Wide Band Gap Semiconductors in Aerospace and Satellite Applications

Wide Band Gap (WBG) semiconductors bring several advantages in power conversion, such as increased power density and efficiency, while reducing system size and weight with higher frequency switching that allows use of smaller passive components. These advantages can be even more important in aerospace and satellite power systems, where size and weight are critically important. In the article, we explore the relative advantages of WBG components such as Silicon Carbide (SiC) and Gallium Nitride (GaN) in these applications.

Power conversion in aircraft

As the world moves to a greener future, attention has focused on methods of reducing emissions from traditional gas-powered aircraft. Some approaches being considered are:

• More Electric Aircraft (MEA): The goal here is to replace some of the mechanically or hydraulically driven engine accessories with electrically driven components (e.g., the fuel pumps).
• More Electric Propulsion (MEP): Here electric generators are used to provide a hybrid-assist to the gas turbine, thereby lowering fuel consumption.
• All Electric Aircraft (AEA): A more ambitious plan where the plane is all-electric. This would start with smaller planes such as helicopters, Urban Air Mobility (UAM) vehicles and Vertical Take-off and Landing (VTOL) planes, such as those planned for use as air taxis.

In modern aircraft, the increased power consumption has necessitated an increase in the incoming voltage generated from the gas turbine to 230 V_AC. This voltage is converted by a rectifier to a DC link voltage of ±270 V_DC, also known as the HVDC voltage. DC/DC converters are then used to generate an LVDC at 28 V, which is used to run equipment such as the flight deck display, the DC fuel pumps, etc. Just as in EV chargers for cars where systems are now being developed for 800 V, the trend in aircraft is to push the voltages up higher to reduce cabling losses. In aircraft, the DC voltage will likely be pushed towards the kV range, especially in hybrid-propulsion and AEA systems. In terms of power, MEA power converters can range from 10 to 100 KW, whereas hybrid-propulsion and AEA power converters must be in the several MW range.

Key requirements and challenges for power electronics in aircrafts

• Size, Weight and Power-loss (SWaP): Lower SWaP metrics are key as fuel consumption, range, and overall efficiency are directly related to them. Consider the example of an AEA. In this case, the battery system is the heaviest component of the electric power generation system. The required battery size depends on the efficiency of the inverter. Even a 1% improvement in inverter efficiency from 98% to 99% can reduce the battery size required for a typical battery with an energy density of 250 Wh/kg by several 100 kg. The gravimetric power density of the inverter module (kW/kg) is another key metric. Similarly, the size and weight of passive components, as well as the cooling system required for converter active devices, can be substantial.
• High-power electronics installed close to the engine in non-pressurized areas face many challenges related to heat and isolation. Active devices need significant de-rating for temperature, and their cooling requirements can place burdens on the overall aircraft's cooling system. At high altitude, partial discharge can occur at lower electric fields, and hence semiconductor and module packaging, as well as isolation components, need to be designed with sufficient margin. Ensuring tolerance to cosmic radiation exposure can also require significant voltage de-rating for the active devices.
• Qualification and reliability standards: The DO-160 is a rule for testing avionics hardware in different environments. Very few commercial off-the-shelf (COTS) components are certified for this, leading OEMs and aircraft manufacturers to qualify and ensure their use.

Advantages in the use of wide bandgap (WBG) power semiconductors in aerospace and satellite

WBG materials, such as SiC and GaN offer many advantages over traditional silicon (Si) based devices as shown in Figure 1.

Figure 1: Comparison of material properties for Si, SiC and GaN. (Image source: Researchgate)

These material advantages translate into many benefits in aircraft power electronics:

• Higher thermal conductivity, especially in SiC, makes it easier to cool parts like those used to control the engine.
• Higher system voltage reduces ohmic losses in cabling. This is especially true for SiC, where commercial devices are available up to 3.3 kV, with active research aimed at extending this further.
• Improved reliability at high temperatures. For example, +200˚C operation in SiC has been demonstrated.
• Lower conduction and switching losses. The higher bandgap allows for a smaller drift region at a given voltage rating, leading to improved conduction losses. In addition, lower parasitic capacitances lead to lower switching losses with faster switching slew rates.
• Lower parasitics also allow for higher-frequency operation. As an example, switching frequencies in a 1-5 kV SiC MOSFET can be in the 100s of kHz, compared to the 10’s of kHz possible with equivalent topologies in Si. GaN HEMT (high electron mobility transistor) devices, though mostly available in the <700 V voltage range, are unipolar and have further advantages with no reverse recovery losses and the ability to switch at several MHz in this 100-volt range. The big advantage of higher frequencies is the ability to shrink the size of magnetics.

