2638 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 6, JUNE 2014

Letters

High Efficiency Dual-Mode Current Modulation Method for Low-PowerDC/AC Inverters

Ahmadreza Amirahmadi, Student Member, IEEE, Lin Chen, Student Member, IEEE,Utsav Somani, Student Member, IEEE, Haibing Hu, Member, IEEE,

Nasser Kutkut, Senior Member, IEEE, and Issa Bartarseh, Fellow, IEEE

Abstract—Boundary conduction mode (BCM) zero voltageswitching (ZVS) current control is a promising soft switchingmethod for microinverter applications. In this letter, different BCMZVS current control modulation schemes are compared based onpower losses breakdown, switching frequency range, and currentquality. Compared to continuous conduction mode current con-trol, BCM ZVS control decreases MOSFET switching losses andfilter inductor conduction losses but increases MOSFET conduc-tion losses and inductor core losses. Based on the loss analysis, adual-mode current modulation method combining ZVS and zerocurrent switching schemes is proposed to improve the efficiencyof the microinverter. The experimental results show that by usingthis proposed current modulation scheme, higher efficiency of 0.5%can be achieved with no additional cost for a 400-W three-phasemicroinverter.

Index Terms—BCM, current modulation, three-phasemicroinverter, ZVS.

I. INTRODUCTION

AN ac PV module, consisting of a PV panel and amicroinverter has many advantages over the conventional

centralized [1], string and multistring inverters such as per panelmaximum power point tracking and lower manufacturing costthrough mass production, as well as safe and simple instal-lation [2]–[5]. In the application of low-power inverters suchas microinverters, increasing the switching frequency has be-come more interesting as it can reduce the size and cost of theoutput filter inductor and capacitor [6], thus achieving higherpower density, lower cost, and better dynamic performance [7].However, higher switching frequency leads to higher switch-ing losses and more severe electromagnetic interference (EMI).To increase the switching frequency, while maintaining a highconversion efficiency and low EMI, soft switching techniques

Manuscript received July 6, 2013; revised September 3, 2013; acceptedOctober 5, 2013. Date of current version January 29, 2014. This work wassupported in part by the U.S. Department of Energy under Grant DEEE0003176and in part by NSF under Grant ECCS-1156633. Recommended for publicationby Associate Editor C. C. Mi.

The authors are with the Florida Power Electronic Center (FPEC), Uni-versity of Central Florida, Orlando, FL 32826 USA (e-mail: amirahmadi@knights.ucf.edu; frankchen@eecs.ucf.edu; somani.utsav@knights.ucf.edu;huhaibingucf@gmail.com; nhkutkut@gmail.com; batarseh@eecs.ucf.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2013.2285624

Fig. 1. Half bridge three-phase dc/ac inverter.

Fig. 2. Zero voltage transition with BCM ZVS current control.

can be employed [8], [9]. Considerable research has been con-ducted on the topologies and control methods of microinvertersto achieve soft switching operation [5], [10], [11].

The ZVS BCM current control is an interesting soft switchingcandidate for microinverter applications due to its low cost, highpower density, and high efficiency. The digital implementationof this soft switching approach known as hybrid BCM currentcontrol has been verified in [5]. Three different current mod-ulation schemes have been introduced for this current controlapproach in [7] and [5]. They are referred to as fixed reversecurrent modulation, variable reverse current modulation, andconstant bandwidth current modulation, respectively. This letter

0885-8993 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 6, JUNE 2014 2639

TABLE IBCM ZVS CURRENT CONTROL MODULATION SCHEMES

Fig. 3. Dual-mode ZVS ZCS current modulation scheme.

presents a detailed comparison of these three current modulationschemes with respect to the converter power losses breakdown,switching frequency range, and output current quality. To fur-ther improve the efficiency and reduce the current stress on thepower components, a dual-mode ZVS ZCS current modulationscheme is proposed.

II. ZVS BCM CURRENT CONTROL

Fig. 1 shows the power circuit of the half bridge three-phasemicroinverter. In this figure, the diode and capacitor in parallelwith the switch are the body diode and the parasitic capacitanceof the MOSFET, respectively. In ZVS BCM control as shown inFig. 2, the inductor current is intentionally operated in both di-rections within a switching cycle. It is controlled to pass throughthe body diode of the MOSFET after discharging its output ca-pacitor to generate ZVS condition during commutations [5].

Implementation of the ZVS BCM control requires settingthe upper and lower boundaries (limits) of the inductor cur-rent. The value of the reverse current during each half line pe-riod is determined by the amount of current required to chargethe inductor such that its discharging current during the dead-time interval is large enough to discharge the parasitic capaci-tance and forward bias the body diode of the MOSFET about to

turn ON, therefore ensuring ZVS operation for the switches [7].The other limit is set to ensure that the average current duringeach switching cycle is equal to the reference current. DifferentBCM ZVS current control modulation schemes with differentupper and lower limits are shown in the Table I [5], [7].

III. PROPOSED DUAL-MODE ZVS ZCS CURRENT

MODULATION

Compared to the CCM current control, BCM ZVS currentcontrol increases the conduction losses and the current stress ofthe switches. In order to improve the efficiency and reduce thecurrent stress on the components, a dual-mode current modu-lation strategy is proposed. Switching between ZVS and ZCSoperation is used during each line half cycle. Using ZVS aroundthe line zero crossing point reduces the switching frequency andswitching losses. Using ZCS at the peak of the current duringeach line half cycle reduces the high-frequency RMS currentthus reducing conduction losses. The idea here is to combinethe ZVS and ZCS BCM current modulation schemes alterna-tively according to the different value of the current during eachline half cycle so that the dominant losses can be optimized andhigher efficiency is achieved. Fig. 3 shows the proposed currentmodulation.

