DC to Single-phase AC Grid-Tied Inverter
using Buck Type Active Power Decoupling
without additional magnetic component
Jun-ichi Itoh, Tomokazu Sakuraba, Hiroki Watanabe, Nagisa Takaoka
Nagaoka University of Technology
Department of Electrical, Electronics and Information
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp, t_sakuraba@stn.nagaokaut.ac.jp, hwatanabe@stn.nagaokaut.ac.jp, ntakaoka@stn.nagaokaut.ac.jp
Abstract— This paper proposes a single-phase AC converterwith an active power decoupling circuit operated in DCM(discontinuous current mode) for PV (photovoltaic) systems. The
proposed converter consists of an H-bridge single-phase AC
converter and a boost converter, and a compensation circuit
employed with a small capacitor for the power ripple. In many
active power decoupling topologies that have been studied, an
additional magnetic component for the voltage control of thebuffer capacitor is required. This worsens the power density andthe converter efficiency. In the proposed converter, the boost
inductor is utilized for both the boost operation and the active
power decoupling operation by using DCM. In this paper, byharmonic analysis, the effectiveness of the active power decoupling
method is evaluated. As a calculating result, the maximum power
density of the proposed converter is 1.26 times higher than that ofthe passive topology. Especially, the boost inductor volume can be
reduced by 42.7% applying by DCM. The fundamental operationis confirmed by a 1-kW prototype. From experimental results, asecond-order harmonics on the input current is reduced by 90.2%
with the proposed active power decoupling method. In addition,
the maximum efficiency of 96.0% is achieved at the output powerof 600 W.
Keywords—High power density systems; Single phase system;
Power decoupling;

I. INTRODUCTION
Recently, photovoltaic (PV) systems have been researched
actively as a solution for sustainable power. Especially, theworld PV market installation reached a high record of 277 GW
in 2015 [1]. In order to utilize the generated power from the PV
panels, power converter system (PCS) are employed in order toconnect the PV system to the single-phase AC grid [2-3]. ThesePCS requires the following capabilities;(1) High efficiency power conversion,(2) “Plug-and-Play” operation,(3) Maintenance-free,(4) Small packaging design.
In order to satisfy these capabilities, a two-stage power
conversion with a boost converter and a voltage source inverter(VSI) is employed due to its simple configuration in generally.
Note that, the instantaneous power ripple component at twice
grid frequency occurs owing to the single-phase AC grid. As aresult, the performance of the maximum power point tracking(MPPT) decays. In order to absorb the power ripple component,a bulky electrolytic capacitor is employed in the DC-link in
generally. However, the bulky electrolytic capacitor limits thelife-time of the converter for PV systems according to Arrhenius
law.
On the other hand, several circuit topologies with active
power decoupling methods have been increasingly studied toeliminate the bulky electrolytic capacitor [4-15]. The active
power decoupling methods can reduce the capacitance for the
instantaneous power fluctuation compensation, which leads tothe use of a film or a ceramic capacitor instead of the bulky
electrolytic capacitor. However, the conventional active power
decoupling methods decrease the converter efficiency due to theloss coming from the additional component. In particular, theadditional magnetic component greatly introduces large volume.
In this paper, a novel active power decoupling topology
which is constructed based on a boost converter is proposed. The
proposed method does not require the additional magnetic
component for the active power decoupling circuit, because theboost inductor is utilized for both the boost operation and thepower decoupling capability applying by discontinuous current
mode (DCM). Furthermore, an inductor applied to DCM can
reduce the inductance [16]. Thus, the proposed topology can befurther achieved the downsizing. In addition, the active power
decoupling circuit can apply the low on-resistor switching
device owing to the low voltage rating devices. Therefore, theproposed converter with DCM can achieve the high efficiency
and the high power density. This paper is organized as follows;in section II, the proposed converter topology for the DC tosingle-phase AC converter are introduced. In section III, theprinciple of the active power decoupling is explained. In section
IV, the control strategy of the proposed converter is explained.
Finally, the validity of the proposed power decoupling isconfirmed by a 1-kW prototype in the experiment. The power
density and the system efficiency of the each circuit topologies
with the power decoupling capability are evaluated by Paretofront optimization and the volume distribution.
II. CIRCUIT CONFIGURATION
A. Conventional Circuit
Figure 1 shows a DC to single-phase AC grid-connected
system with a typical boost converter. The boost converter
pushes up the PV input voltage more than the peak voltage ofthe single-phase grid, and the generated power is supplied to thesingle-phase grid by the VSI. In this case, the bulky electrolytic
capacitor Cdc is employed onto the DC-link in order to absorb
the power ripple component which has twice grid frequency. If
the power ripple component is absorbed by the bulky electrolytic
capacitor at a DC link in the conventional system, the required
capacitance is decided by the energy of the power ripple. Thus,
the capacitor voltage ripple is expressed by

