Comparisons of 6.5kV 25A Si IGBT and
10-kV SiC MOSFET in Solid-State Transformer Application
Gangyao Wang
Xing Huang
Student Member
Student Member
Tiefu Zhao
Jun Wang
Student Member
Subhashish Bhattacharya
Student Member
Member
Alex Q. Huang
Fellow
FREEDM Systems Center,
Department of Electrical and Computer Engineering
North Carolina State University, Raleigh, NC
gwang3@ncsu.edu
Abstract --A 6.5kV 25A dual IGBT module is customized and
packaged specially for high voltage low current application like
solid state transformer and its characteristics and losses have
been tested under the low current operation and compared with
10kV SiC MOSFET. Based on the test results, the switching
losses under different frequencies in a 20kVA Solid-State
Transformer (SST) has been calculated for both devices. The
result shows 10kV SiC MOSFET has 7-10 times higher
switching frequency capability than 6.5kV Si IGBT in the SST
application.
operation is given; in the section III, the theoretical switching
frequency and break down voltage for Si and SiC device is
compared. In section IV, the tested switching loss for 6.5kV
Si IGBT is given and compared with switching loss of 10kV
SiC MOSFET under the same test condition. The switching
loss model of 10kV SiC MOSFET has been reported in [3].
In section V, the switching frequency capability of 6.5kV Si
IGBT and 10kV SiC MOSFET is compared for SST
application when considering the same losses. The
conclusions are presented in Section VI.
Index Terms-- IGBT, Solid state transformer, SiC MOSFET
I.
II.
INTRODUCTION
1
Solid-state transformer (SST) is an AC-AC converter
which is used to replace traditional 60Hz transformer. It has
many advantages compared with traditional transformer like
unity power factor, instantaneous voltage regulation, DC
output etc. as reported in [1]. It converts 7.2kV ac to
120/240Vac for utility applications or vice verse for feeding
back energy to the grid. In order to reduce the size of the
SST, the switching frequency must be very high,
unfortunately the highest voltage for currently available
silicon IGBT is 6.5kV with a minimum current rating 200A
[2]. The switching frequency capability of this kind of IGBT
is typical less than 1 kHz and not suitable for low current
application. For this reason, a 6.5kV 25A silicon IGBT has
been specially packaged and used as switching device of a
seven level rectifier, which is being considered as the rectifier
stage of SST. On the other side, SiC power semiconductor
device has much higher break down voltage, lower switching
loss, and higher operation temperature, so it can operate
under much higher frequency and becomes a good power
device candidate for SST application. In the section II of this
paper, the packaging of 6.5kV 25A IGBT is introduced, its
switching characteristics and losses under very low current
Fig.1, 6.5kV 25A IGBT dual module layout
The final prototype is shown in Fig. 2, for this package
design, the minimum clearance distance in air is 19mm and
creepage distance is 78mm which complies with IEC-600771 standard.
This work was supported by ERC Program of the National Science
Foundation under Award Number EEC-0812121.
978-1-4244-5287-3/10/$26.00 ©2010 IEEE
INTRODUCTION OF 6.5KV 25A IGBT DUAL MODULE
Because of absence of high voltage, low current power
device on the market, a 6.5kV IGBT dual module with antiparallel diode has been packaged. Fig 1 shows the chips
layout on DCB, it contains two 6.5kV 25A IGBT chips and
two 6.5kV 50A diode chips.
100
Vg
I
Vc
inte(Vce*Ic
Vce*I
Fig.2, 6.5kV 25A IGBT dual module prototype
Fig 3 shows an optical signal triggered gate driver
installed on the IGBT, plated slots are specially designed in
order to keep enough creepage distance between high voltage
terminals.
Fig.5 IGBT Turn Off under 3000V 10A
Fig.4 shows the 10A turn off waveform with 3kV bus under
125degree. The Turn off loss is 52.23mJ.