2640 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 6, JUNE 2014

Fig. 4. Power losses breakdown for ZVS BCM current control with four different current modulation schemes at 10%, 50%, and 100% of microinverter ratedpower.

The lower and upper boundaries of the dual-mode currentmodulation scheme can be expressed (1), as shown at the bottomof the page, where iref sin(ωt) is the reference current and B0 isthe reverse current required to achieve ZVS. In (1), the boundarypoint between the ZVS and ZCS operation was defined basedon the reference current and a constant value IB . Decreasingthe amplitude of reference current iref will move this boundarypoint toward the peak of the current during a line half cycle andit will reduce the maximum of the switching frequency rangeand therefore the switching losses at light loads. Increasing theamplitude of reference current iref will move this boundarypoint toward the zero crossing of the reference current and itwill reduce the conduction losses further at heavier loads.

IV. COMPARISON OF THE CURRENT MODULATION SCHEMES

In order to fairly compare the four different current modula-tion methods, all of the design parameters are considered to bethe same. In ZVS BCM operation, the lowest reverse currentover a line half cycle is set to 1 A, since this is the desired ZVScondition.

Fig. 4 shows the calculated loss distribution of a 400-W three-phase microinverter with different current modulation schemes.This figure illustrates the power losses under 100%, 50%, and10% load. The power losses breakdown includes the conduc-

Fig. 5. Switching frequency variation of four different current modulationschemes for half cycle of line frequency at rated power of microinverter.

tion and turn-off switching loss of the MOSFETs, core andcopper loss of the inductors, and anti-parallel diode losses. Itcan be verified from Fig. 4 that the power losses can be reducedsignificantly using the proposed dual-mode ZVS ZCS currentmodulation method.

The switching frequency range comparison of the four cur-rent modulation methods at microinverter rated power is shownin Fig. 5. This figure shows the switching frequency variation

⎧⎪⎪⎪⎨

⎪⎪⎪⎩

iupper = 2iref sin(ωt) + B0

ilower = −B0

iupper = 2iref sin(ωt)ilower = 0

if (iref sin(wt) ≥ 0 and iref sin(wt) ≤ IB )

if (iref sin(wt) ≥ 0 and iref sin(wt) > IB )

(1)⎧⎪⎪⎪⎨

⎪⎪⎪⎩

iupper = B0

ilower = 2iref sin(ωt) − B0iupper = 0

ilower = 2iref sin(ωt)

if (iref sin(wt) < 0 and iref sin(wt) ≥ −IB )

if (iref sin(wt) < 0 and iref sin(wt) < −IB )

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 6, JUNE 2014 2641

Fig. 6. Current waveforms (a) dual-mode ZVS ZCS current modulation,(b) fixed reverse current modulation, (c) variable reverse current modulation,(d) constant bandwidth current modulation.

Fig. 7. Current waveforms (a) dual-mode ZVS ZCS current modulation, in-ductor current, and corresponding output current, (b) three-phase output current.

during each half cycle of line frequency. The constant band-width current modulation has the narrowest switching frequencyrange. The proposed dual-mode current modulation scheme re-duces the switching frequency range compared to that in thefixed reverse current modulation.

V. EXPERIMENTAL RESULTS

To verify the proposed dual-mode current modulation, a400-W three-phase microinverter prototype was built. The spec-ifications are as follows: input voltage: Vdc = 400 V; Gridvoltage: Vac = 208 V; Grid frequency: F = 60 Hz; and themajor components used in the power stage are as follows: in-ductor filter: La = 270 uH; capacitor filter: 1 uF; MOSFETs:FCB20N60. The controller was implemented using a microchipDSP dsPIC33 F.

The inductor current waveforms of the four current mod-ulation methods for the three-phase microinverter under thefull-load condition are shown in Fig. 6. Fig. 7(a) illustrates theinductor current and the output current for the dual-mode cur-rent modulation method. The output current THD is less than2.5% and meets the industrial requirements. The three-phaseoutput current is shown in Fig. 7(b). Comparison of the RMSvalue (high frequency and 60-Hz current) and output currentTHD of the four current modulation methods is given in the Ta-ble II. Notice that the RMS value of the inductor current for the

2642 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 6, JUNE 2014

TABLE IITHD AND INDUCTOR CURRENT RMS VALUE COMPARISON OF FOUR CURRENT

MODULATION SCHEMES

Fig. 8. Converter efficiency versus output power.

dual-mode current modulation scheme is significantly less thanthe other three methods which leads to reduction in conductionlosses and higher efficiency can be achieved.

Fig. 8 shows the measured efficiency of the three-phasemicroinverter with four current modulation methods. Owingto dual-mode current modulation, higher efficiency can beachieved without additional cost. All the efficiency measure-ments were performed using a Yokogawa PZ4000 power ana-lyzer. The measured efficiency does not include auxiliary circuitpower consumption.

VI. CONCLUSION

In this letter, the loss distribution and the efficiency of a400-W three-phase microinverter with ZVS BCM current con-

trol were investigated analytically under different current mod-ulation methods. It was found that fixed reverse current mod-ulation has the highest efficiency and THD, while the constantbandwidth current modulation has the lowest THD and effi-ciency. To further improve the efficiency of fixed reverse cur-rent modulation, a new dual-mode current modulation schemecombining the ZVS and ZCS operations was proposed.

The advantages of the proposed current modulation methodare the reduction of conduction losses, current stress on thecomponents, and inductor core losses. The experimental resultsverified that with dual-mode current modulation the efficiencyis improved by 0.5% with no additional cost.

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