High-power high-fidelity switching amplifier driving gradient coils for MRI systems

Magnetic resonance imaging (MRI) systems use an amplifier to drive the gradient coils with several hundred amperes and voltages in excess of 1500 V. The fast response and extremely low coil current errors needed for good image quality in MRI systems are one of the main challenges for the designer. This paper presents new power stage architecture for the gradient amplifier, and two different control modulation techniques. The power stage consists of three full-bridges in a stack configuration operated with interleaved switching to deliver the required output voltage with high-frequency current ripple. One of the bridges has 400 V input voltage and switches at high frequency to provide high bandwidth, the other two bridges have 800 V and switch at lower frequency for loss reduction. The modulation technique is crucial to achieve the required waveform accuracy; a fully digital control provides great flexibility on the control. The paper explores two modulation options; one that minimizes the switching losses, but that will require bidirectional power supplies for some waveforms, and another with higher losses but more suited for arbitrary waveforms. The experimental results show the waveform accuracy from the amplifier implementation with the selected control.