ΔE =


ΔVc 2
ΔVc 2 1
C dc (Vave +) − (Vave −
) 
2
2
2



Fig. 1. Circuit configuration with typical boost converter. The
bulky electrolytic capacitor Cdc is required for the compensationof the power ripple component in this circuit. As a result, the lifetime of PCS is limited by Cdc.

(1),where E is the energy of the power ripple caused by single-phasegrid, Vave is the average value of the bulky electrolytic capacitor
voltage and ΔVc is the fluctuation voltage of Cdc. In theconventional method with a bulky electrolytic capacitor as
shown in Fig.1, Δ Vc is designed to be small for a stable
operation of the inverter by using the large capacitance of Cdc.
In this configuration, it is necessary to perform a periodic
maintenance of the electrolytic capacitor owing to its short lifetime.
Figure 2 shows a DC to single-phase AC grid-connected
system. This system has a conventional active power decoupling
circuit to compensate the power ripple. In this case, the DC-link
capacitor value can be minimized in comparison with Fig. 1
because the power ripple is compensated by the small buffer
capacitance of Cbuf. Thus, the film or ceramic capacitor can beapplied for the smoothing capacitor. As a result, the converterwith the power decoupling circuit can be eliminated the bulky
electrolytic capacitor. However, an additional magnetic
component Lbuf is required in order to control the buffer
capacitor voltage Vcbuf. Consequently, this additional inductor
increases the volume and the loss of the converter.
B. Proposed circuit
Figure 3 shows the proposed circuit which is constructed byan active power decoupling circuit based on the boost converter.
This converter consists a boost converter, an H-bridge inverter
and an active power decoupling circuit. Similar to theconventional active power decoupling circuit, the proposed
converter can reduce the capacitance of the DC-link capacitor
Cdc because the power ripple is compensated by the buffer
capacitor Cbuf. However, contrast to the conventional method,

Fig. 2. Boost-type crircuit configuration with conventional active
power decoupling circuit. Cdc can be reduced by the active power
decoupling circuit. However, the additional magnetic component
Lbuf is required in order to control buffer capacitor voltage Vcbuf.
Consequently, the converter loss and the volume of the converter
increase due to Lbuf.

Fig. 3. Proposed converter with power decoupling capability. The
boost inductor Lin is utilized for both the boost operation and thebuffer capacitor voltage control for the power decoupling. Thus,
the additional magnetic component is not required. The proposed
converter is operated in DCM in order to discharge of Cbuf.

the additional magnetic component Lbuf as shown in Fig. 2 is not
required for the active power decoupling circuit. This is because
the boost inductor Lin is used in order to achieve the active power
decoupling. Note that, in the active power decoupling circuit, abidirectional switch is required for the prevention of the shortcircuit between the DC link capacitor Cdc and the buffer
capacitor Cbuf. In particular, the active power decoupling circuit

