Design of a One-to-Four Isolated DC-DC Converter Using a 13.56 MHz Resonant Air-Core Transformer

This paper showcases the design and development of a DC-DC converter with one input and four outputs using a high frequency resonant air-core transformer. The transmitter to receivers air-gap is 25 mm. Practical tuning equations were derived for multiple receivers which allow the converter to be optimised for overall efficiency and unity power factor at the transmit coil (i.e. zero reflected reactance). Experiments were conducted using two receive coil structures, one with four equally shaped adjacent coils in a single PCB, and the other with four differently-shaped coils featuring overlapping traces to maximise the coupling factor with the transmitter and minimise the coupling factor between the receivers. The two structures were tested and compared using the same transmitter, driven by a single-ended 13.56 MHz Class EF inverter. Single-ended Class D rectifiers were implemented at the receive side. Experiments were performed, first with equal AC test loads, and afterwards with the addition of the rectifiers and buck converters to regulate each of the four output voltages to 15 V independently. The results of the experiments implementing adjacent coils demonstrate that equal distribution of power can be achieved by modifying the tuning capacitances at the receivers with the AC loads; however, when the voltage-regulating buck converters were introduced at each output, it was only with the coil structure with overlapping traces that the required power of 10 W at each output was achieved.