| Summary: | 碩士 === 中原大學 === 電機工程學系 === 88 === Switching rectifiers with power factor correction (PFC) circuits have given considerable attention due to the increasing demand of power quality improvement. The demands of power quality improvement include unity power factor and high efficiency. Moreover, high power density and low cost are also very important in industrial applications.
The traditional three-phase diode bridge, which is followed by a relatively large L-C filter at the load side, suffers from high content of low order harmonics in supply currents, high dc voltage ripple, low power density with large size of reactive components and improvement of input power factor in narrow operating region. On the other hand, the PFC with six-switch full-bridge topologies can provide excellent performance but needs complicated control with relatively high cost for large number of switches. Therefore, the applications of the PFC with six-switch full-bridge topologies are not popular in industry.
A low-cost single-switch three-phase rectifier with PFC has recently attracted significant interest due to its simplicity. In this thesis, a single-switch three-phase boost rectifier with high power factor, which is operated in discontinuous current mode (DCM) pulse-width-modulation (PWM) with constant switching frequency and variable on-time, is proposed for the design of switching power supplies. The major advantages of this rectifier are that its input current waveform automatically follows the input-voltage waveform, and it can achieve extremely high efficiency because the reverse-recovery-related losses of the boost diodes are eliminated. However, if the rectifier is implemented with the conventional low-bandwidth, output-voltage feedback control at a constant switching frequency, the rectifier input current exhibits a relatively large 5th-order harmonic. Thus, a single-switch three phases boost rectifier cannot be pushed into high power levels due to high total harmonic distortion (THD). As a result, at power levels above 5KW and below 10KW, the 5th-order harmonic imposes sever design, performance and cost trade-offs in order to meet the maximum permissible harmonic-current levels defined by the IEC 555-2 document. Recently, a number of control techniques have been introduced to improve the THD but with low efficiency and complicated control. Therefore, the inject signal method is proposed in this thesis to reduce the THD in order to meet the IEC 555-2 standard.
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