Cree - Performance Evaluations of Hard-Switching Interleaved DC/DC Boost Converter with New Generation Silicon Carbide MOSFETs

Cree - Performance Evaluations of Hard-Switching Interleaved DC/DC Boost Converter with New Generation Silicon Carbide MOSFETs
Performance Evaluations of Hard-Switching
Interleaved DC/DC Boost Converter with New
Generation Silicon Carbide MOSFETs
Jimmy Liu, Kin Lap Wong
Scott Allen, John Mookken
Cree Inc
SiC Application Engineering
Hong Kong, China
[email protected]
Cree Inc
SiC Application Engineering
Durham, NC, USA
Abstract— The emergence of PV inverter and Electric
Vehicles (EVs) has created an increased demand for high power
densities and high efficiency in power converters. Silicon carbide
(SiC) is the candidate of choice to meet this demand, and it has,
therefore, been the object of a growing interest over the past
decade. The Boost converter is an essential part in most PV
inverters and EVs. This paper presents a new generation of
1200V 20A SiC true MOSFET used in a 10KW hard-switching
interleaved Boost converter with high switching frequency up to
100KHZ. It compares thermal and efficiency with Silicon high
speed H3 IGBT. In both cases, results show a clear advantage for
this new generation SiC MOSFET.
Keywords—Silicon Cardbide; MOSFET; Interleaved; Hard
Switching; Boost converter; IGBT
I.
INTRODUCTION
Power converters made with Silicon Carbide (SiC) devices
offer the promise of a higher power density due to its higher
blocking voltage, lower on state resistance and higher thermal
conductivity when compared to their silicon counterparts. Of
the available types of SiC devices, comparing SiC JFET or SiC
transistors, the N-channel enhancement mode SiC MOSFET
offers the most compatibility for a drop in replacement of
conventional Si MOSFET or Si IGBT due to its simple
structure, ease of a design in and low drive losses. Cree Inc.
commercially released the next generation SiC MOSFET
C2M0080120D in March 2013, which has superior parameters
over first generation SiC MOSFET.
99.3% and a lower total BOM cost. It compares the switching
performance, efficiency and thermal performance between this
new SiC MOSFET and Si high speed H3 IGBT. The
experimental results demonstrate that full Cree SiC MOSFETs
with SiC schottky diodes allow increased system frequency
while still improving the efficiency and lowering overall
system cost.
II.
FULL SIC INTERLEAVED BOOST CONVERTER DESIGN
To design this full SiC interleaved Boost converter with
100KHZ operating frequency, the SiC MOSFET and Boost
inductor should be further studied.
Figure 1 gives the circuit block diagram for this full SiC
based interleaved boost converter. In this configuration, each
channel of the interleaved converter’s architecture includes one
SiC 1200V/20A 80mohm MOSFET (C2M0080120D) and one
SiC 1200V/10A schottky diode (C4D10120D) to achieve the
10KW Boost function. Due to all SiC power devices, the
converter can operates at high frequency to achieve high power
density; also, the full SiC converter does not have additional
circuit with soft-switching, such as ZVS, to get high efficiency
and it is just an interleaved topology design with fewer
components, which is a great breakthrough for power
electronics industry.
Solar Panel
D
G
S
Nowadays, PV inverter and EV applications are the
applications where the characteristics of SiC are especially
attractive as high power density with high frequency is
essential for lower overall cost and weight, and also to reduce
cooling requirements. However, there is no clear demonstration
on how much performance improvements can be achieved by
using SiC MOSFET for a hard-switching DC/DC converter
replacing other complicated soft-switching DC/DC converter,
and how much cost it can reduce in the system bill of materials
(BOM) with high frequency and high power density. In this
literature, a 10KW hard-switching interleaved DC/DC
converter is developed with high frequency up to 100KHZ with
full SiC power devices, operating at its highest efficiency of
D
G
S
Fig. 1. 100KHZ interleaved 10KW Boost converter with full SiC
A. New generation SiC MOSFET parameters
The below table compared the key parameters between this
second generation SiC MOSFET C2M0080120D and first
generation SiC MOSFET CMF20120D. From the comparison,
the new generation C2M0080120D has low capacitance with
less switching losses. Meanwhile, its on-state resistance has
better positive coefficient with junction temperature which
allows the new SiC MOSFET to get better thermal dissipation
with multiple devices in parallel. From any comparison, the
new SiC MOSFET will allow the system with high frequency
to achieve high power density and high efficiency.
