Experimental Verification of Single-phase Inverter with Power
Decoupling Function using Boost-up Chopper
Jun-ichi Itoh, Hiroki Watanabe, Kazuhiro Koiwa and Yoshiya Ohnuma
Nagaoka University of Technology
1603-1 Kamitomioka-cho
Nagaoka city Niigata, Japan
Tel. / Fax: +81/ (258) – 47.9533.
E-Mail: itoh@vos.nagaokaut.ac.jp
URL: http://itohserver01.nagaokaut.ac.jp/itohlab/index.html

Keywords«Voltage source inverter», «Grid connected», «Single phase power ripple», «Maximum power point
tracking (MPPT)»

Abstract
This paper discusses a circuit configuration for a single-phase voltage source inverter that features
power decoupling function. Generally, the converter that is connected to a single-phase power grid isrequired to decouple the power ripple with a twice frequency of the power grid. The proposed circuit
compensates the single-phase power ripple by using an active buffer with small capacitors 50µF at 200
W. In this paper, the fundamental operations of the proposed converter are confirmed by experimental
results. Then, the proposed converter is evaluated with the maximum power point tracking (MPPT)
under a simulated photovoltaic (PV) conditions. From the experimental results, the output current
Total Harmonic Distortion (THD) is 3.51% and the output power factor is over 99%. In addition, theinput current ripple is reduced 12.3%. Moreover, from the loss analysis, the maximum efficiency of
the inverter including the active buffer circuit is 95.5%.

Ⅰ.Introduction
The dramatically cost down of photovoltaic (PV) has urged the uses of solar energy becoming
popular in the interest of energy saving. In general, the power conversion system employs a gridconnected inverter with the boost-up chopper to connect the PV modules to a single-phase grid system.
The grid-connected inverter for the PV applications can be categorized into two types, as shown in
Fig. 1. The first type uses a large power capacity inverter to connect multiple numbers of PV modules
in a series connection. The second type employs a small power capacity inverter, which is also known
as the “micro-inverter”, and then it is connected to each of the single PV cell [1]-[4]. The “microinverter” offers better features because of the following reasons; (i) Maximum Power Point Tracking
control (MPPT) can be easily applied in each of the individual PV cell. (ii) Optimal design for control
is simple. (iii) Capacity of the PV system can be easily modified due to the simple structure.
However, micro-inverters have problems in costing and sizing, since the micro-inverter is needed a
lot of numbers for PV system in comparing with the first type system which only employs a large
power capacity single-phase inverter.
Although the lifetime of a PV module is namely 25 years, the lifetime of the electrolytic capacitor istypically 1000-7000 hours at 105 degree Celsius operating temperature. As a result, periodicallymaintenance is required for the conventional micro-inverter. Besides, the volume of the electrolytic
capacitor dominates the total volume of the micro-inverter. Moreover, the initial charging for theelectrolytic capacitor is needed.

In order to remove the large electrolytic capacitor, the power decoupling methods between DC sideand AC side known as the DC link active filters have been studied [5]-[7].
These topologies technically can reduce the capacitance value of the DC link capacitor, however
extra devices and passive components are required. Consequently, the efficiency is degraded and thecost is higher due to the additional circuit.
In the paper, the authors propose a power decoupling method that uses an active buffer circuit in the
micro inverter. The active buffer circuit is employed to achieve boost-up function for PV voltage andto decouple the power fluctuation with a small capacitor at the same time. Due to the circuit of Ref .[5],the proposed circuit has the following advantages; (i) A resonant DC/DC converter can apply the zerocurrent switching (ZCS), and (ii) A proposed circuit can compose the 6 in 1 modules. Thus,
miniaturization is possible. (iii) For the power decoupling, the active buffer can use the switch of theboost-up chopper. Thus, Additional switch for the power decupling is not need.
This paper is organized as follows: first, the constitution of the proposed circuit is shown. Next, theprinciple of the power decoupling control strategy is described. Then, the proposed circuit isdemonstrated by the experiment. From the result, the output current THD is 3.51%, the ratio of the
input current is 14.3% and the output power factor is over 99%. At last, the loss analysis shows thatthe converter can achieve efficiency 95.5%.

