DC-DC power conversion for high voltage applications using piezoelectric transformers

Modern electronic systems increasingly demand power converters that are compact, efficient, and cost-effective. Piezoelectric transformers (PTs) may offer a promising alternative to conventional magnetic-based converters, with demonstrated advantages such as higher efficiency, smaller size, and lower electromagnetic interference (EMI), as evidenced by their widespread use in LCD screens for laptops and TVs during the 2000–2010 decade. This potential is further highlighted by an expanding body of recent academic publications in the literature, reflecting renewed interest in PT-based power supply designs for next-generation applications such as space and medical electronics. With no magnetic core, inductorless PT based converters are of particular interest in medical equipment intended for use in the vicinity of high magnetic fields. However, as a relatively new approach for power conversion, PTs have several issues that need to be overcome. To address an initial issue, a method to control the inrush current of inductorless converters based on PTs with high input capacitance is presented. The proposed method involves applying a reduced gate voltage to the converter MOSFETs during start-up to increase their on-resistance and decrease dv/dt at the switching point. A higher gate voltage is applied once steady state PT conditions are established to provide efficient operation. Design, analysis and experimental results successfully validate the proposed start-up circuit. The main focus of this thesis is on a new application of PT based power converters to generate high-voltage (HV) bipolar pulses for medical electroporation therapy. In particular, PT based power conversion is investigated as an alternative to magnetics-based approaches of generating high-voltage from a relatively low-voltage input source for application in electroporation therapy. A detailed PT based system design and selection of wide-bandgap semiconductor switches such as GaN FETs, high-voltage SiC diodes and SiC MOSFETs, as well as simulation results to demonstrate a proof-of-concept are presented. Preliminary experimental results of a PT based capacitor charger validate the simulation results. Following proof-of-concept, the performance of a PT based capacitor charger is compared with a Flyback transformer converter to be used in bipolar pulsed-power applications, analysing efficiency, capacitor charging time, input power, and component count to determine which is most suitable for HV medical applications. The parallel operation of up to five sample PTs is demonstrated as a means of extending power transfer limits to charge the load capacitor charger faster, and to enable comparison in the power range typically used for capacitor chargers in high-voltage pulse generators employed for medical electroporation therapy. The operation of a PT based charger using parallel operation of three PT samples is demonstrated experimentally and results are compared with simulations to understand the overall trends in performance. Due to comparable impedance levels with the PT output, analysis of the effects of measurement probe impedances and parasitic impedances of HV diodes used for rectification on the output voltage of PT based capacitor charger are shown to be significant. While the work of this thesis is focussed on high voltage generation for use in electroporation therapy, the proposed methods and analysis may be applied and extended to high voltage generators for other medical and industrial equipment, and other PT based power electronic circuits where magneticless operation is required.

Funder

University of Galway

Publisher

University of Galway

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CC BY-NC-ND

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University of Galway Theses (PhD Theses)