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High Voltage, Step-Down Converter Design using
20-kV Silicon Carbide IGBTs
Miguel Hinojosa and Aderinto Ogunniyi
Army Research Laboratory, Adelphi, MD 20783, USA
Abstract — Single-die, silicon carbide (SiC) Insulated-Gate
Bipolar Transistors (IGBTs) fabricated on wide epitaxial drift
regions have been developed reaching avalanche breakdown
voltages greater than 24 kV and rated currents up to 20 A. This
work utilizes similar research-grade, SiC IGBT devices to
investigate their continuous switching capabilities, by integrating
them in a high voltage, non-isolated, step-down DC-DC converter
circuit. The details on the design and construction of the buck
converter test-bed and its components are covered in this
document, as well as initial results on the device capabilities. The
goal of our work was to explore the device capabilities beyond
previously-reported levels with a focus on frequency and
operating voltage. The SiC-based step-down converter was
successfully operated at 10 kHz and 10 kV for a test period of 60
min. While the results found here are not conclusive, the
demonstration of the system shows that SiC technology for power
conversion systems is very promising.
Keywords— UHV IGBT, Buck Converter, Silicon Carbide (SiC),
High voltage solid state switch, 20kV 4H-SiC, insulated-gate bipolar
transistor, pulsed testing.
capabilities beyond previously-reported levels with a focus on
frequency, operating voltage, and duty cycles.
II. BUCK TOPOLOGY
The step-down (buck) converter is one of the basic building
blocks in power electronics and it is commonly used in switchmode DC-DC converter designs. This topology is easily scalable
to high voltages, as presented in Fig.1. The buck converter's
transfer function, which can be written in its simples form as
𝑉𝑜𝑢𝑡
𝑡𝑜𝑛
=
=𝐷
𝑉𝑖𝑛
𝑡𝑝𝑒𝑟𝑖𝑜𝑑
(1)
depends on the on-time, ton, of the switch (U1) and its period
(tperiod), or as a function of duty cycle (D), when the converter is
operating in continuous-conduction mode (CCM) [5]. In CCM
operation, the current through the inductor L1 flows
continuously and it does not fall down to zero.
I. INTRODUCTION
Silicon carbide (SiC) Insulated-Gate Bipolar Transistors
(IGBTs) fabricated on wide epitaxial drift regions are desirable
for pulsed-power and continuous power conversion systems due
to their high avalanche breakdown voltages, as well as their
ability to switch currents at fast repetition rates. For mobile
platforms, the use of SiC-based electronics is advantageous as it
allows for a significant reduction of space in comparison to
silicon and gas switches, in addition to faster switching speeds,
continuous/burst mode of operation, and advanced triggering
capabilities.
This work reports on the design and construction of a stepdown converter test-bed utilized to evaluate the continuous
high-frequency operation of newly-fabricated, research-grade,
20-kV 4H-SiC IGBTs. These devices, manufactured by
Wolfspeed Inc. under the U.S. Army High Voltage Power
Technology Program (HVPT), have wide epitaxial drift regions
greater than 200 µm, reach avalanche breakdown voltages
greater than 24 kV, and have continuous current ratings up to 20
A. This paper expands on previous work reported for similar
devices [1-2], where resistive and inductive switching were
performed, and from [3] where they were used in a burst-mode
solid-state Marx generator. In [4], the previous generation of
4H-SiC IGBTs with epitaxial regions of 140 µm to 160 µm were
switched in a buck-boost converter at 8 kV with an operating
frequency of 5 kHz. The goal of our work is to explore the device
Isolated gate driv er
R3
Rsupply
Dsupply
1
L1
2
2
U1
HVcharger
20kVIGBT
D2
Rload
C_bank
Cout
15kVJBS
1
0
Fig. 1. Non-isolated step-down buck converter circuit.
