Design of Isolated Interleaved Boost DC-DC
Converter based on SiC power devices for
Microinverter Applications
Fahad M. Almasoudi, Khaled S. Alatawi and Mohammad Matin
Daniel Felix Ritchie School of Engineering and Computer Science
University of Denver
Denver, USA
Abstract— The power delivered by a photovoltaic (PV) to the inductors, transformers, heat sink and capacitance of energy
grid must be modified in multiple stages. The operation concept storage. SiC devices have thermal conductivity three times
of the micro-inverter is to increase the low output voltage larger than Si devices. Moreover, the electric breakdown fields
received from a PV panel. Wide bandgap (WBG) power devices of SiC is four times larger than Si [8]-[13]. Those
such as SiC MOSFETs offer multiple advantages over traditional characteristics of wide band gap material make them preferred
silicon power devices when are used in a solar energy field due to to be used in micro-inverter applications where the smaller
their high switching frequency, high thermal conductivity and size of converter and high temperature operation are required.
high power density that led to high efficiency and a smaller size
of a converter. This paper demonstrates the implementation and Boost DC-DC converters offer the optimal solution in a
design of an isolated interleaved boost DC-DC converter using a PV micro-inverter system where low current ripple and high
capacitor voltage doubler to achieve a high voltage step ratio. conversion ratio are achieved [14]-[19]. A high voltage gain
The performance of the design is analyzed and investigated at can be attained with a traditional non-isolated boost DC-DC
high switching frequency using different power switching converter, but the drawback of this kind of converter is that as
devices, Si and SiC MOSFETs. The simulation results show that the duty cycle move toward unity the gain decreases because
a 3% improvement achieved by SiC MOSFET at a higher of the parasitic elements in active and passive components.
switching frequency of 300 kHz. One possible solution is using isolated boost converter where
a high frequency transformer is used to afford isolation and
Keywords—micro-inverter; silicon carbide (SiC); gallium
nitride (GaN); switching characteristics.
supporting the operation of high voltage gain [20]-[22].
In this paper an isolated interleaved boost DC-DC
I. INTRODUCTION converter is designed to achieve high conversion ratio and to
Renewable energy, such as a solar energy field where PV fulfill the requirement of PV micro-inverter. Also, a
is used promises to be one of the most important renewable comparison is performed between Si and SiC power devices to
energy sources due to its emission free, clean and abundant investigate the performance of the converter with respect to
[1], [2]. Power electronics converters have an important role high switching frequency and efficiency.
in improving the performance of PV systems. Recently, there
are three main configurations of a grid-connected PV inverter, The paper is structured as follow. Section II presents the
such as string inverters, central inverters and micro-inverters. structure and operation modes of the system model. The
The micro-inverter configuration has the advantages of easy design parameters of the isolated interleaved boost DC-DC
installation, high efficiency and flexible system extension. converter are presented in section III. In section IV, the
According to these advantages, micro-inverter has become the switching performance and efficiency of SiC and Si
dominant configuration for low power requirement range from MOSFETs are investigated under simulation results. Finally,
200W to 300W for the application in the PV distributed power section V states the conclusion and future work.
generation used in the business district and the residential
area. II. STRUTURE AND ANALYSIS OF DC-DC CONVERTER
Wide bandgap semiconductor devices (SiC and GaN) have A. Structure and Operational Modes of DC-DC Converter
been examined widely in converter applications [3]-[7]. Wide The topology of the DC-DC converter is chosen to be an
bandgap power devices have the capability of operating at isolated interleaved boost DC-DC converter as displayed in
high switching frequencies. Due to their small ON state Fig. 1.
resistance the switching loss is reduced and then the efficiency
is increased. Operating at high switching frequency
contributes to a smaller size of passive components such as
978-1-5090-3270-9/16/$31.00 ©2016 IEEE
TABLE I. DC-DC CONVERTER SPECIFICATIONS
Specification Value
D2 Max. input voltage 40V
C2
T1 Min. input voltage 25V
L2
output power 250W
L1 C4
Rload Max. output voltage 430V
Min. output voltage 370V
MOS2 Switching frequency 100kHz and 300kHz
Vin MOS1
C1
+ D1 C3
- The transformer turns ratio is obtained as
N 2 Vout ⋅ (1 − D)
= (2)
N1 4 ⋅ Vin
Fig. 1. Schematic of the proposed topology.
A. Selection of Inductor
The primary side of this topology consists of two The value of input inductors is given by
switching MOSFETs, two inductors and one input capacitor.
