MRI Compatible DC Modulator for an Envelope Tracking Transmitter

Typical MRI power amplifiers are characterized by the requirement for high linearity and a high peak-to-average power ratio. When implemented as a linear amplifier, such as the commonly used Class-AB architecture this leads to a low efficiency and high heat dissipation, which impacts performance and reliability. Here we present a MRI compatible DC modulator for envelope tracking applications based on a 4-phase interleaved buck converter using air core magnetics and utilizing a FPGA based digital controller. The modulator is capable of delivering 500 W into a 2.5 Ω load at an efficiency of 95.6%, has a 3 dB modulation bandwidth of 50 kHz, and has been tested with a Class-AB linear amplifier operating at 64 MHz to produce a modulated RF envelope with a peak power of 320 W. Due to the higher efficiency of envelope tracking amplifiers and the ability to mount the amplifier directly on the MRI machine itself this system has the potential to decrease both the cost and space requirements of MRI machines.