1
PAPER 35
Power converter with built in Silicon Carbide
technology
Xavier Fonteneau
Loïc Pinson
PhD, Development Engineer
ECAGROUP
24 rue Jan Palach, 44220 Couëron – FRANCE
fonteneau.x@eca-en.com
Export Sales Engineer
ECAGROUP
24 rue Jan Palach, 44220 Couëron – FRANCE
pinson.l@eca-en.com
Abstract—This paper details a methodology for predicting losses
inside a three phase inverter built with SiC MOSFET and SiC
Schottky diode. Different parameters such as the output current,
and modulation index are taken into account. As the SiC
MOSFET is a unipolar device, a reverse current can flow during
the on state. Two strategies- using and not using this capabilityare described and compared. For each strategy, a static and
dynamic model is established. Analytical calculations are used to
compute static and dynamic losses based on the characteristic of
a 5kVA converter. A good agreement is demonstrated between
measurements and the proposed model. Then, the proposed
methodology is performed on the characteristic of a 60kVA
converter to shows the benefit of using SiC devices instead of Si
power module for submarine applications
Keywords- SiC MOSFET, three phase inverter, reverse
conduction
I.
INTRODUCTION
Wide band-gap materials such as Silicon Carbide (SiC) or
Gallium Nitride (GaN) have been subjected to recent intense
research in power electronics. This has led to the
commercialisation of SiC diodes from the beginning of the 21st
century. Recently, SiC controlled switches have also been
released. These new devices provide better switching
performance than Silicon (Si) components leading to a
reduction of losses [1]. This opens new opportunities such as a
reduced cooling requirements [2,3] and/or an increased
switching frequency [4]. These factors play a major role
especially when compactness and/or high acoustic discretion is
required such as in submarine applications.
ECAGROUP is a company specialized in design and
manufacturing of power electronics equipment dedicated to
harsh environments. ECAGROUP take an interest in SiC
technology to develop a new range of converters for submarine
applications. The goal of this study is to evaluate the
advantages and drawbacks of using SiC MOSFET and SiC
Schottky diodes instead of Si-IGBT in power converters. The
converter studied is based on the electrical characteristic of a
5kVA three phase inverter. In Section II, we present a review
of commercialized SiC transistors. The main characteristics are
described in order to select a candidate for our study. In section
III, the design of a SiC three phase inverter is described and the
loss relations are introduced. Section IV details the design of a
5kVA system based on the loss calculations. The required
parameters were extracted from the manufacturer data sheet
and a prototype built to validate the analytical calculations.
Losses and heatsink temperature are measured at different
switching frequencies and different room temperatures. In
section V, a theoretical design of a 60kVA is presented based
on the previous calculations. Finally, conclusions and
perspectives are given in section VI.
II.
QUICK REVIEW OF SIC TRANSISTOR
A. SiC Normally-On JFET
SiC Normally-On Junction Field Effect Transistors (JFET) are
manufactured by Infineon and commercialized as “CoolSiC
transistors” [5]. A Normally-On characteristic is the main
difference to classical switches used in power electronics
(IGBT). Indeed, this device conducts current when no bias
voltage is applied on the gate. As a result, “normally-On”
devices are not prevalent in the industry and require complex
drivers for safe operation [6]. These components have the
particularity to operate at high temperatures [7] (>200°C) and
are already finding applications in the aerospace and mining
industries.
B. SiC MOSFET
SiC Metal Oxide Semi-conductor Field Effect Transistors
(MOSFET) are manufactured by companies such as CREE
[8], ROHM [9], MICROSEMI [10]. Compared to the SiC
Normally-On JFET, the MOSFET has a Normally-Off
behaviour, meaning that no current flows in the absence of
gate bias voltage. However the SiC MOSFET is not suitable
for high temperature applications as it can be affected by gate
oxides [11 at high temperatures (>150°C). The gate driver
required to bias the MOSFET is similar to an IGBT, so a
classic driver can be used.
C. SiC SJT
The “Super Junction Transistor” (SJT) is produced by
GeneSiC. Its structure is similar to a Bipolar Junction
Transistor (BJT) in that a gate/base current must be injected
[12] to keep the transistor in its on state. The use of DC
gate/base current creates extra losses and requires a high
Submarine Institute of Australia Science, Technology & Engineering Conference 2015
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power driver. As for the JFET, the SJT can operate at high
temperature which is an asset for high temperature application.
