Isolated feedback coupling by means of an acoustic channel

This paper describes an ultrasonic signal transmit link through an electronic circuit's PCB which offers isolation and high immunity from EMI. The approach proposed employs ceramic materials with enhanced piezoelectric characteristics, which have become widely available in recent years. This type of coupling may be adopted in applications such as isolated switched-mode power supplies (SMPS), high-voltage current probes and data transfer. A further application might be the acoustic coupling of electrically-isolated circuits. Recent research has investigated using acoustic transfer for electrical isolation by incorporating devices designed specifically for the media through which the acoustic wave propagates. This, in effect, creates a piezoelectric transformer, however notable limitations exist, such as a low common mode rejection ratio (CMRR) and a relatively narrow bandwidth. In the present study, several system configurations with high CMRR, low voltage input and wide bandwidth were analyzed and their relative efficiencies evaluated. A signal was transmitted using piezoelectric devices through conventional PCBs (FR4 material) of different thicknesses. FM modulation proved quite effective for this purpose, revealing a data transfer bandwidth of up to 100kHz for the overall link. The efficient utilization of an ultrasonic link over PCB is demonstrated by applying it in a SMPS feedback signal path. A DSP-implemented digitally-compensated controller was adopted to further enhance the acoustic link and the converter's dynamics. Areas of interest: feedback coupling, signal isolation, piezoelectricity, dc-dc converters, digital power, acoustic signal transferring.