The test chip, which contains both the proposed MPPT circuit and the controller for the boost controller, was fabricated in 0.25μm BiCMOS-DMOS (BCD) 40V 1P4M process. Fig. 5.29 shows the experimental setup, the prototype and the chip micrograph of the controller for the boost converter.
The solar array used in the measurement is Tynsolar TYN-285P6 [38]. TABLE 5.3 gives the technical specifications of the used solar array.
Prototype Oscilloscope
Lamp Prototype Grid Power
PV Module Tynsolar TYN-285P6
7 Bit s U p-do w n C o unt er
Driver Slope Detection
PWM Generator
BG
Bias S/ H
Driver
Fig. 5.29 Experiment prototype and chip micrograph.
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TABLE 5.3 Technical Specifications of TYN-285P6
Maximum Power 285W
Tolerance ±5%
Open circuit Voltage 44.21 V
Short circuit Current 8.32A
Maximum Power Voltage 36.79V
Maximum Power Current 7.75A
Module Efficiency 14.61%
Solar Cell Efficiency 16.37%
Operating Temperature -40°C ~85°C Short Circuit Current Temperature Coefficient +4.500 mA/°C
Open Circuit Voltage Temperature Coefficient -0.1500 V/°C Maximum Power Temperature Coefficient -0.4982 %/°C
The tracking efficiency of the MPPT circuit can be approximated to the following equation [39].
, ,max PV tracked
100%
MPP
PV
P
P
(5.3)PPV,tracked is the actual energy the system captured and delivered from the solar array. PPV,max is the maximum energy from the sun at a specific time instant, temperature and solar irradiation. Fig. 5.30 shows the measured maximum power point tracking efficiency based on the proposed tracking algorithm and circuit.
80 96
97 98 99
95
100 150 200 250 300
50 Tracking Efficiency, ηMPP (%)
Available Power from Solar Panel (W) 94
0
Fig. 5.30 Tracking efficiency of the proposed MPPT algorithm and circuit.
The tracking efficiency is higher than 97.3% in the available power range from 100W to 300W at ambient temperature of 28°C. The tracking efficiency is limited below 98%, compared to contemporary product [40] due to some reasons: the process variation during the silicon fabrication, temperature variation, IC layout optimization, and the limited operation range of the calculation circuit such as the multiplier, the adder and subtractor. An overflow or underflow current may cause the circuit to misinterpret the slope condition. As a result, it undermines the tracking efficiency.
Fig. 5.31 shows the waveforms of the PV system during the power-on period. At the time the PV system turns on, the OCT technique is enabled to adjust the operating voltage close to the value of 0.7×VOC. During this open-circuit voltage detection period, the duty cycle is set to its maximum value to enhance the tracking speed. After that, the SDT technique takes over the tracking procedure to improve the tracking accuracy. When the slope detection circuit detects the slope condition is positive, which means Eslope is high, the duty cycle decreases, as shown in Fig. 5.31(a), to reduce the inductor
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current and therefore increase the operating voltage toward the MPP.
Conversely, as shown in Fig. 5.31(b), when the slope condition is detected as negative, Eslope is set low, the controller increases the duty cycle to the boost converter. As a result, the inductor current increases to lower the operating voltage of the system.
ESlope
VG OCT
Period
SDT Period
OCT Enable
5μs Dmax·T
(a)
ESlope
OCT Period
SDT Period
OCT Enable
VG
Dmax·T 5μs
(b)
Fig. 5.31 The waveforms of PV system during the system power-on period. (a) When the detected slope condition is positive. (b) When the detected slope condition is negative.
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Fig. 5.32 shows the duty cycle transitions when the solar irradiation changes. The detected slope condition changes either from negative to positive or from positive to negative depending on the solar irradiation level. In Fig.
5.32(a), when the solar irradiation level increases, the duty cycle of the control signal, VG, which controls the gate of the power NMOS (MN), gradually decreases to a smaller value. On the contrary, in Fig. 5.32(b), VG changes from small duty cycle to large duty cycle when the detected slope condition transits from positive to negative, which is due to the degradation of the solar irradiation level.
