energies
ArticlePV Micro-Inverter Topology Using LLC
Resonant Converter
Hiroki Watanabe 1 , Jun-ichi Itoh 1, *, Naoki Koike 2 and Shinichiro Nagai 2
1
2*

Department of Electrical, Electronics and Information Engineering, Nagaoka University of Technology,
Kamitomiokamachi, Nagaoka, Niigata 940-2137, Japan
Pony Electric Co., Ltd., Kitasaiwai, Yokohama, Kanagawa 220-0004, Japan
Correspondence: itoh@vos.nagaokaut.ac.jp; Tel.: +81-258-47-9561



Received: 11 July 2019; Accepted: 7 August 2019; Published: 13 August 2019

Abstract: In this paper, a DC–single-phase AC power converter with an LLC resonant converter ispresented for a photovoltaic (PV) micro-inverter application. This application requires the leakage
current suppression capability. Therefore, an isolated power converter is usually combined for DC/AC
systems. The LLC resonant converter is the one of the isolated power converter topologies, and it
has good performance for conversion efficiency with easy control. On the other hand, a double-line
frequency power ripple has to be compensated for in order to improve the performance of the
maximum power point tracking (MPPT). Therefore, a bulky electrolytic capacitor is usually necessary
for the power converter. However, the electrolytic capacitor may limit the lifetime of the micro-inverter.
This paper introduces the PV micro-inverter with a LLC resonant converter. In addition, the active
power decoupling circuit is applied in order to compensate the double-line frequency power ripple
by the small capacitor in order to eliminate the electrolytic capacitor. Finally, the transformer design
is considered in order to reduce the transformer losses. As a result, the conversion efficiency of the
LLC converter is improved by 1% when the litz wire has many strands.
Keywords: PV micro-inverter; LLC converter; high switching frequency; transformer loss

1. Introduction
Recently, photovoltaic systems (PVs) are actively researched as a sustainable power solution due
to their attractive characteristics such as flexibility, high system efficiency, and low manufacturing cost.
In order to enhance the utilization of PVs, the generative power optimization by MPPT is necessary.
However, the decrease of the generative power due to the shade is a problem for PVs with the typicalcentralized power conversion system (PCS). In order to solve this problem, AC module systems using
micro-inverters are attractive in comparison with large capacity inverters [1–4]. This is because themicro-inverters optimize the generation power at each PV module. Note that the micro-inverters
consist of the DC/AC converter and the isolated DC/DC converter because the leakage current due tothe stray capacitance of the PV panel should be avoided for safety reasons and for easy installation.
In this case, a flyback converter or push-pull converter is often chosen because these circuits have someadvantages, e.g., a low number of components.
On the other hand, the LLC resonant converter is also one choice for the isolated DC/DC converter,
because it has good performance to reduce the switching losses utilizing Zero Voltage Switching (ZVS)and Zero Current Switching (ZCS) by series resonance [4–9]. In particular, the parasitic capacitor isdischarged before being turned on by the negative current. On the other hand, each drain current
approximately becomes zero when each switch is turned-off, owing to the sinusoidal resonance current.
That means the turn-on and the turn-off switching losses are drastically reduced by the ZVS andZVS operation. In addition, the volume of the transformer is drastically reduced owing to the highEnergies 2019, 12, 3106; doi:10.3390/en12163106

