Download SiC technology will meet the military`s future needs

DefenseElectronics
SiC technology will meet the military’s
future needs
While reduction size, weight and thermal management requirements enabled by
SiC technology will meet the requirements of future military systems, the military,
as an early adaptor, will speed the development of SiC devices for commercial
applications.
By Marcelo Schupbach and Alexander Lostetter
T
he requirements of modern high-performance power
electronic systems, in particular for the military applications
as shown in Figure 1, are surpassing the power density, efficiency
and reliability limitations set by the inherent properties of widely
employed silicon-based devices. To overcome this, new device
technologies are being explored. The past decade has seen an intense
and steady increase of resources being funneled into the research
and development of wide bandgap devices, such as silicon carbide
(SiC). SiC power devices hold the promise of vastly exceeding
previously constraining restrictions imposed by silicon-based
devices. This new technology is just now beginning to find its way
into the commercial marketplace. While the present focus in the
market is on developing power devices such as high-voltage,
high-current MOSFETs the truly revolutionizing potential of SiC
has yet to be tapped.
The next-generation of SiC-based power systems
The potential applications of SiC are widespread and all
Figure 1. The next-generation fighting force will employ power electronics in every platform and weapons system on the battlefield.
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facturing equipment, the national electrical grid, and mass
transportation systems (trains). Therefore, the future of power
electronics will be greatly influenced by the commercialization of
SiC semiconductor devices.
Advantages of SiC
Figure 2. High-temperature (>250 ºC) SiC 4 kW three-phase MCPM motor
drive developed by APEI.
During the late 1960s, the electronics industry was
revolutionized by the development of silicon integrated circuit (IC)
technology, resulting in microelectronics applications shrinking
by orders of magnitude in comparison with their discrete
component counterparts and, ultimately, leading to vast cost
reductions in electronics markets. A similar revolution will occur
in the power electronics industry, driven by the SiC power switch.
SiC has one-tenth the switching losses of silicon, 10 times the
blocking voltage, four times the thermal conductivity, and 10
times the switching speeds. SiC technology also provides a junction temperature threshold in excess of 600 °C. All of these
physical advantages that SiC has over current silicon technology
will greatly enable increased power density, which is the chief limiting
factor of today’s power electronic systems. It will also significantly
enhance energy efficiency, and shrink the size of power electronics
systems by an order of magnitude. All of these factors will also result
in cost savings. Whereas the IC drove the computer revolution that
shrank mainframes to the size of wall cabinets to fit on a desktop,
so too will SiC technology be the prime mover behind shrinking
wall-sized power electronics systems to the size of a suitcase.
A powerful argument for using SiC power electronics is the size
and weight reductions that can be achieved with high-temperature operation. For example, a silicon-based power module with a
3 kg heat sink can achieve a maximum power of 5 kW assuming
a junction temperature of 150 °C; while a SiC-based power
module with a 0.3 kg heat sink can achieve a maximum power of
encompassing in the area of power electronics. The ability to greatly
increase the power density of current power systems makes the
technology attractive for every branch of the military. The Army’s
Future Combat Systems (FCS) program will require lighter, more
compact power supplies to easily deploy the new communications
and computers systems, networked logistics systems, and intelligence,
reconnaissance and surveillance systems. Moreover, FCS will require
a range of high-efficiency power supplies for the infantry and
new ground vehicles, such as the armed robotic vehicle (ARV),
small unmanned ground vehicle (SUGV), multifunctional utility/
logistics and equipment (MULE) and the infantry carrier vehicle (ICV).
The Air Force, through its More Electric Aircraft (MEA) program,
aims to minimize and replace hydraulic control systems with
light, low-maintenance
electric actuators and motor drives. Last, the Navy’s
next-generation destroyer
DD(X) will require highvoltage and high-powerdensity systems to implement the envisioned compartmentalized powerdistribution architecture.
The ability of SiC-based
systems to operate in harsh
environments or at high
temperatures (up to 600
ºC) also opens the door for
new systems impacting
applications such as space
exploration vehicles and
landers, nuclear power reactors, and petroleum and
geological exploration instrumentation. Ultimately,
any system that would see
improvement from highdensity or high-efficiency
power electronics would
benefit from SiC.
This includes commercial electronics, automobiles (onboard sensors,
electric options), electric
vehicles, household appliances, industrial manu- Figure 3. Ten-horsepower integrated three-phase motor drive MCPM concept.
