1. Solar Electric Propulsion: High Voltage Power Processing
Unit
Sean P. Martin1
Temple University, Philadelphia, PA 19022
The Pennsylvania Space Grant Consortium
Henry B. Fain2
NASA Glenn Research Center, Cleveland, OH 44135
Glenn Research Center’s Solar Electric Propulsion Power Integration Testbed project
requires a brassboard power processing unit to be built that is capable of operating a 12.5 kW
Hall Effect Thruster. The PPU is of an existing design, and SEP POINT will use two of these
brassboard PPUs for early phase testing. The PPU operates off of a high voltage (120V) and
low voltage (28V) power bus and provides the necessary filters, telemetry, and auxiliary and
discharge power needed for thruster operation. Full power efficiencies upwards of 92% can
be achieved. Four 4kW discharge modules can provide up to 800V or a maximum of 20A to
the thruster. The auxiliary power module provides power to the thruster’s two electromagnets
and the cathode heater and keeper. The focus of this paper is on the assembly and testing of
the four power supply modules that make up the auxiliary module. The electromagnet supplies
are half bridge DC-DC converters that operate off the 120V bus. The heater and the keeper
supplies are both full bridge, current regulated DC-DC converters that operate off the 28V
bus. Each was tested for efficiency, line/load regulation, output current ripple and telemetry
accuracy. Thermal data was also collected. Each auxiliary module supply operated to
specification, a major step towards the completion of the brassboard PPU. With 99% of
components used having an identifiable path to flight, this technology can be implemented
relatively quickly. The Asteroid Robotic Redirect Mission is an identified candidate for this
technology; moreover, the lessons learned by developing this high power solar electric
propulsion hardware will benefit NASA’s Journey to Mars.
I. Introduction
ASA’s Solar Electric Propulsion (SEP) project is spearheaded by the Glenn Research Center. There are many
applications for this high power technology,ranging fromnear Earth asteroid exploration to satellite keeping and
ultimately NASA’s journey to Mars. To achieve these goals, higher power solar electric propulsion systems must be
designed,tested and built. This paper focuses on work done on furthering this progress: the building and testing of a
second brassboard High Power (HP) Power Processing Unit (PPU) capable of operating a 12.5kW Hall Effect
Thruster.
The brassboard PPU described in this paper is identical to a finished brassboard unit that has been integrated with
a Hall Effect Thruster in both ambient and vacuum test conditions. The results of this testing and development were
recently presented at the 2016 AIAA Propulsion and Energy Forum.1 A second brassboard PPU is needed to further
the objectives of another GRC project, the Solar Electric Propulsion Power Integration Testbed (SEP POINT).
The ultimate goal of SEP POINT is to provide a high fidelity electrical test environment that will provide insight
into the electrical interactions between SEP hardware. This includes the solar array, a high voltage power distribution
unit, high voltage PPU, and the thrusteritself. Designed for flexibility, the testbed will provide the ability for plug in
verification of flight hardware.2 The first PPUs used in POINT will leverage the recent HP 120V PPU design. The
POINT testbed PPU must still provide the necessary filtering, telemetry and auxiliary and discharge power need to
operate a Hall Effect Thruster.1
1 Electrical Engineering Intern, Power Management and Distribution Branch, NASA GRC
2 Electrical Engineer, Power Management and Distribution Branch, NASA GRC
N