www.mdpi.com/journal/energies

Energies 2019, 12, 3106

2 of 11switching frequency operation being same as the push-pull converter or flyback converter. Note thatin the single-phase AC grid systems, a double-line frequency ripple occurs in the input current due
to the single-phase AC grid, and the current ripple decays the performance of the MPPT because
the PV generative power becomes unstable. Therefore, the bulky electrolytic capacitor is usually
installed on the DC side. However, the bulky electrolytic capacitor may limit the lifetime of the power
converter because the micro-inverter has to operate on the high environmental temperature condition
although the lifetime of the electrolytic capacitor depends on the Arrhenius law. In order to solve
this problem, the active power decoupling topologies have been studied to compensate the current
ripple [10,11]. These topologies utilize the small capacitor such as a firm or ceramic capacitor for the
power decoupling. Thus, an electrolytic capacitor is not necessary.
In this paper, a PV micro-inverter using an LLC converter is presented. In addition, the active
power decoupling circuit based on the boost converter is combined with the micro-inverter in order toeliminate the bulky electrolytic capacitor in order to improve the reliability. Finally, the conversion
efficiency of the LLC resonant converter is evaluated using two type litz wire in order to reduce theskin effect due to the high switching frequency operation. The contribution of this paper is that the
influence of the number of strands to the conversion efficiency is evaluated in order to improve theconversion efficiency of the high frequency power converter. A lot of papers have only considered thetransformer loss or the inductor loss [12], and the relationship between the number of strands and theconversion efficiency is not clarified.
2. Circuit Configuration
Figure 1 shows the PV power generative system. Typically, the centralized inverter, as shown in
Figure 1a, is employed, and the generative DC power is translated to single-phase AC. In this system,
each generative power of the PV modules is regulated by the single inverter with a large power rating.
On the other hand, the micro-inverter as shown in Figure 1b is integrated to the PV modules, and eachpower
converter is connected to the single-phase grid directly. Therefore, it is possible to optimizetheEnergies 2019, 12, x FOR PEER REVIEW
3 of 11
generative power at each of the PV modules.

(a)

(b)

Figure 1.1. Configuration
Configurationofof
power
generative
system.
(a) Centralized
inverter(b) microFigure
PVPV
power
generative
system.
(a) Centralized
inverterand (b)and
micro-inverter.
inverter.

On the other hand, the micro-inverters require a long lifetime and the PV panels, likewise, areideally maintenance-free. However, the electrolytic capacitor may decay over the lifetime of theconverter. In particular, bulky electrolytic capacitor is necessary on the single-phase AC system
in order to compensate the double-line frequency power ripple. In order to solve this problem,the micro-inverter with the active power decoupling circuit is considered in this paper.
Note that the micro-inverter systems are the distributed power supply, and each power convertertransfers the generative power to the single-phase AC grid independently. Typically the all-converter
operation is regulated by the Power Line Connection (PLC), and it is easy to get some informationsuch as the power generation status. In this system, the micro-inverters are operated independently,
and each power converters does not interfere with each other by the circuit operation.
Figure 2. Circuit configuration of DC to Single-phase AC converter using LLC resonant converter.
Figure 2 shows the circuit configuration of the DC to single-phase AC converter which consistsof
LLC resonant
converter
and anactiveDecoupling
power decoupling
3. Operation
Principleof ActivePower
Circuit circuit. The active power decoupling
circuit compensates the double-line frequency power ripple due to the single-phase grid by the small
Figure 3 shows the principle of the power decoupling between the DC and single-phase AC
sides [14]. In this system, the sinusoidal inverter output current is generated by the power converterin order to transfer the power to the single-phase AC grid. In this case, the single-phase grid voltage
and the inverter output current are expressed asvout = Vacp sin ωt(1)

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Energies 2019, 12, 3106

3 of 11capacitor [13]. The LLC resonant converter transfers the input power to the secondary side. After
that, the boost-up of the DC link voltage and the power decoupling operation are achieved by theactive power decoupling circuit.(a)Finally, the grid connection is achieved by(b)the Current Source Inverter(CSI).Figure
Note thatthereverse-blocking
diode
is
not
necessaryon
CSI
becausethe free-wheelingmode is1. Configuration of PV power generative system. (a) Centralized inverterand (b) microimplementedontheactivepower
decoupling
circuit.
inverter.

Figure 2.
2. Circuit
Circuit configuration
configuration of
of DC
DC to
to Single-phase
Single-phase AC
AC converter
converter using
using LLC
LLC resonant
resonant converter.
Figureconverter.

3. Operation Principle of Active Power Decoupling Circuit
3. Operation Principle of Active Power Decoupling Circuit
Figure 3 shows the principle of the power decoupling between the DC and single-phase AC
Figure 3 shows the principle of the power decoupling between the DC and single-phase AC
sides [14]. In this system, the sinusoidal inverter output current is generated by the power converter insides [14]. In this system, the sinusoidal inverter output current is generated by the power converter
order to transfer the power to the single-phase AC grid. In this case, the single-phase grid voltage andin order to transfer the power to the single-phase AC grid. In this case, the single-phase grid voltage
the inverter output current are expressed as
and the inverter output current are expressed asvout == Vacptvoutsinωωtacpsin(1)(1)

