Symposium on Semiconductor Power Conversion, Nagaoka, Japan

DC to Single-phase AC grid-connected inverter with Boost Type
Active Buffer Circuit Operated in Discontinuous Current ModeJun-ichi Itoh＊a), Tomokazu Sakuraba, Hoai Nam Le, Keisuke Kusaka
A circuit configuration for a single-phase inverter with a power decoupling capability operated in discontinuous current mode(DCM) is proposed in this paper. The inverter connected to a single-phase grid requires the power decoupling capability tocompensate a power ripple with twice the grid frequency. Bulky capacitors are required as a DC-link capacitor in a conventionalsystem. In contrast, the proposed active buffer, which operates without an additional inductor for a buffer circuit, uses ceramic
capacitors instead of bulky electrolytic capacitors. In this paper, a control method for the power decoupling circuit operated in
DCM is described. In addition, the validity of the proposed circuit is experimentally demonstrated by a 600-W prototype. As theexperimental results, the input current ripple at twice the grid frequency is reduced by 96.8%.
Keywords : PV inverter, Power ripple compensation, Discontinuous current mode

1.

Introduction

In recent years, single-phase grid connected converters havebeen studied actively as the power conversion systems (PCSs) fora PV system. Instantaneous power of the single-phase grid
oscillates at twice the grid frequency whereas the output power isconstant. As a result, a power ripple with twice the grid frequency
occurs in the DC-link. In order to absorb this power ripple, bulky
electrolytic capacitors are used in the conventional circuits.
However, the electrolytic capacitor limits the lifetime of the PCSs.
As an alternative power decoupling method, an active power
decoupling, which consists of small capacitors, switching devices
and inductors has been proposed [1-2]. The PCS with long
lifetime is expected by using film or ceramic capacitors. However,an additional circuit is necessary in order to control the buffer
capacitor voltage. In particular, an additional inductor leads to low
power density. Moreover, buck type active buffer circuits, whose
the buffer capacitor voltage is lower than the DC-link voltage, areproposed [3-4]. In these circuit topologies, since the maximum
voltage of the buffer capacitor is limited, the reduction of thebuffer capacitance has limitations in order to satisfy the average
stored energy of buffer capacitor CV 2/2.
This paper proposes a novel circuit topology, which requires noadditional inductor for the power decoupling circuit in order toachieve higher power density. The proposed circuit uses
discontinuous current mode (DCM) for the power decoupling
control. In the proposed circuit, the capacitance of the buffer
capacitor can be reduced by using the higher buffer capacitor
voltage than DC-link voltage. The validity of the proposed circuit
is experimentally demonstrated by a 600-W prototype.

2.

S4
S3SupSvp

S2

Lgri

Lfil

Cfil

voutvdc
Linvbuf

Lboost

Vin

Cin

S1

Cbuf

Cdc
Sun

Svn

Active power decoupling circuit

pin

Fig. 1. Active power decoupling circuit operated in DCM.
pbuf Buffer power
pout
Input power
Output power
Vm I mPin discharge
2Constant value0+=t0tcharge

0tToutFig. 2. Single-phase power pulsation compensation.

By using the zero-current period, the DC-link voltage and thebuffer voltage are controlled by only one boost inductor.

3.

Power decoupling and input control strategy

Fig. 2 shows the relationship among the input power pin, theoutput power pout and the compensation power pbuf in the activebuffer. The output instantaneous power is given in (1) when theoutput current is the sinusoidal wave with unity power factor.

Proposed Boost-Type Active-Buffer Circuit
Operated in DCMpout Vm I m1  cos 2out t  ............................................... (1),2where Vm is the peak voltage of the single-phase grid and out isthe angular frequency of a grid. From (1), the power ripple, whose
frequency is twice the grid frequency, occurs in the DC link. Inorder to absorb the power ripple, the instantaneous power pbuf inthe active buffer is controlled according toFig. 1 shows a DC-AC converter integrating a power
decoupling in the DC-DC converter stage. This circuit requires noadditional inductor for the power decoupling circuit. When theboost converter operates in the continuous current mode (CCM),
the discharge mode for the buffer capacitor cannot be realized
because the current direction cannot be changed suddenly in CCM.
Thus, in this paper, the charge and discharge modes are achievedby the discontinuous current mode (DCM). In particular, thezero-current period is utilized for the power decoupling operation.

1pbuf  Vm I m cos 2out t ................................................... (2).
2Fig. 3 shows the proposed control block diagram in the DC-DC
converter stage. The proposed control block consists of theDC-link capacitor voltage, the buffer capacitor voltage and theboost inductor current controls. These current and voltage
regulators are employed by PI controllers.

a) Correspondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.ac.jp＊Nagaoka University of Technology
1603-1, Kamitomioka-machi, Nagaoka, Niigata, Japan, 940-2188
1

Active Buffer Circuit Operated in Discontinuous Current Mode (Jun-ichi Itoh et al.)iL _ ave _ dc i peak2