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This paper discusses the progress made on building the second brassboard PPU, particularly the auxiliary module.
Section II provides an overview of the PPU’s design and functionality. This provides context for the subsequent test
results presented in Section III. Finally, forward work that must be done is discussed along with the implications of
this project’s success.
II. PPU Design Overview
The HP 120V PPU built uses an existing
PPU design, which itself leveraged lessons
learned from the development of a 300V
silicon carbon (SiC) based brassboard PPU.1
By reverting from SiC components to more
conventional electronics, 99% of the
components on the PPU have a qualified path
to flight. The specifications for the PPU also
remained the same: It must be able to deliver
up to 14 kW of power by the discharge
supply, at either 800V or 20A maximu m.
This is required to provide a Hall Effect
Thruster with a specific impulse of 3,000
seconds.1 The PPU also has auxiliary power,
input filtering and a master control board.
Figure 1 provides a block diagram of the PPU
integrated with a thruster.
The PPU receives input power from both
a high voltage (120V) and low voltage (28V)
bus.All four discharge modules as well as the inner and outermagnets are connected to the high voltage bus,assumed
to be representative of a spacecraft’s solar arrays. The thruster’s cathode heater and keeper are each connected to a
supply powered by the low voltage bus. This 28V bus is assumed to always be powered in a spacecraft, meaning that
the thruster’s cathode can be kept hot when
the spacecraft is in eclipse. Thrusting can
then be resumed easier when eclipse is
exited.1
A. Discharge Module
Each discharge module is capable of
outputting 200V. Four modules are
connected in series at the output,producing
the 800V needed. By connecting the inputs
in parallel, higher efficiencies can be
achieved at voltages lower than 400V and
currents less than 20A by only utilizing two
of the four discharge modules for these lower power output conditions.1
Each discharge module is a DC-DC converter that uses a full bridge, transformer isolated topology.At the
primary side on each bridge leg three MOSFETs are parallel to provide both better efficiency (through lower
conduction losses)and redundancy.3 Schottky diodes are used in the full bridge rectifier. The controller is capable of
operating each discharge module in either current or voltage regulation mode. The pulse width modulation controller
operates at a 60 kHz switching frequency. Moreover, current transformers that can accurately sense the current
through each MOSFET leg are used for peak current control to protect the supply from overcurrent events.3 Figure 2
below pictures a single assembled discharge module.
B. Auxiliary Module
The auxiliary module provides power to the thruster’s inner and outer electromagnets as well as the heaterand the
keeper for the cathode.
Figure 1. PPU and Hall Effect Thruster Block Diagram.
Figure 2. Assembled Discharge Module.

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1. Inner/Outer Magnet Supplies
Both the inner and the outermagnet supplies are powered from the 120V bus since they only require power when
the thruster is operating. This is a change from previous PPU designs. By operating the magnet supplies off of the
120V bus instead ofthe 28V bus,the power requirement of the 28V bus can be halved from 1kW to 500W.3 A second
advantage is that the magnet supplies need only a half bridge topology when operated off the 120V bus.Both of these
advantages reduce the number of components for the magnet supplies and ultimately reduce the mass of the PPU
itself.3
The magnet supplies are half bridge DC-DC converters with nominal operating conditions of 120Vin, 10 ADC
out with a 2Ω load. They operate in current regulation mode with an average current control loop. The pulse width
modulation controller operates at a 30 kHz switching frequency. Again, current transformers can sense the current
through the MOSFET legs in the bridge and are used to provide overcurrent detection. Figure 3 pictures a single
assembled magnet module.
2. Heater Supply
The heater supply is powered from the 28V bus. It is a DC-DC
converterwith a full bridge, transformer isolated topology.Much like the
magnet modules, the heater operates in current regulation mode and
current transformers and the controller are utilized for overcurrent
protection. The pulse width modulation controller operates at a 30 kHz
switching frequency. The nominal operating conditions of the heater
supply are 28Vin and 9 ADC out with a 4Ω load.
3. Keeper/Igniter Supply
The keeper supply is near identical to the heater module. The only
difference being the addition of an igniter drive circuit. This provides an
ignition pulse that starts discharge to the cathode. The pulse is a 750V
(±10%) pulse with a ≤ 5µs rise time that repeats ten times per second until
discharge is sustained.3 Figure 5 pictures the assembled keeper module.
The heater is almost identical in appearance save forthe igniter circuitry
which is barely visible under the blue LEM current sensor; therefore,
only the assembled keeper is pictured here.
C. Additional Modules
Additionally, the PPU will contain an input filter module, a master
control module and a system control board. The input filter module
filters the high voltage and low voltage buses and detects the current
and voltage levels at each. Common and differential mode low pass
filtering is used to attenuate noise with a corner frequency of 7 kHz.1
The MasterControl Module (MCM) receives commands from a digital
interface, the System Control Board (SCB). The MCM can receive
telemetry data from the power supply modules and input filters and
relay it to the SCB; moreover, it synchronizes the power modules and
provides the igniter pulse command.1 It must also be noted that the SCB
is being simulated on this brassboard PPU and an actual SCB will be
integrated at a later date.
III. Test Results
Each of the four auxiliary modules was tested individually. A bench top power supply was used to simulate the
high voltage and low voltage buses,as well as housekeeping power. The current command signal the MCB sends to
the auxiliary supplies was simulated with a varying voltage signal from a power supply and potentiometer. Moreover,
resistive load banks were used to simulate different power level load conditions.1 Steady state measurements of
input/output voltage and current as well as telemetry values were measured with calibrated digital multimeters and
dynamic electrical signals were measured with a digital oscilloscope.
Figure 3. Assembled Magnet Module
Figure 2. Assembled Discharge
Module.
Figure 4. Assembled Keeper Supply