iout == IIacpsin ωtiout
acp sin ωt

(2)
(2)

where Vacp is the peak grid voltage and Iacp is the peak inverter current of the single-phase AC grid.
where V acp is the peak grid voltage and Iacp is the peak inverter current of the single-phase AC grid.
When both the output voltage and the current waveforms are sinusoidal, the instantaneous output
When both the output voltage and the current waveforms are sinusoidal, the instantaneous output
power pout is expressed as
power pout is expressed as
V
Vacp
Iacp
acpI acp(3)(12wt2wt) )= v iout
iout ==
(1cos
− cos
(3)
poutp=
out voutout
22
from (3),
(3),
the
power
ripplethat contains
contains double-line
double-line frequency
frequency ofof thethe power
power grid,
grid, appears
appears at
at DC
DC
link.
from
the
ripplethat
link.
Energies
2019,12, powerx FOR PEER
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11
In order to absorb the power ripple, the instantaneous power pbuf should be controlled bypbuf =

Vacp I acp(4)
cos 2ωt2
where the polarity of pbuf is defined as positive when the energy Cfc discharges. Due to the power
decoupling capability, the input power is matched to the average output power of the single-phase
AC. Therefore, the relationship between the input and output power is obtained as
1(5)
pin = Vacp I acp = VPV I PV
2
where Vpv is the PV input voltage and Ipv is the PV input current.

FigureFigure3.3.Principle
Principleofofpower
powerdecouplingdecouplingbetween
betweenDC
DCand
andsingle-phasesingle-phaseAC.
AC.

Figure 4 shows the operation mode of the active power decoupling circuit when the grid voltage
is the positive period. This converter has the charge mode and discharge mode of the small buffer
capacitor Cbuf in order to regulate the capacitor voltage. In order to obtain the buffer power as per
Equation (4), the capacitor voltage is actively fluctuated at the twice of the grid frequency such as
Figure 3.

Energies 2019, 12, 3106

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In order to absorb the power ripple, the instantaneous power pbuf should be controlled by
pbu f =

Vacp Iacp2

cos 2ωt(4)

Figure 3. Principle of power decoupling between DC and single-phase AC.

where the polarity of pbuf is defined as positive when the energy Cfc discharges. Due to the power
decoupling
input power
topower
the average
outputcircuit
powerwhenof thethesingle-phase
AC.
Figure 4 capability,
shows the theoperationmode isofmatched
the active
decoupling
grid voltage
theperiod.
relationshipbetween theand output
is obtained
as of the small buffer
isTherefore,
the positive
This converter
hasinputthe charge
modepowerand discharge
mode
capacitor Cbuf in order to regulate the capacitor voltage. In order to obtain the buffer power as per
1
Equation (4), the capacitor voltage is actively
fluctuatedatPVthepin = VIPV twice of the grid frequency such as(5)acp Iacp = V
2
Figure 3.
TheVoperationof theactivepower
circuitwhere
input
voltageanddecoupling
Ipv is the PV
inputcooperates
current. with the CSI; this achieves both
pv is the PV
powerFigure
decoupling
andthepower
transmission
single-phase
AC grid. In
modewhen
1, thetheswitchSbuf is4 shows
operationmode ofto
thetheactivepower decoupling
circuitgrid voltage
turnedand theinput Thispower
is transferred
to charge
the single-phasegrid. In modeswitchesis the on,
positive
period.
converter
has themode andAC
discharge
mode 2,
ofall
thethesmall
bufferareturned C
off,
the buffer
capacitor
is charged.
In mode3, thetobuffer
is powerdischarged
capacitor
in orderto regulate
the capacitor
voltage.
In orderobtaincapacitor
the buffer
as perbuf andthroughthe(4),
Sbuftheandcapacitor
CSI. Finally,mode
4 is the free-wheeling
mode.
The
capacitor
voltageEquation
voltageis actively
fluctuated at thetwice
ofbuffer
the grid
frequency
suchisas
regulated
Figure 3.to adjust the duty command for modes 2 and 3 on the one period of the triangle carrier.
Sbuf

Sup

Svp

Sun

Svn

Cbuf

Mode 1Mode 2Mode 3Mode 4

FigureOperation
mode
activepower
decoupling
circuit.
Figure4. 4.
Operation
mode
ofof
activepower
decoupling
circuit.

