Development Of A HB-MMC Based Nanosecond Pulse Generator With Adjustable Rise/Fall Time

Cancer has always been a major illness that threatens human life.In recent years,the application of nanosecond pulsed electric field to treat tumor cells has become a research hotspot in related fields.The parameters of the nanosecond pulse,including amplitude,pulse width,repetition frequency,and rise/fall time,have a great influence on the effect.Compared with microsecond pulses,the rise/fall times of nanosecond pulses have a greater influence on the effect.However,due to the difficulty in the development of nanosecond pulse generators with adjustable rise/fall time,few studies have been done on the effects of the rise/fall time on tumor cell viability.Therefore,the development of a nanosecond pulse generator with adjustable rise/fall time is of great significance for the study of nanosecond pulsed electric field treatment of tumors.Firstly,the principle of half-bridge modular multilevel converter(HB-MMC)is described,and the method to generate multilevel waveforms through HB-MMC structure is analyzed.In addition,the principle of adjusting the rise/fall time of pulse through multilevel waveforms is introduced.Then,the topologies of the unipolar generator and the bipolar generator designed in this paper are presented.The working principles of the two generators in charging and discharging mode are analyzed.At the same time,the method to adjust the rise/fall time of positive and negative pulses by setting the switching sequence is analyzed in detail.After that,the simulation models of the proposed topologies are established by PSpice,which verifies the correctness of the two topologies as well as the principle of rise/fall time adjustment.Besides,the influence of stray parameters on the output pulse is simulated and analyzed.Secondly,the pulse generator is designed in this paper.The parameters calculation and selection of important devices in the generator are firstly completed,including solid-state switch,drive chip,storage capacitor,isolation module and DC power supply.After that,the control circuit is designed.FPGA is selected as the control signal source.The electro-optical conversion circuit is designed to convert the electrical signal into an optical signal,which is transmitted through the optical fiber so as to achieve isolation between the high and low voltage sides.Then the HB-MMC sub-module board as well as the baseplate to place and connect the sub-module boards are designed.Then,a test platform is built to test the performance of unipolar and bipolar generators.The test results show that when using a 200? resistor as a load,the generators can stably output a square-wave pulse with adjustable amplitude of 0~4kV and adjustable pulse width of 100~500ns,and the highest repetition frequency can reach 10 kHz.The rise time of the pulse can be smoothly adjusted in the range of 15~130ns,while the fall time can be smoothly adjusted in the range of 30~130ns.Finally,this paper establishes a cell viability experimental platform.The human skin cancer cell A375 is selected to conduct cell viability experiments to preliminarily explore the influence of rise/fall time on the viability of in vitro tumor cells.The experimental results show that under under an electric field strength of 3kV/cm or 4kV/cm,the shorter the rise/fall time of the pulse,the stronger the killing effect and the lower the cell viability.At the same time,the experiments also verifies that the pulse generator developed in this paper can be applied to in vitro tumor cell experiments.