The second mode of operation of the buck converter is
discontinuous-conduction mode (DCM), where the inductor
current flattens upon reaching zero. In this case, the output
becomes a function of duty cycle, inductance (L1), the load
resistance (Rload), and the switching period (Tsw). The transfer
function for this mode was defined by Pressman [6] as
𝑉𝑜𝑢𝑡
=
𝑉𝑖𝑛
2 ∙ 𝐷
𝐷 + √𝐷2 +
8 ∙ 𝐿1
𝑅𝑙𝑜𝑎𝑑 ∙ 𝑇𝑠𝑤
(2)


III. CONVERTER COMPONENTS
A. Power source group
The buck converter circuit shown in Fig. 1 was implemented
in the laboratory utilizing an adjustable 15-kV, 8-kJ/s, 8-kW
TDK-Lambda capacitor charger (HVcharger) that draws its power
from the 480 VAC 3-phase power outlet. In addition to the
manufacturer's internal protection circuits, the power supply is
safeguarded from reflections and unexpected events with two
series-connected, 10-kV silicon puck diodes (Dsupply) and a
low-inductance ceramic resistor (Rsupply). A capacitor bank
(C_bank) follows the protection components and it is comprised
of four, series-connected 256-µF, 10-kV film capacitors with
respective balancing resistors. The bank's equivalent
capacitance is 64 µF, which maintains the high voltage rail
constant during operation.
B. Silicon Carbide IGBTs and JBS diodes
The devices-under-test are the 20-kV SiC IGBTs labeled U1
in Fig. 1. The 4H-SiC IGBTs utilized in this experiment were
vertical power devices with a P-type, 300-µm substrate and an
epitaxially-grown N-type drift region with a thickness of 230
µm. The devices have a double-diffused MOS structure, an
asymmetrical punch-through design, and a field-stop buffer of
approximately 2 µm. Their nominal breakdown voltage was 2224 kV at a leakage current of 10-14 µA. Their active area was
0.28 cm2 and a chip area of 0.81 cm2. Fig. 2 shows the forwardconduction and forward-blocking characteristics of one IGBT.
(Additional device fabrication details can be found in [1]). Fig.
3(a) shows the 9-mm by 9-mm chip die, where the backside
region serves as the die attach to a gold-plated, copper collector
terminal. The topside interface consists of four emitter fingers
and one gate pad, and these pads are connected to their
respective terminals using two 10-mil wire-bonds per pad. Fig.
3(b) shows the SiC IGBT on a high-temperature
polyethertherketone (PEEK) package and it is mounted on an
actively cooled aluminum heat sink. The heat sink circulates
Castrol 399 gas turbine oil, which is pumped by a Mydax
1H100W water-cooled heat exchanger, at a flow rate of 1.6
gpm.
(a)
(b)
Fig. 3. (a) IGBT die and dimensions. (b) IGBT package mounted on an oilfilled, actively-cooled aluminum heat sink.
Silicon carbide Junction Barrier Schottky (JBS) diodes were
utilized in the step-down converter to protect the IGBT from
the inductor's stored energy. These SiC rectifiers, which have
forward-conduction and blocking characteristics shown in Fig.
4, have a chip area of 1 cm2 and are rated for 15 kV and 17 A.
In the 10-mm by 10-mm JBS diode die, the cathode terminal is
located on the backside of the chip and it attaches to a goldplated, copper plate via SnPb solder. The top side of the die
attaches to the anode terminal using multiple strands of 10-mil
bond wires. Similar to the SiC IGBTs, the JBS diodes are
packaged in PEEK and cooled with an aluminum heat sink and
dielectric fluid.
(a)
(b)
Fig. 4. (a) JBS diode conduction I-V plots. (b) JBS diode blocking I-V plot.
(a)
(b)
Fig. 2. (a) IGBT Forward-conduction I-V plots. (b) Blocking I-V plot.
C. Floating gate driver
The SiC IGBT in the buck converter is conFig.d as a high
side switch, which requires a floating gate driver and an isolated
power supply. The gate driver board seen in Fig. 2(b), is based
on the IXYS IXDD430 gate driver chip, it is capable driving
the gate voltage up to 30 V, and it is optically triggered. The
gate board is powered by a custom multi-stage, isolated power
supply that employs an open-loop forward converter and two
regulated buck converters. The gate board was conFig.d to
operate at a voltage range from +20 V to -6 V, based on the
IGBTs forward-conduction I-V curves. Fig.5 shows the block
diagram for the isolated power supply that feeds the gate driver
board.