The operation
ofLLC
the active
power
decoupling circuit cooperates with the CSI; this achieves both
4. Operation
Mode of
Resonant
Converter
power decoupling and power transmission to the single-phase AC grid. In mode 1, the switch Sbuf isFigureshows
the operationmode, and tothe
6 shows
keyInwaveforms
LLC
turnedon, 5andthe input
power is transferred
theFiguresingle-phase
ACthe
grid.
mode 2, allof
thetheswitches
resonant
converter.
In
thiscircuit,theseriesresonanceofC
1, C2, and Lres is utilized for soft switching.
are turned off, and the buffer capacitor is charged. In mode 3, the buffer capacitor is discharged throughthe Sbuf and CSI. Finally, mode 4 is the free-wheeling mode. The buffer capacitor voltage is regulated
to adjust the duty command for modes 2 and 3 on the one period of the triangle carrier.
4. Operation Mode of LLC Resonant Converter
Figure 5 shows the operation mode, and the Figure 6 shows the key waveforms of the LLC
resonant converter. In this circuit, the series resonance of C1 , C2 , and Lres is utilized for soft switching.
The LLC converter regulates the input voltage of the active power decoupling circuit. The detail of theoperation mode is as follows.

Lres resonate as mode 1, and the resonant current occurs.
Lres resonate as mode 1, and the resonant current occurs.
Mode 5 [t4-t5]: S2 is turned off, and the dead-time period is started. At this mode, S2 achieves ZCS
Mode 5 [t4-t5]: S2 is turned off, and the dead-time period is started. At this mode, S2 achieves ZCS
turned-off. In mode5, the parasitic capacitor Cds1 is quickly charged. On the other hand, the parasitic
turned-off. In mode5, the parasitic capacitor Cds1 is quickly charged. On the other hand, the parasitic
capacitor Cds1 is discharged owing to the negative resonant current.
capacitor Cds1 is discharged owing to the negative resonant current.
Mode 6 [t5-t6]: This mode is still dead-time, and the parasitic diode of S1 conducts. Discharge ofMode
6 [t31065-t6]: This mode is still dead-time, and the parasitic diode of S1 conducts. Discharge
Energies
2019, 12,5 of of11
Cds1 should
be completed during modes 5 and 6 for ZVS of S1.
Cds1 should be completed during modes 5 and 6 for ZVS of S1.

Figure 5. Operation mode of LLC resonant converter.
Figure 5.
5. Operation
Operation mode
mode ofof LLC
LLC resonant
resonant converter.
converter.
FigureKey waveforms
waveforms of LLC resonant converter.
Figure 6. Key
Figure 6. Key waveforms of LLC resonant converter.

Mode 1 [t0 -t1 ]: S1 is turned-on and half of the PV input voltage is applied to the transformer.
In this mode, C1 , C2 , and Lres resonate, and the resonance current occurs.
Mode 2 [t1 -t2 ]: S1 is turned-off and the dead-time period is started. At this mode, S1 achieves ZCS
turned off. In mode 2, the parasitic capacitor Cds1 is quickly charged. On the other hand, the parasitic
capacitor Cds2 is discharged owing to the negative resonant current. Cds2 is fully discharged in thismode, and mode 3 is started.
Mode 3 [t2 -t3 ]: This mode is still dead-time, and the parasitic diode of S2 conducts. Discharge ofCds2 should be completed during Mode2 and 3 for ZVS of S2 .
Mode 4 [t3 -t4 ]: S2 is turned on with ZVS when Cds2 is already discharged. In this mode, C1 , C2 ,and Lres resonate as mode 1, and the resonant current occurs.
Mode 5 [t4 -t5 ]: S2 is turned off, and the dead-time period is started. At this mode, S2 achieves ZCS
turned-off. In mode5, the parasitic capacitor Cds1 is quickly charged. On the other hand, the parasitic
capacitor Cds1 is discharged owing to the negative resonant current.
Mode 6 [t5 -t6 ]: This mode is still dead-time, and the parasitic diode of S1 conducts. Discharge ofCds1 should be completed during modes 5 and 6 for ZVS of S1 .