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Figure 4. Block diagram of a high-temperature, three-phase motor drive control electronics.
7.5 kW assuming a junction temperature of 600 °C. This implies that
the use of SiC technology allows for a 50% increase in power and
a 90% reduction in weight and volume. To take full advantage
of the high-power density capabilities offered by SiC electronics,
the development of high-temperature electronics as well as hightemperature packaging technologies and design methodologies are
required. In particular, the integration of high-temperature power
devices and high-temperature control electronics into a single
module greatly minimizes parasitics, allowing for very high frequencies of operation.
Arkansas Power Electronics International has developed several SiC-based power converters to demonstrate the true potential of
the SiC technology. Figure 2 illustrates a complete SiC-based
multichip power module (MCPM) that operates at temperatures in
excess of 250 °C ambient. This highly compact 4 kW three-phase
MCPM inverter integrates SiC power transistors with high-temperature
silicon-on-insulator (HTSOI) control electronics. The high-temperature
operation of the MCPM allows for increased power density by an
order of magnitude when compared to equivalent silicon-based
systems. This opens new possibilities in the design of many power
systems. An example of this is shown in Figure 3, where a 10 hp
induction motor and drive are integrated into a single unit. The
power electronics integrated with the motor is attached to a small
passive cooling heat sink in order to minimize the increase to the
motor’s thermal load. A motor design that allows for increased
thermal loading could even eliminate the heat sink altogether.
Electrical design of a high-temperature MCPM
The electrical design of the MCPM stems from a demonstration
three-phase motor control developed for high-temperature operation.
The high-temperature motor controller operated in an environment
of 250 °C[1-2]. Figure 4 illustrates the circuit block diagram of a
three-phase motor drive MCPM design. The control electronics were
developed using HTSOI SiC components, which are guaranteed to
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702RFDEF1.indd 10
operate at 225 ºC for five years (and one year at 300 °C).
The digital core control block (block 1) contains a microcontroller, latch, SRAM and proprietary software to generate the
control signals required for a three-phase motor drive. The heart
of this digital core control block is the monolithic 8-bit microcontroller that uses the standard MCS-51 instruction set. Key features
include the programmable counter array, watchdog timer, enhanced serial
port for multiprocessor communication and a hierarchical
interrupt structure.
In addition to the digital core control block, the system contains
start-up circuitry to deliver power from the dc bus to the low-voltage control logic (block 2). Another feature included is feedback of
critical conditions such as overvoltage, overcurrent and overtemperature (block 3). Block 4 takes the low-voltage digital signals
from the microcontroller and amplifies the voltage and current to drive
the isolation transformers in block 5. Block 6 can be customized to
drive different types of SiC power switches. In this case, the design
drives the gate of a SiC JFET from -40 V (fully off) to 0 V (fully on).
The SiC JFET gate drive circuitry also ensures that there is adequate
dead time between the high- and low-side switching.
Mechanical design of a high-temperature MCPM
Figure 5 illustrates the cross-section of the MCPM design approach.
The MCPM has two main stages, the control and power stage. The
control stage requires a low-power, high-density substrate in order
to house all of the HTSOI control electronics. The power stage
requires a thermally conductive substrate, with high current-carrying
capabilities and high-voltage isolation, to house the SiC power devices.
Both of these substrates must withstand high temperatures. Other key
components of the mechanical design of a high-temperature MCPM
are the heat spreaders or base plates, wire bonds and die attaches.
The selection of packaging materials for a high-temperature
(>250 ºC) MCPMs is a great challenge because many common packaging materials used in today’s electronic systems cannot be used
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Material
CTE
(ppm/°C)
Thermal
Conductivity
(W/m °K at 25 °C
Max. Use
Temperature
(°C)
Elastic
Modulus
(Gpa)
Tensile
Strength
(Mpa)
different heat spreader materials.
