Dead-time compensation for a high-fidelity voltage fed inverter

The paper presents an improved method to accurately compensate the dead time effects on a voltage fed inverter for a Magnetic Resonance Imaging (MRI) gradient driver. MRI systems require very high accuracy, errors in the milliamps range, and integral current errors of a few tens of μAs, with output currents of several hundred amperes. In order to achieve the bandwidth required and minimize the output filter size, the ripple frequency is 125kHz or higher. The power architecture consists of three full-bridges in a stack configuration with a modulation technique that results on current ripple very dependent on the operating point. The accuracy requirement, combined with high power, 1MVA, high switching frequency, and arbitrary output waveforms, makes compensation crucial and very challenging. The standard techniques for dead time compensation do not meet the required specifications. A new technique based on conventional approaches with a correction based on experimental results is proposed to significantly reduce the transient errors. The new algorithm has been tested on the real gradient driver, and has shown the peak integral error reductions of more than 50%. The system thus configured fully meets the requirements for image quality in the MRI system.