Quasi-linear IGBT inverter topologies

Magnetic resonance imaging (MRI) systems require stronger gradient coil currents with faster ramp times for high-speed image taking and in order to obtain a smaller field of view or a magnification effect. Previous speedup inverter circuits able to provide those higher power levels have been discussed at APEC-92 and APEC-93. They could generate squarewave currents with a constant ramp slope determined by the applied high voltage and the coil inductance. Two new circuit topologies are described here which permit the generation of variable ramp as well as sinusoidal current waveforms for the benefit of pulse sequence designers. The result is a novel high-efficiency quasi-linear power amplifier concept. In one case high-frequency pulse-width modulation schemes are replaced by low-frequency baseband switching. Very good sinusoidal current waveforms (150 A peak, 1150 V) have been obtained