III. THEORETICAL COMPARISON OF SILICON AND SILICON
CARBIDE DEVICES
The limitations of power devices based on silicon and
silicon carbide can be evaluated from Baliga’s Figure of
Merit, the relationship of specific resistance of the ideal drift
region and the breakdown voltage [4]:
Ron −ideal =
Fig.3 6.5kV IGBT Gate Driver Prototype
The packaged 6.5kV 25A IGBT is tested with an 18.5mH
clamped inductive load, the DC bus voltage is 3000V, Rg-on
is 100ohm while the Rg-off is 10ohm, the test is conducted
with 125 ºC case temperature. The Fig.4 gives the 10A turn
on waveform; the turn on loss is 66.26mJ.
4VB 2
ε S μ n EC3
Where, Ron-ideal is the specific on resistance, VB is
breakdown voltage. εS , μn and EC is dielectric constant,
carrier mobility and critical electric field respectively. Fig.6
shows the limitation of two different materials. With the same
10kV break down voltage, the Ron_sp of Si power device is
several hundred times of SiC power device, so silicon power
device needs multiple modules in parallel to achieve
acceptable low on resistance.
Ron-BV
1.00E+03
I
1.00E+02
Vce*I
R on,sp (oh m *cm 2)
Vc
inte(Vce*Ic
1.00E+01
1.00E+00
Silicon
1.00E-01
Silicon Carbide
1.00E-02
1.00E-03
1.00E-04
1000
10000
BV (V)
Fig.4 IGBT Turn On under 3000V 10A
Fig.6 Theoretical limit for silicon and silicon carbide power
devices for trade-off between Ron and BV
Fig.4 shows the 10A turn on waveform with 3kV bus under
125degree. The Turn on loss is 66.26mJ.
101
Another factor that limits silicon devices’ application in
SST is the frequency limitation. From Fig.7, one can see that
at the same breakdown level, silicon carbide device can
operate at a much higher switching frequency [5].
Fig.8 compares the forward I-V curves of these two
devices. It shows that SiC MOSFET has higher forward
voltage drop than Si IGBT at 125 ºC.
B. Comparison of turn-on loss
Fig.9 shows a comparison of the measured turn-on
energy losses of the 10kV SiC MOSFET and 6.5kV IGBT
with an inductive load and a DC-link voltage of 3kV. The
inductive load parallel diode is 6.5kV 50A ABB silicon diode
and 10kV SiC JBS separately for different tests. The test
condition for IGBT is 125 ºC, since the IGBT switching
performance is highly dependent on the operation
temperature, and the switching loss is higher at higher
operation temperature, so 125 ºC can be considered as worse
case operation condition; while for MOSFET, the operation
temperature has very little impact on switching loss, so the
test here is only conducted under 25ºC. The result shows the
6.5kV IGBT has 20 times larger losses compared to 10kV
SiC MOSFET.
The above two formulas predict the maximum switching
frequency for semiconductor devices with minimized power
loss. Fig.6 gives us a comprehensive view about the
difference between silicon and silicon carbide devices’
switching capability. Since device’s high frequency and high
voltage operation is preferred for SST application, silicon
carbide shows far leading characteristics of trade-off limit. At
a frequency of 10 kHz, silicon carbide device can support two
times higher voltage than silicon one.
Fig.9 Turn-on loss of 6.5kV IGBT and 10kV SiC MOSFET
Fig.7 Limitation of switching frequency and breakdown
voltage
IV. LOSS COMPARISON OF 6.5KV SILICON IGBT AND 10-KV
SILICON CARBIDE MOSFET
A. Comparison of forward I-V characteristics
The forward I-V characteristics of a 6.5 kV IGBT has
been measured. The forward I-V characteristics of a 10-kV
SiC MOSFET were measured and reported [3].
C. Comparison of turn-off loss
Fig.10 shows a comparison of the measured turn-off
energy losses of the 10kV SiC MOSFET and 6.5kV IGBT in
the same test circuit and the same test condition as turn-on
test. The result shows the 6.5kV IGBT has 20 times larger
losses compared to 10kV SiC MOSFET at 1A current, and
about 80 times at 5A current.
Fig.8 Forward I-V curves of 6.5kV IGBT and 10-kV SiC
MOSFET at 125 ºC
Fig.10 Turn-off loss of 6.5kV IGBT and 10kV SiC MOSFET
102
the calculation method described above. The results are
plotted in Fig.13.
