Steve Taranovich, Eta Kappa Nu Member and an IEEE Life Senior Member
The commercial space sector led by Space X and Blue Origin, plus companies like Northrop Grumman and Boeing, have given new life and accelerated NASA’s plans for the journey to Mars. The addition of the Artemis program, to establish a Moon base for the first leg of exploration and mining in the mid-2020s, will also be a gateway for the journey to Mars in the 2030s. NASA is now a spaceport with Space X, Blue Origin, and Boeing residing at Kennedy Space Center in Florida.
In this article, I would like to discuss an oft forgotten or little-noticed part of the spacecraft enabling travel into outer space---power management in the space vehicle. Wide bandgap semiconductors like gallium nitride (GaN), silicon carbide (SiC), as well as diamond, are looking to be the most promising materials for future electronic components since the discovery of silicon. These technologies, depending upon their design, offer huge advantages in terms of power capability (DC and microwave), radiation insensitivity, high temperature and high frequency operation, optical properties and even low noise capability. Therefore, wide bandgap components are strategically important for the development of next generation space-borne systems.
I am particularly excited about wide bandgap semiconductors, especially enhancement mode gallium nitride, in this article as the power driver of choice for these critical power supplies in space. I will explain why GaN belongs in space-related power solutions and will be one of the most important elements in the power supply regarding SWaP: Size, Weight, and Power efficiency, the three most important elements in a space vehicle coupled, of course, with reliability (We can’t pull off to the side of the road and call for help in the event of a malfunction in space).
Power sources are usually heavier than most other equipment onboard a spacecraft. GaN power devices can achieve the best efficiency as a power transistor as well as having the smallest size in a power management architecture since these power devices run at very high frequency which reduces the size of power magnetics (transformers and inductors that contain iron/metal cores that add weight) in the design architecture. Lighter weight also means less fuel consumption to escape Earth’s gravitational pull upon launch; this equates to lower costs.
GaN also has EMI benefits because reduced parasitics lead to less energy stored and released in these parasitics during each switching cycle. The circuit architecture has a smaller footprint which will help designers improve loop inductance---which can act as a transmitting and receiving antenna on the board.
Primary power to spacecrafts and on the Moon’s and Mars’ surface: The MMRTG
A Multi-Mission Radioisotope Thermoelectric Generator now powers NASA’s Mars 2020 Rover and also most previous Rovers on Mars. The MMRTG also provides the main power on many present and future spacecraft. This will be one of the primary sources of long-lasting power both aboard a spacecraft or on a planet/moon.
Another Primary Power Source: Solar Panels
The other primary power source would be the Sun, sending energy to a series of photelectric cells. Solar panels gather the Sun’s energy and store it in batteries. This is a preferred way to power satellites. GaN excels here to take the solar panel output and to charge the batteries as well as converting those voltages in a Point of Load (PoL) converter to power instruments and other systems on the spacecraft. See Figure 1.
The International Space Station (ISS) uses nickel-hydrogen batteries to support its solar panels. Spirit, another older Mars rover, also uses batteries paired with Solar.
Mars Insight Lander
The Mars Insight Lander has 2 Solar Panels which are 7 feet in diameter. Their power is stored in two, 23 amp-hour, lithium batteries to power the space vehicle during the Martian night. GaN is also at home in this application.
A 500W Solar Power-based microgrid for Space
This design focuses upon four parameters that characterize Solar Power-based microgrids: battery voltage, PV Maximum Power, PV Maximum Power Point Voltage, and number of panels per string. In the end, the final optimization metric was the ratio of daily average deliverable power to total system mass (W/kg).
A series of different DC-DC micro-converters were investigated for this system including buck, boost, buck-boost, and non-inverting buck-boost (this topology proved the best candidate). See Figure 2.
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Figure 2. The prototype of a 400W DC/DC MPPT, Non-Inverting Buck Boost (NIBB) converter
A Distributed Maximum Power Point Tracking (DMPPT) architecture could be used with a variety of power converters. eGaN FETs from Efficient Power Conversion, or from their partner Freebird Semiconductor, were selected for the DC/DC converter switches in this design because of resilience to high radiation conditions as opposed to Si devices. High efficiency was also a reason to select GaN.
Power conditioning in a system is one of the most critical tasks to control in an optimum way so that the exchanges of power between the Solar generator or MMRTG, the battery and the loads is efficient. In order to achieve this, the power delivered to the loads has to remain within the voltage range that these loads can handle.
