High power PWM amplifier with coupling inductor based parallel structure for magnetic resonance imaging

In Magnetic Resonance Imaging (MRI) system, large current with fast ramp up/down transition are required to obtain strong gradient field and to minimize the imaging duration. In addition high quality image requires extremely high fidelity replication of the current from command signal generated by the spectrometer. 500A peak current and 800V output voltage are required to achieve desired gradient strength and slew rate. The control loop bandwidth needs to be larger than 10 kHz for accurate reproduction of the command signal. This paper presents a topology based on the parallel connection of the half-bridge structure. Current combination is achieved by a center tapped coupling inductor, and phase displacement approach is used to increase the effective frequency of output ripple. The switching harmonics can be greatly reduced at the output of coupling inductor with the proposed structure. Small ripple output can also be achieved by implementing a single stage filter with small LC. High control loop bandwidth can be obtained by minimizing filter delay. A prototype was built and tested. Both simulation and experiment results demonstrate the effectiveness of the proposed structure, and the MRI system requirement is fulfilled.