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As data is presented for the auxiliary modules only, their required electrical specifications are presented in Table
1 below.
Table 1: Auxiliary Electrical Requirements4
Input Voltage
Bus
Output Current Output Voltage Ripple Line/Load
Regulation
Magnet 120 V 1-10 A 2-20 V ≤ 5% ≤ 2%
Heater 28 V 3-9 A 6-36 V ≤ 10% ≤ 2%
Keeper 28 V 1-3 A 10-30 V ≤ 5% ≤ 2%
A. Efficiency
Each of the modules was set at different load and output current settings to gather efficiency data. By comparing
the power at the input of each supply to the power at the output,efficiency could be determined. This was then plotted
as a function of output power. At nominal operating conditions, efficiencies of 90%-92% were observed. Figure 5
shows the operating efficiency of the outer magnet supply. The inner magnet exhibited similar behavior. Figure 6
below shows the operating efficiency of the heater supply. The keeper exhibited similar behavior.
Figure 5. Outer Magnet Effciency Plot
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
0 50 100 150 200 250
Efficiency(%)
Output Power (W)
120V Magnet: Efficiency vs. Output Power
10 amp
setting
7 amp
setting
5 amp
setting
2 amp
setting

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B. Current/Voltage Ripple & Line/Load Regulation
For each supply,output ripple and line and load regulation were measured with input/output multimeter readings
as well as through the use of the oscilloscope. Line regulation was defined to be how well the supply can keep its
output setting as the input voltage is varied. Load regulation was defined as how well the supply can maintain its
Figure 6. Heater Effciency Plot
Figure 4. Assembled Keeper Supply
78%
80%
82%
84%
86%
88%
90%
92%
94%
96%
0 50 100 150 200 250 300 350
Efficiency(%)
Output Power (W)
Heater: Efficiency vs. Output Power
Constant Input, 28 VDC
9 Amp
Setting
7 Amp
Setting
5 Amp
Setting
3 Amp
Setting
Figure 9. Outer Magnet Line Regulation Data

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output setting as the load resistance is varied.1 Figure 9 below shows the line regulation data obtained for the outer
magnet and Fig 10 shows the load regulation data obtained for the heater.
Since the auxiliary supplies are current regulated, the peak to peak ripple in output current was measured with an
oscilloscope and compared as a percentage of the output current setting to obtain a value for ripple. Figure 11 below
shows the ripple oscilloscope reading for the inner magnet.
Figure 11. Inner Magnet Input/Output Ripple Waveform
Figure 10. Heater Load Regulation Data

