Thyristor triggering by coil zero-current crossing detection in a magnetic stimulator for living tissues

This paper describes the design of a zero-current crossing detection circuit for a high-power (650 V, 6000 A) magnetic generator. The magnetic pulses are produced by discharging a capacitor C_d into a coil in resonant mode via thyristors and are used to stimulate living tissues. The generator delivers short (340 μs), high-intensity 2 Tesla magnetic pulses. Half-sine or full-sine waveforms are available. When full-sine current into the coil is selected, a smooth transition between the 2 half-sines is required: the second thyristor must be triggered shortly before the first one switches off to ensure continuity in tissue excitation. The period of the resonant circuit is computed in advance, irrespective of interchangeable coil inductance and damping factors. The system core consists of a toroid sensor encircling the high-current main wire of the coil associated with an integrator circuit. The detection of three main events: the starting time and zero-crossing time of the sensor voltage and the zero-crossing time of the potential across C_d, makes it possible to detect the pulse period right from the beginning of the second quarter period. Electronic design and validation tests are presented in section II. The results and possible improvements are discussed in the final section.