To convert the DC electrical power from the spacecraft into the high DC voltage levels and the high frequency AC voltage required to operate the ion thruster, a power processing unit (PPU) is used. One of the components of the PPU is the radio frequency generator (RFG), which is used to ionize the propellant allowing to control the thrust. The RFG architecture is shown in the following figure, where a DC/DC stage controls the power and the DC/AC stage is resonant inverter that synchronizes with the load’s frequency and phase to ensure a high efficiency power transfer.
Buck converter operating in Triangular Current Mode:
In order to minimize the size of the DC/DC converter, high frequency operation (200kHz-500kHz) is required, being necessary the use of GaN semiconductors and to operate with Zero Voltage Switching (ZVS) to reduce switching losses. The proposed topology, Buck converter operating in Triangular Current Mode, allows for ZVS operation under the whole range, being necessary the use of Digital Control to face the control challenges of the topology. The following picture shows the block diagram of the DC/DC state.
The DC/DC converter has been built and tested at full power in open loop, achieving very high efficiency 98.7% efficiency at 1kW. Currently, the converter is being tested in closed loop operation. The following pictures show the prototype as well as measurements that validate the proposed design.
Picture of the DC/DC prototype
Main waveforms operating at 1kW with ZVS
Temperature of high side GaN switch at 1kW
Closed loop operation: output voltage step
Resonant Inverter for the RF Generator:
The main blocks of the RFG inverter are shown in the following figure.
The proposed solution has been tested at full power (1kW) in closed loop, being self-synchronized the inverter with the load. Therefore, the proposed concept has been fully proven, achieving in the inverter very high efficiency at full load: 97% at 1kW and 700kHz.
The modelling and design of the harness is very challenging since it operates at 700kHz. One of the next steps is to test the inverter with a real harness to operate under the desired conditions.
The following pictures shown the RFG inverter prototype and different measurements that validate the proposed concept in closed loop and at full power.
Picture of the setup to test the inverter
Main waveforms operating at 1kW and 700kHz with ZVS
Temperature of high side GaN switches at 1kW
Harness effect on the admittance seen by the RFG inverter