Parallel operation of switching amplifiers driving magnetic resonance imaging gradient coils

The paper presents a high-current switched driver for gradients coils used in magnetic resonance imaging. The proposed solution driver consists of two 1 W, 2 A amplifiers in parallel. The successful parallel operation of these drivers requires good current sharing during steady state, as well as transients. In addition, MRI system performance requires a bandwidth no less than that of each single driver, and no loss of voltage to drive the gradient coil. To ensure good current sharing only by control means results in an amplifier with unacceptable transient response, either too slow, or with oscillations. A coupled inductor at the output of the amplifiers is the preferred option. The coupled inductor offers the additional option of operating the two amplifiers out-of-phase to reduce the output current ripple with minimal impact on the inductor size. The paper presents the analysis and simulations for all the options, and the design of the coupled inductor for the selected solution. The experimental results in a system prototype confirm the design selection and show the system performance at high currents.