DESIGNfeature
SAM DAVIS,
Editor-in-Chief, PET
1200V SiC MOSFET Poised to
Replace Si MOSFETs and IGBTs
C
ree, Inc. has gained the distinction of producing the industryís first
fully-qualified, commercial silicon carbide (SiC) power MOSFET
(Fig. 1). The company’s SiC power MOSFET is the end result of
many years devoted to materials research, process development
and device design.
The SiC DMOSFET, designated CMF20120D, allows blocking voltages up to 1200V. Its consistency of performance characteristics across operating conditions, along with a true enhancement mode MOSFET
architecture (normally-off), makes it well-suited for power electronics switching
circuits. Compared with available silicon MOSFET or IGBT devices of similar ratings,
the CMF20120D has the lowest gate drive energy (QG <100nC) across the recommended input voltage range. Plus, minimized conduction losses produce a forward
drop (VF) of <2V at 20A.
The SiC MOSFET reduces switches losses compared with silicon MOSFETs and
IGBTs. One reason is that the high voltage SiC MOSFET does not have the tail current losses found with IGBTs. In addition, the SiC MOSFETís high current density
and small die size results in lower capacitance than with silicon MOSFETs. Fig. 2
compares the switching losses of IGBTs and silicon MOSFETs with those of the SiC
MOSFETs.
This SiC MOSFET offers advantages over conventional silicon devices, enabling
high system efficiency and/or reduced system size, weight and cost through its
higher frequency operation. Compared to the best silicon IGBTs, the SiC device will
improve system efficiency up to 2% and operate at 2-5 times the switching frequencies. Higher component efficiency also results in lower operating temperatures.
Combining these lower operating temperatures with the CMF20120Dís ultra-low
leakage current (<1μA) can add significantly to system reliability. Fig. 3 compares the
The new SiC MOSFET will
enable power electronic
system engineers to develop
higher power switching circuits with improved energy
efficiency, size and weight.
Fig. 1. The CMF20120D
SiC MOSFET is housed in a
TO-247-3 package that is
pb-free, RoHS compliant and
halogen free.
1.2
Fig. 2. SiC MOSFET exhibits
lower switching losses than
silicon MOSFETs and IGBTs.
a. Turn-on loss comparison
b. Turn-off loss comparison
2.5
1
Si MOS 8
NPT IGBT
SiC DMOS
TFS IGBT
2
Turn-on Loss
Turn-on Loss
0.8
0.6
1.5
1
0.4
Si MOS 8
NPT IGBT
SiC DMOS
TFS IGBT
0.2
0.5
0
0
0
(a)
36
25
50
75
100
TJ (ºC)
125
150
175
Power Electronics Technology | February 2011
(b)
0
25
50
75
100
TJ (ºC)
125
150
175
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SILICON CARBIDEmosfets
TJ = 150ºC
3.0
1E- 1
Normalized RDS (ON)
1E-2
1E-3
ID, IC (A)
SiMOS RDS(ON) varies
<250% from 25 deg C
to 150 deg C
2.5
Si MOS 8
NPT IGBT
SiC DMOS
TFS IGBT
1E-4
SiMOS 8
2.0
SiSJMOS
1.5
SiCDMOS
1.0
SiC DMOS R subscript DS (ON) varies
< 20% from 25 deg C to 150 deg C
0.5
0
1E-5
0
20 x lower than Si
1E-6
50
100
TJ (ºC)
150
200
Fig. 4. The CMF20120D SiC MOSFET has a nearly flat RDS(ON) with only a 20 variation over a broad temperature range, whereas the silicon MOSFET can vary as
much as 250% over the same temperature range.
1E-7
0
200
400
600
800
VDS, VCE (V)
1000
1200
Fig. 3. SiC’s wide band-gap semiconductor material results in minimal leakage at
elevated reverse voltages and temperatures.
leakage current for silicon MOSFETs and IGBTs. SiC’s wide
band-gap ensures minimal leakage even at elevated reverse
voltages and temperatures.
