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Characterization and Modeling of Silicon Carbide Power Devices and
Paralleling Operation
Article · May 2012
DOI: 10.1109/ISIE.2012.6237089
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Characterization and Modeling of Silicon Carbide
Power Devices and Paralleling Operation
Yutian Cui1
ycui7@utk.edu
Madhu S. Chinthavali2
chinthavalim@ornl.gov
Fan Xu1
fxu6@utk.edu
1
2
The University of Tennessee, Knoxville
Knoxville, TN 37996-2250 USA
I.
INTRODUCTION
Even though the technologies with silicon (Si)-based
power devices are mature, inherent material restrictions limit
their performance in high voltage, high power, high
switching frequency and high temperature applications.
Bipolar power devices, such as insulated-gate bipolar
transistors (IGBTs), can handle high power, but the
switching speed is limited by the devices’ structure [1].
Unipolar power devices, like metal-oxide semiconductor
field effect transistors (MOSFETs), can be switched at high
frequency, but suffer from relatively high on-state resistance.
Furthermore, Si power devices generally can only withstand
operational temperature of 150°C and can require a
substantial cooling system.
Because of these limitations in Si power devices, wide
bandgap (WBG) power devices like silicon carbide (SiC) and
gallium nitride (GaN) are becoming more attractive. SiC
power devices are more developed and have the potential to
replace Si in the near future within certain application areas.
SiC has many superior properties compared to Si, like higher
critical electrical field, relatively high electron mobility, and
higher thermal conductivity. Higher critical electrical field
allows thinner device and higher doping density for the same
blocking voltage. Correspondingly, the on-state resistance
can be reduced. Higher thermal conductivity can increase
thermal dissipation capability and reduce the requirement of
the cooling system. SiC has the potential to be operated at
much higher temperature (> 300°C) compared to Si devices
[2]. These properties make SiC power devices a worthy
substitution for their Si counterparts for high voltage, high
temperature, and high frequency applications [3-6].
In this paper, different types of SiC power devices are
tested and analyzed, and their advantages have been shown
Oak Ridge National Laboratory
Oak Ridge, TN 37831-6472 USA
in the testing results. Furthermore, the issues of paralleling
devices are discussed in simulation as well.
II. UNIPOLAR POWER DEVICES
A. SiC JFET
SiC junction field effect transistor (JFET) is a
commercially available SiC active power device with both
normally-on and normally-off structures. The application of
SiC JFETs in power converters has been under research and
development [7-9].
A 1200 V/ 20 A normally-off SiC JFET has been tested for
characterization. Fig. 1 is the forward characteristics of the
SiC JFET at 3 V gate voltage from 25°C to 175 °C. The onstate resistance of the SiC JFET was calculated at 3 V gate
voltage when the device is fully ON and shown in Fig. 2.
This JFET has a positive temperature coefficient, which is
helpful for paralleling operation. Transfer characteristics
were tested and shown in Fig. 3, which clearly indicates this
is a normally-off JFET as the threshold voltage is positive.
Transcondunctance is calculated from the transfer curves and
the value decreases with increasing temperature as shown in
Fig. 4.
15
25 C
10
Id (A)
Abstract- This paper presents recent research on several silicon
carbide (SiC) power devices. The devices have been tested for
both static and dynamic characteristics, which show the
advantages over their Si counterparts. The temperature
dependency of these characteristics has also been presented in
this paper. Then, simulation work of paralleling operation of
SiC power MOSFETs based on a verified device model in
Pspice is presented to show the impact of parasitics in the
circuit on the switching performance.
Leon M. Tolbert1,2
tolbert@utk.edu
175 C
5
Vgs=3V
0
0
0.2
0.4
0.6
0.8
Vds (V)
1
1.2
1.4
Fig. 1. Forward charcteristic of 1200 V / 20 A SiC JFET.
Fig. 5 shows the switching waveforms of the SiC JFET
tested in the double pulse tester (DPT) with SiC JBS diode as
the free wheeling diode at 400 V and 5 A. The gate circuit
used here was a 15 Ω resistor for steady state in parallel with
a series connection of a 5 Ω resistor and a 22 nF capacitor for
transient. The gate voltage varied from 15 V to -15 V to
inprove the switching speed [10]. The transient time is less
than 40 ns based on current, which shows the fast switching
capability of the SiC JFET.
