a current shaping method for pv-ac module

8/12/2019 A Current Shaping Method for PV-AC Module

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A Current Shaping Method for PV-AC ModuleDCM-Flyback Inverter under CCM Operation

Young-Hyok Ji 1, Doo-Yong Jung 1, Jae-Hyung Kim 1, Tae-Won Lee 2, and Chung-Yuen Won 11 Sungkyunkwan University, Suwon, South Korea

2 Samsung Electro-Mechanics, Suwon, South Korea

Abstract In the grid interconnected photovoltaic powergeneration system including PV-AC module system, theoutput current have to be sinusoidal synchronized with thegrid voltage. Concerning the voltage-controlled current-source modification requirement and the characteristics offlyback topology, boundary or discontinuous conductionmode operations are suitable for the PV-AC module flybackinverter. However, the flyback inverter can be operated incontinuous conduction mode (CCM) due to its operating

conditions in spite of it is designed to operate underboundary or discontinuous conduction mode. Unfortunately,it causes unintended distortion to output current. In thispaper, a current shaping method to reduce the outputcurrent distortion for the DCM-flyback inverter is proposed.Even though the flyback inverter operates under CCM, theoutput current distortion can be reduced by the unfoldingH-bridge switching technique.

Index Terms Photovoltaic AC module systems, Flybackinverter, Unfolding H-bridge.

I. I NTRODUCTION

Since the conventional photovoltaic (PV) powergeneration systems, several photovoltaic modules areconnected in series and parallel to form an array and feedenergy to a single centralized inverter or to a few parallelstring inverters, have some drawbacks on mismatch

problems, the use of several decentralized grid-connectedPV systems is quite more appropriate as they can also beeasily installed on buildings.

The decentralized grid-connected PV systems areusually named PV-AC module. In the PV-AC modulesystems, the output of the PV module is directlyconnected to the utility grid through a module integratedconverter (MIC) [1]. In the AC module system, themaximum power point (MPP) of each PV module can betracked individually. Thereby the overall utilization ofsolar energy can be increased. And the potential arcing

problems and cost problems due to DC wiring can also beavoided.[2][3] The AC module system has become thetrend for the future PV system development and a numberof single-phase power inverter topologies for AC modulesystem have been reviewed recently[4][5].

Among the topologies of various configurations, theflyback inverter topology has been recognized as anattractive solution for low cost application because of thesimplicity of its configuration and control. This topologyallows the required voltage level step-up, the maximum

power point tracking (MPPT) and current shaping all to be performed in a single power conversion stage withminimum component counts. However, to become the

AC module system using flyback type inverter viable,further improvements including the power conversionefficiency improvement, system cost reduction andoverall reliability increment are required. Thereby variousapproaches have been performed by researchers [6]-[8].

The basic idea of flyback inverter for PV-AC modulesystem is to make the flyback inverter as a sinusoidalcurrent source synchronized with grid voltage. It can be

achieved by operating the flyback inverter underdiscontinuous conduction mode (DCM) which is hard toincrease power conversion efficiency due to its largeRMS and peak current values [9]. Even though higherefficiency can be achieved under continuous conductionmode (CCM), the output current of the flyback invertercan be distorted and some specialized techniques forcurrent shaping are required to overcome this problemunder CCM operation [10]-[12].

The flyback inverter which has an unfolding H-bridgeis applied in this paper as shown in Fig. 1. The designconsideration of this configuration will be presented inthe next section. The major objective of the present work

is to reduce the distortion of output current of DCM-flyback inverter by the unfolding H-bridge switching,even if the flyback inverter is operated under CCM due tothe variation of input condition (i.e. photovoltaic outputvoltage and current).

