Evaluation of an isolated DC-DC converter
for a micro inverter
Naoki Koike1, Shinichiro Nagai1
Pony Electric co., ltd. 2-8-4 Kitasaiwai Yokohama Nishi-ku, Kanagawa 220-0004 Japan

1

Abstract
Although it is necessary to increase the frequency of the microinverter, it is difficult to improve effi
ciency at high frequency. In this paper, considerati
on and efficiency evaluation were made on reduct
ion of loss occurred at high frequencies in current
mode resonant DCDC converter for micro inverters. In the following, the operation analysis including the parasitic component in the current mode res
onant DCDC converter was performed and the sw
itching loss was reduced by Zero Voltage Switching(ZVS). In order to reduce the eddy current loss
which causes the loss of the high frequency transf
ormer, we used an extremely fine litz wire to quantitatively show the influence due to the difference in strand diameter and realized a highly efficient D
CDC converter.

1.

Introduction

Solar power generation has attracted attention fora long time as a very useful energy source forJapan with little resource such as petroleum, but it
was difficult to disseminate due to high equipmentcost. However, in recent years, the expectation forsolar power generation has increased due to themomentum of environmental conservation andrenewable energy, and the dissemination policy
including the subsidy system has been proactively
promoted, and the purchase system in 2012 Many
energy trading companies have entered into themarket as a result of being started, and in recentyears the number of installations of solar power
generation system facilities has been rapidly
increasing [1]. As a result, expensive facility costs,
which had been hindered by the spread of solar
power generation systems, have also been
improved [2], and the penetration rate of solar
power generation in Japan has dramaticallyincreased in recent years. However, facilities andmaintenance costs of photovoltaic power
generation systems for general households arestill expensive, so we think that cost reduction is
the most important factor in order to further spread.

In the long term, it is possible to cover thesecosts in the profit obtained by the sale of electricityof the electric power generated by the solar powergeneration. We intend to reduce the costs in solar
power generation systems from two points ofimprovement of power generation efficiency andreduction of maintenance cost by improving
product life and proceed with the development oflong-life microinverter
In solar power generation systems, there aremany systems in which a plurality of solar cellstrings are connected in parallel and input to apower conditioner. In this method, the generated
power of the whole solar light string decreases dueto the solar panel that does not generate electricity
due to shadows and the like, so there is a problem
that the generated power drops extremely.
However, in the micro inverter system connectedin parallel to solar panels, since there is no strings,
even if panels that do not generate electricity exist,
the influence can be minimized, and improvementin power generation efficiency of the system canbe expected.
We have proposed an electrolytic capacitor-lessmicroinverter as a long-life microinverter system.
However, in order to extend the lifetime of themicroinverter, not only the electrolytic capacitor but
also the loss must be reduced in order to reduce
the load on the element. However, in a small
device such as a microinverter, it is necessary to
set the switching frequency high, so it is difficult toachieve high efficiency. We evaluated theefficiency of the high frequency DCDC converter
for the microinverter, and considered it.

2. Circuit configuration of microinve
rter
The circuit configuration of the microinverter isshown in Figure 1. It consists of a Current ModeDCDC conveter

Active buffer
L2inverterD6CT2V3C1Q1
Tr1D1Cc2

D3S1S0S3L3V2V1CT3V5CdcPVC2Q2D2D5D4S2S4Figure 1 Circuit configuration of microinverter

Resonant isolated DCDC Converter, an activebuffer, and a current type inverter.
Normally, most of the electrolytic capacitors
have a lifetime of 15 years at the maximum, but inorder to achieve a long life, the electrolytic
capacitors are not used in this method, and thecircuit is composed only of the ceramic capacitors.
Since a large capacity can not be obtained with aceramic capacitor, an active buffer system isadopted. In the active buffer system, it is possible
to reduce the capacity by controlling the charging /discharging amount of the capacitor of the DC link
voltage (Figure 1,Cc 2) according to the ripple ofthe instantaneous power generated by the gridinterconnection.[3][4]3. Circuit configuration of DCDC converterd at 50% and the switching frequency was fixed to the resonance frequency of Lr, C1, C2, and it w
as always operated with ZVS.