Although this SiC MOSFET has removed the upper voltage limit of silicon MOSFETs there are some differences in
characteristics when compared to what is usually expected
with high voltage silicon MOSFETs. These differences need
to be carefully addressed to get maximum benefit from the
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February 2011 | Power Electronics Technology
37
SILICON CARBIDEmosfets
120
120
VGS = 20 V
100
VGS = 18 V
80
VGS = 16 V
60
VGS = 14 V
40
VGS = 12 V
20
V GS =
V GS =
80
ID (A)
ID (A)
100
0
18 V
V GS =
16 V
60
V GS = 14 V
40
VGS = 12 V
VGS = 10 V
20
VGS = 10 V
20 V
0
0
2
4
6
8
10 12
VDS (V)
14
16
18
20
0
2
4
6
8
10 12
VDS (V)
14
16
18
20
Fig. 5. Typical SiC MOSFET output characteristics for a 25ºC junction temperature.
Fig. 6. At 125ºC junction temperature the SiC MOSFET shows minimal change
compared with 25ºC junction temperature.
SiC MOSFET. In general, the SiC MOSFET is a superior
switch compared with its silicon counterparts, but it should
not be considered as a direct drop-in replacement in existing
applications.
Among the SiC MOSFET advantages over silicon devices
is an RDS(ON) improvement. As shown in Fig. 4, SiC
MOSFETís RDS(ON) increases only 20% over operating
temperature compared with over 250% for 1200V silicon
MOSFETs. The flatness of the SiC MOSFET RDS(ON)
curve eases the design of high efficiency applications. It also
ensures reliable system thermal performance. In addition,
the SiC MOSFET’s positive temperature coefficient allows
easy paralleling to obtain higher operating currents.
mind when applying the SiC MOSFETs:
•Transconductance
•Gate drive pulse fidelity
Modest transconductance requires a VGS of 20 V to
optimize performance. This can be seen in the output and
transfer characteristics shown in Figs. 5, 6, and 7. This also
affects the transition where the device behaves as a voltage controlled resistance to where it behaves as a voltage
controlled current source as a function of VDS. The result
is that the transition occurs over higher values of VDS than
are usually experienced with Si MOSFETs and IGBTs. This
might affect the operation of anti-desaturation circuits, especially if the circuit takes advantage of the device entering the
constant current region at low values of forward voltage.
The modest transconductance needs to be carefully
considered in the design of the gate drive circuit. The first
DRIVING THE SIC MOSFET
There are two key characteristics that need to be kept in
■ SILICON CARBIDE: THE NEW MOSFET IN TOWN
SILICON CARBIDE (SIC) possesses
many favorable properties, making
it useful for high-temperature, highfrequency and high-power applications,
including:
• Wide bandgap
• High thermal conductivity
• High breakdown electric field strength
(about 10X of Si),
• High saturated drift velocity (higher
than GaAs)
• High thermal stability
• Chemical inertness
38
These properties allow a high power
device to block several kilovolts in the
blocking mode and conduct high currents in the conducting mode. Typical
switching devices with these characteristics are conventional silicon MOSFETs
and IGBTs.
A major advantage of SiC-based
switching devices is operation in hostile
environments (600ºC) where conventional silicon-based electronics cannot
function. Silicon carbide’s ability to
function in high
Power Electronics Technology | February 2011
temperature, high power, and high
radiation conditions will enable large
performance enhancements to a wide
variety of systems and applications. For
example, SiC’s high-temperature highpower capabilities can benefit aircraft,
automotive, communications, power,
and spacecraft applications.
While producing SiC commercially
for MOSFETs has proven to be a daunting task, Cree has overcome the problems of producing MOSFET-grade SiC.
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SILICON CARBIDEmosfets
RLOOP
60
LLOOP
Fig. 8. SiC gate circuit
approximation is an RLC
circuit driven by a voltage pulse.
50
CGATE
VPULSE
ID (A)
40
T = 125ºC
30
the product of gate voltage swing and gate charge for the
SiC MOSFET is lower than comparable silicon devices. The
gate voltage must have a fast dV/dt to achieve fast switching
times, which indicates the necessity of a very low impedance
driver.
Gate drive pulse fidelity must be carefully controlled.
The nominal threshold voltage is 2.5V and the device is not
fully on (dVDS/dt≈0) until the VGS is above 16V. This is
a noticeably wider range than what is typically experienced
with silicon MOSFETs and IGBTs. The net result of this is
that the SiC MOSFET has a somewhat lower ‘noise margin’.
Any excessive ringing that is present on the gate drive signal
could cause unintentional turn-on or partial turn-off of the
device. The gate resistance should be carefully selected to
ensure that the gate drive pulse is adequately dampened.