II. F LYBACK I NVERTER CONFIGURATION AND A NALYSIS

PV Grid

m L

pQinC

s Dk L

aQaC

f C f L

Active Clamped Flyback Converter Unfolding H-Bridge

sC

1 sQ

2 sQ

3 sQ

4 sQ1:n

Fig. 1. Configuration of the flyback inverter

spQ s p D

snQ sn D f C

f L

pQinC

k L

aQaC

m L

Fig. 2. Configuration of the conventional flyback inverter

8th International Conference on Power Electronics - ECCE Asia May 30-June 3, 2011, The Shilla Jeju, Korea

978-1-61284-957-7/11/$26.00 2011 IEEE

[WeP1-030]


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The configuration of the flyback inverter used in this paper is shown in Fig. 1. The flyback inverter used in this paper is comprised of two-stages. The active-clampedflyback converter converts the DC output of PV moduleto rectified-DC synchronized with grid voltage. Also, theshape of output current of the flyback inverter is

determined by the switching of flyback converter.An active-clamped circuit which composed of clampcapacitor, C a , and an auxiliary switch, Qa , is applied tothe flyback converter to reduce the voltage stress offlyback main switch, Q p, and to increase powerconversion efficiency [13]. The clamp or snubber circuitis an important factor which determines the performanceof the flyback based circuit. However, the active clamptechnique is not a dominant factor for output currentshaping that focused in this paper. Thus, the active clamptechnique will not be considered in this paper.

The unfolding H-bridge stage determines the directionof output current to be matched with grid voltage polarity.

The unfolding switches Q1 and Q4 are turned-on and theQ2 and Q3 are turned-off during the positive period ofgrid voltage. During the negative period, the unfoldingswitches are operated oppositely.

As already known, power conversion systems for photovoltaic applications have to track the MPP. In theflyback inverter shown in Fig. 1, the maximum power

point tracking (MPPT) can be performed by peak pointvariation of the switching reference. Thus, MPPTdetermines the amplitude of output current reference.Because of the unfolding H-bridge determines only thedirection of the output current conventionally, the flybackconverter stage covers the output current shaping as well

as the MPPT. Thus, the flyback converter switching isdetermined by rectified sinusoidal reference whichsynchronized with the phase of grid voltage.

* FBi t g v

pQ

1 sQ

2 sQ

Lmi

Qpi

1Qsi

2Qsi

g i

Fig. 3. Key waveforms for flyback inverter operation

A. Unfolding H-Bridge ConfigurationAs mentioned above, the flyback inverter used in this

paper is comprised of two stages (i.e. the flybackconverter stage and unfolding H-bridge), while theconventional flyback inverter looks like a single-stage

power conversion system as shown in Fig. 2. However,

fundamental operating theory is same, and the controlschemes are also same to each other. Fig. 3 shows theoperating waveforms.

The flyback inverter composed of two stages has someadvantages to get higher power conversion efficiency dueto its H-bridge based unfolding method, while the single-stage configured flyback inverter is a cost effectivesolution for photovoltaic AC module systems.

In the single-stage flyback inverter in Fig. 2, a threewinding transformer is required. It means that largerwinding area is required even though the turns-ratio ofthe transformer, n, is same in Fig. 1, and Fig. 2. Thus, thetotal loss of transformer used in single-stage flybackinverter can be much larger than that in the two-stageflyback inverter.

On the other hand, the major difference between theunfolding H-bridge used in this paper and theconventional unfolding stage shown in Fig. 2 is appearedin the voltage stress of the secondary switching devices.In the Fig. 2, as already known, the maximum voltage ofthe secondary active switches Q sp and Q sn is equal to2V g ( pk ), where V g ( pk ) is the peak value of the grid voltage.

Actually, the required voltage rating of the secondaryactive switches can be higher than 2V g ( pk ) due to theoscillation or ringing of the secondary current. In case ofthe 220V rms grid applications, the voltage rating ofsecondary switches recommended to be higher than 800V.Unfortunately, higher voltage rating of the switchingdevices causes larger conduction losses.

The unfolding H-bridge requires four active switchingdevices as shown in Fig. 1, and it includes four driverswhich increase system costs. However, the maximumvoltage of the unfolding switching device can be a half ofthat of the conventional unfolding stage shown in Fig. 2.Thus, in the power conversion efficiency point of view,the unfolding H-bridge can be more useful solution.

As known, the output current of the flyback inverter, i g ,can be represented as (1).