DC-link capacitor voltage control
iinv
iL_ave_dc*+++

vdc*+PId1  d 2   P 1  cos 2out  ................... (3),Vmd2d3d44GateSignal
S1~S4Fig. 3. Control block for DC link voltage and buffer voltage.
Table 1. Circuit parameters of the prototype.
Rated power
P
600 W
Input voltage
150 VVinvdc
DC-link voltage
300 V
400 V
Buffer average voltage vbuf_ave
Switching frequency
fsw
20 kHz55.6 mF
Cbuf
CapacitanceCdc
57.2 mF
56.6 mH (Critical condition:87.9 mH)
LboostInductance
5.3 mH (%Z=2.5%)
Lgri
S1~S4Rohm, SCH2080KE
Switching device
Sup~SwnFuji electric, FGW30N60VD
Boost inductor current iL[20 A/div]Experimental verifications

000

Input current iin [5 A/div]

0Buffer capacitor voltage vbuf
[250 V/div]
Output current iout [10 A/div]0iL [20 A/div]iin [5 A/div]
vbuf [250 V/div]
iout [10 A/div]

04 ms/div(a) Without power decoupling

4 ms/div(b) With power decoupling

Fig. 4. Experimental waveforms.
(Cutoff frequency of low pass filter:2kHz)

boost inductor current [%]100% : 100 Hz component
without power decoupling

100Conclusion

96.8% reduces.
Without active power decouplingWith active power decoupling

1010.18 10 12 14 16 18 20
Harmonic numberFig. 5. Harmonics components on the boost inductor current.

References

(2)

vin
vbuf vin

iL_ave_bufv inPI

d1

Carrier 1(20 kHz) 0

This paper proposed a new integrated DC-DC converter whichuses discontinuous current mode for the power decoupling
capability. The proposed circuit requires no additional inductor forthe power decoupling circuit. This contributes to the high power
density of the single-phase inverter. The experimental results
demonstrate that the current ripple of the boost inductor with twice
the output frequency of the inverter is reduced by 96.8% by thepower decoupling control.

(1)

v buf

vbufPIvin
vdc vin

iL_ave_dc

Buffer capacitor voltage control
ibuf*
vbuf_ave*iL_ave_buf*++++

4.1 Operation waveforms
Fig. 4(a) presents the operation
waveforms without the power decoupling control when a small
prototype of 600 W is operated. The boost inductor current
oscillates at twice the grid frequency when the power decoupling
is not applied. On the other hand, in Fig. 4(b), the buffer capacitorvoltage with the power decoupling control is oscillated at twice
the grid frequency. As a result, the single-phase power ripple iscompensated by the active power decoupling, which is confirmedby the significant ripple reduction in the boost inductor current.
4.2 Evaluation for current ripple
Fig. 5 shows theharmonics analysis result of the boost inductor current. When thepower decoupling is applied, the second order harmoniccomponent is reduced by 96.8% compared to that without power
decoupling. Thus, the compensation of the single-phase powerfluctuation is confirmed.

5.

vdc
Vinvdcwhere, P is the rated output power. Thus, the boost inductor
current fluctuates at twice the grid frequency when the power
decoupling control is not applied.
3.2 Buffer capacitor voltage control
In order to absorb
the power ripple by the buffer capacitor, the buffer capacitor
voltage is controlled to oscillate at twice the grid frequency.
However, when the voltage command with twice the gridfrequency is input, the rapid response is required for the voltage
control. Thus, by adding the inductor current command i*buf of thepower ripple compensation to the output of a PI controller of thevoltage regulator, the buffer capacitor voltage is oscillated at twice
the grid frequency. When the power ripple is compensatedcompletely by the power decoupling, the average current of theboost inductor is the constant value. Thus, from (3), the inductor
current command for the power ripple compensation is given byP*ibuf
cos 2outt  ...................................................... (4).
vbuf

4.

PI

Modturation

3.1 DC-link capacitor voltage control
In Fig. 3, thevoltage command vdc* for the DC-link is set to be higher than themaximum output voltage. Fluctuation of the DC-link voltage
causes the increase of the output current THD. Thus, in order tocontrol the constant value of the DC-link voltage, the inverter
input current is feedforwarded to the output of a PI controller ofthe voltage regulator. The average current, which controls theDC-link voltage, is expressed by(3)A. Tokumasu, K. Shirakawa, H. Taki, K. Wada : "AC/DC Converter Basedon Instantaneous Power Balance Control for Reducing DC-Link
Capacitance",IEEJ Journal of Industry Applications,Vol.4,No.6,pp.745-751(2015)
S. Yamaguchi, T. Shimizu : "Single-phase Power Conditioner with a
Buck-boost-type Power Decoupling Circuit", IEEJ Journal of Industry
Applications, Vol. 5, No. 3, pp. 191-198 (2016)(4)

202(100 Hz)

46

Yi Tang, Frede Blaabjerg, Poh Chiang Loh, Chi Jin, Peng Wang :"Decoupling of Fluctuating Power in Single-Phase Systems Through a
Symmetrical Half-Bridge Circuit", IEEE Trans., Vol. 30, No. 4, pp.
1855-1865 (2015)
K. H. Chao, P. T. Cheng, T. Shimizu : "New Control Methods for Single
Phase PWM Regenerative Rectifier with Power Decoupling", IEEJ PEDS,
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