(a) The large capacity inverter.
(b) The small capacity inverter.
Fig.1. A two type grid-connected inverter for the PV applications.

Ⅱ. Circuit topology
A. Conventional circuit
Figure 2 shows the single-phase grid connected inverter with a power decoupling function for PV.
The DC voltage of the single PV cell Vin is relatively low, generally ranging from 25 V to 50 V. On
the other hand, the power grid voltage is typically 110V-240 V. For the reason, the boost-up chopperis necessary for the grid-connected inverter. In addition, the isolated DC/DC converter is connected
the boost-up chopper before.
The power ripple which has twice of the power grid frequency occurs in the input DC current. Dueto the reason of large current ripple, the effeciency of the PV is decreased. In order to suppress thepower ripple, the large electolytic capacitor C is connected in the DC link part. The electrolytic
capacitor requires following; (i) large space and an initial charge circuit. (ii) Thus, due to short lifetime, regular maintenance is required. As a result, the conventional circuit becomes bulky and highcost.

B. Proposed circuit
Figure 3 illustrates the proposed circuit. This circuit is constructed from a resonant DC/DC
converter, an active buffer circuit and a voltage source inverter. The resonant DC/DC converter
applies zero current switching (ZCS) control technique to reduce the switching loss, and then the highfrequency transformer increase the input voltage by five times. The active buffer is constructed from aboost chopper and a buffer capacitor C3. In order to compensate the power ripples. The buffer
capacitor can be employed with a small film capacitor 50µF at 200 W, depending on the capacitor
voltage. Then, the voltage source inverter is used to control the output voltage and the output
frequency. Comparing the circuit in fig.2, the advantage of the proposed circuit is following; (i)Additional components is only a buffer capacitor C3 only for power decoupling. (ii) Due to ZCS, theswitching loss of the DC/DC converter can be reduced. (iii) The propose circuit become smaller a 6 in
1 module.

VL

Fig.2. Conventional circuit.

VC

VL

Fig.3. Proposed circuit.

Ⅲ. Control method
A. Principle of power ripple compensation.
Figure 4 shows the principle of the power decoupling between the DC and AC sides. When both the
input voltage and current waveforms are sinusoidal, the instantaneous output power pout is expressedaspout =

Vout I out(1 − cos 2ωt )2(1)where, Vout is the peak voltage, Iout is the peak current, and is the angular frequency of the output
voltage. From (1), the power ripple, that contains twice frequency of the power grid, appears at the DClink.

In order to absorb the power ripple, the instantaneous power pbuf of an active buffer, should becontrolled bypbuf =

1
Vout I out cos 2ωt2(2)

where, the polarity of the pbuf is defined as positive when the active buffer discharges. Note that themean power of the active buffer is zero because the active buffer does not generate the power [8].

Fig.4. Compensation principle of power decoupling.