TABLE 1.
+20 V
(Adjustable)
Buck
Stage
-6 V
(Adjustable)
Fig. 5. Isolated power supply used for gate driver board.
D. Inductor
Designing the inductor was a challenging task due to the
switching times (frequency up 10 kHz, but pulse widths as low
as 2-10 µs), the intended high operating voltage (~10 kV), and
the initial, low operating current levels (1-10 A range). The
design parameters included a target inductance which had to be
tens to hundreds of mH, an acceptable saturation current, no
winding-to-winding and winding-to-core dielectric breakdown,
and acceptable thermal management. The inductor was
designed using AMCC1000 cores due to their high saturation
flux density, high permeability, window size, and availability.
These cores are manufactured with iron-based METGLASS
2605 SA1 amorphous alloys. The current inductor, seen in the
CAD drawing in Fig. 6(a) uses four, C-shaped cores held
together with steel banding, a 3D-printed polycarbonate
bobbin, and 90 turns of 16 AWG magnet wire enclosed in a 15kV sleeve. DC Hi-pot tests indicated isolation between the core
and the windings to be greater than 30 kV. Dynamic tests were
also conducted, such as the one illustrated in Fig. 6(b) during a
5-kV, 10-kHz test, to verify the transient electrical
characteristics and ensure the core received proper ventilation
and would not overheat.
(a)
C-Cores
Gap
Sleeved
Magnet wire
windings
(b)
Gap
Steel
banding
INDUCTOR PROPERTIES WITH 20 MIL AND 40 MIL GAPS
Properties
n turns
gap
Ls @ 10 kHz
Rs @ 10 kHz
Ls @ 20 kHz
Rs @ 20 kHz
I_sat
Com
Buck
Stage
Forward
Stage
Input 36 V
40-kV
Isolation
Design 1
90
20 mil
46.1 mH
944 Ω
45.3 mH
289 Ω
14 A
Design2
90
40 mil
28.4 mH
37.8 Ω
28.4 mH
392 Ω
25 A
E. Output capacitor and load
The output capacitor is part of the LC filter needed to obtain
a constant voltage level in the output. The output capacitor used
was a 20-kV film capacitor with a measured capacitance of 7.5
µF and an equivalent series resistance of 18 mΩ. This value was
selected based on initial calculations to obtain an output ripple
lower than 100 V, and because its availability in the laboratory.
A reconfigurable resistive load was designed to test the buck
converter using non-inductive bulk ceramic resistors. Each
resistive element is 250 Ω and rated for 22 kV, with peak energy
of 17.5 kJ and a maximum operating temperature of 350 °C.
The test fixture contains 10 slots, in which the units can be
connected in series or parallel configurations. Fig.7 (a) shows
an image of the resistive load prior to its installation. A
circulating fan with an air flow of 550 CFM was installed on
top of the unit to pull air out of the mid-section (not shown in
Fig. 7a). Fig.7 (b) shows a thermal image of the load while
operating the buck converter at 10 kV and 10% duty cycle for
1.5 hr.
(a)
(b)
13 in.
25 in.
Bobbin
Fig. 6. (a) Inductor design using 4 AMCC1000 C-cores. (b) Inductor
temperature during 5-kV tests with 50% duty cycle.
The inductor saturation current (I_sat) was determined using
a pulsed clamped inductive circuit, where the voltage and pulse
width were varied to find the current level where the current
began to increase exponentially. During these experiments,
sheets of 10-mil Mylar were inserted between the gaps to
increase the saturation current, while decreasing the inductance.
Table 1 shows a summary of the results of these tests.
Fig. 7. (a) Reconfigurable resistive load with 10 elements in place (2.5 kΩ).
(b) Load temperature during 10-kV test at 10% duty cycle.
IV. DEVICE EVALUATION
Isolated supply
JBS
diode
Load
Inductor
SiC
IGBT
Capacitor
Bank
Fig. 8. Image of high-voltage, step-down converter system.
