Propulsion has always been relevant topic in space exploration. There are different propulsion methods that can be used and on of these methods that has gained interest in the past decades is electric propulsion. The conventional rocket-propelled spacecraft has some inherent limitations, which is related to the proportionality between the mass of the spacecraft and the thrust that can be obtained. If large amount of the initial mass has to be transferred to the destination, the conventional method starts to exhibit some serious drawbacks. If high velocity increment is required for the mission, an alternative method that has a higher specific impulse than the conventional method is required. Electric propulsion offers just that.

Electric propulsion typically requires relatively large voltage levels to operate. Coming up with power converter design that is able to withstand the large voltages and successfully survive the radiation that is present in the space environment is a complex task. Typically, at large voltages, the semiconductor devices are more sensitive to high energy particles that may result in failure of the device and, ultimately, failure of the power system. Flying capacitor multilevel converter (FCML) has shown good results in applications here on Earth because this type of converter topology allows higher voltages with lower switch stress, which is particularly compelling in the context of space environment. This paper focuses on challenges that are related to the design of FCML boost converters that has to operate in space environment.

GaN switches have shown to be tolerant against total ionizing dose (TID) and single event effects (SEE), which was the purpose of using these type of switches in the research. Another challenge in the FCML design is the many levels, which means that there are many switches that have to be controlled, and their control is time critical. Advantage of using the FCML is the reduction of the inductor size as the frequency increases with the level count. In FCML individual switches experience lower stress, but designing the gate drive circuitry becomes more complicated as the switches are not referenced to ground. The authors had to develop a dedicated gate driver circuitry for each switch, but they used boot-strap method, which was relatively simple and only involved capacitors, diodes and low-dropout regulators (LDOs). Another major concern is the current loops within the FCML design, which can cause serious electromagnetic interference issues (EMI) and severely degrade the performance if not designed properly. The GaN transistors have relatively high switching speed, meaning that there will  by high di/dt present, which can cause high voltage overshoots if the parasitic inductance is large. Also, all the switches should withstand the maximum output voltage even if the individual cells will practically not experience these levels because creepage and parasitics can cause coupling between different parts of the circuit. The authors also explored and implemented different PCB design techniques to reduce and deal with different parasitics.


Figure 1. Top and side views of the hardware prototype. source



A prototype of the system was developed and tested. The performance of the proposed design was validated and the prototype was able to reach maximum efficiency of 99.1% . The input ranged from 20 V to 100 V, the output was able to go from 200 V to 500 V, with maximum output power being 1 kW. All the power losses were summarized and analyzed as well. In conclusion, the authors were able to develop an efficient FCML boost converter design, that is able to output large voltages and power. Many steps were taken to implement proper design techniques that eventually lead to small amount of area consumed and low power losses. The converter demonstrated high efficiency and high power density (24 kW/kg), and components were chosen wisely so that they could operate in high radiation environment, which makes this design suitable for space applications. 


Sources:

  • S. Coday, A. Barchowsky and R. C. N. Pilawa-Podgurski, "A 10-level GaN-based Flying Capacitor Multilevel Boost Converter for Radiation-Hardened Operation in Space Applications," 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), 2021, pp. 2798-2803, doi: 10.1109/APEC42165.2021.9487143.
  • “Electric Spacecraft Propulsion”, 2019, https://sci.esa.int/web/smart-1/-/34201-electric-spacecraft-propulsion
  • Z. Liao, Y. Lei and R. C. N. Pilawa-Podgurski, "Analysis and Design of a High Power Density Flying-Capacitor Multilevel Boost Converter for High Step-Up Conversion," in IEEE Transactions on Power Electronics, vol. 34, no. 5, pp. 4087-4099, May 2019, doi: 10.1109/TPEL.2018.2858184.


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