ESlope
VG
5μs
(a)
ESlope
5μs
VG
(b)
Fig. 5.32 The waveforms of VG according to different Eslope values. (a) When the solar irradiation level increases, Eslope changes from low to high. (b) When the solar irradiation level decreases, Eslope changes from high to low.
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Fig. 5.33 shows some waveforms to investigate the tracking speed during the system power-on period. In Fig. 5.33(a), when the OCT technique is disabled, the settling time (ts) for the system is around 1.1s. When the OCT technique is activated during the startup period, which can rapidly approximate the maximum power point, the settling time can be shorten to 22ms, as shown in Fig. 5.33(b). Therefore, the tracking speed is improved with the proposed MPPT algorithm and circuit.
P
PVV
PVI
PVt
s=1.1s
High irradiation
Low irradiation
(a)
P
PVV
PVI
PVHigh irradiation
Low irradiation
t
s=22ms
(b)
Fig. 5.33 The waveforms of VPV, IPV and PPV during the power-on period. (a) When the OCT technique is disabled. (b) When the OCT technique is activated.
84
Fig. 5.34(a) shows the waveforms at the output of DC-AC inverter which is connected to the boost converter. Fig. 5.34(b) demonstrates that when the grid-connected PV system is enabled, the output voltage VAC works in phase with the grid power VGrid. TABLE 5.4 summarizes the measurement results.
The proposed AMPPT technique can achieve a tracking efficiency of 97.3%.
V
AC 112.5V/60HzI
AC 1.75A/60Hz(a)
V
GridV
AC12.5ms 60Hz
(b)
Fig. 5.34 The waveforms of the DC-AC inverter. (a) Output voltage and current of the inverter. (b) Output waveforms when VAC is connected to VGrid.
85
TABLE 5.4 Design Specifications
Fabrication Process 0.25μm BCD 40V 1P4M
Chip area 4.22mm2
Inductor L1=5mH, L2=5mH
Capacitor C1=1500μF, C2=50μF
Switching frequency 120kHz
Nominal AC output voltage 112V
Power factor 0.998
Tracking efficiency >97.3% (@PPV=100W~285W)
Overall efficiency 88.97%
Input Capacitor Cpv=4700μF
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Chapter 6
Conclusion and Future Work
Several MPPT algorithms and the characteristics of solar arrays are discussed first in this thesis to help understand the behaviors of solar array, which is useful while designing the maximum power point tracking circuit.
The proposed maximum power point tracking algorithm, which basically combine two tracking algorithms, can not only achieve fast tracking speed but also maintain high tracking efficiency in a solar electricity system.
Furthermore, with analog circuit implementation, power consumption can be reduced and high power efficiency of the system is ensured as a result. A wide-range current multiplier circuit is implemented to meet the requirement of the tracking circuit. The proposed MPPT circuit can determine the slope condition with fast speed and high accuracy. In the end, simulation and experiment results verify the efficiency and the tracking speed of the proposed MPPT algorithm. The experiment results, moreover, demonstrate that the PV system can also convert solar energy into grid electricity with synchronized phase response.
The tracking speed and tracking efficiency of the proposed tracking technique can be further improved when the variable perturbation steps are included. When the operating point is far away from the maximum power point, the perturbation step can be set as a larger one to increase the tracking speed. While the operating point is near the maximum power point, the perturbation can be switched to a smaller one to ensure the minimum oscillation and therefore a high tracking efficiency.
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Reference
[1] H. Chun-Yu and C. Ke-Horng, "Boost DC-DC Converter With Fast Reference Tracking (FRT) and Charge-Recycling (CR) Techniques for High-Efficiency and Low-Cost LED Driver," Solid-State Circuits, IEEE Journal of, vol. 44, pp.
2568-2580, 2009.
