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DARPA SPCE improving efficiency of radiation-tolerant Power convertors for Military LEO Satellites

The space industry is set to expand to over $8.8 billion dollars by 2030 fueled by the rapid increase in the number of small satellite launches and decreased costs resulting from rideshare companies and programs

 

A satellite present in an orbit should be operated continuously during its life span. All the satellites require internal power in order to operate various electronic systems and communications payloads that are present in it. The Electrical Power System (EPS) is a vital subsystem whose primary role is to supply satellite systems with the necessary electrical power to operate effectively.

 

The source of the power is mainly the energy collected from the solar panels which are exposed to direct solar radiation or to indirect radiation from albedo. Batteries are installed alongside the solar panels to store energy which can then be used when the satellite regularly passes through the shadow of the Earth. Batteries can also help to provide sufficient power during periods of peak demand by the payload onboard the satellite. As of 2020, approximately 85% of all nanosatellite form factor spacecraft were equipped with solar panels and rechargeable batteries.

 

The EPS is a major, fundamental subsystem, and commonly comprises up to one-third of total spacecraft mass and volume. The electrical power system (EPS) encompasses electrical power generation, storage, and distribution. The Electrical Power Supply, or EPS of the CubeSat is composed by three modules which are the PCC (Power Control Circuit), the PV (photovoltaic panel) and the BAT (Battery). The role of the EPS is to generate, store and distribute the electricity produced by the solar panels.

 

The collected and stored power must then be distributed to other systems throughout the satellite as needed by the EPS. The satellite itself may need multiple voltage levels for different sensors and sub-systems. Managing these levels is another function of the system; the EPS houses a power conditioning unit that is able to deliver the required amount of electrical power at several voltages.

 

With these more complex and ambitious experiments launching into orbit, a need exists for high-efficiency, compact, and inexpensive converters to power these missions. Converter power losses create significant heat and large package sizes waste valuable space that could be used for additional computational or remote sensing capabilities. These problems often require complex thermal and structural design trade-offs that increase mission complexity and cost.

 

The high levels of radiation in space can cause a”single event latch-up” in the semiconductor devices on the satellite. This can damage some of the components on the satellite if the power is not turned off quickly enough, so the EPS is also required to protect the satellite and its sub-systems against over-currents.

 

When energetic protons/heavy ions in a space environment strike the high electric field regions of these transistors, a large number of carriers are generated. This causes a large current pulse between the high voltage and ground, potentially leading to single event burnout in which the switch fails from catastrophic Joule heating.

 

DARPA launched the  Space Power Conversion Electronics program to look at novel devices, designs, and engineering to improve the conversion of usable power in radiated space environments. POL power efficiency declines to as low as 60 percent because of radiation hardening, a form of thermal management to offload heat generated from space-based power consumption, the agency noted.

 

Due to the stringent design constraints of these small-scale missions and the growing volume of CubeSats, there has been a call for radiation-tolerant, power dense, high-efficiency
point-of-load (PoL) converters.

 

Today’s most advanced space point-of-load systems use discrete radiation-hardened silicon lightly doped-drain MOSFET (LDMOS) transistors, which limit the overall point-of-load efficiency to less than 60 percent.

 

Typically, radiation-hardened (rad-hard) components have large
footprints of greater than 300 mm2 , switching speeds of less than 1MHz, and efficiencies of less
than 92 percent at max load.

 

Point of load power conversion represents a severe bottleneck for space, particularly for
proliferated low Earth orbit (LEO) applications. In today’s space power systems, radiation tolerance is achieved by derating the operating voltage by more than a factor of two to permit
sufficient margin from single event burnout (SEB) in the high-voltage transistors that serve as
the power switches.

 

To avoid single-event burnout, these LDMOS transistors operate at a maximum voltage substantially lower than the device breakdown voltage, which leads to degradation in performance. For a space-qualified 100-volt LDMOS, the figure of merit is more than 3 times worse than for an LDMOS transistor for non-radiation environments. This results in a significant increase in the switch power loss, and a reduced point-of-load conversion efficiency.

 

As a result, space POL converters are characterized by low (< 60 %) efficiency, severely limiting the satellite available power, capabilities, and battery lifetime. POL power conversion relies on high voltage transistors to serve as power switches.

