The Modular Multilevel Magnetic Stimulator: Energy-Efficiency, Pre-Charging and Overlap Protection

The technology of transcranial magnetic stimulation (TMS) enables to break new ground in medical research. It is suitable for different diagnostics, as well as therapeutic purposes, such as the treatment of depression or epilepsy, which are not fully explored yet. TMS devices that are currently available on the market offer a limited possibility for a targeted stimulation, due to their predefined voltage pulse shape. This paper presents a novel TMS device based on a multilevel inverter technology. Utilizing several cascaded sub-modules (built as H-bridges), arbitrary output waveforms can be discretely generated. At first, a theoretical part discusses how the energy efficiency of such a device can be increased by applying suitable control strategies and how it is possible to charge multiple sub-modules with only one auxiliary power supply. In addition, it is shown how to determine the required capacitance rating of a sub-module. Next, in the practical part, a designed prototype is presented and its current and voltage capabilities are verified, conducting 2.3 kA (peak-to-peak). In addition, the influence of different dead-times on the output voltage waveform is investigated. It is shown that the precise adjustment of the dead-time is crucial to ensure the proper switching of the required current without damaging the hardware.