Development of Micro DC-DC Converter Realizes
High Efficiency and Temperature-Tolerancein CPV Module
Koji Orikawa, Nagisa Takaoka, Jun-ichi Itoh

Kazuya Okamoto, Noboru Yamada

Department of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Japanorikawa@vos.nagaokaut.ac.jp

Department of Mechanical Engineering
Nagaoka University of Technology
Nagaoka, Japanokamotok@stn.nagaokaut.ac.jp

Abstract—This paper describes the micro DC-DC converter
realizing high efficiency and temperature-tolerance inconcentrator photovoltaics (CPVs) applications. Fundamental
experiments are carried out to verify the feasibility of the micro
DC-DC converters in high concentration CPV module. As aresult, it is experimentally demonstrated that the prototype
circuit can automatically starts the operation of the converter
according to the rise of the CPV cell voltage. In addition, it isanalytically confirmed that the designed circuit has a potential
to achieve the efficiency over 90.0%.

I.

INTRODUCTION

Recently, concentrator photovoltaics (CPVs) are one ofpromising next-generation photovoltaic technologies owing totheir excellent conversion efficiency [1-4]. According tothermodynamics theory, solar concentration can be exploited
to increase the theoretical conversion efficiency limit. In theCPV module, current mismatch among the CPV cells due tooptical performance difference of misaligned individual cells
and partial shading aggravates module efficiency. On the otherhand, suitable installation site is generally limited only in theregion of high direct solar irradiation. “CPV+ concept” hasbeen recently introduced to broaden the potential market ofCPVs [5]. In this concept, low-cost solar cells and CPV cells
are combined in a high-concentration CPV module. Most ofdiffuse solar radiation is captured and harvested by the lowcost solar cells; in contrast, direct solar radiation isconcentrated on the CPV cell. As a result, the DC conversion
efficiency of CPV module based on global solar radiation canbe significantly improved. Motivated by above challenging
research issues, exploration of suitable method for electricalintegration of CPV cells and\or two different types of cells in
CPV module is of interest. The use of micro inverter
technology will be one of the suitable methods.
For flat PV systems, research and development of themicro inverters connected to each single PV module are

becoming the trend for the future grid-connected PV systems[6-10]. In conventional method, a single large power capacity
inverter is connected to multiple PV modules in a seriesconnection. Therefore, the output power of PV panels
drastically changes depends on shading and weather
conditions. In contrast, micro inverters can extract the energyof PV cells efficiently by minimizing such effects and canimprove the system reliability by reducing output voltage ofPV cells. This advantage is attractive for not only theconventional flat PV but also CPV. However, to date, limited
to the author’s knowledge, no studies applying micro inverters
to the CPV module has been reported.
This study aims to develop micro inverters to the CPV
module especially with “CPV+ concept” proposed by one ofthe authors. In order to implement the micro inverters to theCPV module, further downsizing, high efficiency, andaffordable temperature-tolerance are required for micro DCDC converters. In the CPV module micro DC-DC converters
are closely connected to each solar cell or each small groupsof solar cells, thus the converter should operate at hightemperature because the temperature of surroundings of CPV
cell generally becomes high due to the concentrated beamsolar radiation. Micro DC-DC converters should also operate
at high switching frequency using gallium nitride (GaN) andsilicon carbide (SiC) and so on. In addition, micro DC-DC
converters should automatically start up and shut down
depending on the output voltage of CPV cells. Toward therealization of such micro converter, in this study, designconcept is introduced and fundamental experiments are carriedout to verify the feasibility of the micro DC-DC converters tocontrol the output voltage of a typical CPV cell. As a result, it
is experimentally demonstrated that the output voltage of theCPV cell is controlled based on the electricity generation
characteristic of the cell to achieve the maximum power point.
Furthermore, the result of loss analysis indicates that thedesigned circuit can achieve the efficiency over 90.0%.