4. Operation mode analysis including parasitic components
In the DCDC converter of Figure 2, soft switchi
ng of current resonance is performed. As the swit
ching frequency increases, the parasitic compone
nt of the switching element greatly affects the resonance. In Figure 2 ,C1, C2 and Lr resonate andCoss1
C1

Q1

D1

Lr

Cj1

Tr1

Coss1

D3

LmCin

C1

Cj3
Cdc

Q1

C2
Cj2

Coss1
Q1

Cj4

Cj1

Tr1

D4
Cj2

Coss1

D3

C1

Cj3

Lm

Cdc

Cj4

Q1

D1

LrCin

Cj1

Tr1

D3
Cj3
Cdc

Lm

C2
D2

Coss2

Cj2

Q2Coss1
Q1

Cj4

Cj1

Tr1

Coss1

D3

C1

Cj3

Lm

Cin

Cj2

Q2

D4
Cj4(g)dead time2

D1

Lr

D2

Coss2

D4(b)Q2 turn offCdc

Q1

D1

LrCin

Cj1

Tr1

D3
Cj3
Cdc

Lm

C2

C2
D2

Coss2

Cj2

Q2

Coss1
Q1

Q2

Cj4CinD4
Cj2

Cj4(f)Q1 turn off

D1

Lr

D2

Coss2

D4(b)dead time1

C1

D2

Q2

C2

The circuit configuration of the evaluated DCD
C converter is shown in Figure 2. The switching fr
equency of the DCDC converter was 200 kHz, and the input DC voltage was 40 V assuming the maximum output voltage of the PV panel. In order to make the output voltage twice the input DC voltage, the winding ratio of the transformer was 1: 4.
In this DCDC converter, the output voltage is notcontrolled, and the voltage is controlled by the act
ive buffer and the inverter. The duty ratio was fixe

Cj3(d)Q2 turn on

D1

Lr

Cin

Figure 2 Circuit configuration of DCDC converter

D3

Cdc

Coss2

D4(a)Q2 ONC1

Cj1

Tr1
Lm

C2
D2

Coss2
Q2

C1

D1

LrCin

Cj1

Tr1

Coss1D3

C1

Cj3
Cdc

Lm

Q1Cin

C2

D1

Lr

Cj1

Tr1

D3
Cj3
Cdc

Lm

C2
Coss2
Q2

Figure 3

D2

D2

Coss2

D4
Cj2

Cj4(d)Q1 turn on

Q2

D4
Cj2(e)Q1 on

Operation mode analysis including parasitic
components

Cj4

Q1 and Q2 become zero current switching (ZCS).
However, since the parasitic capacitance is charged and discharged at the moment when Q1 andQ2 turn on, switching loss occurs. As the switching frequency increases, this loss also increases.
Figure 3 shows an operating mode analysis incl
uding parasitic components affecting soft switching operation. The parasitic component consideredis the output capacitance (Coss) of Q1 and Q2, and the junction capacitance (Cj) of D1 to D4. Eac
h operation mode will be described below.
<a>Q2 ONQ 2 are in the ON state. Current resonance occ
urs at the load currents C1, C2 and Lr to the secondary side diode, and reactive current flows throu
gh the excitation inductance Lm. The reactive current depends on the input voltage and does not depend on the load power. Consequently, on the primary side, a conduction loss due to a certain amo
unt of reactive power occurs regardless of the loa
d.
<b>Q2 turn off
When Q 2 turns off from the state of Figure 3(a),it shifts to the mode of Figure 3(b). In this mode, t
he load current is interrupted as Q2 turns off, theexciting current flows due to the back electromotiv
e force of the exciting inductance of the transformer, the output capacity Coss of Q1 and Q2 is char
ged and discharged, and when charge and discha
rge are completed, Q1 The body diode turns ON.

