The Flying Capacitor Multi-Level (FCML) converter is heralded as enabling the utilization of low-voltage switches within a high-voltage converter by evenly distributing the voltage stress on a series string of switches through the use of "flying" capacitors. However, this advantage of the converter requires that the flying capacitors do not deviate too far from their ideal voltage distribution regardless of transients, load disturbances, or other parasitics to ensure the switches are not overvolted among other concerns. Previous works have identified certain theoretic combinations of duty cycle and level count where the flying capacitor voltages do not naturally balance and instead diverge. Although seemingly problematic, in practice this behavior is not observed in practical implementations. To resolve this discrepancy between FCML theory and experiment, we analyze the impact of switch output capacitance on the flying capacitor voltage balancing dynamics, and illustrate this capacitance has a naturally balancing effect.
Electric aircraft require power dense and efficient powertrains to become practical and break out of the laboratory and into the air. This work describes a full electric drivetrain, comprising six interleaved 10-level flying capacitor multi-level (FCML) dc-ac converters with high (38.4 kW/kg) specific power density and efficiency (98.95%). A global controller implements field-oriented control, and communicates through digital optical links to a local FPGA controller on each dual-interleaved FCML module (each module comprising two dc-ac converters). Successful demonstration on a full dynamometer test-bed with twin 250 kW electric aircraft machines is demonstrated, along with an active feedforward cancellation technique to reduce higher order harmonics.
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