NASA GRC successfully designed, built and tested a technology-push power processing unit for electric propulsion applications that utilizes high voltage silicon carbide (SiC) technology. The development specifically addresses the need for high power electronics to enable electric propulsion systems in the 100s of kilowatts. This unit demonstrated how high voltage combined with superior semiconductor components resulted in exceptional converter performance

Birchenough, Arthur G.

Bozak, Karin E.

Pinero, Luis R.

Santiago, Walter

Scheidegger, Robert J.

NASA Technical Reports Server

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The Electrochemical Society - GaN & SiC Power Technologies 
 
High Power Silicon Carbide (SiC) Power Processing Unit Development 
 
 
Robert J. Scheidegger1, Walter Santiago1, Karin E. Bozak2, and Luis R. Piñero3 
NASA Glenn Research Center, Cleveland, OH 44135 
and 
Arthur G. Birchenough4 
Vantage Partners, LLC, Brook Park, OH 44142 
 
 
NASA GRC successfully designed, built and tested a technology-push power 
processing unit for electric propulsion applications that utilizes high voltage silicon 
carbide (SiC) technology.  The development specifically addresses the need for high 
power electronics to enable electric propulsion systems in the 100s of kilowatts.  This 
unit demonstrated how high voltage combined with superior semiconductor 
components resulted in exceptional converter performance. 
 
 
 I. Introduction 
 
The NASA space flight program can be divided into manned and unmanned programs,  
setting the stage for advanced space transportation systems which includes, future space 
habitats, and spacecraft that can send humans deep into the solar system. Most spacecraft 
use solar arrays as the main source of power, except for those with relatively short flight 
duration using batteries or fuel cells or deep space probes designed for very long flight 
duration using radioisotope thermoelectric generators.  As the vision for space exploration 
evolves power required for, spacecraft, space stations, and space habitats increases 
exponentially, thus creating the need for more efficient and revolutionary power 
technologies. NASA is investing in cutting-edge power technologies to enable human space 
exploration beyond its current limitations. 
In 2011, the NASA Human Exploration Framework Architectures Team1 (HEFT) 
introduced a capability-driven framework with a 300kW Solar Electric Propulsion (SEP) 
vehicle concept.  Spacecraft buses with increasing power and decreasing mass will enable 
advanced electric and plasma propulsion spacecraft and decrease trip times to Mars and 
beyond. Solar array systems combined with solar electric propulsion have shown many 
potential benefits to the spacecraft developer when compared to chemical propulsion 
systems. The high specific impulse provided by electric thrusters substantially reduces the 
amount of propellant required to achieve a given mission. This mass savings can translate to 
reduction of spacecraft mass, reduced launch vehicle requirements, increased on-orbit 
lifetime, or increased payload. 
Advances in solar array technology that produce more efficient and lighter solar 
arrays have enabled the notion of high power spacecraft as large as 300 to 1000kW class.  
Power systems of this magnitude will require higher voltage distribution and thus power 
                                                        
1 Senior Electrical Engineer, Power Management and Distribution Branch, 21000 Brookpark Rd,  
2 Electrical Engineer, Power Management and Distribution Branch, 21000 Brookpark Rd., 
3 Senior Electrical Engineer, Electric Propulsion System Branch Branch, 21000 Brookpark Rd., 
4 Senior Electrical Engineer, Power Management and Distribution Branch, 3000 Aerospace Pkwy. 
 
https://ntrs.nasa.gov/search.jsp?R=20150023084 2019-08-31T04:59:48+00:00Z
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The Electrochemical Society - GaN & SiC Power Technologies 
 
components capable of operating at high voltage.  In the case of solar electric propulsion, 
high voltage power converters are needed to process the power required by the thrusters. 
In response to these technology needs, NASA’s Office of Chief Technologist (OCT) 
approached the Glenn Research Center to study technologies with the potential to bring high 
voltage to reality.  The study focused on semiconductor devices, mostly switching devices for 
power conversion.  The study team investigated several switching technologies, both old and 
new.  The results identified silicon carbide (SiC) MOSFETS as the most promising.  At that 
time, commercial SiC switching devices were relatively new with little understanding of how 
they would perform when configured as a power converter.  The team felt that the wide 
band-gap SiC device would be better suited for a radiation environment, therefore an easier 
transition to flight qualification.  Armed with this information, a plan for a 15kW converter 
design was accepted by OCT to develop a breadboard high voltage, high power converter 
utilizing this new technology and demonstrate how high power propulsion could be realized.    
At the time this development program started, the first practical, commercial-grade 
silicon carbide MOSFETs became available.  CREE SiC MOSFET devices were selected as the 
switching device in combination with a SiC schottky diode to be used in the converter 
rectifier circuit.  The MOSFET devices selected were 1200V, 42Amp CMF 20120D and 1200V, 
54A schottky diodes C4D40120D. The power processor would operate from a nominal 
300VDC power bus.  The power processor is designed to operate a discharge circuit for a high 
power Hall Effect Thruster.    
II. Breadboard Development 
 
