Analysis, modeling, and design of low-harmonic rectifiers for avionics applications

Future commercial aircraft will employ variable frequency (360 to 800 Hz) power generation for increased reliability and reduced maintenance. Front-end (ac-dc) converters in the distributed power systems of these aircraft will be required to have a low total harmonic distortion (THD) of the input current, and a high efficiency. This thesis addresses the issues of the low-harmonic rectifiers operating at high line frequencies for the first time and accomplishes the challenging task of meeting the stringent requirements with innovative solutions.; The application demands a high switching frequency for the converter involved. This in turn affects the conversion efficiency; hence a soft-switching technique has to be used. A zero-voltage-switching active-clamped isolated SEPIC (single-ended primary-inductance converter) topology is selected. This topology is demonstrated in an ac-dc application for the first time in this thesis. Additional efficiency improvement is brought about by a single-layer design for the isolation transformer that minimizes its high-frequency eddy current losses.; The high switching frequency operation enables us to design a high-bandwidth feedback loop to achieve a low THD. Active-clamped converters have not been modeled in detail earlier; hence this thesis proposes the use of averaged-switch modeling approach for this task. Implementation of the resulting model in PSpice is described in detail. It is discovered that the converter has a well-damped dynamic response compared to its parent topology, which simplifies the controller design. Importantly, it is shown that this modeling approach can also be extended to other popular active-clamped converters.; The active-clamped SEPIC rectifier achieves an efficiency of 90% and a THD of less than 5% in an experimental 100 W prototype and outperforms the previously reported SEPIC based rectifiers for utility applications.; Fast digital control methods can be used for low-harmonic rectifiers with high line frequencies. A novel variable-frequency predictive digital current control method is proposed in this thesis. This approach enables a faster, single-cycle current control, which is also simple to implement. The digital controller is modeled in MATLAB/Simulink and is implemented using a DSP evaluation board. Excellent results for the THD (∼3%) are obtained with a boost rectifier for the entire frequency range.