Comparison of direct and transformer drive high voltage ultrasonic pulser topologies

Ferroelectret films are a relatively new material. Highly anisotropic polymers can be polarized resulting in a piezoelectric response. Advances in manufacturing techniques and polarization have made this technology a viable option in various applications: pressure sensors, air transducers (0.05 MHz-0.6 MHz) and in configuration of patch transducers as pulse sensors and for Lamb waves. Previous experience with this material indicates that this type of material also possess the electrostatic traction effect in ultrasound generation. Yet, excitation with voltages exceeding 1000 V is needed. Probing using rectangular pulse is expected. Production of high voltage pulses beyond 1000 V is problematic, because corresponding semiconductor devices are available only up to 1700 V. Accounting that ferroelectret films have high input impedance and capacitance is low it is reasonable to apply a transformer in order to provide both impedance matching and voltage step-up. Though, higher transformer turns ratio can increase the leakage inductance and driving impedance limiting the attainable bandwidth in such way. Investigation presented was aimed to compare the direct drive and transformer drive excitation topologies when load is the ferroelectret film ultrasonic transducer. Dedicated ferroelectret transducer was used to study two excitation topologies.