Asymmetric Modular Pulse Synthesizer: A High-Power High-Granularity Electronics Solution for Transcranial Magnetic Stimulation with Practically Any Pulse Shape for Neural Activation Selectivity

Abstract

Noninvasive brain stimulation can activate neurons in the brain but requires power electronics with exceptionally high power in the mega-volt-ampere and high frequencies in the kilohertz range. Whereas oscillator circuits offered only one or very few pulse shapes, modular power electronics solved a long-standing problem for the first time and enabled arbitrary software-based design of the temporal shape of stimuli. However, synthesizing arbitrary stimuli with a high output quality requires a large number of modules. Systems with few modules and pulse-width modulation may generate apparently smooth current shapes in the highly inductive coil, but the stimulation effect of the neurons depends on the electric field and the electric field becomes a burst of ultra-brief rectangular pulses. We propose an alternative solution that achieves high-resolution pulse shaping with fewer modules by implementing high-power wide-bandwidth voltage asymmetry. Rather than equal voltage steps, our system strategically assigns different voltages to each module to achieve a near-exponential improvement in resolution. Compared to prior designs, our experimental prototype achieved better output quality, although it uses only half the number of modules.