Proper sizing of the Solar Array must be a primary goal since the battery will need to be replenished during the time that the satellite equipment is being powered. Designers must ensure that the battery does not experience current- or voltage-related overcharging. The ability to disconnect some non-essential spacecraft functions, so as to avoid a battery full discharge, is critical to the safety of the spacecraft.
Distributed Power Architecture (DPA)
Driving modern high-speed digital processors, FPGAs and ASICs with a DPA could help system efficiency as well as dynamic response from negative effects of parasitic impedance. An Intermediate Bus Converter (IBC), with good transient response and followed by Point-of-Load (PoL) regulators, will help create a stable power architecture under various load fluctuations, especially with load voltages dropping below 1VDC with ever increasing current needs.
The Intermediate Bus Converter (IBC)
The IBC is usually the first conversion stage after an MMRTG or Solar Panel array and may be regulated or unregulated. The IBC is typically a DC/DC converter with a typical spacecraft power bus input of 28VDC. The designer will have to determine whether the IBC output source is regulated well enough, while also checking the PoL input range needs.
Point of Load (PoL) converter
Here is where GaN comes into play right at the load. There will typically be many of these PoLs with different output voltages that the loads would need and ultimately be directly driven by Space qualified GaN power transistors.
See how Data center power in 2019 demonstrates one such architecture on Earth; however, that architecture would also apply in Space.
eGaN Technology for Power Electronics enters the scene: Power needs in Space
Power for larger spacecraft such as telecom satellites or the International Space Station (ISS) need tens of kW. GaN designs can easily handle this.
Electrical loads in a satellite can vary widely, depending on which instruments/subsystems are running at a particular time.
The power system in a satellite must be protected against failures of the supplied units that could degrade it and even take it out of service, especially during short-circuits. This is a centralized distribution architecture and will have circuit breakers or fuses to eliminate uncontrolled current surges. Aboard a spacecraft, both fuses or electronic circuit breakers are commonly used.
Key areas in which GaN has typically been used in satellites is with RF and switching. The Space community has taken notice that the enhancement mode GaN FET now has the availability of integrated GaN FET driver modules (See Freebird section of this article)
Batteries are needed in satellites
Satellites will have orbits that may block the Sun behind the Earth, another planet, or Moon. For this reason, satellites and spacecraft need rechargeable secondary batteries to keep them powered. These batteries may be the sole power source available just after launcher separation and until the solar generators are deployed and properly pointed towards the Sun.
A Bi-directional DC/DC topology has been proposed for such Space applications in Figure 3.
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Figure 3. Solar Panels providing power to the Spacecraft load with an intermediate Bi-directional DC/DC converter for continuous power flow to the load at all times.
Weinberg’s conventional topology by which a bi-directional topology is created with the addition of the switching device (GaN is perfect here) and a diode. This design has two working modes: boost (created with the Weinberg topology) and buck (designed as a conventional circuit). This design enables a smaller unit with higher energy density and low weight. See Figure 4.
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Figure 4. A bi-directional chopper circuit for the Weinberg converter
Small Satellites and CubeSats
CubeSats typically only need a few watts of power.
Smallsats are a bit bigger and require a little more power, operate in Low Earth Orbit (LEO), and can deliver low-cost internet access around the world. These satellites have a 3 to 5 year lifetime. GaN is perfectly suited for these systems.
eGaN FETs provide the radiation tolerance, fast switching speeds, better efficiency, leading to smaller, lighter power supplies (smaller magnetics and reduced heat sink sizes or even elimination of heatsinks in many cases). Power supply designers have their choice of increasing the frequency to allow for smaller magnetics or increase efficiency or design a satisfactory balance of both. eGaN FETs are also smaller than equivalent MOSFETs. Increasing the switching frequency also speeds up the feedback loop. Faster transient response can reduce capacitor sizes too.
Maximum gate voltage allowed is 6V, but is derated to 5.0V in satellite applications.
Mars Rover 2020
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Figure 5. Solar energy powers the Mars 2020 spacecraft during its 9 month journey to Mars, but while on the Mars surface, the MMRTG will be the prime source and heat pumps need to be used as well due to the extreme temperature conditions. (Image courtesy of NASA)
Solar heating on Mars would be difficult for the electronics of the Rover since it would take a great deal of power. The mission is planned to last one Mars Year (about 687 Earth days), but as we have seen in the past, the Rovers usually go far beyond their planned missions.
The MMRTG, with a 14 year operational lifetime, supplies 110W of power (60W of that is for the Avionics on the journey to Mars) There are two rechargeable Lithium Ion backup batteries, each have an energy capability of 43 Ah.