It was determined that the power module was going to consist of two 7.5kW 
converters connected in parallel and operating with an input from 270 to 330VDC and a 
variable output voltage from 300 to 500VDC.  The overall strategy for power converter 
breadboard design employs the full-bridge topology shown in Fig. 1.  This topology is good 
for high power applications because it has low electrical stress on the power transistors and 
very good transformer utilization. Two (2) versions of the discharge were developed, the 
first design used single MOSFET devices in the full-
bridge.  Due to the thermal characteristics of the 
devices, a second version parallels the MOSFETs on 
each leg of the bridge.  This increased the bridge 
power, reduced the overall parts count, and 
reduced the conduction losses on the transistors.  
Since these devices had never been used in this 
application, it was not clear how well the circuit 
would perform in either version.  
 
Version 1 was built with the inverter stage 
using a single FET in each position and identical 
isolated drivers for each of the 4 FETs, essentially 
a 2.5kW converter design.  Multi-layer stacked 
ceramic capacitors were used across the DC power 
input to absorb the current pulses, and a Hall 
Effect current sensor provided isolated sensing of  
A 
B 
C 
D 
Input Voltage 
x n 
x n 
x n 
x n 
Output 
Voltage 
Figure 1. Full-bridge topology 
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The Electrochemical Society - GaN & SiC Power Technologies 
 
the total bridge current.  This version required 3 of these inverter stages in parallel through 
a single power transformer.    
Version 2 uses three MOSFETs in 
parallel on each leg of the bridge to 
reduce conduction losses, reduce the 
parts count, and meet the power needs for 
the 7.5kW circuit. A galvanically-isolated 
gate drive circuit provides 25 V and -5 V 
to the SiC MOSFETs.  The gate drive 
voltages were used to fully turn on and 
turn off the SiC MOSFETs, therefore 
reducing switching and conduction 
losses.3  The main power transformer was 
built with a ferrite U-core using split 
windings, and filter inductors were built 
on amorphous metal C-cores.     
The breadboard discharge circuit 
was thoroughly exercised with both a 
resistive load and a test thruster.  The team was encouraged by the performance of the 
breadboard.  The performance was sufficient to warrant the development of a higher fidelity 
brassboard design with a thermal rejection design.  Figure 2 shows both versions where 
Version 1 is on the lower coldplate and Version 2 is on the upper coldplate. 
 
III. Brassboard Design 
 
Design version 2 was chosen as the go forward architecture.  A full bridge with 
parallel MOSFETS with a switching frequency of 30kHz. The overall strategy for the 
brassboard discharge was a modular design, and if more power were required, converters 
would be operated in parallel.  The primary goal for the design was to focus on a robust 
electrical design, and less on the mechanical design or optimizing packaging.  The next goal 
was to optimize the efficiency by making design choices that could provide the best efficiency 
performance.  The unit has a thermal design using the chassis baseplate as the primary 
thermal path for cooling.  Staying with the full bridge provides low voltage stress on the SiC 
transistors, good transformer utilization, and is easy to scale. 
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The Electrochemical Society - GaN & SiC Power Technologies 
 
The brassboard converter design features include PWM control, peak-current-mode 
control, cycle-by-cycle current limit and shoot-
thru protection, voltage and current mode 
output regulation, and soft start control.  SiC 
Schottky diodes were used as the output 
rectifier for their high voltage capability and 
reverse recovery.  Ideally since the unit is 
designed for a space application, parts 
selections would be parts with path-to-space 
qualified devices.  In this case, with the SiC parts 
being so new, there are no space-qualified 
components, although all other parts have a 
path to flight.  Figure 3 shows the end product of 
the modular brassboard SiC power unit.  This 
unit is designed to operate a Hall effect thruster 
(HET) and is electrically fully functional.     
It is anticipated that a packaged flight design will be reduce the mass by 25% 
compared to state-of-the-art (SOA) units.   
 
III. Brassboard Testing 
 
Testing was conducted to validate the operation and performance of the 300 V SiC 
brassboard unit.  Performance parameters such as regulation, ripple, telemetry and setpoint 
accuracy were validated for various input and output voltages and power levels.  Inputs and 
outputs were instrumented to measure DC performance.  A digital oscilloscope with current 
probes was used to measure and characterize input and output ripples.  Temperature was 
monitored using thermocouples to measure operating temperatures.  The converter load 
was a resistive load bank to simulate steady-state operation and transients.  Total efficiency 
was calculated as the ratio of all power outputs to all power inputs.  
 