[2] A. Richelli, et al., "A 0.2V-1.2V DC-DC Boost Converter for Power Harvesting Applications," Power Electronics, IEEE Transactions on, vol. 24, pp. 1541-1546, 2009.
[3] E. Carlson, et al., "20mV input boost converter for thermoelectric energy harvesting," in VLSI Circuits, 2009 Symposium on, 2009, pp. 162-163.
[4] C. Zhe, et al., "A Review of the State of the Art of Power Electronics for Wind Turbines," Power Electronics, IEEE Transactions on, vol. 24, pp. 1859-1875, 2009.
[5] W. Rong-Jong and W. Wen-Hung, "Grid-Connected Photovoltaic Generation System," Circuits and Systems I: Regular Papers, IEEE Transactions on, vol.
55, pp. 953-964, 2008.
[6] G. J. Bauhuis, et al., "26.1% thin-film GaAs solar cell using epitaxial lift-off,"
Solar Energy Materials and Solar Cells, vol. 93, pp. 1488-1491, 2009.
[7] M. A. Green, et al., "Solar cell efficiency tables (version 36)," Progress in Photovoltaics: Research and Applications, vol. 18, pp. 346-352, 2010.
[8] T. Esram, et al., "Dynamic Maximum Power Point Tracking of Photovoltaic Arrays Using Ripple Correlation Control," Power Electronics, IEEE Transactions on, vol. 21, pp. 1282-1291, 2006.
[9] S. Jain and V. Agarwal, "A Single-Stage Grid Connected Inverter Topology for Solar PV Systems With Maximum Power Point Tracking," Power Electronics, IEEE Transactions on, vol. 22, pp. 1928-1940, 2007.
[10] F. Boico, et al., "Solar Battery Chargers for NiMH Batteries," Power Electronics, IEEE Transactions on, vol. 22, pp. 1600-1609, 2007.
[11] T. Esram and P. L. Chapman, "Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques," Energy Conversion, IEEE Transactions on, vol. 22, pp. 439-449, 2007.
[12] C. Rodriguez and G. A. J. Amaratunga, "Analytic Solution to the Photovoltaic Maximum Power Point Problem," Circuits and Systems I: Regular Papers, IEEE Transactions on, vol. 54, pp. 2054-2060, 2007.
[13] K. Jung-Min, et al., "Three-Phase Photovoltaic System With Three-Level
88
Boosting MPPT Control," Power Electronics, IEEE Transactions on, vol. 23, pp. 2319-2327, 2008.
[14] N. Khaehintung, et al., "FPGA Implementation of MPPT Using Variable Step-Size P&O Algorithm for PV Applications," in Communications and Information Technologies, 2006. ISCIT '06. International Symposium on, 2006, pp. 212-215.
[15] W. Wenkai, et al., "DSP-based multiple peak power tracking for expandable power system," in Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE, 2003, pp. 525-530 vol.1.
[16] L. Zhigang, et al., "A new cost-effective analog maximum power point tracker for PV systems," in Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, 2010, pp. 624-631.
[17] P. Mattavelli, et al., "A simple mixed-signal MPPT circuit for photovoltaic applications," in Applied Power Electronics Conference and Exposition (APEC), 2010 Twenty-Fifth Annual IEEE, 2010, pp. 953-960.
[18] K. K. Tse, et al., "A novel maximum power point tracker for PV panels using switching frequency modulation," Power Electronics, IEEE Transactions on, vol. 17, pp. 980-989, 2002.
[19] H. S. H. Chung, et al., "A novel maximum power point tracking technique for solar panels using a SEPIC or Cuk converter," Power Electronics, IEEE Transactions on, vol. 18, pp. 717-724, 2003.
[20] R. Leyva, et al., "MPPT of photovoltaic systems using extremum - seeking control," Aerospace and Electronic Systems, IEEE Transactions on, vol. 42, pp.
249-258, 2006.
[21] P. Joung-Hu, et al., "Dual-Module-Based Maximum Power Point Tracking Control of Photovoltaic Systems," Industrial Electronics, IEEE Transactions on, vol. 53, pp. 1036-1047, 2006.