performs the buck-type operation. Therefore, the voltage applied
to this bidirectional switch is lower than the DC link voltage
because the maximum value of this voltage is either thedifference between the DC link voltage and the buffer capacitor
voltage, or the low capacitor voltage. Therefore, low on-resistor
switching device can be applied to S3 and S4 and the loss of S3and S4 can be reduced to compare the boost-type active buffer
circuit [17].
When the boost converter is operated in continuous current
mode (CCM), the current pathway is limited in one direction.
Therefore, with the application of CCM, the buffer capacitor
control cannot perform both the charging mode and thedischarging mode. In order to overcome this, the proposed
converter is operated in the discontinuous current mode (DCM)
in order to discharge the buffer capacitor Cbuf. In particular, thebuffer capacitor is discharged during the negative period of theinductor current on the DCM operation. In general, the inductor
value applied with DCM can be reduced compared to CCM.
Thus, the volume of the boost inductor is reduced by DCM.
III. RRINCIPLE OF POWER DECOUPLING OPERATION
Figure 4 depicts a principle of the power decoupling
operation with the active buffer. When the output current has
sinusoidal unity power factor, the instantaneous output power
pout is expressed by

Fig. 4 Principle of power decoupling. The buffer power to absorb
the power ripple component is charged/discharged by the buffer
capacitor. Consequently, the DC current without the power ripple isobtained.

pout = Vacp sin (ωt ) ⋅ I acp sin (ωt )=Vacp I acp
2(1 − cos 2ωt )(2).

where Vacp is the single-phase grid voltage (peak), Iacp is theinverter output current (peak), and ω is the output angularfrequency. As shown in (2), the power ripple, which has twice
grid frequency, occurs at the DC-link. In order to compensate
the power ripple at DC side, the instantaneous active buffer
power pbuf should be controlled by1
pbuf = − Vacp I acp cos 2ωt2(3)

where the polarity of instantaneous power of an active buffer isdefine as positive when the active buffer discharges. When the
power decoupling is applied, the input power pin is matched tothe average value of pout.

1
pin = Vacp I acp = Vin I in
2

Fig. 5. Operation mode of proposed converter. The proposed
converter achieves the boost operation and the active power
decoupling operation through four modes.

(4)

IV. CONTROL STRATEGY OF PROPOSED CIRCUIT
A. Control strategy
Figure 5 illustrates the operation mode of the proposed
converter which has four modes. Mode 1 and Mode 2 are theboost operation, where the PV input voltage Vin is pushed up to
the DC link voltage Vdc higher than Vacp. Note that, S3 and S4 isalways off in order to separate the power decoupling operationfrom the boost operation. In Mode 3 and Mode 4, the buffer
capacitor is charged and discharged for the compensation of thepower ripple component.

Figure 6 shows the control block diagram of the proposed
converter which consists of the buffer capacitor voltage control
and the DC link voltage control. The current and voltage control
in the proposed method are implemented by PI controller. The
DC link voltage is regulated more than the peak grid voltage by
the voltage control (AVR). Equally, the buffer capacitor voltage
is fluctuated at twice grid frequency for the active power
decoupling. If the fluctuation of the buffer capacitor voltage iscontrolled by a voltage controller (AVR), the bandwidth of the
AVR has to be high. This can be avoided by regulating only theaverage voltage by the AVR in the buffer capacitor voltage
control. Meanwhile, the charge and discharge value of the buffer
capacitor is controlled by the boost inductor current control(ACR).

vdc
Vin

vin
vdc −vin

v buf

vin
vbuf −vin

v inFig. 6. Control block diagram of proposed converter.

Figure 7 shows the boost inductor current waveforms in
DCM. In order to separate the DC-link voltage control from thebuffer capacitor voltage control, the zero-current period isinserted into each current mode.
Note that, in order to achieve the proposed active power
decoupling, the boost inductor average current iL_ave, which is thesum of the DC link current and the buffer capacitor current, has
to become constant.
(a) Charge mode for buffer capacitor

The boost inductor current is expressed asi L _ ave = i L _ ave _ dc + i L _ ave _ buf(5)where iL_ave_dc is the DC link average current, iL_ave_buf is thebuffer capacitor average current.
On the other hand, the DC link average current iL_ave_dc is
expressed asiL _ ave _ dc =i peak2(d1 + d 2 ) = Pout [1 − cos(2ωt )]
Vin

