit (RTRU) for more electric aircrafts (MEA), requiring a high step-down in voltage conversion. Unlike the conventional two-stage AC-DC isolated converters, the DC-DC primary side full/half-bridge is eliminated in the proposed structure and three-phase bridge itself is utilized for generating high-frequency pulsating voltage, appearing across the transformer primary side. Additional advantages of this topology include zero voltage switching (ZVS) in four PFC MOSFETs, connected to the high-frequency tank, whereas all six transistors are hard-switched in case of conventional two-staged AC-DC converter. A sine-PWM based control scheme is applied with common mode duty ratio injection method to minimize the current harmonics without affecting the power factor. A LC filter is used after the PFC semi-stage to suppress the line frequency voltage ripple. Also, the PFC output capacitance requirement is greatly relaxed by allowing a larger swing in its voltage without affecting the converter operation. Simulation studies are carried out for a rated load of 5kW, with a three-phase 230V RMS, 400Hz phase voltage input and the output voltage set to 30V DC. The results demonstrate a power factor of 0.992 (lag) with an efficiency of 96.4%, and a total harmonic distortion (THD) of 3.4%. To validate the hardware implementation, considering various design constraints and limitations, experimental results are also included for a scaled down laboratory proof-of-concept.