 

A GaN HEMT interfaced with a radiation tested commercial off-the-shelf (COTs) controller uses printed circuit board (PCB) space more effectively and is a significantly cheaper solution in comparison to a rad-hard converter. These radiation-tolerant solutions using wide bandgap devices such as GaN have emerged as the preferred choice in low voltage space applications due to their improved efficiency, capability of higher switching frequencies, and a wider bandgap allowing better performance in radiation environments

 

To achieve the program objectives, DARPA launched SPCE  program in Sep 2022 which will overcome two key Technical Challenges (TCs). The SPCE program will exploit technology advances in, for example, wide bandgap and other novel semiconductor power
transistors, defect-free wide bandgap semiconductor growth, and advanced heterogeneous
integration technology to overcome the TCs.

 

TA1 seeks to develop a device technology for realizing radiation-tolerant high efficiency POL
DC-DC converters. To achieve the program goals, the performer(s) is expected to realize
radiation-tolerant high-voltage transistors with switching performance better than SOTA high-voltage transistors for non-radiation environment and the integration technology needed for
realizing high-efficiency, high-energy-density POL converters.

 

Program Description

The SPCE program seeks to develop radiation-tolerant high-voltage transistors with high
performance and the integration technology to enable compact, high conversion ratio POL
converters for space applications. Specific SPCE program goals are: realizing integrated
radiation-tolerant high-voltage transistors with performance better than state-of-the-art (SOTA)
wide bandgap semiconductor (WBGS) high-voltage devices for non-radiation environments,
employing these transistors to demonstrate a 48 V-to-1 V radiation-tolerant POL converter with
over eighty five percent efficiency at 50 A output current, and achieving power density greater
than 500 W/in3.

 

Phase 1 (Base) – The goal of Phase 1 is to verify the device approach(es) to realize radiationtolerant high voltage transistors with switching performance better than the SOTA high-voltage devices for non-radiation environments.

 

Phase 2 (Option) – The goal of Phase 2 is to demonstrate device integration technologies which
enable compact radiation-tolerant POL converters with over 75 % efficiency for 48 V-to-1 V
conversion, 50 A output current and power density larger than 250 W/in3. Another key part of
this Phase 2 is further development and optimization of devices realized in Phase 1 for improved
device characteristics and performance.

 

Phase 3 (Option) – The goal of Phase 3 is to realize further device optimization, and to demonstrate radiation-tolerant POL converters with > 85 % efficiency for 48 V-to-1 V conversion, 50 A output current and power density greater than 500 W/in3.

 

To achieve the SPCE program objectives, performers in this program should be prepared to address two key Technical Challenges (TCs):

TC 1: Achieving a high-performance high-voltage transistor that is radiation-tolerant.

SOTA space POL systems use discrete, radiation-hardened, silicon lightly-doped-drain
MOSFET (LDMOS) transistors, limiting the overall POL efficiency to < 60 %. This is because,
to avoid SEB, these LDMOS transistors are operated at a voltage, Vmax, substantially lower than
the device breakdown voltage. This derating leads to degradation in performance, reflected in the
poor transistor figure-of-merit (FOM = 1/(RonQG)).1 For a space-qualified 100 V LDMOS, the
FOM is more than 3X worse than for an LDMOS transistor for non-radiation environments. This
results in a significant increase in the switch power loss, and consequently a reduced POL
conversion efficiency.

 

Recently, wide bandgap semiconductors such as GaN and SiC have been investigated for space applications. With their larger breakdown voltages and higher carrier mobilities, WBGS transistors promise a 10X-100X better FOM than LDMOS. However, WBGS transistors are also known to be prone to SEB in radiation environments and require substantially higher (up to 4X) voltage derating, which has limited their performance when implemented into space power converters.

 

TC 2: Achieving a low-loss, high-voltage integrated circuit technology that is radiation tolerant.

The most common terrestrial monolithic integrated power electronic technology,
Bipolar-CMOS-DMOS (BCD),2 is not suitable for demanding space missions due to the poor
radiation reliability and performance of older node CMOS, and degraded DMOS capabilities
after derating. In addition, SOTA space-qualified high voltage transistors have large junction
sizes and footprints (tens of square nanometers) due to voltage derating, preventing monolithic
integration with advanced microelectronics. SOTA space POL systems are therefore based on
discrete implementations of the LDMOS devices and passive components, which results in large
POL size (power density of < 50 W/in3) with long and lossy electrical connections that
contribute as much as five percentage points reduction in POL efficiency due to power loss in
wiring under high output current conditions.

About Rajesh Uppal

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