Experimental Verification of DC to Single-phase
AC Converter with Power Decoupling Capability
using 1.2 kV SiC-MOSFET Module
Hiroki Watanabe
Nagaoka University of Technology
Nagaoka, Niigata, Japanhwatanabe@vos.nagaokaut.ac.jp

Jun-ichi Itoh
Nagaoka University of Technology
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper presents a DC to Single-phase ACconverter with 1.2 kV SiC-MOSFET power module for PV
power conditioning systems (PCSs). The authors are aiming todevelop a SiC-MOSFET power module with low on-resister inorder to improve the conversion efficiency. In addition, theactive power decoupling topology with small buffer capacitor
has been considered in order to improve the system reliability.
the active power decoupling circuit compensates the double-line
frequency power ripple by the small firm or ceramic capacitor.
However, the high voltage rating devices is required due to thevoltage ripple of the buffer capacitor. In this paper, a first proto
type SiC-MOSFET power module is tested by the experiment.
This module has the high voltage rating as 1.2 kV in order toreduce the buffer capacitor. As the experimental result, it wasconfirmed that the fundamental operation. In addition, themaximum efficiency of 95.0 % was obtained at the rated outputpower.
Keywords—Photovoltaic, Power decoupling, AC module, SiCMOSFET

I. INTRODUCTION
Recently, photovoltaic (PV) systems have been actively
researched as a sustainable power solution [1]-[3]. In the PV
systems, power conditioning systems (PCSs) are necessary
for utilization of the PV generated power. Typically, thePCSs are consisted of a DC/DC converter and a voltage
source inverter (VSI) in order to connect the PV panels to thesingle-phase AC grid. These PCSs require the following
capabilities: (1) high efficiency power conversion, (2) highreliability, and (3) small packaging design.
The switching devices based on a wide band-gap devices
such as silicon carbide (SiC) or gallium nitride (GaN) may
become a trend instead of IGBT and silicon MOSFET (SiMOSFET) because these devices have a good performance
which include fast switching and features low on-voltage
drop compared with the silicon devices [4]. Therefore, it havebeen reported that the improvement of the power conversion
efficiency by using SiC or GaN devices.
On the other hand, double-line-frequency power ripple
occurs in the DC side due to the single-phase AC grid. Thispower ripple leads to a decrease in a performance of themaximum power point tracking (MPPT). Therefore, a bulky
electrolytic capacitor is usually installed on the DC link in

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Akio Iwabuchi
SANKEN ELECTRIC CO., LTD
Niiza, Saitama, Japanaiwabuch@ms2.sanken-ele.co.jp

order to absorb the power ripple: i.e., a passive power
decoupling method is used. However, passive power
decoupling method limits the life time of the power converter
due to the Arrhenius law.
Active power decoupling topologies have been researched
actively in order to enhance the system reliability of thesingle-phase systems [5]-[10]. In this method, the double-line
frequency power ripple is compensated by the small capacitor.
Therefore, it is possible to remove the electrolytic capacitor
from the PCSs, and a small capacitor such as a firm orceramic capacitor is utilized for compensation of the power
ripple. However, the some components require a high voltage
rating when the small capacitor is applied by the active power
decoupling method because the voltage ripple with low
frequency component becomes increase. In this case, it isdifficult to apply the Si-MOSFET with 500 or 600 V voltage
rating because the diode recovery characteristics does not
good. On the other hand, switching speed of IGBT does notfaster than the Si-MOSFET. Therefore, the volume of thepassive components such as the inductor increases.
According to this problem, SiC-MOSFET is the key
component because it has the high speed switching speed
characteristics and high voltage rating more than 650 V. Theauthors have been develop the SiC-MOSFET modules forthis topology.
In this paper, a first proto type SiC-MOSFET power
module is tested by the experiment. The test circuit consistsof the active power decoupling circuit, and a current source
inverter (CSI). The active power decoupling circuit
compensates the double-line -frequency power ripple by thesmall capacitor.
The remainder of the paper is organized as follows. Insection II, the circuit topology of the DC to single-phase AC
is explained. In addition, the developing SiC-MOSFET
power module is demonstrated. In section III, thecompensation principle of the double-line frequency of thepower ripple is explained. Finally, in section IV, thesimulation and the experimental result are demonstrated.
II. CIRCUIT TOPOLOGY
Fig.1 shows the DC to single-phase AC grid-connected
converter with a typical DC/DC converter. The DC/DC

converter boosts the PV input voltage above the peak valueof the grid voltage, and the generated power is supplied to thesingle-phase grid by the VSI. In this case, a bulky electrolytic
capacitor Cdc is required in the DC link due to the doubleline-frequency power ripple of the single-phase grid.
Fig. 2 shows the active power decoupling circuit [11]. The
active power decoupling circuit consists of the typical
DC/DC converter and additional switching device Sbt. The
active power decoupling circuit regulates the buffer capacitor
voltage Vcbuf in order to compensate the double-line
frequency power ripple. In addition, the reverse-blocking
diode does not necessary on CSI because the free-wheelingmode is achieved on the active power decoupling circuit. Inthis paper, the SiC-MOSFET power module is used for CSI
and the part of active power decoupling circuit.
Fig. 3 shows the 1.2 kV SiC-MOSFET power module
which consists of 3 phase configuration. In this module, aSiC-schottoky barrier diode is connected in parallel to each
SiC-MOSFET in order to reduce the recovery loss. In
addition, an aluminum nitride ceramic circuit board is applied
to reduce the ground capacity. Furthermore, the rise ofjunction temperature is suppressed because it has highthermal conductively such as 67 W/mK.
III. COMPENSATION PRINCIPLE OF POWER RIPPLE

Fig. 1 DC to single-phase AC grid connected converter with typicalboost converter.