Table 1 circuit constants
Itemsymbol

value unit

input voltage

Vin40

V

output voltage

Vout80Vduty cycle

D

0.5switching frequency

fsw

200k

magnetizing inductance

Lm25u

H

resonant inductor

Lr400nHresonant capasitor

C1,C2750n

F

DCcapacitor

Cdc

15u

F

junction capasitance

Cj500pF

output capacitanceCoss

1500p F

Hz

Figure 4 current resonant waveform<c>dead time1
Figure 3(c) shows the state in which the body diode of Q1 is turned on. In this state, since a voltag
e is applied to the exciting inductance Lm in the opposite direction, the exciting current flowing through Lm rapidly decreases and the body diode turns OFF.
<d>Q1 turn on
When Q1 is turned on just before the body diode turns OFF in Figure 3(c), it goes to the mode ofFigure 3(d). When the transistor Q 1 is turned on, t
he polarity of the transformer is switched and the junction capacitances C j 1 to C j 4 of the seconda
ry side rectifier diodes D 1 to D 4 are instantaneo
usly charged and discharged.
At this time, when Q1 turns on before deadtime
is small and charge / discharge of Coss is compl
eted, Q1 generates not only its own parasitic cap
acitance Coss but also turn-on loss correspondin
g to the sum of the junction capacitances of Q2 and D1 ~ D4, so deadtime Is very important for theFigure 5 waveform @turn on (ZCS)

timing adjustment.

5. evaluation of DCDC conveter
5.1. Analysis of switching waveform
The evaluation results of the DCDC converter are s
hown below. The circuit constants are shown in Table1. Figure 4 shows the waveform of current resonance.
A sin wave due to the resonance of Lr and Cr can be confirmed. Figure 5、Figure 6 is the enlarged wavefor
m of the switching at the moment Q2 turns on.
Figure 5 shows the waveform when the dead time is 200 ns. Q 2 of Vgs is turned ON before the output capa
city of Q 2 is discharged after the gate of Q 1 is turned
OFF. In this case, since ZVS has not been achieved, aloss due to a short circuit of the output capacity occurs.
Figure 6 shows the waveform when the dead timeis extended by about 100 ns from Figure 5. In thiscase, since the gate of Q1 falls and the output capacity of Q2 is fully discharged, ZVS is achieved
and switching loss does not occur.
Figure 7 shows the waveform of the same deadtime as in Figure 5 at light load. The triangular wav
e current of 200 kHz is the exciting current flowing
through the transformer. It can be seen that a large peak current flows at the instant of switching.
This current flows by charging / discharging the junction capacitance of the diode on the secondaryside via Q1 as in the mode in Figure 3 (d). Figure 8
shows the waveform in the case of light load inFigure 6. Compared to Figure 7, it can be seen thatthe peak current flowing at the instant of switching decreases. This shows that switching loss can be reduced by ZVS.
Figure 9 compares the efficiency evaluation res
ults with and without ZVS. Efficiency difference ofup to 1% occurred at 100 W operation. In addition, it can be seen that the efficiency difference decreases as the load increases, since the switchingloss generated in the MOSFET is caused by the short circuit of the output capacitance, it is not dep
endent on the load and is almost constant .

Figure 6 waveform @turn on (ZCS&ZVS)

Figure 7 waveform (ZCS) (@light load)

5.2. The study of the high frequency transformer

We studied for transformer high frequency. isola
ted DCDC converter increases loss in transformer
as frequency increases. One of the causes is theeddy current loss due to the leakage of the transformer (Figure 11)。Therefore, we used the litz wire for the transformer of the high-frequency insulated
DCDC converter and measured how the differenc
e in the wire diameter influences the efficiency. In
the following, we examined the change in efficienc
y at high frequencies, the dependence of strand diameter and frequency on changing the wire diame
ter of Litz wire to φ0.1 and φ0.05.
Table 2and Figure 10 show the characteristics anFigure 8 waveform (ZCS&ZVS) (@light load)d outline of the isolated transformer Tr1. In order to reduce the eddy current loss due to leakage, thecore gap was eliminated, the litz wire was used for the winding and the ferrite was used for the corematerial because of the high frequency.

efficiency [W]

Table 2 specifications of transformer

95
94.5
94
93.5
93
92.5
92
91.5
91
90.5
90

trans former ① trans former ②
number of turns[T]primary2

secondary8

primary

150/Φ0.1×4p 600/Φ0.05×4p

ritz wiresecondary

150/Φ0.1

2

ZCS&ZVS

050

100

150
200
power [W]

250

ZCS

300

350

600/Φ0.05

current density[A/mm ]4.24

core material

ferrite(PQ)

gap [mm]0magnetizing inductance [uH]