The secondary side is composed of two rectifiers’ schottky Vin (min) ⋅ DSW(max)
L= (3)
diodes and three capacitors. The last capacitor used as output ǻIind ⋅ fSW
filter and the other two used withe rectifier diodes to perform
voltage doubler and increase the output voltage of the high The values of the inductor at 100 and 300 kHz are 224 and
frequency transformer. 149 μH, respectively. It is clearly that the value of the inductor
decreased when the switching frequency increased.
B. The Steady State Operational Analysis
The converter exhibits four modes of operation. In the first B. Selection of capacitors
mode, both of the switching MOSFETs switched on and the Output capacitors values are obtained as
input current is split equally between the two MOSFETs. The
Io ⋅ D ⋅ TSW
energy during this mode is stored in both of the input C≥ (4)
inductors. In the secondary side, output capacitors supplied the ǻVmax
load where there is no current passing through rectifier diodes Where Io is the output current. The values of the output
and the secondary winding. In the second mode, MOS 2 is off capacitors at 100 and 300 kHz are equal to 6.75 and 2.25 μF
while MOS1 is still on. Therefore, the preserved energy in L2 respectively. It is noticed that as the switching frequency
is transmitted to the secondary winding. In the secondary side increases the value of the capacitor decreases. The value of the
the current is passing through D2 and charging C2. The input capacitor is calculated as
operation mode3 is the same as mode1. In the last mode
ǻi c
MOSFET1 is turned off while MOSFET2 is still on, so the Cin = (5)
energy preserved in L1 is transmitted to the secondary side. fSW ⋅ ǻVc
Therefore the secondary current passing D1 and charging C3. The values of the input capacitors are 3.375 and 1.125 μF
at 100 and 300 kHz respectively.
III. PARAMETER DESIGN OF DC-DC CONVERTER
The steady state characteristics of the boost DC-DC C. Loss Analysis of MOSFET and Diode
converter working in a continuous conduction mode (CCM) is The losses of MOSFET and diode can be classified into
stated below. The converter specifications are given in Table I. two parts: conduction loss and switching loss.
The output voltage is given as The switching loss of the MOSFET is given as
VDS ⋅ I D ⋅ t sw (off )
2 ⋅ Vin N 2 PSW = (6)
Vo = ⋅ (1) 2 ⋅ Tsw
1 − D N1
Where VDS represents the voltage from a drain to a source,
Where Vin denotes to input voltage, D stands for duty cycle tsw is the switching off time and ID is the drain current.
and N1 and N2 are the transformer turns ratio. The conduction loss of the MOSFET is obtained as
Pcondution = 1.6 ⋅ R DS(on ) ⋅ I 2 rms (7)
Where RDS (on) is the drain source ON resistance. The
switching loss of the diode is calculated as
1 VBR ⋅ I RM ⋅ t rr TABLE II. CHARACTERISTICS OF SWITCHING DEVICES
PSW(Diode) = ⋅ (8)
2 TSW Parameter Si SiC
STB11NM80 C3M0065090D
Where VBR is the maximum break down voltage, the Breakdown Voltage 800 V 900 V
reverse recovery current is IRM and trr is the reverse recovery
time. Continuous 11 A 23 A
Current
The conduction loss of the diode is given by
RDS(on) 0.4 0.065
2 Qrr (reverse recovery 11.25 μC 0.134 μC
PCon (Diode) = 1.16 ⋅ I D(ave) + 0.053 ⋅ I D(rms) (9) charge)
Trr (reverse recovery 970 ns 30 ns
Where ID(ave) and ID(rms) are the average and root mean time)
square of the diode current. Diode
C3D16060D
D. High Frequency Transformer Design DC Blocking 600 V
The number of primary and secondary turns are calculated Voltage
IF (continuous forward 22 A
as follow
current)
Vin ⋅ D max QC (Total Capacitive 42 nC
N1 = (10) Charge)
fSW ⋅ ǻB ⋅ A C
N 2 = N1 ⋅ n (11)
Where ǻB denotes to the flex density and is assumed to be
equal to 0.2T.
The area product of the cross sectional area and window
area is expressed as follow
0.5 ⋅ Po
AP = (12)
2 ⋅ K W ⋅ J ⋅ f s ⋅ Bm
Where Ap is the area product, KW is the window factor, J
denotes to the current density and Bm is the flux density.
IV. SIMULATION RESULTS
The design of the 250W isolated interleaved boost DC-DC (a) Turn-on
converter with the specifications given in Table I was
simulated using LTSpice IV. The performance of the
converter is compared and evaluated at two di erent
switching frequencies 100 kHz and 200 kHz with respect to
two different switching power devices Si MOSFET and SiC
MOSFET. The characteristics of the two switching MOSFETs
are presented in Table II.