160
0.08
140
0.075
Total Energy Loss (uJ)
Resistance (Ohm)
0.085
0.07
0.065
0.06
0.055
0.05
120
100
80
60
40
0.045
20
40
60
80
100
120
Temperature(C)
140
160
180
Fig. 2. On-state resistance of 1200 V / 20 A SiC JFET.
20
2
300
50
Total Energy Loss (uJ)
30
25 C
20
175 C
4
5
6
7
Current (A)
8
9
10
25 C
75 C
125 C
150 C
250
40
3
Fig. 6. Switching losses of 1200 V / 20 A SiC JFET at 400 V.
60
Id (A)
25 C
75 C
125C
150 C
200
150
100
10
Vds=5V
0
0
0.5
1
1.5
Vgs(V)
2
2.5
50
3
3
Fig. 3. Transfer characteristics of 1200 V / 20 A SiC JFET.
Transconductance(S)
6
7
Current (A)
8
9
10
Fig. 7. Switching losses of 1200 V / 20 A SiC JFET at 600 V.
SiC MOSFET is an attractive unipolar power device,
capable of high switching frequency, high temperature, and
high efficiency operation [11, 12]. A 1200 V / 30 A SiC
MOSFET has been tested here.
45
40
35
30
25
20
40
60
80
100
120
Temperature (C)
140
160
180
Fig. 4. Transconductance of 1200 V / 20 A SiC JFET.
16
14
Current (A)/ Voltage (V)
5
B. SiC MOSFET
50
Voltage/40
12
10
8
6
4
2
Current
0
-2
4
Fig. 8 shows the forward characteristics of the SiC
MOSFET over temperature. On-state resistance is calculated
around 10 A of conducting current and shown in Fig. 9.
Unlike the SiC JFET, the on-state resistance of the SiC
MOSFET dropped at temperatures near 50°C.
The
resistance of a power MOSFET mainly comes from three
parts: channel resistance, RCH, JFET region resistance, RJFET,
and drift layer resistance RDRIFT [13]. RCH has a negative
temperature coefficient, while the other two components
have positive temperature coefficient. At temperatures less
than 50 °C, the change of RCH is dominant, which makes the
whole resistance decrease with increasing temperature. At
higher temperature, RJFET and RDRIFT changes more than RCH,
which lead to the positive temperature coefficient [13,14].
Fig. 10 shows the transfer characteristics over temperature.
It can be seen that the threshold voltage decreases with
temperature increasing.
30
1.6
1.8
2
2.2
2.4
2.6
Time (s)
2.8
3
3.2
x 10
-6
25
Fig. 5. Switching waveform of 1200 V / 20 A SiC JFET.
Current (A)
20
The switching losses of the SiC JFET have also been
obtained from experiments at 400 V and 600 V DC voltage
with different conducting current and are shown in Figs. 6
and 7. The switching losses change little with temperature,
while increasing with conducting current and voltage.
15
25C
50C
75C
100C
125C
150C
175C
200C
10
Vgs=20 V
5
0
0
0.5
1
1.5
2
2.5
Voltage (V)
3
3.5
4
Fig. 8. Forward characteristics of 1200 V / 30 A SiC MOSFET.
700
0.125
600
0.12
500
Energy losses (uJ)
R (Ohm)
0.13
0.115
0.11
400
300
0.105
0.1
0.095
25 C to 175 C
200
100
Vgs=20 V
0
50
100
T (C)
150
0
200
Fig. 9. On-state resistance of 1200 V / 20 A SiC MOSFET.
2
4
6
8
10
12
Id (A)
14
16
18
20
22
Fig. 12. Switching losses of 1200 V / 30 A SiC MOSFET at 400 V.