Fig. 2 Test circuit configuration. The active power decoupling circuit
compensates the double-line frequency power ripple by small capacitor
Cbuf. The SiC power module is applied to CSI and active power
decoupling circuit.

Fig. 4 shows the principle of the power decoupling
between the DC and single-phase AC sides. When both theoutput voltage and current waveforms are sinusoidal, theinstantaneous output power pout is expressed asp out =V acp I acp
2(1 − cos 2ωt )(1)(a) Top view

where Vacp is the peak voltage, Iacp is the peak current, and ω
is the angular frequency of the output voltage [12]. As shownin (1), the power ripple at twice the frequency of the singlephase power grid appears at the DC link.
In order to absorb this power fluctuation, the instantaneousbuffer power pbuf should be controlled by1pbuf = Vacp I acp cos 2ωt2(2)where the polarity of pbuf is defined as positive when theflying capacitor Cbuf discharges. Note that the active power ofCbuf is zero. As a result of the power decoupling, the input
power is matched to the output power. Thus, the relationshipbetween the input and output power is expressed as follows:1
pin = Vacp I acp = V IN I IN
2(b) Power module configuration.
Fig. 3 1.2 kV SiC-MOSFET power module. The SiC-Shotokey barrier
diode is connected in parallel to SiC-MOSFET.

(3)

where Vin is the DC input voltage, Iin is the DC input current.
In the active power decoupling circuit, the buffer
capacitor Cbuf becomes small when the fluctuation voltage
increase as shown in Fig.3. The buffer capacitance isexpressed as

Cbuf =2 Poutω (Vcbuf _ peak 2 − Vcbuf _ min 2 )(4)where Vcbuf_peak is the peak voltage of the buffer capacitor,
Vcbuf_min is the minimum voltage of the buffer capacitor.

Fig. 5 shows the relationship between the fluctuation
voltage ΔVcbuf and the buffer capacitor Cbuf. Note that, Cbuf iscalculated from (4). In addition, the minimum voltage of thebuffer capacitor should set more than the peak grid voltage
Vacp in order to avoid the short circuit current [13]. Finally,
this application supposes the PV micro-inverter [14].
Therefore, the output power is set around several handled
watt. According to Fig.4, the buffer capacitor decreases when
ΔVcbuf becomes large. Therefore, the voltage rating conditionis the one factor to decide the minimum capacitance of Cbuf.
In this paper, the SiC-MOSFET power module which has
high voltage rating is applied in order to avoid this problem.
IV. SIMULATION AND EXPERIMENTAL RESULT

Fig. 4 Principle of power decoupling between DC and single-phase AC.

Buffer capacitor Cbuf [ F]

Table 1 shows the simulation parameter, Fig.6 shows thesimulation result of the active power decoupling circuit withsmall buffer capacitor. In this simulation, the buffer capacitor
Cbuf is set to 2.5μF.
According to Fig. 6, the constant input voltage is obtainedby the active power decoupling circuit. In addition, thesinusoidal inverter output current is obtained. Finally, thebuffer capacitor maximum voltage is match to Fig.5.
According to these result, the buffer capacitance becomes
small when the fluctuation voltage of Vcbuf is set to large.
Especially, a firm capacitor has the small energy density incomparison with the electrolytic or ceramic capacitor.
Therefore, it is important to improve the power density of theactive power decoupling circuit when the firm capacitor isused for the decoupling capacitor.
Fig.7 shows the experimental result. Note that, the output
power is 300 W, switching frequency is 20 kHz, buffer
capacitor is 44μF, and the grid voltage is 200 Vrms. In thisexperiment, the buffer capacitor is set to large in comparison
with the simulation result in order to confirm the fundamental
operation of the SiC-MOSFET power module.
According to Fig.7, the constant DC voltage and thesinusoidal inverter output current are obtained. In addition, it
is confirmed that the buffer capacitor voltage is fluctuated by
the double-line frequency by the power decoupling control.
Fig.8 shows the efficiency characteristics of theexperimental circuit. According to Fig.8, the maximum
efficiency is 95.0% at the rated power of 300 W. Note that,the efficiency curve still increases because the power module
has a large capacity such as 1200 V/ 40 A although the rated
power of the experimental circuit is 300 W.

Fig. 5 Relationship between fluctuation voltage and buffer capacitance.
Table1 Simulation parameter

SymbolQuantity

value

VinPinCbufVacVcbuf_min
facInput voltage
Input power
Buffer capacitor
Grid voltageBuffer capacitor minimum voltage
Grid frequency

50 V
350 W
2.5 F
200 Vrms
300 V
50 z

V. CONCLUSION
In this paper, DC to single-phase AC converter with power
decoupling capability using 1.2 kV SiC-MOSFET isdemonstrated. The active power decoupling circuit requires
the high voltage rating devices in order to reduce the buffer
capacitance. From the simulation result, the constant input
voltage is obtained by the 2.5μF of the buffer capacitor. Fromthe experimental result, the constant DC voltage and thesinusoidal inverter output current are obtained. Finally, it wasconfirmed that the maximum efficiency is 95.0%.
In the future work, the switching characteristics of the SiCMOSFET module will be considered.
Fig. 6 Simulation result.

ACKNOWLAGEMENT

Table 2 Experimental parameter

This study was supported by New energy and Industrial
Technology Development Organization (NEDO) of Japan.
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Fig.7 Experimental result. Sinusoidal output current waveform andconstant DC input voltage are obtained. Note that, buffer capacitor voltage
is fluctuated by the power decoupling control.

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