Comparison of resonant topologies for step-down applications using piezoelectric transformers

A 1 to 5 Watts wide-range power supply demonstrator with a transoner-type piezoelectric transformer (PT) has been built in class E topology to show the full functionality of an off-line power supply. A basic design target was the ZVS or near-ZVS behaviour, which was achieved for 3 Watts and 6 V output at input voltages between 85 and 260 V AC, and for 5 Watts and 6 V output at Vin = 150 to 260 V AC, respectively, using one and the same PT sample. The PT was 2.3 mm thick, and had a diameter of 17 mm. The used fieldstop-IGBT 1 A-type shows losses less than 700 mW over the full operating range for constant output voltage optimization, and less than 250 mW, for maximum efficiency optimization, respectively, at frequencies of 155 to 172 kHz. The measured overall worst case efficiency of the converter (without rectifiers) has been accordingly 70.5%, and 82.4%, respectively. The application is suited for smart-card format power supplies below 5 mm thickness. Compared to the half-bridge, the class E concept, as a current fed topology, promises better EMI suppression by the input inductor than a half-bridge, lower switch current peaks, and a larger control bandwidth at the same frequency. Further, it was demonstrated with the class E concept, that it is possible to keep the output voltage constant, using the resonant tank with the output capacitance. Thus, output charging capacitors can be made very small, compared to fly-back converters or other hard-switching conventional topologies. The measurements had been in good agreement with the differential equations analysis and the SPICE simulation, as well. These results led to current projects for mass market applications using the class E topology, and alternatively the half-bridge for higher power levels.