B. Control block diagram
Figure 5 shows the control block diagram. Automatic voltage regulator (AVR) is implemented tocontrol the voltage of the buffer capacitor C3. Thus, high speed response is required in the AVR
because the control reference signal frequency is twice of the single-phase power grid frequency. In
order to solve this problem, controlling the capacitor voltage which is implemented on the rotational dq frame, has been discussed in Refs. [9] [10]. Although the AVR does not require a high speed
response in the method, the control structure is complicated due to using the rotational frame. The
active buffer circuit is applied with automatic current control (ACR) only, and then the inverter isapplied with AVR to control the output voltage.
In the paper, the current reference iamp* which intendeds to compensate the power ripple is added
into the buffer reactor current reference of an ACR, so as the high speed AVR is not required in the
proposed circuit. The instantaneous buffer reactor current reference iL* is expressed by (3)
*
*
*
*
*
i L = iamp + iin ＝ic + iin(3)where, iamp* is the power ripple compensation current reference, and iin* is the DC component in thebuffer reactor current.
The power ripple compensation current reference iamp* is calculated by output power pout and buffer
capacitor voltage vc Thus, the power ripple compensation current is expressed by (4)p**iamp = ic = out cos(2ωt )(4)vc
On the other hand, the DC component in the buffer reactor current iin* is expressed by (5)p*iin = in(5)
vinwhere, pin is the input power, and vin is the transformer secondary voltage. Then, the current
reference iamp* and the DC current iin* are calculated by (4) and (5).
Relationship among the DC link voltage vinv, transformer secondary voltage vrec and buffer capacitor
voltage vc can be expressed by (6).
vinv = vrec + vc
(6)
Note that, the inverter input voltage vrec is fluctuating due to the power ripple from PV panel and theimplementation of maximum power point tracking (MPPT). In order to control the DC link voltage vinv,
reference value of AVR is set more than the grid voltage. However, the buffer capacitor voltage vc

fluctuates by twice frequency of the single-phase grid. As a result, the output current THD is decay.
The detection value of the DC link voltage is applied with a band eliminate filter (BEF) in order tosolve this problem. Then the AVR controls output voltage by referring to the average value of the DC
link voltage. Since the output is a grid, phase locked loop (PLL) is applied to ensure that the phaseangle of the inverter current is identical to the grid.
*

Vin
Iin
Vout

Vc

*

iamp

*

*

V

*

iL

iin

iout_cmd

*
inv

Vedc
iout

*

Vout I out
cos[2θ ]
Vc

θ

*

*

Vconv

*

iout _ cmd Sin[θ ]*iout _ cmd Sin[θ ]

*

Vout

Vinv_lpf

PWM transform

Vout
*
Iout

iL
Fig.5. Control block diagram.

C. Maximum power point tracking (MPPT)
Figure 6 shows the maximum power point tracking (MPPT), known as the hill climbing method.
The MPPT is controlled in four modes as illustrated as Phase1, Phase2, Phase3, and Phase4.
In Phase1, the input current is increased to observe the maximum point of input power. In Phase2,
the present value will be compared with the last value, if the present value of the input power is lower
than 20% of the previous maximum power point, then the input current will be increased in order tomaintain the point of maximum power. In Phases3 and 4 are the repeating modes similar to phases 1
and 2. As a result, the input current is increased or decreased by (7)(7)

iin = ±0.8impp

Fig.6. Method of the maximum power point tracking.

IV. Design method of boost-up reactor and buffer capacitor
In this section, the design method of the parameters for boost-up reactor and buffer capacitor are
discussed. At first, the buffer capacitance Cf is expressed from Wc which is the electric storage energy
to compensate the power ripple, by (8),

Cf =

Wc
V I
= out out2
ωVc rc 4ωVc 2 rc

(8)

where, rc is the ratio of the voltage ripple that is expressed by (9),rc =ΔVc / 2Vc

(9)On the other hand, the boost-up reactor Lf is obtained by (10),Lf ≥Vc _ maxVin
2rl I in (Vc _ max + Vin ) f s(10)where, Vc_max is the buffer capacitor voltage at maximum point. Additionally, rl is the ratio of thecurrent ripple reactor current iL that is expressed by (11),rl =ΔI L / 2
I in

(11)

From (8), the buffer capacitor Cf is 50µF when the rated power is 200W, and the grid voltage
frequency is 50Hz. Where the amplitude of the buffer capacitor voltage is expressed by (12),
Wc
ΔVc =
(12)
ωC f Vave
From (10), the buffer reactor Lf is 6mH. Here, the design value of the current ripple is 30%.