Table 1 shows the experimental parameters. Figure 7 shows the experimental result. Note thatthe output power is 300 W, the switching frequency is 20 kHz, the buffer capacitor is 44 µF, and thegrid voltage is 200 Vrms.
According to Figure 7, the constant DC voltage and the sinusoidal inverter output current with
Energies 2019, 12, 3106
6 of 11the low THD are obtained. In addition, it is obtained that the buffer capacitor voltage is fluctuated by
the double-line frequency by the power decoupling control. However, the low frequency component
is5. still
remainingResulton theinput
voltage. Thisis because the mismatch is occurring during theExperimental
with
Fundamental
Operationcompensation value and the actual power ripple component. The compensation value for the power
5.1. Active Power
Decoupling
andpower.
CSI However, the input power is included the conversion
decoupling
is obtained
from Circuit
the input
loss, which
means
the
error
betweenthecompensation
the actual output
power
Table 1 shows the experimental parameters.
Figure 7powershowsandthe experimental
result.
Note occurs.
that theThis
problem
calculation
the compensation
to usetheoutputpowerisis solved
300 W, by
the changing
switchingthe
frequency
is 20process
kHz, theofbuffer
capacitor is 44valueµF, andthe grid
outputpower
condition.
voltage is 200 V .
rms

Table
Table 1.
1. Experimental
Experimentalparameters.
parameters.

Symbol
Vin
V in
Pin
Pin
buf
CC
buf
VVacac
f ac
fac
f sw
fsw

SymbolQuantity
Quantity
Input Voltage
Input Voltage
Input Power
Input Power
BufferCapacitor
BufferCapacitor
Grid
Voltage
Grid
VoltageGridfrequency
Grid frequency
Switchingfrequency
Switchingfrequency

Input voltage Vin 100 [V/div]

Value
90 V
90 V
300
W
300 W
44 µF
F
44
200VV
rms
200
rms
50
Hz
50 Hz
20
20kHz
kHz
Value

10 [ms/div]0
Buffer capacitor voltage Vcbuf 100 [V/div]
Grid voltage Vac 500 [V/div]00Inverter output current Iac 5 [A/div]

Figure7.7.Experimental
Experimentalresult
resultof
ofactive
activepower
powerdecouplingdecouplingcircuit
circuitand
andcurrent
currentsource
sourceinverter
inverter (CSI).
(CSI).
FigureAccordingFigure 7, the constant DC voltage and the sinusoidal inverter output current with the
5.2. LLC
ResonanttoConverter
low THD are obtained. In addition, it is obtained that the buffer capacitor voltage is fluctuated by the
Table 2 shows the experimental parameters and Figure 8 shows the experimental result. Note
double-line frequency by the power decoupling control. However, the low frequency component is stillthat the resonant frequency is designed to 205 kHz, and the switching frequency is set to below theremaining on the input voltage. This is because the mismatch is occurring during the compensation
resonant frequency as Table 2. In addition, the gate signals of S1 and S2 is provided by the 50% of duty
value and the actual power ripple component. The compensation value for the power decoupling issignals from the controller. According to Figure 8, the sinusoidal resonant current is obtained by theobtained from the input power. However, the input power is included the conversion loss, which meansseries resonance.
the error between the compensation power and the actual output power occurs. This problem is solved
Figure 9 shows the efficiency characteristics and the loss analysis result of the LLC resonantby changing the calculation process of the compensation value to use the output power condition.
converter. According to Figure 9, 94.3% of the maximum efficiency is obtained when the output
power
is Resonant
100 W. In
order to improve the conversion efficiency, the utilization of the synchronous
5.2. LLC
Converter
rectifier is simple solution. On the other hand, the reduction of the transformer losses is important
Table 2 shows the experimental parameters and Figure 8 shows the experimental result. Note thatfor the high switching frequency DC/DC converter because the copper loss becomes largely due tothe resonant frequency is designed to 205 kHz, and the switching frequency is set to below the resonant
the skin effect and the proximity effect [15].
frequency as Table 2. In addition, the gate signals of S1 and S2 is provided by the 50% of duty
signals from the controller. According to Figure 8, the sinusoidal resonant current is obtained by theseries resonance.

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12,x3106

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11

Table 2. Experimental parameters.
Table 2. Experimental parameters.