(6)

where d1 and d2 are the duty reference of mode 1 and mode 2,
Pout is the output power. When the active power decoupling is
not applied, the inductor average current is fluctuated at twice
grid frequency as shown in (6). In order to obtain the constantcurrent by the power decoupling, the buffer capacitor current has
to be fluctuate in order to compensate the power ripple
component.
When the active power decoupling is applied, the buffer
capacitor average current iL_ave_buf is expressed asi peak(d3 + d4 ) = Pout cos(2ωt )iL _ ave_ buf =2
Vin(b) Discharge mode for buffer capacitor
Fig. 7. Current waveform of boost inductor with DCM for active
power decoupling circuit.

(7)

where d3 and d4 are the duty reference of mode 3 and mode 4.
According to (6) and (7), the boost inductor average current
iL_ave becomes constant, and the buffer capacitor voltage isfluctuated at twice grid frequency owing to the proposed control.
Thus, the proposed converter does not required the additional
magnetic component for the active power decoupling.
B. Designing method of boost inductor
In Figure 7 (b), in order to prevent an interference between
the two voltage control loops, each duties necessary to meet theequation (8).

d1 + d 2 + d 3 + d 4 ≤ 1(8)When the sum of each duty is equal to 1, the operation modeof this proposed is the CCM. The current ripple of the boost
inductor becomes the minimum, which leads to the minimum of
the conduction loss in the critical condition. Thus, the inductanceof the boost inductor Lin is designed based on the criticalcondition at the rated power, and is calculated by

Lin =

Vin2 (α di − 1)α bd α di − 1 
4 f sw Pout α di 1 +

2 α di α bd − 1 V
α di = dcVin

α bd =

2

Vbuf _ ave
V dc

Table I. Simulation and experimental condition.

(9)

(10)

(11)

where fsw is the switching frequency, αdi is the boost ratio of theDC-link voltage to Vin, αbd is the boost ratio of the averagevoltage of Cbuf to the DC-link voltage, and Vbuf_ave is the averagevoltage of Cbuf.
V. SIMULATION AND EXPERIMENTAL RESULTS
A. Simulation Results in Steady State
Table I presents a simulation conditions in order to verify thefundamental operation of the proposed converter with the active
buffer. Note that the output voltage is 100 Vrms, the dead time
and the input filter is not replaced for simplicity. In order toremove a switching ripple, the boost inductor current is filteredwith a LPF with the cut-off frequency of 2 kHz.
Figure 8 shows the input and output waveforms of the
proposed converter. Fig. 8 (a) shows a result without the
proposed active power decoupling. As shown in Fig. 8 (a), theboost inductor current is distorted by the power ripple
component. This cause is the proposed circuit has a small
capacitance of Cbuf which is not completely compensated thepower ripple at 100 Hz. In contrast, Fig. 8 (b) shows a resultwith the power decoupling. The power ripple component isreduced in Fig. 8 (b), owing to the proposed active power
decoupling.
Figure 9 shows extended waveforms of area A and area B in
Fig.8. Fig. 9 (a) shows the charging mode with the buffer
capacitor, whereas Fig. 9 (b) shows the discharging mode of thebuffer capacitor. The boost inductor current becomes
discontinuous, i.e. DCM. As shown in Fig. 9 (a), the firstcurrent ripple as a smaller side performs the operation, whereas
the secondary current ripple as a bigger side performs the active
power decoupling, i.e. the charge mode of the buffer capacitor.
Similarly, as shown in Fig. 9 (b), the positive performs the boost
operation, whereas the negative current ripple performs theactive power decoupling, i.e. the discharge mode of the buffer
capacitor. The boost current ripple Therefore, the proposed
active power decoupling compensates the power ripple
component on the input current without any additional
magnetic component.
B. Experimantal results in Steady State
In order to evaluate the fundamental operation, a 1-kW
prototype circuit is tested. It should be noted that the

60
40
20
0

Boost inductor current iL [A]

A

50

B

0

Input current iin [A]
10
0

6.8

Buffer capacitor voltage [V]

300

300

250
200

250

10

200

Inverter output current [A]
5
0
-5

0
-10

10ms/div.