V. Experimental results
A. Fundamental Operation
In order to demonstrate the validity of the proposed circuit, a 200 W class prototype circuit wastested. In this experiment, the proposed circuit is connected to 200 V grid voltage. Table 1 shows theexperimental parameters.
Figure 7(a) shows the experimental results without the power decoupling control. Note that, theactive buffer operates as the boost-chopper does not compensate the power ripple. According to Figure7(a), the input current ripple is fluctuated at approximately 5 A (peak to peak). Figure 7(c) shows thebuffer capacitor voltage and buffer inductor current waveforms without power decoupling control.
According to 7(c), the buffer inductor current is not fluctuated twice frequency of a single phasepower grid.
Figure 7(b) shows the operation waveforms that the power decoupling control is applied. As a result,the input current fluctuation is reduced to less than 1 A (peak to peak). In addition, the output power
factor of the rating operation is 0.99, and a clean the sinusoidal waveform is obtained at the output
current. Figure 7(d) shows the buffer capacitor voltage and buffer inductor current waveforms withpower decoupling control. According to Figure 7(d), the buffer capacitor voltage VC is fluctuated at
approximately 63.6 V (peak to peak) due to the power decoupling control. The result illustrates thatthe buffer reactor current is fluctuating because the active buffer circuit is decoupling the powerfluctuation.
Figure 8 shows the result of harmonic analysis. The second order harmonic component is reducedby 87.7% compared to that without the power decoupling control. The reason that the second orderharmonic components exist is because; (i) The compensation value is not enough at 200 W. (ii) The
phase of buffer power pbuf is not matching to the single-phase power grid. This will be improved in the
future work.
Figure 9(a) show the efficiency and power factor of the proposed system in subjecting to the output
power. According to Figure 9(a), the maximum efficiency of the DC/DC converter is 96.2%, and the
maximum efficiency of the inverter including the active buffer is 95.5%. However, when the output
voltage is 50W, the efficiency becomes low due to the low power factor. Figure 9(b) shows the output
current THD and input current ripple of the proposed system in subjecting to the output power.
According to Figure 9(b), the input current ripple is 12.3% when the output power is 150 W. In
addition, the output current THD of the inverter is less than 5% when the output power is more than100 W.
Table.1. Experimental parameters.

(a) Without power decoupling control.

(b) With power decoupling control.

(c) Without power decoupling control.
(d) With power decoupling control.
Fig.7. Experimental results.

Fig.8. Harmonic analysis.

(a) Efficiency and power factor.
(b) THD and current ripple.
Fig.9. Load characteristics.

B. Loss analysis
Figure 10 shows the loss analysis of the proposed system at 200 W. The loss ratio of the DC/DC
converter is equal to 45% (6.84 W) of the total losses. Due to the implementation of the ZCS, theswitching loss of the DC/DC converter can be reduced to nearly zero. However, the isolatedtransformer contains of large amount copper and iron losses.
On the other hand, the loss ratio of the active buffer is equal to 40% (6.08 W) of the total losses.
The largest losses are dominated by the 6mH reactor, which are the copper and iron losses. The copper
loss can be reduced once a higher switching frequency is applied to the active buffer circuit as thewiring can be reduced.
The inverter loss is equal to 15% (2.28 W) of the total losses. Schottky-Barrier-Diode (SBD) is usedin this prototype therefore the recovery loss is not considered. From the result of loss analysis, thedownsizing of the buffer reactor is important in order to further improve the efficiency.

Fig.10. Property of the loss simulation.

C. Experimental result of the maximum power point tracking.
Figure 11 shows the experimental result of the MPPT. In this paper, the MPPT is demonstrated inthree conditions. When the maximum input power is changed at 100 W, 150 W, and 200 W, the input
power will track the maximum power for each of the point by MPPT. In order to simulate the PV
output power as illustrated early, the resistor is connected to the DC power supply in series. The input
current is increases when the load is increased. Thus, the voltage drop of the resistor is increased andresult that the input voltage of the converter decreases. From this reason, the maximum power point
can be observed. However, when the maximum input power is 200W, the measurement value differs
in regarding to the previous condition, due to the input current limit is set to 8 A. This problem can be
improved by increasing the input current limit. However, the fluctuation of the input power is large,
because the permissible fluctuation range of the input power is large. Thus, the fluctuation of the input
power can be decreased by setting the permissible fluctuation range of the input power smaller.