1 1 g Lf Cf g f f

i v dt v v dt L L (1)

where L f is filter inductance, v Lf is voltage of the filterinductor L f .and vg is the grid voltage. During the positive

period of grid voltage, the voltage of filter capacitor, vCf ,is equal to link voltage, vCs, due to the unfolding bridgeoperation. If the voltage drop of unfolding H-bridge isneglected, the i g can be written as;

1 g Cs g

f

i v v L

dt (2)

Consequently, it is obvious that the output current can be controlled by the link voltage, vCs, under the normalunfolding bridge operation.


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B. Link Voltage Comparison between DCM and CCMConcerning the operation of the unfolding H-bridge,

the output current can be controlled by the link voltage.As already known, the voltage transfer function of theflyback converter is affected by the conduction mode ofmagnetizing current.

Fig. 4 (a) and (b) shows the magnetizing current, i Lm,of the transformer during DCM and CCM operation,respectively. Concerning the operation theory of theflyback topology, the secondary currents, is, due to theconduction modes can be represented as Fig. 4 (c) and (d)where the I o is the average output current of the flybackstage during one switching cycle. In here, the effect ofactive clamp circuit is neglected for simplicity of analysis.

The average output current of the flyback stage, I o , foreach conduction mode can be represented as follows;

2,

12

pv DCM oo DCM

L m s

V DV I

R L f V o

(3)

,1

12

oo CCM CCM A B

L

V I D i i

R (4)

where the V o means the average link voltage across thelink capacitor, C s, during one switching cycle. From thevoltage-second balance, and the above equations, theaverage link voltage for each conduction mode can berepresented as follows;

, 2 g

o DCM pv DCM m s g

V V V D

L f I (5)

, 1 pv CCM

o CCM CCM

nV DV

D (6)

where the V g and I g mean the root-mean-square value ofthe grid voltage and current, respectively. The f s isswitching frequency of the flyback main switch Q p andthe Lm means the magnetizing inductance.

Lmi

t 0 sT DCM D r D w D

Lmi

t 0 sT CCM D 1 CCM D(a) DCM magnetizing current (b) CCM magnetizing current

si

t 0 sT

sp I

o I

si

t 0 sT

Ai

o I Bi

r D w D (c) DCM secondary current (d) CCM secondary current

Fig. 4. Magnetizing and secondary currents under DCM and CCM

Design of the magnetizing inductance is an importantfactor to determine the conduction mode of flybackoperation. Refer to DCM requirements, the magnetizinginductance, Lm, have to be satisfied by (7) for PV-ACmodule application.

2

4 pv DCM

m g g s

V D LV I f

(7)

The turns-ratio of the transformer, n, is also adominant factor which determines power conversionefficiency as well as the voltage stress of the flybackmain switch Q p. More detailed design consideration willnot be taken into account in this paper. However, theflyback inverter used in this paper has been designedthrough careful analysis and designed consideration. Thedesign parameters are listed on Table I.

TABLE IFLYBACK I NVERTER DESIGN PARAMETERS

Parameter Symbol Value UnitMax. Handling Power P max 250 W Nominal PV Power P pv 200 W

Nominal Input Voltage V pv 30.3 V Nominal Input Current I pv 60.6 A Nominal Grid Voltage V g 220 V rmsTransformer Turns-ratio n 1:4 -Magnetizing Inductance Lm 11 uH

Switching Frequency f s 50 kHzLink Capacitance C s 150 nF

Output Filter (Ind./Cap.) L f /C f 5.7 / 33 mH / nF

Through the above equations from (5) to (7), and thespecified design parameters on Table I, the difference of

link voltage between DCM and CCM operation can berepresented as Fig. 5. As shown, the link voltage underDCM operation is proportion to the duty ratio of the mainswitch, linearly.

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

L i n k V o l

t a g e , V o

[ V ]

Duty Ratio, D

DCM CCM

Fig. 5. Voltage transfer function for DCM and CCM

Concerning the linear characteristics of link voltageunder DCM operation, the sinusoidal output current can

be fed to the grid through the rectified sinusoidal dutyratio variation in DCM operation. If the power loss isneglected, required duty can be simply determined by (8).

2 sin pv m s DCM pv

I L f D t V

t (8)


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From the consideration of boundary duty ratio and theequation (8), following condition has to be satisfied toensure the DCM operation.