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The line regulation, load regulation and output current ripple data is presented in Table 2 below. The specifications
laid out in Table 1 are met in all four cases.
Table 2: Ripple and Regulation Results
Line Regulation Load Regulation Ripple
Inner Magnet 0.08% 1.90% 1.26%
Outer Magnet 0.075% 1.42% 1.28%
Heater 1.38% 0.2% 2.32%
Keeper 0.04% 0.35% 0.78%
C. Other Results
Each supply was demonstrated to be able to provide overcurrent detection and protection as well as be able to
continue operation when either the positive output lead or negative output lead is shorted to the chassis. Thermal
analysis was performed by placing thermocouples on the rectifier diodes, switching FETs, transformer and baseplate.
No abnormally hot parts were found.
The reliability of the telemetry data of each supply was also recorded. Both the voltage and telemetry data for each
module was measured to be within 2% of what the actual output current and output voltages were. This is within
design specifications. However, the voltage telemetry between the magnets and the heater/keeper must be measured
at different places. The heater and the keeper used a dedicated telemetry board which sends the voltage telemetry
signal to J6 pin 3 of the controller. The magnets generate the telemetry signal on the controller itself and it can be
measured at J7 pin 1.
IV. Next Steps
The completion of the assembly and testing of the auxiliary module is a major step in the completion of the
brassboard PPU. To remain on schedule with SEP POINT, this brassboard PPU must be ready for integration with a
thrustersimulator, PDU and solar array simulator by October 2016. Then Phase 2A of POINT can begin integration
and testing.2
This will require the auxiliary module to be integrated with the other modules of the PPU. On the previous
brassboard PPU, instability was observed at unique operating conditions when integrated with a Hall Effect Thruster.
Oscillatory interactions between the PPU’s discharge modules and the thruster were observed that can propagate to
the power bus.At 500V and 20A discharge output,low frequency oscillations can be seen at the high voltage bus.1 A
simulation model of the discharge module has been built in PSIM, and the discharge control loop is being investigated
to see if a design change can mitigate this.
V. Conclusion
The development of this brassboard PPU is essential to the project success ofSEP POINT. Moreover, work done
in tracking revision changes and instability analysis serves to provide minor improvements to the PPU design as a
whole. SEP POINT aims to provide a solar electric propulsion testbed which can provide drop-in, high fidelity testing
of solar electric propulsion hardware. Developing this technology is valuable to advance future mission goals of the
agency as higher and higher power solar electric propulsion becomes feasible. Due to the high percentage of path to
flight components utilized on this brassboard PPU, it has near-term flight viability with particular relevance to the
agency’s Asteroid Robotic Redirect Mission (ARRM).
Acknowledgments
The author would like to thank Henry Fain for his mentorship and guidance throughout the summer. Gratitude
must also be expressed for the following engineers who have helped immeasurably with this work: Arthur
Birchenough, Dragos Dinca, Robert Scheidegger, Michael Aulisio, Walter Santiago and Frederick Wolff. The author
also extends thanks to the team of dedicated engineers and technicians at NASA GRC’s Power Distribution Branch.
References
1
Santiago, W., et al., “High Input Voltage Power Processing Unit Performance Demonstration,” AIAA Propulsion & Energy
Conference, Salt Lake City, UT, July 25-27, 2016

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2
Bozak, K., Crable, V., Herman, A., Scheidegger, R., “Solar Electric Propulsion Power Integration Testbed (SEP POINT)
Concept of Operations Document,” NASA GRC SEP DOC 0010, 2016
3
Birchenough A., “120V PPU Design,” NASA Engineering Directorate/Electrical Systems & Electric Propulsion Division,
2014.
4
Scheidegger, R., Birchenough, A., Aulisio, M., Gonzalez, M., Pinero, L., “HP-120/800V Power Processing Unit (PPU)
Subsystem Development Specification Document,” NASA GRC729200-003, 2014.