Fig.11. Experimental result of MPPT.

VI. Conclusions
This paper proposed a circuit topology for the single-phase voltage source inverter that features
power decoupling function. The advantages of the proposed circuit are following;z The proposed circuit compensates the single-phase power ripple by the active buffer. Thus,
large electrolytic capacitor is not needed on the DC link. In addition, the buffer capacitor can beapplied small capacitor.
z The proposed circuit can be miniaturization by using the 6 in 1 module.
z The active buffer is configured the boost chopper and buffer capacitor.
z The proposed circuit can apply with the MPPT.
z Long life-time due to the use of a film or ceramic capacitor.
The proposed circuit was experimented by using a 200W prototype. From the experimental results,
the output current THD is 3.51%, the ratio of the input current is 14.3% and the output power factor isover 99%. The maximum efficiency of the DC/DC converter is 96.2%, active buffer and inverter is
95.5%. In addition, the loss analysis is obtained. As a result, the loss ratio of the DC/DC converter isequal to 45% (6.84 W).
On the other hand, the loss ratio of the active buffer is equal to 40% (6.08 W), the inverter loss isequal to 15% (2.28%). In addition, the input power which is controlled approximately maximum power
point on theory is obtained when the input power is 100 W, 150 W, 200 W.
In future work, reduction of the capacity of the buffer capacitor and buffer reactor will beconsidered.

References
S. B. Kjaer, JK Pedersen: "A review of single-phase grid-connected inverters for photovoltaic modules",IEEE Trans., Vol. 41, No. 5, pp. 1292-1306 (2005).
[2] H. Hu, S. Harb, N. Kutkut, I. Batarseh, Z. J. Shen: "Power Decoupling Techniques for Micro-inverters in
PV Systems-a Review", ECCE2010, Vol. 32826, No. 2, pp. 3235-3240 (2010)s", IEEE Trans., Vol. 41, No.
5, pp. 1292-1306 (2005).
[3] T. Shimizu, K. Wada, N. Nakamura: "Flyback-Type Single-Phase Utility Interactive Inverter With Power
Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System", IEEE Trans., Vol. 21, No. 5,
pp. 1264-1272 (2006)[4] D. Cao, S. Jiang, X. Yu: "Low-Cost Semi-Z-source Inverter for Single-Phase Photovoltaic Systems", IEEE
Trans., Vol. 26, No. 12, pp. 3514-35-23 (2011)[5] F.Shinjo, K.Wada, T.Shimizu:"A Single-Phase Grid-Connected Inverter with a Power Decoupling Function" PESC 2007, pp.1245-1249, (2007)[6] Kuo-Hen Chao, Po-Tai Cheng: "Power decoupling methods for single-phase three-poles AC/DC converters",
ECCE 2009, pp3742-3747,(2009)[1][7] F. Schimpf, L. Norum: "Effective Use of Film Capacitors in Single-Phase PV-inverters by Active Power Decoupling",
IECON 2010, Vol. , No. , pp. 2784-2789 (2010)[8] Y. Ohnuma, J. Itoh: "A Control Method for a Single-to-three-phase Power Converter with an Active Buffer and aCharge Circuit", ECCE2010, Vol. , No. , pp. 1801-1807 (2010)[9] FANG Xiong, WANG Yue, LI Ming, LIU Jinjun: "A novel frequency-adaptive PLL for single-phase grid-connected
converters", ECCE2010, Vol. , No. , pp. 414-419 (2010)[10] K. Toyama, T. Shimizu: "Study on a Control Method of a Single-Phase Utility Interactive Inverter with Power
Decoupling Function", 2012 KJJTWS, Vol. , No. IEEJ-P1-07, pp. 75-76 (2012)