High Density Power Converter Design for Pulsed NMR Applications

Nuclear Magnetic Resonance (NMR) spectroscopy utilizing pulsed radio frequency (RF) signals has been widely used in several applications such as subsea (or downhole) formation analysis, magnetic resonance imaging (MRI), etc. Since pulsed RF signal’s instantaneous power can be over ten times of its average power, converters designed in peak power rating causes lower power density and higher cost, mainly due to the large capacitor bank requirement for energy transfer. The proposed architecture adopts a two-stage topology to reduce the required capacitance by over ten times, leading to a four-fold improvement in power density. The first stage is an isolated converter only suppling average power, therefore input filter and transformer sizes can get reduced. The second stage is a fast response boost converter followed by a RF transmitter to produce pulsed RF signal, so that the mid-point voltage after the first stage can be allowed to droop considerably, leading to much smaller size of capacitors. Design methodologies for a 150 V DC to 400 V RF, 20 kW peak power converter are described. Simulation results and preliminary experimental results are provided to prove the concepts.