SymbolQuantity
Value
SymbolQuantity
Value
Vin
Input Voltage
40 V
VVout
OutputVoltage
Input
Voltage
4080
V V
in
Vfsw
Output Voltage
80
V kHz
Switchingfrequency
200
out
Energies 2019, 12, x FOR PEER REVIEW
fLsw
Switching frequency
kHzm
Magnetizing
inductance 20025
H
Lm
Magnetizing inductance
25 µH
Lres Table 2. Resonant
inductance
400 nH
Experimental
parameters. 400
Lres
Resonant
inductance
nH
C
1, C2
Resonant
capacitance
750
C1 , C2
Resonant capacitance
750
nF nF
SymbolQuantity
Value
Cdc
dc
DC
capacitance15
DC capacitance
15 µFFV
in
Input
Voltage
40pF
V
Junction
capacitance
500
pF
Cj
Junction
capacitance
500
V
out
OutputVoltage80
V
C
Outputcapacitance
1500
pF
Coss
oss
Output capacitance1500 pF
fsw
Switching frequency
200 kHz
Lm
Magnetizing inductance 25 H
Lres
Resonant inductance
400 nH
C1, C2
Resonant capacitance
750 nFCdc
DC capacitance
15 F
Cj
Junction capacitance
500 pF
Coss
Output capacitance1500 pF

7 of 11

Figure8.8.Switching
Switchingwaveforms.
waveforms.
FigureEfficiency [%]Efficiency [%]Figure 9 shows the efficiency characteristics and the loss analysis result of the LLC resonant
converter. According to Figure 9, 94.3% of the maximum efficiency is obtained when the output power
is 100 W. In order to improve the conversion efficiency, the utilization of the synchronous rectifier issimple solution. On the other hand, the reduction of the transformer losses is important for the highswitching frequency DC/DC converter because the copper loss becomes largely due to the skin effectand the proximity effect [15].
Figure 8. Switching waveforms.

Figure 9. Efficiency curve.

6. Evaluation of Conversion Efficiency Using Different Litz Wire
In this section, the conversion efficiency of the LLC converter is evaluated in terms of a change
the transformer. The high switching frequency operation increases the transformer loss including thecopper loss due to the skin effect and the proximity effect, eddy loss, and hysteresis loss. Especiallythe skin effect and the proximity effect Figure
reduce9.theeffective
conduction area to surface of the wiring.
Efficiency
curve.
As the result, the alternative wire resistance becomes large. In order to avoid this problem, the litz
wire
is usuallyof
used.
In this paper,
the twotypes
of transformer
are evaluated when the number of
6. Evaluation
Conversion
Efficiency
Using
Different
Litz Wire
strands is changed.
In this section, the conversion efficiency of the LLC converter is evaluated in terms of a change
the transformer. The high switching frequency operation increases the transformer loss including thecopper loss due to the skin effect and the proximity effect, eddy loss, and hysteresis loss. Especiallythe skin effect and the proximity effect reduce the effective conduction area to surface of the wiring.
As the result, the alternative wire resistance becomes large. In order to avoid this problem, the litz
wire is usually used. In this paper, the two types of transformer are evaluated when the number ofEnergies 2019, 12, x FOR PEER REVIEW