50
1200
1100
40
1000
Energy losses (uJ)
Current (A)
900
30
20
800
700
600
500
25 C to 200 C
10
25 C to 175 C
400
300
0
0
2
4
6
8
Voltage (V)
10
12
200
14
Fig. 10. Transfer characteristics of 1200 V / 30 A SiC MOSFET.
Fig. 11 is the switching waveform of the SiC MOSFET at
400 V and 15 A. A similar parallel structure between gate
driver and the MOSFET’s gate was used here like that used
for the JFET earlier. A 15 Ω resistor paralleled with a series
connection of a 3 Ω resistor and a 30 nF capacitor was implemented
for the switching test [15]. The testing circuit for this SiC
MOSFET was not targeted for high switching speed and
some parasitics were added to the circuit for modeling
purpose intentionally [15], yet the transient times of the
MOSFET were around 60 ns based on current as shown in
Fig. 11.
When specially designing circuits for high
frequency operation, the transient time can be even shorter,
and SiC MOSFETs can be used for high frequency operation
without using soft switching techniques [16, 17].
6
8
10
12
14
Id (A)
16
18
20
22
Fig.13. Switching losses of 1200 V / 30 A SiC MOSFET at 600 V.
III. BIPOLAR POWER DEVICES
A. SiC BJT
SiC bipolar junction transistor (BJT) is a current-driven,
normally-off power device, which could be an alternative to
a Si IGBT [10]. A SiC BJT rated at 1200 V and 6 A has
been tested and characterized.
The forward characteristics of the SiC BJT were obtained
at different temperatures from 25°C to 175°C when the base
current is 350 mA as shown in Fig. 14. It can be seen that
the SiC BJT has a positive temperature coefficient.
7
6
30
5
Voltage/20
25 C
4
20
Ic(A)
Current (A)/ Voltage (V)
25
Current
3
15
175 C
2
10
1
Base Current =350mA
5
0
0
-5
1.2
1.4
1.6
1.8
2
2.2
Time (s)
2.4
2.6
2.8
x 10
-6
Fig. 11. Switching waveform of 1200 V / 30 A SiC MOSFET.
The losses of the SiC MOSFET were also measured and
shown in Figs. 12 and 13. It was tested under 400 V and 600
V DC voltage at multiple temperatures. As expected, the
losses are quite consistent at different temperatures, and
obviously increased with current and voltage.
0
0.1
0.2
0.3
0.4
0.5
0.6
Vce (V)
0.7
0.8
0.9
1
Fig. 14. Forward characteristics of 1200 V / 6 A SiC BJT.
Another important parameter of BJTs is current gain. Fig.
15 shows the current gain of the BJT over a wide
temperature range. The gain was calculated when the BJTs
were in the forward active region [10]. Fig. 15 illustrates
that the gain decreases as the temperature increases while
increasing with collector current.
frequency, which may cause the parasitics to be even more
critical. Gate signal and current sharing are important issues
for paralleling operation [10]. Most SiC power devices have
a positive temperature coefficient, which helps current
sharing during paralleling operation.
70
65
60
25 C
55
Gain
50
45
Several papers discuss paralleling operation of SiC power
devices, most of which are for power modules [19-22].
Some papers mention the paralleling of discrete SiC power
devices [10, 23, 24]. The results presented in these papers
showed relatively good current match, however, some
mismatch of transient current did exist. In this paper, some
mechanism of mismatching is simulated and discussed.
40
175 C
35
30
Vce=5V
25
20
0
5
10
15
20
25
Ic (A)
Fig. 15. Gain of 1200 V / 6 A SiC BJT.
B. SiC SJT
SiC super junction transistor (SJT) is a current-controlled
normally-off device rated at 1200 V/ 10 A [18]. The devices
tested were experimental samples. Fig. 16 shows the
forward characteristics of the SJT at different temperatures
with a 350 mA base current recommended by datasheet. The
gain of the SJT was calculated correspondingly for different
base currents and shown in Fig. 17, which decreases with
temperature and collector current.