A. Switching Performance Analysis Using Si MOSFET
The behavior of switching characteristics of Si MOSFET
during switching on and off is shown in Fig. 2 and Fig. 3.
From the simulation results the switching turn-on (ton) and
turn-off (toff) time of 100 kHz switching frequency are
measured at 52.57ns and 44.1ns respectively. From the turn on
time the energy loss Eon is found to be equal to 6.82 μJ.
Similarly, the energy loss during the turn off Eoff is equal to
5.87μJ. The switching frequency is increased to 300 kHz and
ton and toff time were found to be equal to 50ns and 44ns (b) Turn-off
respectively. Moreover, the value of the energy loss during the
Fig. 3. Switching charateristcs of Si MOSFET at 100 kHz.
ton measured at 8μJ which is greater than Eoff that found at 3
μJ.
(a)Turn-on (a) Turn-on
(b) Turn-off
(b)Turn-off Fig. 4. SiC MOSFET switching behavior at 300 kHz
Fig. 3. Si MOSFET switching behavior at 300 kHz.
B. Switching Performance Analysis Using SiC MOSFET
The switching characteristics at 100 kHz of SiC MOSFET
are shown in Fig. 4. The switching behavior during ton and toff
were found to be smaller than that of Si MOSFET and are
measured at 37.3 and 31.8 ns respectively. As a result, the
values of switching energy losses Eon and Eoff are reduced
compared with Si MOSFET and are found to be equal to 2.7
and 2.0 μJ. Similarly, the turn-on and turn-off time at 300 kHz
were measured at 45 and 40 ns with the energy loss of 3 and
2.2 μJ respectively. Consequently, it is clear that SiC
MOSFET exhibits faster switching speed and low energy loss.
It is noticed that from the data sheet the reverse recovery time
trr of Si MOSFET is 30 times larger than SiC MOSFET which
explains the high switching speed of the SiC MOSFET.
Moreover, the reverse recovery charge QC of Si MOSFET is
found from the data sheet to be around 80 times larger than (a) Turn-on
SiC MOSFET which results in a high energy loss of Si
MOSFET.
Fig. 6. Output power versus efficiency of Si and SiC MOSFETs at 100 kHz.
(b) Turn-off
Fig. 5. SiC MOSFET switching behavior at 300 kHz.
C. Performance Comparison
The proposed design of the boost DC-DC converter was
simulated at two switching frequency 100 and 300 kHz using
two switching power devices Si MOSFET and SiC MOSFET.
Table III presents the total switching losses of the two
switching power devices. The SiC MOSFET total losses at
100 kHz was found at 9.3 W which is approximately 30% less
than Si MOSFET. The DC-DC converter efficiency increased
by 1.5 % when SiC MOSFETs used. SiC MOSFET shows
superior performance over Si MOSFET when operating at a
high switching frequency of 300 kHz. The total losses are
decreased by around 38% and the efficiency is increased by
approximately 3%.
TABLE III. SWITCHING LOSS COMPARISON Fig. 6. Output power versus efficiency of Si and SiC MOSFETs at 300 kHz.
Si MOSFET
Fig. 4 and Fig. 5 compare the efficiency of the Si MOSFET
Switching frequency (kHz) 100 300 and SiC MOSFET with output power at two switching
frequencies 100 kHz and 300 kHz. It is clear that SiC
Total switching loss (W) 13 18.84 MOSFET efficiency is much better than Si MOSFET when
operating at higher switching frequency.
Efficiency (%) 94.53 92.5
V. CONCLUSION
SiC MOSFET
In this paper, a 250W 400V isolated interleaved boost DC-
Switching frequency (kHz) 100 300
DC converter was designed. Two capacitors were used at the
secondary side with two rectifier diodes to perform as
Total switching loss (W) 9.2 11.9 capacitor voltage doubler to increase the conversion ratio. The
performance of the converter was tested using two switching
Efficiency (%) 96 95.2 power devices Si and SiC MOSFETs. Two switching
frequencies were selected to test and compare the performance
of the two switching devices. It is noticed that the efficiency
of the converter increased when using SiC MOSFET instead
of Si MOSFET. Moreover, the benefits of using SiC power
devices can be clearly demonstrated when it is operated at
high switching frequencies where the efficiency of the
converter increased from 92.5% to 95.2% at a switching
frequency of 300 kHz.
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