Ripple cancellation filter for magnetic resonance imaging gradient amplifiers

A ripple cancellation technique used on a magnetic resonance imaging (MRI) gradient switching amplifier eliminates switching frequency ripple of the output voltage and current. A new concept requiring only passive components for ripple cancellation in dc power supplies (M.J. Schutten and R.L. Steigerwald, 2003) has been modified for application to an MRI gradient amplifier. The paper presents the circuit changes and the design procedure to accommodate an almost purely inductive load, and arbitrary output waveform of the amplifier. The experimental results show attenuation more than 10× greater than what could be achieved with comparable size conventional filtering techniques. The proposed circuit greatly improves the performance of a switched amplifier for gradient coils in MRI systems. The application of this concept is not restricted to gradient amplifiers; it will also directly apply to other type of amplifiers driving inductive loads such as switched audio amplifiers driving a speaker coil.