10
25C
75C
125C
175C
Current (A)
8
6
A. Experimental Results of Paralleling Operation
Double pulse testing of two paralleled devices was
performed with the SiC JFET, BJT and MOSFET discussed
earlier. Figs. 18 to 20 were the experimental switching
waveforms for these devices. The gate circuit for BJTs and
JFETs was a 10 Ω resistor in parallel with a 3 Ω (for BJTs)
or a 7 Ω resistor (for JFETs) in series connection with a 100
nF capacitor. For the paralleled MOSFETs, an external 5 Ω
resistor was used. Seen from these figures, the current
sharing during turn off process is relatively even; however,
the transient currents are slightly mismatched, yet they
become even after achieving steady state during turn on [10].
Some reasons for the mismatch are discussed in [10].
Simulation work for reasons causing mismatch is presented
in the following section.
4
0
Ib=350 mA
0
1
2
3
Voltage (V)
4
5
Turn off Current (A)
6
2
4
2
0
-2
Fig. 16. Forward characteristics of 1200 V / 10 A SiC SJT.
0
0.5
1
1.5
2
2.5
time(s)
3
3.5
4
4.5
5
-7
x 10
Turn on Current (A)
10
75
25C
75C
125C
175C
70
65
Gain
60
5
0
55
2
2.05
2.1
2.15
2.2
2.25
2.3
time (s)
2.35
2.4
2.45
2.5
-6
x 10
50
Fig. 18. Turn-on and turn-off waveforms of two
paralleled BJTs at 400 V and 10 A.
45
40
6
30
0
5
10
15
Collector Current (A)
20
25
Fig. 17. Gain of 1200 V / 10 A SiC SJT.
Turn off Current(A)
35
4
2
0
-2
IV. PARALLELING OPERATION ISSUES
5.5
6
6.5
7
time(s)
Turn on Current(A)
In order for power converters to be operated at high
current and high power, paralleling of power devices is
necessary. Paralleling devices can ensure each device is
operated in its linear region, which can reduce the
conduction losses.
It is a complex procedure when
paralleling devices as all the parasitics in the circuit and
junction capacitances within the devices will be involved.
SiC power devices are expected to operate at high switching
5
7.5
-7
x 10
10
5
0
1.6
1.65
1.7
1.75
1.8
time(s)
Fig.19. Turn-on and turn-off waveforms of two
paralleled JFETs at 400 V and 10 A.
1.85
-6
x 10
10
Vds (100 V/div)
Zero external drain inductance
10 nH external drain inductance
8
6
Current (A)
Vgs (10 V/div)
Ids of two paralleled
MOSFETs (5 A/div)
4
2
0
(a)
-2
Turn off transient.
3
3.1
3.2
3.3
Time (s)
3.4
3.5
-6
x 10
Fig. 21. Current sharing with 10 nH difference of external Ld.
Vgs (10 V/div)
8
Ids of two paralleled
MOSFETs (5 A/div)
100 mOhm external gate resistor
Current (A)
6
Vds (100 V/div)
(b)
Zero external gate resistor
4
2
Turn on transient.
0
Fig. 20. Switching waveforms of two paralleled MOSFETs at 680 V and 20 A.
B. Current Sharing Issues
Parasitics in the power device and testing circuit can cause
current mismatching during transients and will be discussed
in this paper through simulation in Pspice. The device model
used here is a 1200 V, 30 A SiC MOSFET, which has been
validated [15]. To simplify the simulation, the gate driver
was not included in the simulation, and an ideal voltage
source with 40 ns transient time was implemented to supply
gate signal.
Fig. 21 is the current comparison of two paralleled
MOSFETs. Except for a 10 nH inductance difference
between drain of one MOFET and the free-wheeling diode,
all the other elements were the same.
Under ideal
conditions, each MOSFET should carry half of the load
current. The influence of the external 10 nH inductance is
clearly shown in Fig. 21. The overshoot value dropped
dramatically and the transient time became longer with larger
drain inductance during turn on transient.
Fig. 22 shows the influence of the gate resistance. An
additional 100 mΩ resistor was inserted between gate signal
and the gate of one MOSFET. The turn off current sharing is
almost even, however, the turn on current was mismatched in
overshoot current value, current turn on delay time, and
current rising time
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CONCLUSION
The characterization of several SiC active switches was
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