2 2 pv m s g

pv pv g

I L f V

V nV V (9)

If the (9) is not satisfied, the flyback inverter isoperated under CCM. And it causes geometricalincrement to the link voltage due to the exponentialcharacteristic of the voltage transfer function for CCM.Consequently, it can cause the serious distortion to theoutput current of the flyback inverter using unfoldingtechnique.

C. Design Point Consideration for DCM FlybackWhen the flyback inverter for PV-AC module system

is designed, the characteristics of PV module should beconsidered. As shown in Fig. 6, there are many PV

modules in the industry, and the PV modules come in a broad range of its output characteristics (i.e. MPP voltage,MPP current, open circuit voltage and short circuitcurrent) depending on manufacturers, and materialcharacteristics even though the maximum output powerratings are same to each other. Fig. 6 shows the MPPvoltage distribution of about 1800 commercial PVmodules rated from 140W to 260W power of about 90manufacturers.

For the general-purpose flyback inverter design, theresponsible MPP voltage range can be much wider

because the flyback inverter should cover various PVmodules which have lower power ratings than designed.Unfortunately, it is very difficult to extend the inputvoltage range because it can cause power losses. It meansthat the input operating current can be limited by thecharacteristics of PV module. The input operating currentis limited by (10).

2

2 2

pv g pv

m s pv g

V V I

L f nV V (10)

0

10

20

30

40

50

60

70

140 150 160 170 180 190 200 210 220 230 240 250 260

M P P V o l

t a g e

[ V ]

Maximum Output Power[W]

Fig. 6. MPP voltage distribution of commercial PV modules

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

P h o t o v o

l t a i c V o l

t a g e ,

V p v

[ V ]

Photovoltaic Current, I pv, [A]

D = 0

. 5

D = 0 . 4

D =

0 . 3

D = 0

. 2

D =

0 . 1

Fig. 7. Operating region comparison for PV modules A and B for theidentical output power

This limitation is reported in Fig. 7. If the PV moduleconnected to flyback inverter is not satisfied with abovelimitation, the flyback inverter can be operated underCCM, even though the maximum power of connected PVmodule is lower than the power handling capacity of theflyback inverter. For example, if a flyback inverter whichhas 250W power rating is designed, it should cover 200WPV modules. However, if the design is performed refer toPV module A shown in Fig. 6, the flyback inverter can

be operated in CCM when the PV module B isconnected to the flyback inverter despite of the maximumoutput power of PV module B is 200W (i.e. lower thanthat of PV module A). Actually, it is because the MPPcurrent of PV module B is larger than that of A.

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

P h o t o v o

l t a i c V o l

t a g e , V

p v [ V ]

Photovoltaic Current, I pv, [A]

1 0 0 m W / c m

8 0 m W / c m

6 0 m W / c m

4 0 m W

/ c m

D = 0

. 5

D = 0

. 5

D = 0 . 4

D = 0 . 4

D = 0 . 3

D = 0

. 3

D = 0

. 2

D =

0 . 1

Fig. 8. Effects on the operation mode by the design point selection

On the other hand, the design point is forced lower bythe designers who want to get higher European-efficiency.Because of the European-efficiency has the maximumweight on the half load, sometimes, designers set thedesign point to about 75~80% irradiance. Concerning themaximum efficiency of the PV power conditioning


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system is appeared on 70% of the design point, themaximum efficiency can be achieved on the vicinity of ahalf load. Actually, because of the full power irradiancecondition (i.e. 100mW/cm 2) is not common, thistechnique is widely used.

In the DCM based flyback inverter design, however,

the design point selection technique has to be carefullyconsidered. Because of the magnetizing inductance isdetermined by the design point selection, the design pointvariation can cause the CCM operation when the full

power irradiance is applied to the PV module. Forexample, as shown in Table 1, the nominal PV power isset as 200W, in spite of the maximum handling power is250W. The magnetizing inductance for 200W design isabout 11uH, and for 250W design, the magnetizinginductance has to be set as lower than 9uH. In case of thedesign point is set as 200W, the flyback inverter can beoperated under CCM when the 100% irradiance isapplied. This can be confirmed from the Fig. 8.

