| Three new dc-to-dc converter topologies aimed at high-power high-frequency applications are introduced. Major system parasitics, namely, the leakage inductance of the transformer and the device output capacitance are efficiently utilized. All circuits operate at a constant switching frequency, thus simplifying design of the reactive elements. Of the three circuits the single-phase and three-phase versions of the dual active bridge topology demonstrate minimal electrical stresses, better utilization of the transformer, bi-directional and buck-boost modes of operation. The power transfer characteristics and soft-switching regions on the Vout-Iout plane are identified. Wide control range can be achieved for a dc conversion ratio of unity for the dual active bridge topologies.; For the proposed topologies, relatively low magnetizing inductance helps widen the region of lossless control under lightly loaded conditions. However, increased snubber capacitance, for reduced switching losses, diminishes region of lossless control.; Given the converter requirement of high-power densities and low, distributed leakage inductance, coaxially wound transformers are seen to be a potential candidate. Two coaxial transformers with different cross-sections have been built for a rating of 50 kVA. The measured leakage inductance at 50 kHz is seen to be in the vicinity of 150-250 nH, with power density of approximately 0.1 kg/kW.; Based on the single-phase dual active bridge topology, a 50kW, 50kHz converter operating at an input voltage of 200Vdc and an output voltage of 1600Vdc was fabricated. The actual power density of 0.243 kg/kW, meets the target specification of 0.2-0.3 kg/kW. Experimental results indicate that the converter can handle peak power near rated conditions at an overall efficiency in the range of 84-90%. Bi-directional mode of power transfer at low power levels is also demonstrated. Limitations in the performance of the converter is shown to mainly arise from the device package inductance. The characteristics of current-fed output, make the dual active bridge topologies amenable to paralleling and hence extension to megawatt power levels. |