The control substrate can
Alumina
6.0
24 to 33
1600
310
130
be implemented in a variety
Aluminum Nitride
4.6
150 to 180
1000
310
310
of ways, depending upon the
maximum temperature of opBeryllium Oxide
7.0
270
1093
345
230
eration. For temperatures less
Silicon Carbide
4.6 to 5.1
120
1000
412
17
than 225 ºC, Isola P96 (Tg > 260
Silicon Nitride
3.0
70
1000
314
96
°C), Rogers 4000 (Tg = 280 °C)
or Arlon 527 (Tg =350 °C) could
Table 1. Material properties of power substrates.
be used reliably. For higher
CTE
Thermal
Elastic
Tensile
Max. Use
temperatures of operation, APEI
Temperature
(ppm/°C)
Conductivity
Modulus
Strength
has developed a new approach
Material
(GPa)
(MPa)
(W/m°K@ 25 °C
(°C)
that allows for high circuit density
as well as continuous operation
Aluminum
23.4
222
660
70
455
at temperatures as high as 400 ºC.
Copper
17.3
398
1083
131
220
The control substrate is normally
AlSiC
7 to 14
170 to 200
-188
499
gold-plated (Au-plated) since
many of the control components,
BeBeO
6 to 9
210 to 240
---such as HTSOIs and passives,
Table 2. Heat spreader material comparison.
have Au-plated pads. Some of
(Note: dashed entries mean that that data was not available from the vendor or manufacturer.)
these components do not have
at these temperatures. Therefore, new packaging materials must be a backside electrical connection; therefore, non-conductive (or
selected or developed. An important critical step in choosing packag- highly resistive) epoxies may be used as well as high-temperature
ing materials is matching the coefficient of thermal expansion (CTE) solder to attach the components to the control substrate. Finally,
for adjoining parts. Since SiC transistors will be used in the power the control components are wire bonded to the control substrate
stage, it is important to closely match the CTE of both the power using small diameter (0.75 mil, 1.0 mil or 3.0 mil) Au wire bonds.
substrate and heat spreader to that of SiC (between 4.6 ppm/°C and APEI has performed experiments for the optimization of these
5.1 ppm/°C) to reduce thermal stress failures. Table 1 compares the wire bonds showing reliable operation at temperatures beyond
characteristics of common substrates used in power electronics to that of 500 °C[3-4].
To verify the MCPM design for high-temperature operation, a
SiC. In the design of Figure 5, the power substrate is an aluminumnitride (AlN) or alumina ceramic substrate, bonded on either side with thermal model was generated in FLOTHERM for detailed 3-D thermal
10-mil to 12-mil copper (direct bond copper [DBC]), which allows analysis. The goal of the thermal simulations was to closely estimate
for excellent thermal and electrical conductivity. The copper is and simulate the actual conditions experienced during operation.
nickel-plated or gold-plated in order to enhance surface solder- In these simulations, a worst-case thermal load of 500 W was
ability and long-term resistance to thermal oxidation. The bare-die modeled, which represents a 10 hp motor drive with an overall
SiC components are attached using high-temperature solders, and efficiency of 93%. The results of these simulations in Figure 6 show
then wire bonded, completing the electrical circuits. These wire excellent thermal spreading and an effective removal of generated
bonds are normally large diameter (10 mils) aluminum (Al) bonding heat through the heat sink. Figure 6 also shows that the maximum
wire; but can be gold, depending on the surface metallization. APEI die junction temperature is 240 °C (or a 190 °C rise over the ambiInc. has performed thermal cycling experiments for the optimization ent of 50 °C), and the control electronics’ maximum temperature is
of Al wire bonds showing reliable operation at temperatures beyond close to 200 ºC.
250 °C[3-4].
Since most of the SiC power devices are vertical devices, the Future developments
The penetration and widespread use of SiC devices and systems
connection between the die and the substrate are not only important
for heat transfer, but also for electrical or current transfer as well. will be heavily dependent on the cost and availability of different
A metallurgical process such as soldering can accomplish this attachment. Since the operating temperature
of the MCPM will be approximately 250 °C, solders
above 300 °C solidus are used. Last, the power substrate is attached to a heat spreader. The selection of
a heat spreader is important to the package as it provides mechanical strength and is in the direct line of
heat transfer from the power substrate to the heat sink
or heat exchanger. Heat spreaders are often made out
of metals such as copper, aluminum or metal
matrix composites (MMCs). The heat spreaders
used by APEI are often an AlSiC MMC. The
selection of the heat spreader is made from three
main criteria. First, the CTE of an MMC can
be adjusted to match the CTE characteristics of
the rest of the package to reduce the stresses of
thermal expansion within the MCPM. Second,
the thermal conductivity of AlSiC is excellent for
heat transfer. Third, AlSiC is available commercially. Table 2 compares the characteristics of Figure 5. Cross-section of MCPM design approach.