 
Figure 4. 300V SiC Converter efficiency vs. total output power for several voltages 
 
Figure 3. 300V SiC Power Unit 
 
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The Electrochemical Society - GaN & SiC Power Technologies 
 
Figure 4 shows an efficiency plot of two high power converters in parallel, as seen in 
Figure 3, as a function of output power for several output voltages.4  Testing demonstrated 
converter efficiencies greater than 98%, well beyond the expected results.  The efficiency 
was better than 96.5% for the entire power range from 3 to 15kW.   
Ripple, regulation and telemetry were within 
the expected goals. Measurements revealed a 
temperature rise of about 30 ˚C in the main 
power transformer and output inductor for the 
power converter which is within derating 
guidelines.  The temperature rise of other 
components was less than 20 ˚C.  
Performance tests were repeated in a vacuum 
environment to validate the thermal design.  
Figure 5 shows the unit installed in the GRC 
vacuum facility.  The power unit was mounted on 
an aluminum water-cooled baseplate to 
conductively reject heat.  The converter was 
operated at baseplate temperatures at 5, 25, 50 
oC.5 
 
IV. Conclusion 
 
Combining high input voltage and high performance switching components resulted in a 
power converter with a peak efficiency greater than 98% with an expected reduction in mass 
of 25% from state-of-the-art.  The development demonstrated a fully functional 15kW 
converter that can be utilized for solar electric propulsion system with the development of 
flight qualified SiC parts. 
What started out as a high power converter design demonstration resulted in a high 
power, high efficiency converter design.  The extremely high efficiency has additional 
positive effects that ripple through spacecraft platform.  For a proposed 300kW propulsion 
system, the thermal load produced by the SiC unit would equate to 6kW as opposed to a 
24kW load with a state-of-art efficiency.  This is a huge reduction in thermal system mass.  In 
addition to the mass savings from the thermal system is the mass saving from the power 
generation system.  The spacecraft can carry 18kW less of solar array.  All the mass saving 
then reduces the wet mass in the form of less propellant.     
This development has demonstrated the mass and efficiency advantage by the use of 
silicon carbide technologies combined with high voltage in power conversion applications.  
These gains can be greatly utilized for space power applications if radiation hardened space 
rated components were available.  NASA Glenn Research Center (GRC) had performed 
preliminary heavy ion testing of select commercial SiC MOSFET and diodes.  The outcome of 
the testing produced failures in all the test samples.  NASA is currently investing resources 
to further test and studying the component failures to better understanding the failure 
mechanisms.  Understanding the failure mechanisms can lead to recommendations for 
modifications in the manufacturing of the components to make them less susceptible to 
failures.  It is our hope that the manufacturing community can help develop qualified parts 
for space. 
 
Figure 5. 300V SiC PPU in vacuum testing 
 
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The Electrochemical Society - GaN & SiC Power Technologies 
 
For NASA to pursue high power systems such as high power solar electric propulsion then 
high voltage and high efficiency power distribution and power processing is crucial.  
Therefore the development of qualified high performance space rated electronic 
components as discussed in this paper will be essential.  
 
 
 
 
  
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The Electrochemical Society - GaN & SiC Power Technologies 
 
 
Acknowledgements 
 
The authors would like to acknowledge the GRC PPU Team that supported the design, 
analysis, modeling, documentation, fabrication and testing of all the hardware developed for 
this project including: Danis Arthur, Michael Aulisio, Thomas Balogas, Michael Capelety, 
Edward Chisolm, Randy Clapper, Leon Collins, Justin Elckert, Henry Fain, Andrew 
Fausnaugh, James Fleet, Don Fong, Michael Garrett, Marcelo Gonzalez, Gary Gorecki, Elmer 
Griebeler, Ahmad Hammoud, David Hauser, Halbert Hoyett, Nedyalko Ivanov, Terrell Jansen, 
Hani Kamhawi, Kevin McCormick, Michael McVetta, Beth Osborn, Michael Pastel, Thomas 
Ralys, David Sicking, Marcus Studmire, Paul Trimarchi, Long Truong, and Brandon White.      
 
References 
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2014-3718, 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, July 28-30, 2014.   
2Smith, B., Nazario, M. L., and Manzella, D. H., “Advancement of a 30kW Solar Electric Propulsion 
System Capability for NASA Human and Robotic Exploration Missions,” IAC-12-C4.4.2. 
3Piñero, L., Scheidegger, R., Aulisio, M., and Birchenough, A., “High Input Voltage Discharge Supply for 
High Power Hall Thrusters Using Silicon Carbide Devices,” IEPC 2013-388, 33rd International Electric 
Propulsion Conference, George Washington University, USA, October 6-10, 2013. 
4Bozak, K., et al., “High Input Voltage, Silicon Carbide (SiC) Power Processing Unit Performance 
Demonstration,” AIAA 2015-TBD, 51th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Orlando, 
FL, July 27-29, 2015. 
5 Piñero, L.R., Bozak, K.E., Santiago, W., Scheidegger, R.J.,”High Power SiC Power Processing Unit 
Development”, AIAA 2015-TBD, 51th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Orlando, 
FL, July 27-29, 2015. 


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