Figure 2 compares the efficiency of GaN and Si-based 100 kHz boost converters.

Figure 2: Comparison of efficiency between Si and GaN for a 100 kHz boost converter. (Image source: Nexperia)

All of the above benefits directly lead to better SWaP metrics and higher power densities. For example, higher DC link voltages from the use of higher-rated voltage devices create a smaller capacitance RMS current in the converter DC link capacitor, which can reduce its size requirement. A higher switching frequency allows for the use of smaller form factor, high-frequency planar magnets. In a traditional power converter, the magnetic components can represent as much as 40-50% of the total weight, and with the use of WBG active devices operating at higher frequencies, this percentage is decreasing. Looking at this in terms of the gravimetric power density of an inverter, Si based air-cooled converters have ranged around 10 kW/kg. With the use of WBGs, this metric has exceeded 25 kW/kg in many system demonstrations, and the achievement of densities as high as 100 kW/kg is shown to be theoretically possible with optimized topologies, DC link voltages, and switching frequencies.

Challenges in the use of wide bandgap (WBG) power semiconductors and potential solutions

The above advantages of WBGs do, however, translate to many challenges that need to be addressed. Below are some of these challenges and possible solutions currently being explored:

• Higher power densities directly translate to increased heat generation. High temperatures decrease the efficiency of power conversion and can also be a reliability concern, especially when temperature cycling involves high-temperature changes. Thermo-mechanical stress can impact the power module packaging reliability by making heat spreaders, such as Thermal Interface Materials (TIM) like thermal grease that connect active device substrates to the heat sinks, become unstable as well as increasing their thermal resistance. Some solutions being explored include:
  • Improved packaging: Packages that offer double-side cooling with directly cooled aluminum nitride (DBA) substrates with silver sintering achieve improved heat removal. Other approaches include Selective Laser Melting (SLM) of powder alloy heat sinks directly onto the DBA substrates.
  • As the active die size increases due to increased power requirements, using parallel dies to achieve the same net active area can be advantageous for heat spreading.
• The faster switching transitions with WBG, while good for reducing switching losses, do create more of an Electro Magnetic Interference (EMI) risk. Solutions for this include:
  • Distributed filter cells offer improved performance and can provide redundancy.
  • Use of hybrid active-passive filters using amplifiers to boost the low frequencies can reduce the net filter size and improve performance.
• As the rated voltage increases, the specific resistance of the power device (R_DS(ON) x A, R_DS(ON) being the on-state resistance and A the active area) increases due to the necessity of a thicker drift region. For example, while the high temperature-specific resistance of a 1200 V SiC MOSFET can be 1 mOhm-mm², it can reach 10 mOhm-mm² for a 6 kV-rated device. Larger devices or more devices in parallel are needed to meet an R_DS(ON) target, meaning higher die costs, more switching losses, and more cooling requirements. Some solutions include:
• Using 3 or multi-level converter topologies allows the use of lower-rated devices than the DC link voltage. This can be especially relevant in the sub kV rated GaN devices, where a series in, parallel out (SIPO) configuration distributes the incoming voltage across many devices, thereby allowing their use.

GaN and satellite communications

In terms of how well it can handle radiation, the GaN HEMT device is better than both Si and SiC MOSFETs:

• The AlGaN layer under the gate electrode doesn't collect charge like the SiO₂ gate oxide does in MOSFETs. As a result, the total ionizing dose (TID) performance of e-mode GaN HEMTs is significantly improved, with reports of operation exceeding one Mrad (megarad), whereas in Si/SiC this is typically in the hundreds of krads (kilorads).
• The Secondary Electron Effects (SEE) are also improved with the GaN HEMT. A lack of holes minimizes the risk of secondary electron upsets (SEU), while the risk of gate rupture seen on Si and SiC (SEGR) is also minimized.

GaN-based Solid State Power Amplifiers (SSPAs) have largely replaced vacuum tube devices in many space applications, such as in Low Earth Orbit (LEO) satellites, especially in frequencies from the C to the Ku/Ka bands.

Conclusion

WBG semiconductors like SiC and GaN have a lot of benefits when used in aerospace and satellite communication. As their technology development, usage, and reliability standards mature in terrestrial power conversion applications; greater confidence will be built in their use in aerospace and satellite systems as well.