10ms/div.

(a) W/o active power decoupling (b) With active power decoupling

Fig. 8. Fundamental operation waveforms. The boost current
waveform with DCM is deiced by the mode of the buffer
capacitor voltage.

(a) Area A of Fig. 8(b) Area of Fig. 8

Fig. 9. Extended current waveform of area A and B in Fig. 8
with power decoupling method using DCM. This figure shows
the buffer capacitor in the charging mode as shown in area A
and the discharging mode as shown in area B of Fig. 8.

experimental condition is similar table I. In this paper, insteadof the grid-connection, the RL load is employed. However, theoccurrence of the instantaneous power ripple at the twice gridfrequency is still the same. In this experiment, the buffer
capacitor value of 80 μF, the DC link capacitor of 54 μF, and theboost inductor of 48 μH are employed.
1) Fundamental operation
Figure 10 shows the experimental result without the
proposed active power decoupling. As shown in Fig. 10, thebuffer capacitor voltage is constant. Therefore, the input current
iin fluctuates at twice grid frequency due to the power ripple
component. In this result, the ripple component in the input
current iin is 54.7% with reference to the average input current.
Figure 11 shows the experimental result with the proposed
active power decoupling. In comparison with Fig. 10, the input
current iin becomes almost constant. The ripple component in
the input current at 100 Hz is 5.36% with reference to theaverage input current. Moreover, the buffer capacitor voltage isfluctuated at the twice grid frequency to absorb the power ripple.

Fig. 10. Experimental result without active power decoupling.
The input current iin fluctuates at twice grid frequency.

It is also confirmed that the inverter output current becomessinusoidal which has low distortion. From these results, theproposed active power decoupling with a buffer capacitor of
80 μF is achieved.
Figure 12 shows the harmonic analysis of the boost inductor
current. Note that the harmonic number is based on the gridfrequency of 50 Hz and boost inductor current is based on the
100 Hz component of the boost inductor current. The proposed
active power decoupling method reduces the 2nd-orderharmonics (100 Hz) by 90.2% in comparison with no adopting
the proposed active power decoupling. From this result, thevalidity of the proposed converter with the power decoupling
capability is confirmed without the additional magnetic
component for the proposed circuit.

Fig. 11. Experimental result with active power decoupling. The
input current iin becomes constant, and the buffer capacitor
voltage is fluctuated at twice grid frequency.

Figure 13 shows the efficiency system characteristics of the
proposed converter when the active power decoupling isapplied. The maximum efficiency reaches 96.0% at the output
power of 600 W. In order to achieve the high power density, thelow on-resistor switching devices could be applied into theactive power decoupling circuit in the proposed inverter. As
shown in Fig.13, the proposed inverter using the buck-type
active buffer is confirmed an improvement in the switching lossof the buffer circuit compared to the proposed inverter using theboost-type active buffer [17].
2) Volume evaluationa) Comparison of Pareto-front for DC-DC converter
The fundamental operation and the active power decoupling
capability of the proposed inverter using the buck-type active
buffer are confirmed in the previous section. In order to confirm
the converter efficiency and the power density of the proposed

Fig. 12. Harmonic analysis of boost inductor current. The active
power decoupling method reduces the second-order harmonics
of the power ripple component by 90.2%.

inverter with DCM, the each converter topologies are comparedusing the pareto-front optimization.
Figure 14 shows the pareto-front of the power density andthe converter efficiency at the range of the switching frequency
from 1 kHz to 100 kHz.
Table II presents the selected components. In particular, thepower density ρpower is calculated from the total volume Voltotalby (12)-(13). Moreover, the efficiency η of the each converters
is provided by (14).

ρ power =PoutVoltotal(12)

Voltotal = VolLbuf + VolCbuf + Volsw + Volheatsink

η=

(

Pout

Pout + Ploss + Ploss _ Cbuf + Ploss _ Cdc

)

(13)

Fig. 13. Efficiency characteristics of proposed converter. The
maximum efficiency is 96.0% when output power is 600 W.