MICRO DC-DC CONVERTER IN CPV+ MODULE

Diffuse solar radiation

Direct solar radiation

Fig. 1 shows the conceptual image of combination of“CPV+ concept” and micro DC-DC converters. Concentrator
element such as Fresnel lens concentrates beams solar
radiation onto the CPV cell, e.g., multi-junction cell. The lowcost cell such as silicon (Si) cell captures diffuse solar
radiation incoming through the lens. A micro DC-DC
converter is installed into the concentrator module as well as
solar cells and connected to each CPV cell.

A. Configuration of micro DC-DC converter
Fig. 3 shows the circuit configuration of the micro DC-DC
converter, which is a boost chopper. A technique ofsynchronous rectifier is adopted for the switch S1 in order toreduce the drop voltage and the conduction loss of powerdevices compared with that of a diode. A shunt resistor is usedin order to detect the inductor current instead of a current
sensor.
B. Control circuit
The power supply of the control circuit is a part of theoutput voltage of the CPV cell. As a result, once the CPV cellstarts to generate its power from the sunlight, the control
circuit starts the operation to drive the main circuit. General
purpose ICs are used for the control circuit. The power supplyICs (REG710NA-5, REG710NA-3.3: Texas Instruments) areused in order to obtain 5 V and 3.3 V for the power supply ofICs. PI regulators are implemented for the control of current
and voltage, which are an auto voltage regulator (AVR) andan auto current regulator (ACR). In addition, ICs operated bysingle supply are used in order to minimize IC components.
III.

SPECIFICATION AND EXPERIMENTAL RESULTS

Table I shows the experimental conditions. In order toverify the feasibility of the proposed concept, fundamentalexperiments are carried out. It is noted that the conventional Si
devices are used instead of GaN and SiC for the first-stepexperiments. The switching frequency is set to 50 kHz.
Fig. 4 shows the photograph of the fabricated prototype
micro DC-DC converter. All components are mounted on a
PCB board sized 7070 mm2. In order to eliminate the effect

Micro
DC-DC converters

Si solar cell
Multi-junction solar cell

Figure 1. Conceptual image of the use of micro DC-DC converter inCPV module based on CPV+ concept.
Interconnected inductor
DC-DC converter Module(1)CPV
Cell(1)

Micro (1)DC/DC
converter

Vout(1)

Cell(2)

Micro (2)DC/DCconverter

Vout(2)Cell(n)Micro (n)DC/DCconverter

Vout(n)Module(2)Fig. 2 shows example circuit configuration for the present
concept. A micro DC-DC converter is connected to each solarcell, forming a converter module. The converter module isconnected in series in order to increase voltage because theoutput voltage of the CPV cell and low-cost cell is generally
small. That of the CPV cell used in this experiment isapproximately 3 V. An inverter is connected to the output ofthe strings of DC-DC converter modules in order tointerconnect to the grid. For the micro DC-DC converter, SiCor GaN should be employed in order to achieve hightemperature operation of micro DC-DC converters. In addition,
those wide band semiconductors can contribute to thedownsizing of the micro DC-DC converters due to fastswitching. The unique feature required to the micro DC-DC
converter is automatic start of the operation utilizing lower
voltage of the CPV output.

Fresnel lensModule(m)II.

GridInverter

Micro DC-DC converter

Figure 2. Circuit configuration of the CPV system using micro DC-DCconverters.

2Iin
CellL

2

VinRshI/V2

Vpsout

AVR

Cout
Vout

S2

2Control circuit

ILPower
supplyIC

S1

ILACR

Micro DC-DC converter

Figure 3. Circuit configuration of micro DC-DC converter.

of line-inductance and achieve downsizing of the micro DCDC converter, all components are closely connected. Note that,heat sink is not used for the MOSFETs; besides somecomponents are not optimal materials for simplifying
fabrication of the prototype circuit. The size of the circuitboard will be less than 30  30 mm2 by replacing thecomponents to chip components.

TABLE I.