A small number of SiC IGBTs were evaluated in the
laboratory with the buck converter test bed shown in Fig. 8. As
mentioned previously, the goal of this exercise was to
demonstrate the SiC IGBT's performance while switching
continuously at high voltage and high frequency. The test
procedure consisted of an iterative sequence of steps where the
lowest operating voltage and switching frequency were selected
and the operating current and duty cycle were slowly increased.
These tests were then repeated with a higher voltage, and then
eventually with higher switching frequencies. Due to the limited
number of devices, the tests would vary in time from 10 min and
up to 1-1.5 hr as the extreme cases. The targeted maximum
voltage was 10 kV and the maximum operating frequency was
10 kHz. The limits were based on previous simulations and
experimental double-pulse clamped-inductive tests.
The power stage of the DC-DC converter was successfully
operated in an open-loop configuration as seen in the snippet of
the input and output voltage waveforms versus time in Fig. 9.
The output voltage follows the transfer function given in
equation (2), for discontinuous-conduction mode. For an input
of 10.5 kV, for example, the measured output was 1.98 kV, at
an operating frequency of 10 kHz, 10% duty cycle, an inductor
value of 28.4 mH, and a load resistance of 2500 Ω. In this case,
the output power was 1.5 kW with a voltage ripple of
approximately 30 V using the 7.5 µF output capacitor.
Fig.10 illustrates the collector-to-emitter voltage (Vce) and
the collector current (Ic) as they were driven with a gate-toemitter voltage swing from -6 V to +20 V. The voltage drop
across the IGBT was calculated by measuring at two locations
with PVM-12 Northstar high voltage probes, and then finding
the difference between the two measurements. The collector
current was measured using an insulated Pearson current
transformer. Fig. 10 shows a decaying voltage as the IGBT
cycles on and off. The early drop in Vce is attributed to the
DCM nature of the converter and it begins to fall when the
inductor current approaches zero. This behavior is expected for
DCM because the output capacitor must support the load for
that short period of time. Fig. 10 also shows the current
amplitude of Ic as a function of time when the IGBT cycles in
the converter. The current waveforms indicate that the capacitor
is charging steadily at about 3.5 A when the IGBT is on. The
instantaneous peak current of 22.5 A is caused by the Miller
capacitance of the IGBT, and it can be dampened by utilizing
different resistors for turn-on and turn-off. This feature will be
implemented in future test evaluations. For this 10-kHz case
with a supply voltage of 10.5 kV, the turn-on losses in the IGBT
were 1.47 mJ and the turn-off losses were 10.6 mJ. Fig. 11
shows the inductor current (I_L) and its respective power losses
as a function of time. These waveforms confirm the DCM
operation as the inductor current falls below zero.
Fig. 10. Collector-to-emitter voltage (Vce) and collector current (Ic) transient
characteristics. The supply voltage was 10 kV at an operating frequency of 10
kHz and 10% duty cycle.
Fig. 11. Inductor current (I_L) and inductor loss as a function of time.
Fig. 9. Snippet of the input and output voltage waveforms as a function of
time. The supply voltage was 10 kV at an operating frequency of 10 kHz and
10% duty cycle.
Table 2 highlights the salient results of the buck converter
with a high operating frequency of 10 kHz. The input voltages
were increased from 5 kV up to 10 kV with testing times
ranging from 10 min up to 1.5 hrs. At the lower voltages, the
duty cycles were increased up to 50% and at the higher voltages
the duty cycles were kept at a conservative value of 10%.
