High power, wide bandwidth linear switching amplifier using IGBTs

A linear switching amplifier rated 4.5 MVA (1500 V, 3000 A) with a bandwidth of 1 kHz at full current and up to 10 kHz at reduced power (1500 V, 300 A) has been designed, built, and commissioned in four examples for the JET Joint Undertaking (Abingdon, UK) to generate a high-frequency, high-intensity magnetic field for plasma control in the JET tokamak. These linear amplifiers were designed to generate, in special magnets, current waveforms which accurately follow the reference, with a phase delay not greater than 30 electrical degrees at 10 kHz (and much less at lower frequencies). To do this they employ 1200 V, 200 A IGBTs (insulated-gate bipolar transistors) (24 in parallel, 2 in series) and use sophisticated control techniques, such as multilevel delta-modulation with variable hysteresis control, to achieve a linear behavior with constant gain from 1% to 100% of the output current capability. Special circuits for minimization of the commutation frequency of each IGBT, accurate commutation timing (within 100 ns), and predictive compensation of IGBTs switching delay were required to meet the high performance requirements