Hybrid Direct Drive PPU with Extended Operating Range

High-power electric propulsion with Hall thrusters has been proposed as a strong candidate for Electric Path missions, but conventional power processing units (PPUs) are complicated and the mass of the discharge power converters needs to be reduced. Direct Discharge Power Processing Units (DDUs) have been proposed as an alternative due to their simplicity and low mass, but the achievable operating range of thrust and ISP is significantly limited because power regulation for DDUs is only achieved through gas flow control, array offpointing or shunting. This proposal presents a compromise between PPUs and DDUs called a Hybrid Direct Drive Power Processing Unit (HDDU) that provides a wider operating range than DDUs while reducing the mass and increasing the efficiency compared to conventional PPUs. An HDDU provides filtering like a DDU, but it can additionally raise or lower the discharge voltage over a limited range. An HDDU only processes the power necessary to raise or lower the discharge voltage. We propose using a soft switching non-inverting interleaved buck-boost circuit similar to what was built for Phase I, but with improved control circuitry and a modular chassis. Straight-through direct drive operation is possible by leaving the buck switches on continually while having the boost switches off. The proposed HDDU would operate from an input voltage of 150 V to 300 V, and would provide 15 kW output power from approximately 200 V to 500 V. The HDDU approach is readily scalable to higher power levels by connecting modules in parallel because the proposed circuit naturally shares output currents. The HDDU will also include a digital control interface unit (DCIU), heater, keeper and magnet supply modules and driving circuits for a VACCO gas flow controller. The DCIU will be controlled through a Windows GUI and a MIL-STD-1553 communication to USB adapter. The modular approach and enhanced operating range promote design re-use and reduce life-cycle costs.