[22] D. Brunelli, et al., "Design of a Solar-Harvesting Circuit for Batteryless Embedded Systems," Circuits and Systems I: Regular Papers, IEEE Transactions on, vol. 56, pp. 2519-2528, 2009.
[23] A. Tariq and J. Asghar, "Development of an Analog Maximum Power Point Tracker for Photovoltaic Panel," in Power Electronics and Drives Systems, 2005. PEDS 2005. International Conference on, 2005, pp. 251-255.
[24] D. Sera, et al., "Improved MPPT Algorithms for Rapidly Changing Environmental Conditions," in Power Electronics and Motion Control Conference, 2006. EPE-PEMC 2006. 12th International, 2006, pp. 1614-1619.
[25] C.-x. Liu and L.-q. Liu, "An improved perturbation and observation MPPT method of photovoltaic generate system," in Industrial Electronics and
89
Applications, 2009. ICIEA 2009. 4th IEEE Conference on, 2009, pp.
2966-2970.
[26] G. J. Yu, et al., "A novel two-mode MPPT control algorithm based on comparative study of existing algorithms," in Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE, 2002, pp.
1531-1534.
[27] C. Dorofte, et al., "A combined two-method MPPT control scheme for grid-connected photovoltaic systems," in Power Electronics and Applications, 2005 European Conference on, 2005, pp. 10 pp.-P.10.
[28] C. Yaow-Ming, et al., "The AC Line Current Regulation Strategy for the Grid-Connected PV System," Power Electronics, IEEE Transactions on, vol.
25, pp. 209-218, 2010.
[29] P. Maffezzoni and D. D'Amore, "Compact Electrothermal Macromodeling of Photovoltaic Modules," Circuits and Systems II: Express Briefs, IEEE Transactions on, vol. 56, pp. 162-166, 2009.
[30] M. G. Villalva, et al., "Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays," Power Electronics, IEEE Transactions on, vol. 24, pp.
1198-1208, 2009.
[31] N. Femia, et al., "Optimization of perturb and observe maximum power point tracking method," Power Electronics, IEEE Transactions on, vol. 20, pp.
963-973, 2005.
[32] M. A. S. Masoum, et al., "Theoretical and experimental analyses of photovoltaic systems with voltageand current-based maximum power-point tracking," Energy Conversion, IEEE Transactions on, vol. 17, pp. 514-522, 2002.
[33] T. Shimizu, et al., "Generation control circuit for photovoltaic modules,"
Power Electronics, IEEE Transactions on, vol. 16, pp. 293-300, 2001.
[34] H. Patel and V. Agarwal, "Maximum Power Point Tracking Scheme for PV Systems Operating Under Partially Shaded Conditions," Industrial Electronics, IEEE Transactions on, vol. 55, pp. 1689-1698, 2008.
[35] X. Weidong, et al., "Topology Study of Photovoltaic Interface for Maximum Power Point Tracking," Industrial Electronics, IEEE Transactions on, vol. 54, pp. 1696-1704, 2007.
[36] J. C. Wiles and D. L. King, "Blocking diodes and fuses in low-voltage PV systems," in Photovoltaic Specialists Conference, 1997., Conference Record of the Twenty-Sixth IEEE, 1997, pp. 1105-1108.
[37] M. A. de Blas, et al., "Selecting a suitable model for characterizing photovoltaic devices," Renewable Energy, vol. 25, pp. 371-380, 2002.
90
[38] Tynsolar, "http://www.tynsolar.com.tw/upload/TYN-285P6.pdf."
[39] H. Patel and V. Agarwal, "MPPT Scheme for a PV-Fed Single-Phase Single-Stage Grid-Connected Inverter Operating in CCM With Only One Current Sensor," Energy Conversion, IEEE Transactions on, vol. 24, pp.
256-263, 2009.
[40] RefuSol. (2010) Königsmörder. Photon Profi, Photovoltaik-Fachwissen für die Praxis. 51. Available: http://www.photon.info