Magnetic Resonance Imaging Power: High Performance MVA Gradient Drivers
In high performance magnetic resonance imaging systems, the gradient driver is required to supply the gradient coil with a large current >600 A) and a high voltage >2000 V) to achieve a strong gradient field and a fast slew rate. In addition, extremely high fidelity for reproducing the current command from the central system is very critical to imaging quality. This paper presents the solutions for the different elements of the driver: 1) high bandwidth (BW) power inverter; 2) ripple cancelation filter; 3) multioutput power supply (PS) and 4) digital control. A high BW power inverter requires a stacked bridges structure to achieve a high output ripple frequency with the existing commercial power semiconductor modules. The high voltage and the high frequency for large power modules can be obtained easier and with lower loss using the new silicon carbide devices. The control needs a digital implementation and a very fast processor. Digital control provides compensation and feedforward to improve the response. A capability improvement is obtained by reducing the switching frequency when large currents with a very low frequency variation are needed. The control can handle it very well, but the filter has to be designed to eliminate more than one ripple frequency. Finally, many PS solutions have been used for multiple isolated outputs, but digital control compensation permits the use of much simpler unregulated PS and keeps the performance. A 2 MVA, 900 A/1200 V, platform has been built and fully tested. The experimental results proved the validity of the proposed structure and the modulation technique.
IEEE Journal of Emerging and Selected Topics in Power Electronics