grc martin 08 16

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. IntroductionASA’s Solar Electric Propulsion (SEP) project is spearheaded by the Glenn Research Center. There are many applications for this high power technology, ranging from near 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.

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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 thruster itself. 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

1 Electrical Engineering Intern, Power Management and Distribution Branch, NASA GRC2 Electrical Engineer, Power Management and Distribution Branch, NASA GRC

NASA – Internship Final Report

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

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 OverviewThe 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 maximum. 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 outer magnets 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 ModuleEach 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

NASA Glenn Research Center Page 1 8/11/16

Figure 1. PPU and Hall Effect Thruster Block Diagram.

Figure 2. Assembled Discharge Module.


The auxiliary module provides power to the thruster’s inner and outer electromagnets as well as the heater and the keeper for the cathode.

1. Inner/Outer Magnet SuppliesBoth the inner and the outer magnet 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 of the 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 120V in, 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 SupplyThe heater supply is powered from the 28V bus. It is a DC-DC

converter with 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 SupplyThe 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 for the igniter circuitry which is barely visible under the blue LEM current sensor; therefore, only the assembled keeper is pictured here.

C. Additional ModulesAdditionally, 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 Master Control 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 ResultsEach 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.

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.


0 50 100 150 200 25050%

55%

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120V Magnet: Efficiency vs. Output Power

10 amp setting7 amp setting5 amp setting2 amp setting

Output Power (W)

Efficie

ncy

(%)

Figure 5. Outer Magnet Effciency Plot


B. Current/Voltage Ripple & Line/Load RegulationFor 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 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.


0 50 100 150 200 25070%

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Heater: Efficiency vs. Output PowerConstant Input, 28 VDC

9 Amp Set -ting7 Amp Set -ting5 Amp Set -ting3 Amp Set -ting

Output Power (W)

Efficie

ncy

(%)

Figure 6. Heater Effciency Plot

Figure 9. Outer Magnet Line Regulation Data


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.

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.


Figure 11. Inner Magnet Input/Output Ripple Waveform

Figure 10. Heater Load Regulation Data


Table 2: Ripple and Regulation ResultsLine 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 ResultsEach 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 StepsThe 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 thruster simulator, 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. ConclusionThe development of this brassboard PPU is essential to the project success of SEP 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).

AcknowledgmentsThe 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.

References1Santiago, W., et al., “High Input Voltage Power Processing Unit Performance Demonstration,” AIAA Propulsion & Energy

Conference, Salt Lake City, UT, July 25-27, 20162Bozak, 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



3Birchenough A., “120V PPU Design,” NASA Engineering Directorate/Electrical Systems & Electric Propulsion Division, 2014.

4Scheidegger, R., Birchenough, A., Aulisio, M., Gonzalez, M., Pinero, L., “HP-120/800V Power Processing Unit (PPU) Subsystem Development Specification Document,” NASA GRC729200-003, 2014.


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