Direct-drive architecture allows to connect the solar arrays with the thrusters thereby representing significant technological progress for space electric propulsion : 98% power chain efficiency, cost reduction thanks to simplification of power electronics and thermal subsystems and weight savings (i.e more payload available for spacecrafts).

To make it a reality, some difficulties must be overcome, namely :

  • Primary bus voltage must fit thruster voltage : The 300-400V bus voltage for Direct Drive is 3 to 4 times higher than the current ones (100V)
  • No isolation between thruster and bus : need of Cathode Reference Potential (CRP) bias supply
  • Modification of thruster operating point has a direct impact on the bus voltage.

Conventional architectures are based on a Power Processing Unit which includes an anode module converting the 100 V bus voltage into 300 to 400V (thruster voltage), with 95% efficiency, and providing galvanic isolation between the primary bus and the anode of the thruster. This is not the case in the case of direct-drive architecture where the anode is directly supplied by the bus. Consequently, the cathode needs a specific power supply (called Cathode Reference Point bias supply converter) to avoid any parasitic current flowing through the external plasma due to this new topology.

Conventional Maximum Peak Power Tracking (MPPT) architectures use modules inside the Power Conditioning Unit (PCDU) which are implementing research algorithms on the solar array features and driving the power conversion with respect to the bus voltage at a 95% efficiency rate.[1]

EDDA needs to develop a MPPT architecture without any voltage conversion inside the PCDU to optimize the power chain efficiency (lesser thermal losses and costs) and adapt the Xenon flow to modify the thruster power and operating point. This operating point will be tunable thanks to a specific innovative regulation control loop inside the PCDU to acquire the absolute maximum solar array power generation capability.

[1] MPPT module is costly and heavy with 0,5 kg per kW at 100V. Likewise, the generated heat on the 5% loss has to be managed and dissipated by the thermal subsystem.

Direct-drive benefits