| Abstract [eng] |
The use of ultrasound in non-invasive research and measurements is a common practice in material science. To generate ultrasound ultrasonic transducers are needed, which are commonly made of piezoceramics. Because of the properties of piezoceramics, ultrasonic transducers made from it have a complex input impedance. Not all ultrasonic transducers are made of piezoceramics and alternative materials, such as ferroelectretics are used. The use of ferroelectretics allows to reduce the capacitance of the ultrasonic transducer. In many non-invasive ultrasonic tests, it is of the highest importance to achieve the greatest possible resolution. To achieve high resolution shorter ultrasonic wavelengths must be used, which means higher ultrasonic signal frequency. In many materials ultrasonic wave penetration depth decreases as the frequency increases. To be able to measure greater depth with a high frequency ultrasound, a higher energy signal is needed, which usually means high ultrasonic transducer excitation voltage. Another reason why it is important to have high voltage excitation signal is the high input impedance of the transducer. It is the main problem for ferroelectretic ultrasonic transducers, where the input impedance is higher than in piezoceramic transducers. In order to produce the same amount of acoustic energy as with piezoceramic ultrasonic transducers, excitation voltage must be increased, or a resonant frequency of the transducer used. The aim of this work is to investigate the high voltage pulsers used for ultrasonic transducer excitation. One of the tasks is to determine which of the pulser topologies is best suited to achieve high voltage, high frequency output signal. The comparison of the topologies will be made using several key parameters. Out of the tested topologies one will be chosen to be the main contender for achieving high voltage, high frequency output signal. It is also one of the tasks to evaluate the usage of gallium nitride transistors over silicon transistors. Gallium nitride transistors excel at having higher electron mobility, breakdown voltage and lower main channel conductance. By implementing gallium nitride transistors, the maximum pulser output frequency can be increased to 37 MHz for lower capacitance load of 470 pF and to 15,7 MHz for 2 nF load. |