D. Device selection
Power converters require safe operation which excludes the
use of SiC Normally-On JFET. Compared to an SJT, a SiC
MOSFET can be controlled by a basic Si-IGBT driver and has
low gate losses. For those reasons, a SiC MOSFET is selected.
Figure 2. Structure of a three phase inverter using CCS050M12CM2
III.
DESIGN OF A THREE PHASE INVERTER
The device studied is a full SiC integrated three phase inverter
from CREE named “CCS050M12CM2” (see fig.1). The
module is rated at 1200V and conducts a 59A direct current at
a case temperature equal to 90°C. It contains six switches built
with one SiC MOSFET plus an antiparallel Schottky diode
(see fig. 2). During the on state, the channel between the drain
and the source can be approximated by a pure resistor, RDSON,
so the transistor can conduct a positive or negative current
[13,14]. Therefore, compared to a Si-IGBT, different
strategies (using or not using this capability) can be evaluated.
Those strategies influence static losses, therefore the classic
methods used to predict losses cannot always be applied with
SiC MOSFET.
A. Static losses in a SiC inverter Leg
The operation of an inverter leg is explained in order to
establish a model of static losses. The model takes into
account the static characteristics of the MOSFET and diode,
the modulation index, the output current amplitude and the
load power factor. The considered hypotheses are as follows:
· The
output
current
is
sinusoidal:
I out (q ) = I * 2 * sin (q )
·
The duration when the MOSFET conducts a positive
current divided by the switching period is:
TON = (1 + M * sin (q - j )) / 2
Where M is the modulation index and j is the angle between
the current Iout and the fundamental frequency of the voltage
Uout.
· The static model of the SiC MOSFET is given by:
V DS = R DSON * I D
· The static model of a SiC Schottky diode is given by:
V D = - (V 0 + R D * I D )
Where V0 is the threshold voltage of the Schottky diode and
RD is the diode series resistance. The study is done for a
positive output current Iout>0. When Iout is negative, results are
simply obtained by swapping M1 by M2 and D1 by D2.
1) Step 1: M1 is ON
When M1 is turn on and the output current is positive, Iout
flows through the MOSFET M1 referred as Fig. 3. The mean
power during a period of the current is given by (2).
Figure 3. M1 is ON and M2 is OFF
P DC
_M 1
=
1
2p
p
òT
* R DSON * I out (q
ON
)2
* dq
(2)
0
2) Step 2: M1 is OFF
When M1 is OFF and the load current is positive, Iout flows
through D2. Two strategies are also possible.
a) M2 is turn OFF:
The output current flows through D2 (Fig. 4). The static losses
during a period of the output current are given by equation 4.
Figure 4. M1 is OFF and M2 is OFF
PDC _ D 2 =
1
2p
p
ò (1 - T )* (V
ON
0
0
+ R D * I out (q )) * I out (q ) * dq
(3)
b) M2 is turn ON:
For a low load current level, it can be supposed that the
current flows through the MOSFET M2 only using the reverse
capability (Fig. 5a). If the current is large enough, the voltage
across the MOSFET M2 exceed the threshold voltage of the
diodes and the load current is divided between the MOSFET
and the diode (Fig. 5b).
Figure 1. View of the power module CCS050M12CM2 [13]
Submarine Institute of Australia Science, Technology & Engineering Conference 2015
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C. Junction temperature estimation
When semi-conductors have losses, their junction temperature
increases and can reach thermal runaway. Currently, a cooling
system is added to insure safe operation of the converter.
Figure depicted a simple thermal circuit associate to a
MOSFET and a diode inside the 5kVA prototype.
IV.
Losses and temperature relations were computed to design a
5kVA three phase inverter. The theoretical result is compared
to an existing 5kVA built with IGBT to shows the superiority
of the SiC MOSFET over Si-IGBT technology.
(a) M2 only
(b) M2 and D2 Diode
Figure 5. M1 is OFF and M2 is ON
The calculation of the static loss is therefore complex (4).