(14)

where VolCbuf is volume of Cbuf and Volsw is the volume thepackage of the switching device. Volheatsink is the volume of thecooling system, VolLbuf is volumes of the buffer inductor or theboost inductor. Ploss is the sum of the conduction loss andswitching loss. Ploss_cbuf and Ploss_cdc are the loss of the buffer
capacitor and the DC link capacitor Cdc. These parameter of(12)-(14) is calculated by reference from [17-21]. The power
density ρpower reaches the maximum value at the range of theswitching frequency from 1 kHz to 100 kHz. Note that Fig. 14
shows a calculation result, when the components as shown intable II is used in the each converters with the power decoupling
circuit. In particular, in the proposed converter, the maximum
power density of 7.2 kW/dm3 is achieved with the efficiency of
98.6%. In contrast, in the passive topology, maximum power
density of 5.7 kW/dm3 is achieved with the efficiency of 98.8%.
Thus, the proposed system with DCM is 1.26 times higher thanthe passive topology.
b) Comparison of each volumes
Figure 15 shows the volume ratio of DC-DC converters withthe power decoupling capability at the maximum power density
point as shown in Fig. 14. The components volume isnormalized with the volume of the bulky electrolytic capacitorsof the passive topology as 100%. In the conventional buck-type
converter reduces the volume of the buffer capacitor by 74.3%
compared to the bulky electrolytic capacitors in the passive
topology. However, Volheatsink and Volsw, and the volume of theboost inductor are 61.0% and 42.2% compared to the passive
topology, respectively. The boost inductor size can be reduced
by increasing the switching frequency in generally. However,the cooling system size increases due to the increase of theswitching loss. Thus, the conditional active buffer method beachieved the downsizing of the smoothing capacitor as a Cbuf
while at the same time increasing the volume of the active buffer

Fig.14. Pareto-front of DC-DC converter with active power
decoupling capability. The proposed converter using the bucktype active buffer topology is achieved the maximum power
density at 40 kHz of the switching frequency.

circuit. In contrast, the proposed converter reduces the buffer
capacitor size by 48.7% compared to bulky electrolytic
capacitors in the passive topology. Moreover, in proposed
converter with active power decoupling, the sum of Volheatsink
and VolLbuf are reduced by 47.5% and 42.7% compared to theconventional buck-type converter.
VI. CONCLUSITON
This paper proposed a novel active power decoupling
topology based on the boost converter without any additional
magnetic component. The proposed converter utilized the boost
inductor for both the boost operation and the buffer capacitor
voltage control. In addition, the power decoupling circuit
performs the buck-type operation, and the low on-resistor
switching device could be applied into the active power
decoupling circuit. Thus, the conduction loss could become
small. As the experimental result, the proposed active power
decoupling method reduced the power ripple component at 100
Hz on the input current iin by 90.2%. In addition, the maximum
system efficiency of 96.0% was achieved at the output power of[4]Table II. Selected components.
Circuit
topology

Part

Marking

Maximum
rationPassiveCin

Nippon Chemi-Con
EKMZ451VSN181MP30S

450 V
1.0 Arms
180 F

Cbuf
Step down
type
active buffer SW1
SW2

Murata Manufacturing
EVS20329S2G306MS09

400 V
30 F

ROHM
SCT2120AF

650 V
29 A

Murata Manufacturing
EVS20329S2G306MS09

400 V
30 F

ROHM
SCT3017AL

650 V
60 A

Cbuf
Buck typeactive buffer SW1
with DCM SW2
SW3
SW4

140
120
100
80
60
40
20
0

Fig.15. Volume distribution of DC-DC converter. The volumeof the proposed converter with DCM is 0.42 dm3.

600 W. In addition, the total components volume of the DC-DC
converter in the proposed topology was less than 20.7%
compared to the passive topology. Especially, the boost inductor
volume can be reduced by 42.7% using DCM. Thus, the validityof the proposed converter with the active power decoupling was
confirmed.
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