EXPERIMENTAL CONDITIONS
Input

Surface area of CPV lense

200 (mm)

200 (mm)Surface area of CPC cell

7 (mm)

7 (mm)

Switching frequency

50 (kHz)

Rated output voltage of CPV cell

2.5 (V)

Shunt resistance
Boost inductor

Inductance L
Winding resistance RL

Output capacitance Cout
MOSFET(TPCA8028-H)

10 (mW)

Boost inductor(Back side)

MOS-FET

OutputShuntresistance
S2

S1

Power supplyIC70mm

9.4 (mH)
5 (mW)
100 (mF)On resistance RDSon

2.3 (mW)

Rise time tr

5.0 (ns)

Fall time tf

9.8 (ns)

Gate drive ICControlcircuit
70mm

B. Output voltage control
Even if the load of grid side is changed, the output voltageof DC-DC converter should be controlled to be nearly
constant level by the control circuit without using a large
electrolytic capacitor. This contributes to high-temperature
tolerance and long lifetime of micro DC-DC converters.
C. Starting of prototype circuit
Fig. 6 shows the waveforms of prototype circuit. Thepower supply IC automatically starts the operationsimultaneously with the rise of the CPV cell voltage. Whenthe prototype circuit starts the operation, the chattering ofvoltage occurs. However, the control circuit and the main
circuit start stably the operation after 60 ms.
D. Efficiency and power loss
Fig. 7 shows the experimental results for the efficiency ofthe prototype micro DC-DC converter without thesynchronous rectifier of the switch S1. The efficiency ismeasured from the output power of the CPV cell to the powerof the load. It is noted that the power consumption of thecontrol circuit is included as a loss component. A maximum
efficiency of approximately 65% was obtained in theexperiment in case of Vin = 3.0 - 3.5 V.

Figure 4. Photograph of the prototype micro DC-DC converter.

Variation
2DNI=704.1 W/m
FF=0.79

8P-V curve

36I-V curve

24

1

200

0.5

1.5
2.5
12
3
CPV output voltage Vcpv (V)CPV output power Pcpv (W)4

CPV output current Icvp (A)A. Characteristic of CPV cellFig. 5 shows a I-V curve and a P-V curve of the CPV cellat direct normal irradiation (DNI) of 704 W/m2. From Fig. 5, it
is seen that the output voltage of the CPV cell which achieves
maximum power is approximately 3 V. Generally, thevariation of this voltage is not very large in the CPV cell due
to high fill factor for broad range of DNI. Therefore, thesimplest control strategy is to control the output voltage of theCPV cell to be nearly constant in order to perform
approximate maximum power point tracking (MPPT). The
present fundamental experiment follows this strategy.
However, in actual system, multi-step voltage control strategy
should be employed to perform more precise MPPT
depending on electrical characteristic of the CPV cell andother types of solar cell in case with CPV+ concept.

03.5

Figure 5. Current – voltage (I-V) curve and power – voltage (P-V) curveof CPV cell.

Vin

5 (V/div)0(Vin)Vout
5 (V/div)0(Vout)Vpsout5 (V/div)0(Vpsout)

50 (ms/div)
60 (ms)Figure 6. Waveforms of micro DC-DC converter at the starting of circuit
operation.

Fig. 8 shows the loss analysis using theoretical equation
for the next prototype circuit. The validity of the theoreticalequation was confirmed by the circuit simulator Piece-wise

REFERENCES

[2]

[3]

[4]

[5]

[6]

[7]

[8]

N. Bosco, T. J. Salverman and S. Kurtz: "Modeling Thermal Fatigue in
CPV Cell Assemblies", IEEE Journal of photovoltaics, Vol. 1, No. 2,
pp. 242-247 (2011)
G. S. Kinsey, A. Nayak, M. Liu and V. Garboushian: "Increasing
Power and Energy in Amonix CPV Solar Power Plants", IEEE Journalof photovoltaics, Vol. 1, No. 2, pp. 213-218 (2011)
I. R. Cole, T. R. Betts and R. Gottschalg: "Solar Profiles and Spectral
Modeling for CPV Simulations", IEEE Journal of photovoltaics, Vol. 2,No. 1, pp. 62-67 (2012)
K. Araki, T. Yano and Y. Kuroda: “30 kW Concentrator Photovoltaic
System Using Dome-shaped Fresnel Lenses”, Optics Express, Vol. 18,
Issue S1, pp. A53-A63 (2010)
N. Yamada, T. Ijiro, W. Goto, K. Okamoto, K. Dobashi and T.
Shinobara: "Development of Silicone-Encapsulated CPV Module
Based on LED Package Technology", 39th IEEE Photovoltaic
Specialist Conference, Tampa (June 16-21, 2013), (to be published)
H. Hu, S. Harb, N. Kutkut, I. Batarseh and Z. J. Shen: "A Review ofPower Decoupling Techniques for Microinverters With Three Different
Decoupling Capacitor Locations in PV Systems", IEEE Trans. PE, Vol.
28, No. 6, pp. 2711-2726 (2013)
N. Sukesh, M. Pahlevaninezhad and P. Jain: "Analysis andImplementation of a Single Stage Flyback PV-Micro Inverter with Soft
Switching", IEEE Trans. IE, Vol. PP, No. 99, pp. 1 (2013)
D. Hamza, M. Qiu and P. K. Jain: "Application and Stability Analysisof a Novel Digital Active EMI Filter Used in a Grid-Tied PV
Microinverter Module", IEEE Trans. PE, Vol. 28, No. 6, pp. 2867-2874(2013)