successfully at high voltages with low duty cycles, which is
very encouraging for devices with thick epitaxial regions
greater than 200 µm. In order to proceed further with higher
TABLE 2. RESULTS SUMMARY AT 10 KHZ
voltage, duty cycle, and current levels, a series of tools need to
be incorporated to prevent device destruction. First, thermal
Vin Test Time Dty Vout P_load I_LMAX E_turn-on E_turn-off E_cond
(kV)
(min)
(%)
(kV)
(kW)
(A)
(mJ)
(mJ)
(mJ) models and simulations need to be included to predict structure
heating, packaging, and heatsink interaction. Continuing work
5.1
90
50
3.1
3.84
1.03
0.12
2.5
includes the development of Silvaco, ANSYS, and PSPICE
6.1
10
25
2.4
2.30
2.81
0.14
5.3
models to better understand the heating and cooling
8.5
10
10
1.58
1.00
1.7
0.83
5.3
mechanisms. Second, a correlation must be made between the
junction temperature and the saturation on-state voltage (Vsat),
9.6
15
10
1.78
1.27
1.93
0.91
7.32
to determine when the device reaches its peak operating
10.5
60
10
1.96
1.54
2.13
1.74
10.6
temperature. Future evaluations will include feedback from the
Vsat probe to the switching controller, which can stop the gate
signal in case of device overheating and degradation. In
Operation in discontinuous-conduction mode (DCM) was not
addition to developing thermal simulations and adding
a limitation in these tests because the output current levels were
feedback to the IGBT controller, future evaluations will also
relatively low. At the moment, the 7.5 µF output capacitor
require the design and fabrication of liquid-cooled testing loads
collects enough charge to support the 2500 Ω load while the
and high power inductors.
inductor is off, but this might become an issue in future tests
requiring higher output power. As more current is drawn by the
load, the capacitor discharges at a higher rate, which means the
ACKNOWLEDGMENTS
output capacitance will have to be increased or the output
The authors would like to thank Dr. Sei-Hyung Ryu and Dr.
voltage ripple will increase. The operation of the buck converter
Edward Van Brunt of Wolfspeed Inc., a Cree Company, for
in continuous-conduction mode (CCM) is possible, for example
fabricating the devices and for their technical support. Special
in the 10-kHz, 10-kV operation case, if the load resistance is
thanks to Mr. Oladimeji Ibitayo of ARL for his help with the
reduced to 250 Ω. At this point, both the SiC IGBT and the load
inductor development.
would be operating at a significant higher power levels which
could be detrimental to the components. These conditions will
be considered in future tests once thermal model and
REFERENCES
simulations are incorporated with the device evaluations.
E.V. Brunt, L. Cheng, M. J. O’Loughlin, J. Richmond, V. Pala, J.
Palmour, C. Tipton, and C. Scozzie, “ 27 kV, 20 A 4H-SiC n-IGBTs”.
Silicon Carbide and Related Materials. Materials Science Forum. 2014
[2] M. Hinojosa, A. Ogunniyi, S. Bayne, and C. Scozzie. “Evaluation of High
Voltage, High Power 4H-SIC Insulated-Gate Bipolar Transistors”, IEEE
International Power Modulator and High Voltage Conference. Santa Fe,
NM. 2014.
[3] M. Hinojosa, H. O'Brien, E. Van Brunt, A. Ogunniyi and C. Scozzie,
"Solid-state Marx generator with 24 KV 4H-SIC IGBTs," 2015 IEEE
Pulsed Power Conference (PPC), Austin, TX, 2015, pp. 1-5.
[4] A. Kadavelugu and S. Bhattacharya, "Design considerations and
development of gate driver for 15 kV SiC IGBT," 2014 IEEE Applied
Power Electronics Conference and Exposition - APEC 2014, Fort Worth,
TX, 2014, pp. 1494-1501.
[5] N. Mohan, T.M. Undeland, & W.P. Robbins, Power electronics-Converters, Applications and Design (Hoboken, New Jersey: John Wiley
& Sons Inc., 2003).
[6] A. Pressman, Switching Power Supply Design, McGraw-Hill, Inc., New
York, NY, 1997.
[1]
Gate Driver
Board
SiC IGBT
Package
Gate
Collector
Fig. 12. Thermal image of the SiC IGBT during operation with a supply
voltage of 10 kV and an operating frequency of 10 kHz and 10% duty cycle.
V. CONCLUSION AND FUTURE WORK
A step-down buck converter was designed and constructed to
evaluate state-of-the-art SiC IGBTs at high voltage, high
frequency and low duty cycles. The converter's continuous
switching performance was demonstrated up to 10 kV and 10
kHz, which goes beyond the highest limit published in
literature. Results show that the SiC IGBTs operated