Direct Three-Phase AC-DC Rectifier with High-Frequency Open-Delta Transformer and Closed Loop Output Voltage Regulation

This paper proposes a high current direct three-phase AC-DC rectifier with high-frequency transformer isolation configured as open-delta. The three-phase input voltages are modulated by two AC to AC converters to generate a high-frequency link at 20 kHz. The high-frequency link is then interfaced to two three phase rectifiers configured in 12-pulse by means of a high-frequency open-delta transformer arrangement. With the transformer core operating at high frequency operation, it is shown that the proposed configuration operates as a conventional line-frequency 12-pulse, resulting in high quality input currents with low THD. Furthermore, a phase shifted modulation is introduced to achieve zero voltage switching and to regulate the output DC voltage. An added benefit of the approach is the ability to regulate the output DC voltage/current by means of a simple duty cycle control of the high frequency (20kHz) modulating signal. The overall three-phase input currents are of high quality and near unity power factor due to the cancellation of 5th and 7th harmonics. The proposed approach substitutes the bulky line-frequency transformer with a high-frequency transformer, thereby achieving higher power density. Derivation of a small signal model of the converter is discussed for the design of DC closed loop. A 208 V 3-phase laboratory prototype demonstrates the validity of the concept.