Optimized Switching-Frequency Control for Improved Efficiency in Three-Phase Isolated Y-Rectifiers ^†

The single-stage three-phase isolated Y-rectifier (iYR) merges power-factor correction and galvanic isolation into two conventional B6 bridges. Its power is conventionally regulated by the phase-shift angle alone at a fixed switching frequency. At light load, the transformer current then forms high-peak pulses whose rms value is large relative to the transferred power, and the efficiency falls. This paper proposes a hybrid phase-shift and variable-frequency control that raises the switching frequency as the load falls, without altering the internal space-vector timings. At the lightest tested load, about 300$\mathrm{W}$, the measured rms current falls by roughly 50 when the frequency is raised from 25$\mathrm{k}$$\mathrm{Hz}$ to 80$\mathrm{k}$$\mathrm{Hz}$. The loss-optimal frequency is identified from hardware measurements rather than from simulation, and is stored as a two-dimensional look-up table indexed on the power reference and the dc output voltage. The strategy is validated on a 3$\mathrm{k}$$\mathrm{W}$ prototype over a 350$\mathrm{V}$–450$\mathrm{V}$ dc-bus range and a 25$\mathrm{k}$$\mathrm{Hz}$–80$\mathrm{k}$$\mathrm{Hz}$ frequency range. At 400$\mathrm{V}$ and 300$\mathrm{W}$, the measured loss falls by $38.6$, raising the efficiency from $85.1$ to $90.3$ . Two transformer designs differing only in the magnetizing inductance are compared, which sets the frequency range over which the strategy reduces the loss.