The Rover’s main onboard Power supply is essentially 22-36VDC in operation, a nominal 28VDC. GaN here would be an enormous benefit to lower weight, less wasted heat due to GaN’s high efficiency, and smaller physical size in this type of power converter. See Figure 6.
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Figure 6. Mars 2020 Rover MMRTG power source circled in blue (Image courtesy of NASA)
The European Space Agency (ESA)
The ESA realizes that power systems in space need power generation, conditioning, storage, distribution, and conversion. The ESA is investing heavily in GaN technology.
Global Support Technology Program (GSTP)
There is a focus upon high-voltage and high-switching speed DC/DC Converters based upon GaN technology for next generation power systems. The main goal of this activity is to develop high performance, space-compatible enhancement mode GaN power switching transistors while establishing a European industrial manufacturing route.
International Space Station (ISS) power
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Figure 7. The ISS Electrical Power Channel (Image courtesy of NASA)
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Figure 8. The Main Bus Switching Unit can be seen here. This unit had once malfunctioned on the ISS---all the more necessary for designers to learn lessons in improving and making designs in Space more robust (Image courtesy of NASA)
Energy from the Sun (solar power) is collected by the ISS solar arrays, and is roughly conditioned by the Sequential Shunt Unit (SSU), then tightly regulated by the Direct Current (DC) to DC Converter Unit (DDCU), and stored in Lithium Ion batteries.
Electric Power System (EPS) onboard the International Space Station (ISS) provides all the power vital for the continuous, reliable operation of the spacecraft. NASA Glenn Research Center’s Space Operations Division is leading the sustaining engineering and subsystem integration of EPS hardware. Glenn also manages the integration of the EPS with ISS International Partners’ elements.
The EPS consists of many hardware components called Orbital Replacement Units (ORU). Every different ORU is considered a subsystem of the entire EPS and astronauts can replace them upon failure either robotically or by Extra-Vehicular Activity (EVA). The components collectively provide power generation, power distribution and energy storage for the ISS.
DC-to-DC converter units supply the secondary power system at a constant 124.5 VDC, allowing the primary bus voltage to track the peak power point of the solar arrays. 200 V and 350 V eGaN FETs are perfect in this kind of design.
Starting in 2016, the nickel-hydrogen battery ORUs were being replaced by Lithium-ion (Li-ion) batteries. Each Li-ion battery weighs about 430 pounds, and each adapter plate weighs about 65 pounds, for a weight savings of over 200 lbs. as compared to Nickel-Hydrogen batteries.
It has been more than nine years now that eGaN® devices have shown very high reliability in both laboratory testing and customer applications such as lidar for autonomous cars, 4G base stations, vehicle headlamps, and especially in satellites for this article. See eGaN FET Reliability for more details.
I had the pleasure of interviewing Jim Larrauri Chief Strategy Officer who co-founded Freebird in 2015.
He told me that his company name was like a satellite, sometimes called ‘a bird that is freed into space to provide a service’.
Back in 2016 Freebird took Efficient Power Conversion’s (EPC’s) commercial enhancement-mode GaN (eGaN) product and eliminated the variability that exists in the commercial eGaN and went on to develop that technology for Space.
They also take enhancement mode GaN and provide a package structure along with circuit structures, with patents held by Freebird Semiconductor, that puts them in a strategic position with their multi-function circuit pack in Freebird’s modular part as drivers for the eGaN power transistors. It is the packaging in particular, that has been designed to help the end-user successfully transition from conventional silicon-based semiconductors into the high-reliability performance GaN arena. Part of Freebird’s core strategy is to provide building-block solutions to make the implementation of eGaN HEMTs more successful, taking the guesswork out of the design process. These lower cost, easy to implement, modular solutions are used in space-borne and launch vehicle power systems. The modules provide the end-user with circuit flexibility integrated into a single module: from a half-bridge configuration to two “independent” low- and high-side switches, all incorporated and using “GaN-driving-GaN” technology.
Launch costs need to be reduced from about $200M to $250M per launch to a target like $50M, depending upon the specific launch vehicle and supplier, in order to be cost competitive for such launch vehicles as Delta or Atlas or even SpaceX low cost recovery units. Also, in the satellite industry, satellites can range in cost from $500M to $1B; these costs need to come down as well.
Nowadays there are also more large Low Earth Orbit (LEO) constellation satellite delivery systems for data transmissions. These systems have power budgets and need a technology to support these needs. eGaN devices were selected by Freebird over SiC because SiC does not have adequate baseline radiation hardness assurance capability built into it like eGaN does.
