High power density power electronic transformers using ISOS multi-cell full-wave rectifier with GaN TCM virtual capacitor converters

An open-loop controlled multi-cell rectifier based on ISOS (Input Series Output Series) connection topology has been newly proposed. The proposed multi-cell rectifier consists of the ac-dc line frequency rectifiers, the unregulated isolated dc-dc cell converters and the open-loop controlled buck-boost converters mainly. The isolated dc-dc converters are connected in series to realize a highly scalable high-voltage multi-cell rectifier, and the open-loop controlled buck-boost converters achieve the voltage sharing by injecting reactive power without any sensors and the feedback control systems. Features of the proposed multi-cell rectifier are that the buck-boost converters behave as the virtual capacitor converters (VCCs) to equalize the input cell voltages and the TCM (Triangular Current Mode) control is applied to achieve the soft-switching operation with GaN power transistors in VCCs. The TCM control minimizes the switching losses of the GaN transistors and achieves high efficiency under the high frequency operation. A prototype of an ac 100 V, 450 W ISOS multi-cell rectifier using three cell converters and two VCCs has been fabricated and the potential for the efficiency over 96.0% and the power density over 3.0 W/cm3 has been also estimated. The proposed circuit topology is effective for developing the high power density power electronic transformers, and this approach will contribute to the cost down and the short time to market of the power electronics equipment taking its widespread use into account.