20
T = 25ºC
10
0
0
2
4
6
8
10 12
VGS (V)
14
16
18
20
Fig. 7. Typical transfer characteristics of the SiC MOSFET at 25ºC and 125ºC.
obvious requirement is that the gate be capable of a >22 V
(+20 V to -2V) swing. The recommended on-state VGS
is +20 V and the recommended off-state VGS is between
-2 V to -5 V. Even though the gate voltage swing is higher
than the typical silicon MOSFETs and IGBTs, the total gate
charge of the SiC MOSFET is considerably lower. In fact,
www.paktron.com
Mission
Critical
Applications
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February 2011 | Power Electronics Technology
39
SILICON CARBIDEmosfets
IC Sense
Load Current
VCC
Gate Drive
Input
+
20 V
Gate Driver
SiC DMOSFET
–
R Gate
VEE
Fig. 9. A two-stage current transformer can be used sense gate current when
driving the SiC mOSFET.
To a first order, the gate circuit can be approximated as a
simple series RLC circuit driven by a voltage pulse as shown
in Fig. 8.
As shown, minimizing LLOOP minimizes the value of
RLOOP needed for critical damping. Minimizing LLOOP
also minimizes the rise/fall time. Therefore, it is strongly
recommended that the gate drive be located as close to the
SiC MOSFET as possible to minimize LLOOP.
The internal gate resistance of the SiC MOSFET is 5Ω.
An external resistance of 6.8Ω was used to characterize this
device. Lower values of external gate resistance can be used
so long as the gate fidelity is maintained. In the event that
no external gate resistance is used, it is suggested that the
gate current be checked to indirectly verify that there is no
ringing present in the gate circuit. This can be accomplished
with a very small current transformer.
A recommended setup is a two-stage current transformer
as shown in Fig. 9. The two-stage current transformer first
stage consists of 10 turns of AWG30 wire on a small high
permeability core. A Ferroxcube 3E27 material is recommended. The second stage is a small wide bandwidth current
transformer such as the Tektronix CT-2. Finally, a separate
source return should be used for the gate drive, as shown in
Fig. 10a and b.
CHOOSE GATE DRIVER CAREFULLY
5Careful consideration must be given to the selection of a
gate driver. A typical application error is selection of a gate
driver that has adequate swing, without careful consideration
of output resistance and current drive capability. Therefore,
an appropriate gate driver must have these characteristics:
• High peak current capability
• Low output resistance
• Adequate voltage swing
A significant benefit of the SiC MOSFET is the elimination of the tail current observed in silicon IGBTs. However,
it is very important to note that the current tail does provide
a certain degree of parasitic dampening during turn-off.
Additional ringing and overshoot is typically observed when
silicon IGBTs are replaced with SiC MOSFETs. The additional voltage overshoot can be high enough to destroy the
40
Power Electronics Technology | February 2011
SiC DMOS
Drive
Load Current
L Stray
(a)
20 V
0211_Cree_F10a
R Gate
SiC DMOS
Drive
L Stray
(b)
Fig. 10. Affect of stray inductance (a) Stray inductance on the source lead
causes load di/dt to be feedback into the gate drive, causing oscillations and
limiting di/dt. (b) Kelvin gate connection with separate source return is recommended.
device. Therefore, it is critical to manage the output interconnection parasitics (and snubbing circuitry ) to keep the
ringing and overshoot from becoming a problem.
COmpOnEnT COST VS. SYSTEm pERFORmAnCE
Obviously, because it is a new technology, the SiC
MOSFET will have a higher component price tag than it’s
silicon counterpart. However, overall performance characteristics of a power conversion system using the SiC
MOSFET and SiC Schottky diodes can be superior to a
traditional all-silicon system.
SiC devices allow power circuits to operate at higher
switching speeds, which reduces the cost of magnetics as
well as system size and weight. In addition, thermal management considerations such as heat sinking and cooling
are less stringent, because these systems are more thermally
efficient.
A cost tradeoff based on component price alone does not
provide a realistic comparison of long-term operating costs.
SiC MOSFET prices will eventually go down because Cree
has been using 4-in. wafers and will eventually go to 6-in.
wafers, which offer greater economies of scale.
Cree intends to produce SiC MOSFETs in an N-channel
configuration rated higher than 1,200V, in a variety of current ratings.
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