PDC _ M 2 =
1
p
q1
ò (1 - T ) * R
ON
5KVA PROTOTYPE
* I out (q ) * dq +
2
DSON
0
1
2p
p -q1
ò (1 - T ) * R
q
ON
* I M (q ) * dq
2
DSON
1
PDC _ D 2 =
1
2p
p -q1
ò (1 - T ) * (V
ON
0
+ RD * I D (q )) * I D (q ) * dq
q1
Figure 6. Thermal circuit of SiC devices for the 5kVA protoype
(4)
With:
V + RD * I out (q )
I M (q ) = - 0
RD + RDSON
I D (q ) =
·
- V0 + RDSON * I out (q )
RD + RDSON
·
·
RDSON * I out (q 1 ) = V0
B. Dynamic losses
When M1 is turned off, diode D2 is simultaneously turned on
to ensure the output current continuity. During this phase,
switching losses are generated inside M1 and D2. If the
MOSFET M2 is turned on, we can consider that the voltage
across the switch is close to null, so no switching losses are
generated for M2 (Zero Voltage Switching). So for a positive
output current, the switching losses are located inside M1 and
D2.The switching loss for the transistor is calculated by (5).
PSW _ M 1 = FSW * ETS *
VDC * 2 *I
p *VETS * I ETS
With:
·
(5)
Where, ETS is the total switching Energy evaluates at specified
voltage (VETS), current (IETS). The switching losses are more
complex to determine. Indeed, there is no recovery charge
inside a SiC Schottky diode. The switching losses can be
approximated through the capacitive charge of the SiC
components.
·
·
·
·
·
·
RTHjc_M: Thermal resistance of a MOSFET between
junction and case.
RTHjc_D: Thermal resistance of a diode between
junction and case.
RTHch: Thermal resistance of thermal paste applied
between power module and heatsink.
RTHha: Thermal resistance of the heatsink in natural
convection.
PDIODE: Total diode losses.
PMOSFET: Total MOSFET losses.
PTOT: Total power module losses.
Tamb: Ambient temperature.
Tj: Junction temperature of a MOSFET.
Td: Junction temperature of a Diode.
Based on this result, a prototype was built. Power losses and
heatsink temperatures were measured to confirm the initial
hypotheses.
A. Preliminary design
Table 1 gives the electrical characteristics of the converter and
SiC devices to compute the losses. In order to maximize the
static losses, the parameters RDSON, ETS, V0 and RD have been
evaluated at 150°C junction temperature. The cooling system
is elaborated with an aluminium heatsink and operates at
natural air convection; its thermal resistance has been
evaluated at 0.35°C/W. Figure 7 depicts the total losses inside
the power semiconductor for the 5kVA three phase inverter
Submarine Institute of Australia Science, Technology & Engineering Conference 2015
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built with Si and SiC components. The results show that using
SiC MOSFET module instead of Si IGBT module leads to a
massive reduction of the losses from 137.4W to 63.6W. Also,
when the MOSFET conduct reverse current, the total losses
are reduce and a 98.4% efficiency converter can be achieve.
Figure 8 show the estimated heatsink and junction temperature
for the converter. The use of SiC devices leads to a reduction
of the junction temperatures from 112°C to 86°C for a diode
and from 113 to 89°C.
Figure 8. Theoretical temperature in steady state
TABLE I.
ELECTRICAL CHARACTERISTIC FOR THE 5KVA SIC
CONVERTER USING CCS050M12CM2
Value
Unit
Input voltage, VDC
System characteristics
450
VDC
Ouput voltage, UOUT
180
VRMS
Index modulation, M
0.653
Ouput current, I
16
Power Factor, PF = cos (φ)
0.8
Switching frequency, FSW
12
kHz
Ambient temperature, Tamb
60
°C
Heatsink thermal resistance, RTHha in
free air convection
0.35
°C/W
TIM thermal resistance, RTHch
0.02
°C/W
Value
Unit
On state resistance, RDSON
63
mΩ
Switching Energy, ETS at 600V/50A
1.7
mJ
MOSFET Thermal resistance, RTHjc_M
0.4
°C/W
Threshold voltage, V0
0.7
V
Serie resistance, RD
26
mΩ
Total capacitive charge, QC at 600V
280
nC
Diode thermal resistance, RTHjc_M
0.37
°C/W
Device characteristics (150°C)
ARMS
B. Pratical mesurements
To validate the analytical study, a 5kVA three phase inverter
has been elaborated (figure 9). A gate driver is used to bias the
SiC MOSFET. It provides a 20V/-5V as VGS to turn-on and
turn-off the transistor. The control unit provides
complementary gate signals to the driver and enables the
reverse capability of the SiC MOSFET. A temperature sensor
(CTN – 10kΩ) is placed close to the power module and
monitors the heatsink temperature. Converter losses are
measured through a KinetiQ wattmeter referred as PPA2530.