TABLE I.
SIC MOSFET PARAMETERS’ COMPARISON
SiC MOSFET
Parameters
Typ. On Resistance (Tj = 25 °C)
CMF20120D
C2M0080120D
80 mΩ
80 mΩ
95mΩ
123 mΩ
Typ. On Resistance (Tj = 125 °C)
Die size
-5V/25V
-10V/25V
0.78 mJ
0.56 mJ
Gate Charge
91 nC
49 nC
Input Capacitance, Ciss
1915 pF
950 pF
Output Capacitance, Coss
120 pF
80 pF
Reverse Transfer Capacitance, Crss
13 pF
6.5 pF
Thermal resistance, Juntion to case
0.5ºC/W
0.6ºC/W
B. High frequency Boost inductor design consideration
Due to the desire to have high switching frequency
(100KHZ) with hard-switching, the inductor design is very
important to reduce cost and improve power density. High
speed IGBT switching losses limit the conventional silicon
systems to a maximum 20KHz-40KHz. However, SiC
MOSFETs enable increased frequency up to 100KHz without
sacrificing system efficiency. At 100KHz effective frequency
with SiC MOSFET, the inductance of Boost inductor is
reduced according to equation (1), thus the inductor size,
weight and cost are reduced significantly while maintaining
overall efficiency superior to the IGBT’s 20kHz performance.
Table 2 gives parameters of a 5KW inductor with Si IGBT at
20KHz and SiC MOSFET at 100KHz. Two inductors are used
for the 10KW design.
Vin min⋅ D max
fs ⋅ Δi
5KW INDUCTOR PARAMETERS AT 20KHZ AND 100KHZ
Solutions
Frequency
Si IGBT
20KHz
SiC MOSFET
100KHz
Inductor (uH) @ rated Current 1100
400
Core Material
Fe-Si
Fe-Si-Al
Coil Type
AWG8*1*98Ts AWG12*1*55Ts
Size (mm)
140x108x68
OD:63 x HT:26
Weight (Kg)
2.3
0.4
DCR (mohm)
22
25
Coil Losses (W)
6.1
7.5
Core Losses (W)
13.0
15.8
Reference Price( USD)
31
12
-35%
Max gate voltage (VGS)
Switching Loss
(Tj = 150°C, VDS = 800V)
L=
TABLE II.
(1)
Fig. 2. 5KW Inductors at 20KHz and 100KHz frequency
C. EMI design consideration
The EMI design should be given more attention with high
frequency full SiC power devices. In this design, some
practical approaches were used to limit the influence of noise
when switching frequency is high.
• With high switching frequency and fast switching times
of SiC MOSFETs, drain voltage ringing is potentially
much higher due to parasitic oscillation, especially due
to parasitic capacitance of the inductor. When the
circuit switches are turn-on and turn-off, there is high
frequency resonance between the parasitic capacitance
of inductor and stray inductance in the switching power
loop, which will lead to excessive ringing. In order to
reduce the ringing at high frequency, a single layer
winding design for the inductor is highly recommended.
Figure 3 shows the parasitic capacitance difference
when using two layer winding versus single layer
winding. A single layer winding can dramatically
reduce the parasitic capacitance of the inductor with
good flux coupling. The result is reduced ringing within
the Vds switching node thus minimizing the electrical
noise from the ringing.