Vout =10 (V) fsw=50 (kHz)Without synchronous rectifier
Vin=3.5 (V)Vin=3.0 (V)Vin=2.5 (V)

60
55
50
45
400123456CPVoutput power (W)789Figure 7. Expeimental efficiency of the prototype micro DC-DCconverter.

CONCLUSION

0.5

fsw=250kHz Vin=2.5 V Vout=10 V

100

Efficiency
0.4
0.3

900.2850.1800258
CPV output power (W)Gate drive consumption
Shunt resister loss

95

Inductor copper loss

Efficiency (%)

In this paper, the use of micro DC-DC converter in CPV
system was introduced and the technical requirements for theconverter and possible solutions were described. Highefficiency, long term temperature-tolerance, and automatic
starting and shutdown of the converter operation are important
requirements to realize the present concept. The first-step
experiment with the fabricated converter circuit and analytical
study for the next prototyping demonstrated that the required
performance can be achieved by the better designed circuit
configuration. Further research and development will be
conducted to improve the feasibility of the use of micro DCDC converter in CPV systems.

[1]

65

Power loss (W)

IV.

70

Efficiency (%)

Linear Electrical Circuit Simulation (PLECS, Plexim GmbH).
In particular, the power loss is calculated from theinstantaneous voltage and the instantaneous current in thesimulation circuit, based on the table of switching frequency
data in the appropriate datasheet. In the present analysis, thepower loss includes that of the MOSFETs, the copper loss ofthe boost inductor. The core loss of the boost inductor and thepower consumption of control ICs are not considered. Notethat the synchronous rectifier is adopted in order to achieve
high efficiency although it was not used in the presentexperiment. In addition, the switching frequency is set to 250
kHz in order to reduce the size of the micro DC-DC converter.
The loss analysis result shows that the gate drive power
consumption and the switching loss of MOSFETs are
dominant part in total power loss. Therefore, MOSFETs which
has a fast switching characteristic should be selected in orderto achieve high-efficiency of prototype micro DC-DCconverter even if the switching frequency is high.

Turn off loss (Lower switch S2)Turn on loss (Lower switch S2)
Conduction loss (Lower switch S2)Turn off loss (Lower switch S1)Turn on loss (Lower switch S1)
Conduction loss (Lower switch S1)

75

Figure 8. Loss anaylysis results of the micro DC-DC conveter circuits
desigined for next prototyping.
[9]

S. Jiang, D. Cao, Y. Li and F. Z. Peng: "Grid-Connected Boost-HalfBridge Photovoltaic Microinverter System Using Repetitive Current
Control and Maximum Power Point Tracking", IEEE Trans. PE, Vol.
27, No. 11, pp. 4711-4722(2013)[10] H-J. Chiu, Y-K. Lo, C-Y. Yang, S-J. Cheng, C-M. Huang, C-C.
Chuang, M-C. Kuo, Y-M. Huang, Y-B. Jean and Y-C. Huang: "AModule-Integrated Isolated Solar Microinverter", IEEE Trans. IE, Vol.
60, No. 2, pp. 781-788(2013)