III. C URRENT SHAPING METHOD UNDER CCM

As mentioned above, the DCM flyback inverter can beoperated under CCM due to the operating point variationof the connected PV module. And the CCM operationcauses unintended output current distortion. To reducethis output current distortion, a current shaping methodusing the unfolding H-bridge modulation is proposed inthis paper.

A. CCM Fault Detection MethodTo ensure the performance of the proposed current

shaping method, a detection method for CCM fault isrequired. To avoid additional cost increase, the CCMdetection is performed by software without any additionalcurrent detection circuit.

Because of the output of DCM flyback inverter isshown like as a current source, the output current iscontrolled by the switching reference for flyback mainswitch. From the duty-ratio equation shown in (8) theswitching reference, i* FB , for flyback main switch can berepresented as (11).

* 2 sin pv m s FB m pv

I L f i t K t

V (11)

where the K m indicates that the peak value of the carrierwaveform for the flyback main switch. In general, the K mis determined by switching frequency in the software.

Concerning that only the three detection points (i.e. V pv, I pv for MPPT and v g for grid phase detection) are requiredin the flyback inverter, the CCM detection has to be

performed by the three detected-values and the value ofdesign parameters. From (9) and (11), it is obvious thatthe CCM operation can be detected when the (12) issatisfied.

* m g

FB

pv g

K vi

nV v

(12)

where the n is turns-ratio of the implemented transformer.

B. Unfolding H-Bridge Modulation MethodIn general, during the unfolding H-bridge switches are

turned on and off synchronized with the grid frequency,the voltage of output filter capacitor is equal to the linkvoltage. Thereby the link voltage is a dominant factor todetermine the output current of the flyback inverter.

In this paper, however, once the CCM operatingcondition is detected by (12), the unfolding H-bridge isallowed to be operated as PWM inverter with highfrequency switching. In this state, because of PWMinverter characteristic is same as a buck converter, theaverage value of filter capacitor voltage, V Cf,UBS can berepresented as (13).

, , 1 pv FB

Cf UBS o CCM UB UB FB

nV DV V D D

D (13)

where D FB is the duty-ratio of flyback converter mainswitch, DUB is the duty-ratio of unfolding H-bridge.

Once the (13) is equal to (5), the distortion of outputcurrent can be rejected even though the flyback converterstage is operated under CCM. Thus, required duty-ratioof unfolding H-bridge can be derived as (14).

12

g UB FB

m s pv pv

V D

n L f V I D (14)

From (8) and (14), the variation of unfolding H-bridgeduty-ratio according to the time variation can be derivedas (15).

*

1

FBC

UBm pv pv

i t K D t

K V I (15)

where the V pv and I pv are average values of detected inputvoltage and current, respectively. And the K C is a designcoefficient which is determined by the design parametersas defined in (16).

2 g

C

m s

V K

n L f (16)

From (15) and (16), the required PWM reference forthe unfolding H-bridge can be represented as (17).

* * C UB m FB pv pv

K i t K i t V I

(17)

As represented in (17), the PWM reference forunfolding H-bridge can be determined by the detectedinput voltage, current and the switching reference forflyback main switch which is determined by currentcontroller. The relationship between the unfolding H-

bridge switching signal and the flyback switching signalcan be represented as Fig. 9. The carrier signal forunfolding H-bridge modulation is not perfectly same asthe carrier signal for flyback switching. The phasedifference has to be existed to avoid the drastic increaseof the link voltage due to the simultaneous turn-off of theflyback main switch and the unfolding switch.


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m K

C m

pv pv

K K

V I

t

* UBi t

* FBi t

/2

pQ

, S1 S4Q Q

, S2 S3Q Q

t

Fig. 9. Unfolding H-bridge modulation method

The unfolding H-bridge modulation method proposedin this paper can be easily implemented by softwarewithout any additional hardware components. The overallcontrol block diagram for proposed flyback inverterincluding the current shaping method is shown in Fig. 10.

+ _

MPPT

| sin |

Carrier

, S1 S4Q Q

pv I

pvV * pv I PI

control

* FB

I

PLL sin t t g v

* FBi

pQ

* UBi

pv I

CCM FaultDetection

eq. (12)

, S2 S3Q QCarrier

Fault Signal

UnfoldingModulation

Methodeq. (17)

Line frequencySwitching

* FBi

Fig. 10. Overall control block diagram including unfolding H-bridgemodulation method proposed in this paper

IV. S IMULATION R ESULTS

To verify the validity of proposed current shapingmethod, PSIM simulation is used in this paper. Thesimulation parameters are same as the design parametersshown in Table I.