Fig. 3. Two layers (a) and single Layer (b) winding’s inductor
• Another important consideration is minimizing the
switching loop for the PCB layout. Figure 4 gives the
key switching loops of the boost converter. Loop 1 and
loop 3 are the main switching power loops with high
dv/dt. Minimizing the layout loops will help to
minimize the stray inductance in these loops, thus
reduce the ringing on the switching node. In this design,
the inductor is placed under PCB board and can closely
connect to SiC MOSFET and output diode to minimize
the power loop 1 and loop 3. Since SiC MOSFET is a
fast switching device, the loop 2 for gate drive is also
critical for PCB layout. Kelvin gate connection with
separate source return is highly recommended. The gate
driver daughter boards are located near the heatsink of
MOSFET so that the gate drive signal can directly drive
SiC MOSFET as close as possible. Also, the ground of
gate driver daughter is independently connected to the
source of SiC MOSFET. The SiC MOSFET has low
transconductance when compared with silicon switches.
Due to this characteristic, the turn-on and turn-off times
and the switching losses of the SiC MOSFET are
closely coupled to the transition time of the gate
voltage. Driving the gate harder, by reducing the
external gate resistance will directly result in lower
switching losses and increased efficiency. An external
gate resistor can be used as damping resistor to
minimize the influence of fast rise/fall time on the gate.
However, there is a trade-off for the external gate
resistor between EMI performance and efficiency.
Lower gate resistor can help to improve the efficiency
but reduce the damping effects for the ringing of gate
signal.
• Minimize parasitic gate-drain board capacitance.
Particularly care must be taken on the coupling
capacitances between gate and drain traces on the PCB.
As fast switching MOSFETs are capable of reaching
extremely high dv/dt values any coupling of the voltage
rise at the drain into the gate circuit may disturb proper
device control via the gate electrode. As the SiC
MOSFET reaches extremely low values of the internal
miller Cgd capacitance (Crss in datasheet), The design
keeps layout coupling capacitances below the internal
capacitance of the device to exert full device control via
the gate circuit. As shown in Figure 5, the drain pin and
gate pin are separately located at top layer and bottom
layer without parallel trace between them, and it can
avoid high parasitic capacitance between drain and
source.
Fig. 5. PCB layout guide line of SiC MOSFET with fast switching
• Interleaving operation for two channel Boost converter.
Figure 6 shows the Differential Mode (DM) noise
difference between two phases with interleaving
operation and single phase with non-interleaving
operation. Due to interleaving operation, the first order
DM noise will occur at 2fs (two times of switching
frequency) and the input/output ripples can be
cancellation. Thus, the EMI filter frequency will be
higher for interleaving operation, which means less
attenuation is required for EMI filter and smaller EMI
filter can be used to meet the standard.
Fig. 6. DM noise difference with 1 phase non-interleaved and 2 phase
interleaved
Fig. 4. High switching layout guide line with stray parameters for a Boost
converter
III.
EXPERIMENTIAL RESULTS
To verify the second generation 1200V/20A SiC MOSFET
characteristic, a 10KW hard-switching interleaved Boost
DC/DC converter is developed as shown in Figure 7. A silicon
best high speed 1200V/40A IGBT IGW40N120H3 is also
evaluated to compare the performance with 1200V/20A SiC
MOSFET. The PCB board size is 240mm x 140mm x 90mm.
Controller is TI interleaved PWM control UCC28220 and gate
drive IC is IXYS IXDN609.
TABLE III.
100 KHz SiC solution still matched the 20KHz performance of
the silicon system, which confirms that SiC MOSFET has very
low switching losses. By any comparison, both efficiency and a
frequency advantage can be gained by moving silicon IGBT
based power converter designs to SiC.