8 of 11

Figure 10 shows the overview of the transformer, and Table 3 and Table 4 show the parameterof the two transformers. Note that the air gap is not provided on this transformer in order to eliminate
theleakageflax
from the air gap, and the ferrite core with low eddy current loss is selected for8 oftheEnergies
2019, 12,3106
11high switching frequency operation. The difference between two transformers is only number of
strands, with transformer 2 having many strands on the wiring.
6. Evaluation of Conversion Efficiency Using Different Litz Wire
Figure 11 shows the comparison of the conversion efficiency when the number of the strands ischanged.
Notethat theand blueline means
approximate
curveof eachinplot.
Theofswitching
In thissection,theorange
conversion
efficiencyof thethe
LLC
converter isevaluatedterms
a changefrequency
is set to200
kHz,and the frequency
same transformer
corematerial
used. In thisloss
experiment,
the transformer.
Thehighswitchingoperation
increases
theistransformer
including thetheconversion
efficiency
of
the
LLC
converter
withdifference
numberanddiameterofstrands
asshown
copper loss due to the skin effect and the proximity effect, eddy loss, and hysteresis loss. Especially theinTableis compared.
According
to Figure11, it conduction
is obtainedarea
thattothe
conversion
efficiency
isskineffect3andthe proximity
effect reduce
the effective
surfaceof the wiring.
As theimproved
1% whenwire
the numberof strand
is increased
the towiring.
because thethe litzincreasing
result, thebyalternative
resistancebecomeslarge. In on
order
avoidThis
this isproblem,wire isAC
resistanceto theskintheeffectsuppressed,
and thearecopperloss iswhen
reduced.
Therefore,
the litz
usually
used. dueIn thispaper,
twoistypesof transformer
evaluated
the numberof strands
wirewith a lot of strands has a good performance in order to improve the conversion efficiency foris changed.
the high
switchingfrequency
converter.
that Figure
12Tablesshows3thecomparison
the conversion
Figure10 shows
the overview
of theNote
transformer,
andand
4 show theof
parameterof theefficiency
when theNote
switchingfrequencychanged.
Notethat transformer
the orange andblue tolines
represent
two transformers.
that theair gap isisnot
providedon thisin ordereliminatethetheapproximate
ofgap,
eachandplot.theAccording
to Figure
12,eddy
it is obtained
thatistheimprovement
rateleakageflax fromcurve
the air
ferrite corewith low
current loss
selected
for the highofthe conversion
efficiency
is increased
at 200 between
kHz. From
thisresult, the numberof strands
shouldswitchingfrequency
operation.
The differencetwotransformers
is only numberof strands,
bechosenaccordingly
becauseit strands
dependson
onthethewiring.
switching frequency.
withtransformer
2 having
many

Figure 10. Overview of high frequency transformer.
Figure 10. Overview of high frequency transformer.

SymbolSymbol
N1
N1
N2N2
-

- -

- Lm
Lm

Table 3. Parameters of transformer 1.
Table 3. Parameters of transformer 1.
Quantity

Quantity
Primary side number of turns
Primary sideof turns
Secondarysidenumber
number ofSecondaryturns
side number of turns

Value

Value
2
8
8
Litz wire specification
150/φ0.1150/
Litz
wiredensity
specification
2
Current
4 A/mmCore material
Ferrite (PQ)
Current density
4 A/mm2
Air gap
0 mm
Coreinductancematerial
Magnetizing
21.3FerriteµH (PQ)
Air gap
0 mm
Magnetizing
inductance
21.3
H
Table 4. Parameters of transformer 2.
2

SymbolQuantity

Value

N1

Primary side number of turns
Secondary side number ofturns
Litz wire specification
Current density
Core material
Air gap
Magnetizing inductance

2

N2
Lm

8
600/φ0.05
4 A/mm2
Ferrite (PQ)
0 mm
21.5 µH

Energies 2019, 12, 3106

9 of 11

Figure 11 shows the comparison of the conversion efficiency when the number of the strands
isEnergies 2019, 12, x FOR PEER REVIEW
9 of 11
Energies 2019, 12, x FOR PEER REVIEW
9 of 11changed.
Note that the orange and blue line means the approximate curve of each plot. The switchingfrequency is set to 200 kHz, andTable
the sametransformer
core material
is used. In this experiment,
4. Parameters
Parametersof transformer
transformer
2.
Table 4.
of2.
the conversion efficiency of the LLC converter with difference number and diameter of strands as
Symbol According to Figure
Quantity
Value
SymbolQuantity
Valueshown in Table 3 is compared.
11, it is obtained that theconversion efficiency isN
1Primaryside
numberofturns
improved by 1% when thestrand issideincreased
on oftheturns
wiring. This is22because the increasing
N1 number of Primary
numberN
2
Secondaryside
numberofturns
AC resistance due to theNskin
effect issuppressed,
the copper
loss is reduced.
Therefore, the litz
2
Secondarysideandnumberof turns
88
600/
wire with a lot of strands- has a good performancein order to improve the conversion
efficiency for
600/
Litz wire
wire specification
specification
Litzof the conversion
the high switching frequency converter. Note that Figure 12 shows the comparison
2
Current Notedensity
A/mm-Current
density
44 A/mm
efficiency when the switchingfrequency is changed.
that the orange andblue2 lines represent theCore materialmaterial
Ferrite(PQ)-- plot. According to Figure
Core
(PQ)
approximate curve of each12, it is obtained thatFerrite
the improvement
rate of the
Air
gap
0
mm
Airthisgap
mm should be chosen
conversion efficiency is increased
at 200 kHz. From
result, the number of0strands
Lmm
Magnetizing
inductance
21.5 H
H
L
Magnetizing
inductance
21.5
accordingly because it depends
on the switchingfrequency.