Fig. 11 shows the output current distortion of theconventional flyback inverter. As shown in Fig. 11(c) and(d), the link voltage of the flyback inverter is drasticallychanged by the conduction mode of the magnetizingcurrent. In this region, the output current is distorted asshown in Fig. 11(b).

(a)

(b)

(c)

(d)

Output current distortion

CCM

Fig. 11. Output current distortion due to CCM operation(a) Grid voltage, (b) Output current of flyback inverter,(c) Link-voltage, (d) Magnetizing current

Fig. 12 shows the performance of CCM fault detectionmethod and switching reference generation techniquewhich are represented as (12) and (17), respectively. Inthe Fig. 12, the unfolding H-bridge modulation is notapplied to confirm the performance of CCM faultdetection method. From the Fig. 12(c), it is confirmedthat the proposed method can detect the CCM fault

properly. Fig. 12(d) shows the switching references forflyback main switch, and for unfolding H-bridgemodulation. The peak value of both carrier waveformsdetermined by the switching frequency, K m, is also

presented in Fig. 12(d).

(a)

(b)

(c)

(d)

CCM

m K

* UBi t * FBi t

Fault Flag

Fig. 12. Performance of the CCM detection method(a) Output current, (b) Magnetizing current(c) CCM fault flag, (d) Switching references

Fig. 13 shows the performance of the unfolding H- bridge modulation proposed in this paper. In this method,once the fault flag shown in Fig. 12 (c) is set, theunfolding H-bridge can be operated as PWM inverter asshown in Fig. 13(b). Fig. 13(c) and (d) is link-voltage andoutput current, respectively. From the comparison

between Fig. 12(a) and Fig. 13(d), it is obvious that theoutput current distortion can be reduced through theunfolding bridge modulation.


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(a)

(b)

(c)

(d)

CCM

m K * UBi t

* FBi t

Reduced current distortion

Unfolding H-bridge Modulation

Fig. 13. Performance of the unfolding H-bridge modulation(a) Switching references, (b) Q s1 switching signal(c) Link voltage, (d) Output current

However, the link voltage can be increased by themodulation method. This can be obviously confirmedthrough the comparison between Fig. 11(c) and Fig. 13(c).Thus, some overvoltage protection scheme for the linkvoltage is recommended to ensure the safety of flybackinverter.

Fig. 14. Output current FFT comparison. (under CCM condition)(a) Conventional unfolding, (b) Proposed modulation method

Fig. 14 shows the comparison result between theconventional unfolding method and the proposed unfold-ing modulation method. From this figure, it is shown thatthe 3 rd harmonic component is considerably reduced byusing the proposed unfolding modulation method.

V. C ONCLUSIONSIn this paper, DCM flyback inverter which is

composed of an active-clamped flyback converter stageand an unfolding H-bridge stage is applied. The DCMoperated flyback inverter has been recognized as anattractive solution for PV AC module system due to itssimple controller and circuit configuration. However,despite of the flyback inverter is designed to operateunder DCM, the flyback inverter can be operated underCCM due to the operating condition for the connected PVmodule. Unfortunately, it causes unintended distortion tooutput current.

In this paper, before the proposed unfolding H-bridgemodulation is presented, a performance comparison ofDCM and CCM is performed for a close analysis for the

problem. As shown, the problem can be occurred in caseof not only connected PV module has lower voltage andhigher current characteristics than expected, but also theflyback magnetizing inductance is designed focused onthe European-efficiency. To overcome the unintendedoutput current distortion, an unfolding H-bridge

modulation method is proposed. Once the CCM conditionis detected, the unfolding H-bridge is operated like as aPWM inverter. The CCM detection and switchingreference determination techniques are included and

presented. The validity of proposed current shapingmethod is verified by the simulation results.

ACKNOWLEDGMENT

This work was supported by Energy Power ResearchCenter of Samsung Electro-mechanics Co. Ltd.

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a current shaping method for pv-ac module

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