KEY PARAMETERS OF THE DESIGN
Items
Parameters
Input Voltage
450Vdc
Output voltage
650Vdc
Rated output power
10KW
Frequency
100KHZ for SiC MOSFET
20KHZ for Si IGBT
PCB Board Size
240mm x 140mm x 90mm
Inductor size
OD:63mm x HT:26mm for 100KHZ
140mm x108mm x68mm for 20KHZ
Fig. 8. 10KW efficiency comparisons at different frequencies with Gen1 &
Gen 2 SiC MOSFET and Si IGBT
B. Eon and Eoff
Lower switching losses are the key benefits for SiC
MOSFET when operating frequency is high. Figure 9 and 10
are the turn-on waveforms for C2M0080120D and
IGW40N120H3, the Eon is 54.5µJ for SiC MOSFET and
115.1µJ for Si IGBT. Figure 11 and 12 are the turn-off
waveforms for C2M0080120D and IGW40N120H3, the Eoff is
83.3µJ for SiC MOSFET and 911.5µJ for Si IGBT, which is
about ten times Eoff compared to SiC MOSFET. From the
testing waveforms, the total switching losses for SiC MOSFET
at 100KHZ is about 13.8W, while the total switching losses for
IGBT at 20KHZ is about 20.5W, which is 7W higher than SiC
MOSFET. It shows that SiC MOSFET can have lower
switching losses than Si IGBT, even when the SiC MOSFET
operating frequency is five times of Si IGBT, especially for
turn-off; The Si IGBT has large losses of turn-off due to IGBT
current tailing issue, even though it is a high speed type Si
IGBT.
Fig. 7. Full SiC based 10KW Interleaved Boost converter
A. Efficiency
The below shows efficiency testing data with SiC
MOSFET at 100KHZ (first generation SiC MOSFET
CMF20120D and second generation SiC MOSFET
C2M0080120D) and Si IGBT (IGW40N120H3) at 20KHZ.
Output diodes used for both switch devices were Cree 1200V
SiC schottky diode C4D10120D assuring a fair comparison
and all data are based on the external gate resistor at 2Ω. From
the test results it is clear that even with five times the switching
frequency, the SiC solution was able to achieve a maximum
efficiency of 99.3% at 100KHz reducing losses by 18% from
the best efficiency of the IGBT solution at 20KHz. At light
loads, where the two designs exhibit the poorest efficiency, the
Fig. 9. C2M0080120D turn-on waveforms at100KHZ (time: 100ns/div)
lower losses and thus a 40⁰C lower operating case temperature
by more than 40% versus the Si IGBT, which means SiC
MOSFET can use lighter and thinner heat sink with low cost.
Also, it shows a large inductor size for 20KHZ Si IGBT
solution with low power density compared to SiC MOSFET at
100KHZ.
Fig. 10. IGW40N120H3 turn-on waveforms at 20KHZ (time: 200ns/div)
Fig. 13. Thermal performance at full load with C2M0080120D at 100KHZ
Fig. 11. C2M0080120D turn-off waveforms at100KHZ (time: 100ns/div)
Fig. 14. Thermal performance at full load with IGW40N120H3 at 20KHZ
IV.
Fig. 12. IGW40N120H3 turn-off waveforms at 20KHZ (time: 200ns/div)
C. Thermal
In Figure 13 and 14, thermal performance is compared
between the SiC MOSFET C2M0080120D and the silicon
IGBT IGW40N120H3 implementations. Test results are shown
with input voltage of 450Vdc and output voltage of 650Vdc
with 2x5KW full load. The ambient temperature was 25⁰C
without cooling system for heat sink and the board is tested
without an enclosure. Output diodes used for both switch
devices were Cree SiC schottky diode C4D20120D assuring a
fair comparison for both competitors. The SiC MOSFET had
SUMMARY
The 10kW hard-switching interleaved boost converter
design described in this article clearly demonstrates the
advantages of using SiC power MOSFETs and diodes in high
power systems. The benefit of using SiC’s inherent switching
efficiency is highlighted by the reduction of energy losses,
small system size, weight, lower bill-of-materials for the
system and impressive reduction in device operating
temperature. This full SiC converter design can open doors to
new energy applications, which in turn will result in more and
more SiC devices and packaging options making their way to
the commercial market. With the increase in availability and
options many of the design restrictions created by the
limitations of Si will make way to increased design flexibility
at the system level for the designers of high frequency power
conversion systems from hundreds of watts to hundreds of
kilowatts.
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