Coreless DC Transformer With Near-Unity Power Factor by a Reactive Power Injector

With the development of lightweight power electronic devices, converters with coreless designs are gradually receiving attention due to their high gravimetric power density (GPD). This article presents a reactive-power-injector-based coreless DC transformer (RPI-CLDCX) with a near-unity power factor. The RPI-CLDCX has three inverters, two of which transfer only active power, while the third one provides reactive power for the air-core transformer (ACT). By changing the phase difference between the output voltages of inverters, the amplitude and direction of transfer power can be adjusted freely. Benefiting from the reactive power injector (RPI), the RPI-CLDCX can achieve zero-voltage switching and very low turn-off currents in the whole range of load. The operating principle, steady-state characteristics, control strategy, and design procedure are clarified. To demonstrate the feasibility and performance, a 2.5-kW prototype applied for connecting 270V/28V DC buses is built. The experimental results show that the efficiencies at full load in forward and backward mode are 94.4% and 93.9%, respectively. Owning to the coreless design, the GPD of the transformer is as high as 7.96 W/g. Finally, detailed comparisons with other ACT-based converters and magnetic-core-transformer-based converters are made to show the advantages of the RPI-CLDCX.