Figure 11.
11. Comparison
Comparison ofof conversion
conversion efficiency
efficiency when
when number
number ofof strands
strands is
is changed
changed on
on wiring.
wiring.
Figureonwiring.

Figure Figure
12. Comparison
Comparisonof conversion
conversion
efficiency
when switchingswitchingfrequency
is changed.
changed.
12. Comparisonof conversion
efficiency
when switchingfrequency
is changed.
Figure12.
ofefficiency
whenfrequencyis7.
Conclusions
7. Conclusions
Conclusions
7.
In
a DC–single-phase
AC power
converterwith anwith
LLCanresonant
converter converter
is presented
In thisthispaper,
paper,
DC–single-phase
AC powerpower
converter
LLC resonant
resonantisIn
thispaper,
aa DC–single-phaseACconverterwith anLLC
converter isforaphotovoltaic(PV)micro-inverter
application.
This
converter
consistsoftheLLC
resonant
presented
foraphotovoltaic(PV)micro-inverter
application.
This
converter
consistsoftheLLC
presented for a photovoltaic (PV) micro-inverter application. This converter consists of the LLC
converter,
the active thepower
decoupling
circuit, andcircuit,the CSI.and
In addition,
theaddition,
conversionofresonant converter,
converter,
activepower decouplingdecoupling
the CSI.
CSI. InIn
theefficiency
conversion
resonant
the active
power
circuit, andtheaddition, theconversion
the
LLC converter
is evaluatedthe number
strands
is changed.
the experimental
efficiencyof thethe LLC
LLC
converter whenis evaluatedevaluated
whenofthethenumberof strands
strandsThroughis changed.
changed.
Through thetheefficiency
ofconverteriswhennumberofisThrough
result,itwasobtainedthatthedouble-line
frequency
rippleiscompensatedbythesmall capacitor
experimental
result,itwasobtainedthatthedouble-line
frequency
rippleiscompensatedbytheexperimental result, it was obtained that the double-line frequency ripple is compensated by theusing
an active using
powerandecoupling
circuit.
Finally,circuit.
the conversion
efficiencyof the
LLC converter
issmall
capacitor
activepower
decoupling
Finally,theconversion
efficiency
of
the
LLC
small capacitor using an active power decoupling circuit. Finally, the conversion efficiency of the LLC
improved
by
1%
when
the
litz
wire
have
a
lot
of
strands.
converter is
is improved
improved byby 1%
1% whenwhen thethe litz
litz wire
wire have
have aa lot
lot ofof strands.
strands.
converterInthefuturework,
theoptimaldesignofthehighfrequencytransformer will
will bebe considered.
considered. In
InIn the future work, the optimal design of the high frequency transformer
particular, thethe optimal
optimal number
number ofof thethe strand
strand should
should be
be considered
considered inin orderorder toto minimize
minimize thetheparticular,transformer
loss,
and
it
depends
ontheswitchingfrequency.
Therefore,
therelationshipbetweenthetransformer loss, and it depends on the switching frequency. Therefore, the relationship between theEnergies 2019, 12, 3106

10 of 11In the future work, the optimal design of the high frequency transformer will be considered.
In particular, the optimal number of the strand should be considered in order to minimize thetransformer loss, and it depends on the switching frequency. Therefore, the relationship between thenumber of strands and the switching frequency will be analyzed. Finally, the optimal transformer
design focusing on the minimum transformer losses will be demonstrated based on the loss analysis.
Author Contributions: Conceptualization, H.W. and J.-i.I.; methodology, H.W.; software, N.K.; validation, N.K.,
S.N. and J.-i.I.; formal analysis, H.W.; investigation, H.W.; resources, H.W.; data curation, H.W.; writing—originaldraft preparation, H.W.; writing—review and editing, H.W.; visualization, N.K.; supervision, J.-i.I.; project
administration, J.-i.I.; funding acquisition, J.-i.I.
Funding: No external funding.
Acknowledgments: This study was supported by New Energy and Industrial Technology Development
Organization (NEDO) of Japan.
Conflicts of Interest: The authors declare no conflicts of interest.

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