Enhancement-mode GaN is not natively Rad Hard; it has to be made Rad Hard. However, it is Radiation Tolerant in the technology sense due to its immunity to Total Ionizing Dose (TID). But from a Heavy Ion Single Event Effects (SEE) perspective you need to control and tweak the process with design to obtain the desired radiation hardness from the GaN. Studies have been done for the many other supplier design processes for GaN, demonstrating at rated voltage, that they cannot pass the SEE requirements for the (Au) Gold Heavy Ion standard. There are other lesser heavy ions with which you can successfully bombard this technology in simulation of alternative space borne application environments, but only if you can pass the gold standard, and at rated voltage, have you achieved true rad hard product capability. Freebird Semiconductor uniquely performs 100% Radiation Hardness Assurance against MIL- STD-750, Method 1080 for (SEE) on “every wafer” of eGaN product supplied, conducted at a typical rated Au (15 MeV beam) with a linear energy transfer (LET (Si)) ~ 84.6 at Energy =2365 MeV & Range = 124µm with typical fluences of 3e5/1e7 as standards. As all of Freebirds modular products employ “GaN-driving-GaN” technology, this radiation hardness assurance pedigree is carried on through the entire product portfolio.
eGaN devices are High Mobility Electron Transistors (HEMT) which prove themselves to be excellent candidates for the radiation hardened market to replace Rad Hard MOSFETs in space. MOSFETs are the present incumbent supply base from military support programs down to small satellite systems. They all need Rad Hard MOSFETs. The pressing issues that need to be overcome is cost (there is essentially one major single source supplier with excellent products, along with a few secondary suppliers). MOSFETs however are an ‘old’ technology with large die sizes and a performance Figure of Merit (FoM= Rds(ON) * Ciss) that is much higher than that of eGaN FETs (lowering the Figure of Merit provides for better efficiencies).
The on-resistance of a Rad Hard MOSFET is much higher than an eGaN FET of the same die size. Freebird Rad Hard eGaN HEMT devices are majority carrier devices in which the channel conducts via a Two-Dimensional Electron Gas (2DEG) with a lateral channel current flow---there is NO charge storage in the channel. The eGaN switching speeds are determined solely by the R’s and C’s of the Gate and Drain nodes. Switching times can reach sub-nanosecond levels, so different thinking must be employed for both the design and PCB layout phases of development when using these high-performance devices.
The driving of a MOSFET or eGaN HEMT is highly in favor of eGaN with a 10x to 40x reduction in gate charge over the best Rad Hard MOSFET available.
GaN HEMT also wins in the size metric over MOSFETs. These devices can be mounted directly to a ceramic substrate (needing no external package), thus eliminating wire bonds. The elimination of wire bonds in our eGaN designs allows the true speed performance of the eGaN HEMTs to shine through, as wire bonds bring with them inductance, which can cause all manner of transient-related issues such as voltage overshoot and current ringing.
Doing business in the space community means being able to supply that market sector with a radiation hardness assured product each and every time. That is the initial roadblock to overcome for GaN technology use in space borne applications. Freebird, along with EPC, can state and prove that they can offer a radiation hardened version of EPC’s design and process as a result of a proprietary sourcing agreement. GaN commercial products by themselves cannot claim and then provide that fact.
There are space community applications and programs such as evolving Internet needs across the globe nowadays that use large LEO constellations; Airbus OneWeb is one of these programs with their 900 satellites required in Space. There is also Maxar Technologies (In one of the first steps of the NASA’s Artemis lunar exploration plans, NASA announced in May 2019 the selection of Maxar Technologies, to develop and demonstrate power, propulsion and communications capabilities for NASA’s lunar Gateway), Northrop Grumman, or Honeywell. These companies have satellite needs with one thing in common: A low-cost power delivery system. GaN is specifically used in these satellite systems now, just about across the board, in the Power Distribution Unit (PDU) within the satellite system. Each satellite company has their own PDU, Solar Array manufacturer, motor controller, etc. They all require efficient reliable power delivery.
Freebird supplies the basic power device building blocks to realize the PDU. These PDU systems can use discrete-packaged eGaN HEMTs or even the Freebird Die Adapter (FDA) devices which may then form the basic elements of the higher-level PDU. Freebird utilizes high reliability eGaN FET die and creates driver circuits for the eGaN power transistors. The result is a complete, fully-guaranteed Radiation Hardened power section. Designers can now have the radiation hardened building blocks to create their final PDU system using standard hardened products, not custom designed products.
