Balancing control of paralleled full-bridge converters in high-current gradient amplifiers for MRI applications

To achieve a high current (>500 A) in gradient amplifiers, two full-bridge (FB) converters are paralleled such that each FB converter carries equal share of the gradient-coil current. However, achieving equal current sharing is very challenging because it requires perfect synchronization between the gate pulses of the paralleled FB converters. To achieve equal current sharing, coupled inductors with a large value of magnetizing inductance LM can be added between the paralleled FB converters. However, a large value of LM requires large volume of the coupled inductors. In this paper, to achieve equal current sharing, a combined hardware-software approach is proposed in which, coupled inductors with a small value of LM are used along with a balancing control. Detailed analysis of causes of unequal current sharing between paralleled FB converters is provided. In addition, digital control of two paralleled FB converters with the proposed balancing control is described in details. Finally, simulation and experimental results obtained without and with the proposed balancing control for two paralleled FB converters with 75-A and 400-A maximum gradient-coil currents are presented.