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devices allows designers to include them in
a larger number of applications. In turn, the
cost of these devices will be reduced as production levels are ramped up and confidence
is gained in the new technology. Finally,
other markets such as the rapidly growing
SiC LED market, will help the commercialization of SiC power devices, since technologies
developed for this market (such as the growing
of SiC wafers) can be directly transferred to
the SiC power device manufacturing process.
When the cost of SiC begins to drop, the technology will have the potential to become ubiquitous
in the power electronics industry.
References
Figure 6. Thermal analysis of a 10 hp three-phase MCPM motor drive.
types of devices. Since the first SiC power device was released to
the commercial market (a SiC Schottky diode in 2001), there has
been an extensive effort to commercially develop other SiC power
devices[5]. These devices include the metal-oxide-semiconductor
field-effect transistor (MOSFET), junction field-effect transistor
(JFET), static induction transistor (SIT), gate-turn-off (GTO) thyristor and bipolar junction transistor (BJT). A large selection of SiC
ABOUT THE AUTHORS
Roberto Schupbach is the senior engineering manager of APEI.
He received his B.S.E.E. at the Universidad Nacional de la Plata
(Argentina) in 1998, and his M.S.E.E. and Ph.D. at the University of Arkansas in 2000 and 2004, respectively. Schupbach’s
expertise lies in the design and development of state-of-the-art
extreme environment SiC electronic systems, in which he has
more than two dozen internationally refereed conference and
journal publications. Schupbach is the technical lead of internal
projects and commercial contracts for the development of highpower density SiC-based power electronic systems. He has also
lead work in creating SiC-based dc-dc converters for extreme
environment applications involving the Army’s Future Combat
Systems program. Schupbach heads the development of several
commercial and military SiC programs at APEI. He can be contacted at [email protected].
Alexander Lostetter is the president of APEI. He received
his B.S.E.E. and M.S.E.E. degrees from Virginia Polytechnic
Institute and State University in 1996 and 1998, respectively, and
his Ph.D. in Microelectronics from the University of Arkansas
in 2003. He was employed as a reliability and failure analysis
engineer in the Semiconductor Technology Center of the Space
Electronics Division for Lockheed Martin. Lostetter joined APEI
in 1999, where he led the extreme environment power electronics
research initiative. He was promoted to chief operations officer in
August 2002 and promoted to president in October 2003. He has
published more than 40 articles and journal papers in the area of
power electronics systems, design, miniaturization and packaging,
including a chapter in the IEEE released Advanced Packaging,
2nd Edition textbook with the chapter entitled “Power Electronics
Packaging.” He may be reached at [email protected].
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702RFDEF1.indd 13
1. A. Lostetter, J. Hornberger, S. Magan
Lal, K. Olejniczak, A. Mantooth, and Aicha
Elshabini, “Development of Silicon-Carbide
(SiC) Static-Induction-Transistor (SIT) Based
Half-Bridge Power Converters,” Proceedings
of the 2003 IMAPS Conference, Boston, MA,
October 2003.
2. J. Hornberger, A. Lostetter, T. McNutt, S. Magan Lal, and A.
Mantooth, “The Application of Silicon Carbide (SiC) Semiconductor Power Electronics to Extreme High-Temperature Extraterrestrial
Environments,” Proceedings of the 2004 IEEE Aerospace Conference,
MT, March 2004.
3. J. Hornberger,
A. B. Lostetter, K. J.
Olejniczak, S. Magan
Lal, and A. Mantooth,
“A Novel Three-Phase
Motor Drive Utilizing Silicon on Insulator (SOI) and Silicon
Carbide (SiC) Semiconductor Power Electronics for Extreme
High-Temperature Environments,” IMAPS
37th International Symposium on Microelectronics, Long Beach,
CA, November 2004.
4. H.A. Mustain,
A.B. Lostetter, W.D.
Brown, “Evaluation
of Gold and Aluminum
Bond Performance for
High Temperature (500
° C) Silicon Carbide
(SiC) Power Modules,”
the 55th Electronic
Components and Technology Conference,
Lake Buena Vista,
Florida, 2005.
5. Phlippen and
Burger, “A New High
Voltage Schottky Diode Based on Silicon
Carbide (SiC),” 2001
EPE, Graz, Austria.
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