Freebird DC/DC PoL Modular Converter Building Block
Freebird GAM Adapter series are modular building blocks containing high-reliability small signal eGaN FETs configured as high-speed gate drivers as well as high-powered eGaN power switches (i.e. GaN-driving-GaN) in surface mount package sizes ranging from 0.75” x 0.38” x 0.125: for a single gate driver, up to 1.00” x 0.75” x 0.125” for higher level functions. These larger modules can have low-side drivers, high-side drivers, as well as a complete multi-function module containing a half-bridge---the power stage for a Point of Load (PoL)as featured in the FBS-GAM02. See Figure 9.
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Figure 9. FBS-GAM02 10A/50V Multifunction Module block diagram composed of all eGaN devices except for diodes, capacitors, and resistors. No silicon-based switcher or monolithic ICs are employed inside these devices eliminating low-dose rate radiation effects, and more. (Image courtesy of Freebird Semiconductor)
All FBS-GAM0X devices contain Freebird Semiconductors flight proven US Patent #10,122,274 B2 circuitry, pioneering eGaN driving eGaN technology building blocks from which users can create a wide variety of power supplies: Forward, Flyback, Boost, Full-bridge, Buck, Weinberg, Cuk, non-Isolated, Isolated on the Primary side or isolated on the Secondary side.
The GAM discrete and modular products may be used for many applications other than power supplies such as actuators, power switches, squib drivers, load dump switches, single-phase or three-phase motor drivers.
Today’s space community works mainly in the digital arena regarding electronic circuitry. Every FPGA, every ASIC, every processor in use today are effectively digital. As such we can look at eGaN as a Digital Power, +5V logic-level driven front-end power transistor device!
Even though the space community is slow to change, eGaN devices have been proven compelling as a solid technology for space and they are gradually being considered and accepted. A large part of this acceptance is that fact that Freebird uses MIL-PRF-19500 as the baseline for its space level standard device qualification methodology for their eGaN discrete technology. MIL-PRF-19500 has a long history-within the high reliability industry to ensure effective screening and conformance qualifications for silicon semiconductor transistors including MOSFETS and IGBTs.
As a testament to their efforts in the high-reliability, rad-hard space electronics arena Freebird has their eGaN HEMTs and modular devices presently successfully flying in space, accumulating valuable operational history for this technology! Their commercial space product is presently offered in their unique, proprietary epoxy over-molded GAM (GaN Adaptor Module) technology packaging (See Figure 10)
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Figure 10. FBS-GAM02 10A/50V Multifunction Module (L) and FBS-GAM01-PSE eGaN HEMT Gate Driver (R) compared to US Dime. (Image courtesy of Freebird Semiconductor)
Many power supplies used in space applications are hard-switched architectures. Freebird’s slowest commercial space eGaN multi-function modules are capable of running up to 500kHz (fully de-rated), and 1MHz (with power/thermal de-rating), with their independent drivers capable of speeds up to 3Mhz+ .
Further advancements, in the area of radiation-hardness-assured conversion products, are being developed by industry participants such as SET Group (working in partnership with NASA) whose founding partner Dr. Raul Chinga Alvarado provides the example of a high power, high-frequency, wide-range LLC resonant converter capability utilizing Freebird Semiconductor Rad Hard eGaN device technologies:
SET Group company specializes in the design and development of high-density power converters, leveraging state-of-the-art technology. SET group has achieved a module specific power of 15-20kW/kg as of 2019.
In 2017, SET group began work with NASA on the design, fabrication and demonstration of a gallium nitride (GaN)-based high-power, high-frequency, wide-range LLC resonant converter (GaN-LLC) capable of handling high-power and high-frequency operation. The GaN LLC converter operates at an input voltage of 95V - 150V and can output 600V – 1.8kV, specifically utilizing space-grade Freebird Semiconductor’s GaN HEMT (rated up to 300 kRad) and uses a novel additive-manufactured thermal management solution.
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Figure 11a & b. SET group 1-kW GaN-LLC Converter (Image Courtesy of Setgroup.us)
The LLC topology provides high efficiency and also the advantage of handling a wide input voltage range. Together with Freebird Semiconductor devices and NASA, SET group has successfully developed a 1.25 kW GaN-LLC converter in a half-brick form factor (2.4in x 2.3in x 0.5in) with an input voltage of 70V – 150V and an output voltage of 200 – 600V. SET group is currently continuing to push the limits of DC-DC power conversion topologies by leveraging space-grade GaN devices from Freebird for new and existing space applications.
EPC (Efficient Power Conversion)