21.3

21.5

Figure 9 power vs frequency @ ZCS ,ZCS&ZVS

5.2. Evaluation of high frequency transformerLeakage flux

efficiency [W]Figure 10

ritz wire Φ0.1
ritz wire Φ0.05
50

100

150
200
power [W]

250

300

350

Figure 13 power vs efficiency
94.9

ritz wire Φ0.1
ritz wire Φ0.05

94.7
94.5
94.3
94.1
93.9
93.7

cross-section

93.5
50

Eddy current

Figure 12

Leakage flux

Figure 11

High frequency transformer

95
94.5
9493.59392.5
92
91.5
91
90.5
900efficiency [W]The transformer of Table 2 was connected to the DCDC converter and evaluated.
Figure 13 shows the conversion efficiency when
the load power of the converter is changed from 50 W to 350 W at the switching frequency of 200 k
Hz. We compare the case where the wire diameter of the Litz wire of the transformer is φ0.05 and φ0.1. At this time, transformers with current density,
core material and core size under the same condi
tions were used.
In this case, the efficiency of the converter using
the transformer with the wire diameter φ 0.05 wasimproved by about 0.5% at the maximum.
Figure 12 compares the conversion efficiency ofthe converter when the switching frequency is vari
ed from 100 kHz to 200 kHz.
At a high frequency of 200 kHz, efficiency of φ 0.
05 is about 0.8% better. However, the difference
became smaller as the frequency became lower. At 100 kHz or less the difference between the two w
as reduced to 0.2%. Figure 14 shows the analysis
results of the loss at the switching frequency of 200 kHz. In this regard, it was found that transformer
loss can be reduced by 1.5 W by changing the Litz wire. Since the operating conditions of the converter are the same, the loss of 1.5 W is considered
to be an eddy current loss due to leakage.

Eddy current loss of winding

100

150
frequency [kHz]

200

250

switching frequency vs efficiency(@200W)

References

30

1.5W

loss [W]25
20

Hysteresis loss
Copper loss

Trans
9.6W

n of new energy etc.

Diode12W

Switching loss
Conduction loss

Diode12W

neration system etc

FET3.5W

Switching loss
Conduction loss

FET3.5W

φ0.1

Litz wire

φ0.05

50Fiscal year 2012 Survey on promotion of introductio

Eddy-current loss

Trans11.1W

15
10[1] Ministry of Economy, Trade and Industr,ySurvey on diffusion trend of photovoltaic power ge[2] Association, Japan Photovoltaic Energy,

Figure 14 loss analysis (@200kHz,300W)

Residential photovoltaic power generation subsidy
applications received.pdf[3] J. Y.Ohnuma,
A Single-phase Current Source PV Inverter with
Power Decoupling Capability using an Active Buffer,

6. Conclusion
We investigated and evaluated the loss reduction
which becomes noticeable at high frequencies in
the current resonant DCDC converter for micro inverters. We performed operating analysis consideri
ng the parasitic component and achieved ZVS byexperiment. A loss of 0.3% to 1% was improved compared with ZCS. In addition, the usefulness ofa transformer using a litz wire with a small wire dia
meter at high frequencies was quantitatively shown using an insulated DCDC converter. Miniaturizat
ion of the microinverter is required. For miniaturization, it is necessary to increase the frequency. We
believe that this evaluation is a very useful resultfor miniaturization. In the experiment, when the wi
re diameter of the Litz wire was changed from φ0.
1 to φ0.05, the conversion efficiency of the converter improved by about 0.8% at the maximum at 20
0 kHz. Moreover, it was found that the loss in thecase of the Litz wire strand diameter φ 0.05 is red
uced in the region of 100 kHz or more. In the future, we will study the influence of transformer leaka
ge on the Litz wire and the method of supplementi
ng the resonance inductance with the leakage of t
he transformer on a real machine basis and we will strengthen the guidelines for transformer designin the high frequency range.

Acknowledgement
This study was supported by New Energy and In
dustrial Technology Development Organization (NEDO) of Japan.

IEEJ Journal of Industry Applications ,pp169-172 ,2012.
[4] K. J. I. Y. S. H.Watanabe,
Miniaturization of the Boost-up type Active Buffer
Circuitina

Single-phase

Inverter, The2014

International Power Electronics Conference, No.
19,P1-4, pp. 84-91 , 2014.