The three phase charge is composed of a RL load. Figure 10
shows the schematic of the set up for testing the 5kVA
converter. The converter is tested at 12 kHz switching
frequency, 60°C ambient temperature inside a thermal
conditioner. Under same conditions a 24 kHz is tested to
demonstrate the high frequency potentials of the SiC devices.
Fig. 11 shows a comparison between the theoretical losses and
the measurements for each switching frequency. For a 12 kHz,
a difference of 6W is observed on the total losses between
theoretical analyse and practical measurements. When the
switching frequency is increased from 12 to 24 kHz, the
difference is reduced from 6 to 4W. Therefore it can be
considered that the theoretical model fits the measured losses.
Fig. 12 details a comparison of the calculated and measured
heatsink temperature of the prototype operating at 12 and 24
kHz. According to the thermal measurement the heatsink
temperature remains lower than the expected value for each
switching frequency.
Figure 7. Distribution of the calculated losses at 60°C
Submarine Institute of Australia Science, Technology & Engineering Conference 2015
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Figure 12. Comparison between measured and theoretical heatsink
temperature for a 5kVA SiC converter operating at 12 and 24 kHz
V.
Figure 9. 5kVA prototype made by ECAGROUP
Figure 10. Schematic of the set up
R&D IN ECAGROUP
Currently, a SiC 60kVA converter is being manufactured in
ECAGROUP based on the loss relations developed in section
III. This converter is designed to complement the actual Si
60kVA converter embedded in a submarine. Due to the
superior performance of SiC devices, switching is increased
from 10 to 24 kHz in order to evaluate the benefit on the
acoustic discretion. Complementary qualifications will be
performed to provide converters for submarine applications.
Table 2 presents the electrical characteristics of the SiC
60kVA converter. Among the SiC modules on the market, a
1200V/300A half- bridge module has been selected. To reduce
static losses, two modules per phase are put in parallel. Fig. 13
details a calculated loss comparison between Si and SiC
converters. Despite an increase in switching frequency, a
reduction of 50% on total loss is possible and a 97.1%
efficiency is expected.
TABLE II.
ELECTRICAL CHARACTERISTIC OF THE 60KVA CONVERTER
System characteristics
Figure 11. Comparison between measured and theoretical losses for a 5kVA
SiC converter operating at 12 and 24 kHz
Value
Unit
Input voltage, VDC
504
VDC
Ouput voltage, UOUT
140
VRMS
Index modulation, M
0.454
Ouput current, IOUT
250
Power Factor, PF = cos (φ)
0.8
Switching frequency, FSW
24
kHz
Ambient temperature, Tamb
60
°C
Submarine Institute of Australia Science, Technology & Engineering Conference 2015
ARMS
6
[8]
[9]
[10]
[11]
Figure 13. Theoretical losses inside a 60kVA converter using Si-IGBT and
SiC MOSFET
VI.
CONCLUSION
This paper details a methodology to design SiC converters for
power electronics applications. Analytical tools are developed
based on the behavior of SiC device to determine the losses
and junction temperature for a three phase inverter. A 5kVA
prototype has been elaborated to validate the theoretical
model. A good agreement is demonstrated. According to this
study, a 60kVA SiC converter has been designed for
submarine application. This converter is currently in the
manufacturing process.
[12]
[13]
[14]
CREE, commercial brochure, “CREE power products”, available online:
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ower_Product_Brochure.pdf, 2015
ROHM, commercial brochure, “SiC Power Devices”, available online:
http://rohmfs.rohm.com/en/products/databook/catalog/common/catalog_
SiC_power_device-e.pdf, 2015
Microsemi, commercial brochure, “Silicon Carbide semiconductor
products”,available
online:
http://rohmfs.rohm.com/en/products/databook/catalog/common/catalog_
SiC_power_device-e.pdf, 2015
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Submarine Institute of Australia Science, Technology & Engineering Conference 2015
Xavier Fonteneau was born in 1985. He graduated
from a master’s degree in Electrical Engineering at
Polytech’Nantes, France in 2011. In 2014, he passed a
Ph.D degree in Electrical Engineering at Ampere Lab
INSA Lyon, France. In 2014, he joined ECAGROUP,
Couëron, France as a member of the Power Electronics
team. He is currently working on the Silicon Carbide
technology.
Loïc Pinson was born in 1976. He graduated in
Electronics in 1999 and in Business development in
2001. Loïc Pinson has started to contribute to the export
growth of European companies in 2003, in maritime
and naval business. He joined ECAGROUP, Couëron,
France as Export Sales Engineer, in 2011.