V. SWITCHING LOSS AND FREQUENCY CAPABILITY OF 6.5KV
SILICON IGBT AND 10-KV SILICON CARBIDE MOSFET FOR SST
The basic configuration of a proposed 20 kVA SST
interfaced to the grid is shown in Fig.11. It contains three
stages, AC/DC rectifier, DC/DC dual active bridge (DAB),
and DC/AC inverter. The input voltage is 7.2kV, 60Hz ac.
For 6.5kV Si IGBT based SST, there will be three cascaded
H-bridges for rectifier as shown in Fig.11, each high voltage
DC bus is 3.8kV. But for 10kV SiC MOSFET based SST,
only two cascaded H-bridge are required, in this case, the dc
bus voltage will be 6kV.
Fig. 12 Loss simulation circuit for 10kV MOSFET
While for 10kV MOSFET, a Pspice model has been
reported in [3] and verified by test data, the switching
characteristics of 10kV SiC MOSFET with 6kV dc bus can
be simulated through the circuit shown in Fig.12 and the
fig.13 gives the simulated turn on and turn off waveforms.
Vd
Vds(V)
Fig. 11 Topology of Solid State Transformer
Id*100
For the AC/DC rectifier, the switching device is turned
on and off under the hard switching condition. The total
power loss for one IGBT in the SPWM modulated rectifier
can be calculated by using the following equations[5]:
tn =
f
T
n ,
N sw = sw , n = 1,2,..., ( N sw − 1)
×
f grid
2 N sw
Time(us)
(1)
I n = 2 I rms ⋅ sin(2π × f grid × t n )
(2)
1
Dn = (1 + m ⋅ sin(2π × f grid × tn ))
2
N sw −1
V × I × Dn
Ptotal = ∑ ( on ,n on ,n
+ Eon , n + Eoff , n ) × f grid
f sw
n =1
(3)
Vgs*10
Fig. 13 Switching Characteristics of 10kV MOSFET with
6kV dc bus
So the switching loss can be simulated and the results for
both turn on loss and turn off loss under different current are
plotted in Fig.14.
(4)
For the DC/DC dual active bridge converter, the switch can
achieve ZVS for turn-on, so only conduction loss and turn off
loss is calculated and given by equation (5),
π − φ Von (φ ) I on (φ ) φ / 2
+
×
×
+ Eoff × f sw (5)
2π
2
2
2π
where, ϕ is the phase shift angle between DAB primary and
secondary voltage under the rated power output.
Since the 6.5kV IGBT loss model has been obtained
based on test data, the IGBT losses under different switching
frequencies for both rectifier and DAB can be calculated via
Ploss = Von (φ ) × I on (φ ) ×
103
Fig. 14 Switching Loss of 10kV MOSFET with 6kV dc bus
Similarly by using the same calculation method, the SiC
MOSFET losses under different switching frequencies can be
obtained for both rectifier and DAB and plotted in Fig.15. It
can be found SiC MOSFET can operate at much higher
frequency than Si IGBT if considering the same power loss.
Take 50W loss for example, 6.5kV IGBT can operate under
1750Hz for rectifier and 2650Hz for DAB, while 10kV
MOSFET can operate under 12.5 kHz for rectifier and 18.2
kHz for DAB.
[3]
[4]
[5]
Fig.15 Power loss of 6.5kV IGBT and 10-kV SiC MOSFET
for rectifier and DAB under different switching frequency
VI. CONCLUSIONS
In this paper, the switching characteristics of 6.5kV 25A
IGBT have been tested, and switching loss behavior under
very low current operation has been investigated for high
voltage silicon device. Loss characteristics has been
compared between 6.5kV IGBT and 10kV MOSFET.
Moreover, the switch loss for both devices under different
switching frequencies in the 20kVA SST application has been
calculated, the results illustrate that the 10kV MOSFET has
much higher frequency switching capability.
ACKNOWLEDGMENT
The authors gratefully acknowledge the Powerex Inc. for
packaging 6.5kV IGBT dual modules.
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