Ultrasonic coupling of electrically isolated circuits using piezoelectric devices

Ceramic materials with enhanced piezoelectric characteristics became widely available in recent years, which have led to an ever-increasing employment of these devices in various applications such as: sensing, energy harvesting and ultrasonic transducers, among others. This paper describes an acoustic link on an electronic circuit's PCB that offers isolation and high immunity from EMI. This type of coupling may be adapted to such applications as isolated switched-mode power supplies, high voltage current probes and data transfer. A further application might be the acoustic coupling of electrically isolated circuits. A recent research article reported an attempt to use acoustic transfer for electrical isolation incorporating devices designed specifically for the media through which the acoustic wave propagates i.e. piezoelectric transformer. However, they observed notable limitations such as a low common mode rejection ratio (CMRR) and a relatively narrow bandwidth [1]. In the current research, several system configurations with high CMRR, low voltage input and wide bandwidth were analyzed and their relative efficiencies evaluated. A signal was transmitted through conventional PCBs (FR4) with different thicknesses using piezoelectric devices. FM modulation proved quite effective for this purpose. The study includes: an analysis of the electromechanical processes involved as well as a model of the system, simulation results, and a comparison of the resulting bandwidth for different modes and materials. We also optimized a design for a relevant practical application - a switched mode power supply feedback signal coupling. The simulation was carried out using PSPICE. As there was almost no performance impairment compared to conventional electronic links, the investigation shows the benefits of the proposed acoustic link technique in systems in which a bandwidth of up to 100kH is required.