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Based on the MPPT solar battery charger SPCE061 design

In Electronic Infomation Category: B | on November 23,2011

Abstract: non-linear curve of solar cell output characteristics of the traditional solar charger for the low efficiency of solar cells. After the article in the mathematical model based on the analysis made by changing the duty cycle so that the maximum charge current of MPPT tracking strategy, a substantial increase in solar cell efficiency. Meanwhile SPCE061 by BUCK circuit with microcontroller to monitor the charge process, using three-step algorithm to ensure battery performance, increase its life. Finally, by comparing the experimental data validates the practicality and SN74LS669N datasheet and effectiveness of the program.

0 Introduction

green and SN74LS669N price and renewable solar energy features, so that in low-carbon and SN74LS669N suppliers and energy shortage today of concern. Lithium batteries for high energy, low self-discharge characteristics, and gradually replace the lead-acid batteries into the mainstream. Currently used by the solar cell output characteristics shows that solar cells in a certain light intensity and temperature, neither the constant current source, nor a constant voltage source, its maximum power by the load effects. The lithium battery can be seen as a small load voltage source. If not controlled directly connect the two, then the solar cell voltage clamp in the lithium battery voltage, not efficient use of energy.

SPCE061 this paper, single-chip, using MPPT technology to make solar cells work at maximum power point, and to the lithium battery charging process control, to extend battery life, charging to ensure safety.

1 principle of maximum power point tracking (Maximum Pow < STRONG> er Point Tracking referred to as MPPT)

photovoltaic solar cell has a non-linear characteristics, so even in the same light intensity, the output will be different because of different load power.

its voltage, current and power in the light intensity 1 kW / m 2 , T = 25 under the conditions of the output curve shown in Figure 1. Short-circuit current i sc and open-circuit voltage u oc is given by the manufacturer, P mpp for the conditions of maximum power point.

as solar cells by light intensity, incident angle of light, temperature and other factors, the corresponding change in the maximum power, maximum power point corresponding to the output voltage, output current, and resistance are constantly change. Therefore, adjustable PWM-based DC / DC converter, the load corresponding to change in order to make solar cells work at maximum power point.

Figure 1 A typical solar cell output curve

2 circuit works

Figure 2 shows a block diagram of the solar charger. Sunplus microcontrollers which produced SPCE061A microcontroller, the microcontroller contains seven 10-bit ADC (analog - digital converter) and PWM function built, greatly simplify the circuit complexity, increase stability. Voltage and current sampling circuit sampling circuit voltage by the ADC value and current value into the MCU, MCU based MPPT algorithm to calculate the PWM control circuit BU CK complete control of the charging process.

Figure 2 Block diagram of the overall charger

Figure 3 for the BUCK converter circuit. By the MOSFET, Q3, following the inductor L1 and diode D1 form a typical buck BUCK DC / DC converter, Q1 and Q2 form the MOSFET driver circuit, U out output to the lithium battery cathode.

Figure 3 BUCK converter circuit

Figure 4 for the current sampling circuit. Rsense precision resistors with a resistance as a small sampling resistor, the voltage across the resistor by using the differential amplifier SPCE061 transported to the A / D sampling client. For accurate sampling, to avoid interference with the power line and ground, using a linear optocoupler HC NR200 isolation.

current sampling circuit in Figure 4

Figure 5 shows the voltage sampling circuit. SPCE061 because the A / D input range of side 0 ~ 3 V, while the solar cell output is often higher than 3 V, so a reverse ratio of the amplifier, the input to match the scope of the AD sample.

voltage sampling circuit in Figure 5

3 system software design

in BUCK, the existence of U arr D = U bat relationship. It can be seen:

where, U bat at both ends of the battery voltage; D is the duty cycle; U arr is the voltage across the solar cell. The equation (1) into equation (2) available:

see from Figure 1, when taking the maximum power point, dP arr / dU arr = 0, into (3), ( 4) shows:

Accordingly, the P / D of the curve is convex, and if P has a unique take maximum D value corresponding.

as DC / DC converter connected to the battery output voltage across a short little change in a short time can be considered constant. Therefore, the design of the maximum power point tracking can be simplified to adjust the current through the PWM to the maximum, that is, that the solar cell maximum power output.

characteristics known by the lithium battery, charging in order to ensure safe and efficient, requires the use of pre-charge, constant current, three-stage trickle charge. System by testing the voltage across the battery to determine charge state, and then take the appropriate charging strategies.

when the light intensity to reduce the program to determine the power generated by solar cells cost less than the system itself when entering sleep mode.

4 Experimental results and conclusions

Based on the above principle and circuit diagram above, the production and use of the MPPT solar charger solar charger diode structures of the traditional test data such as shown in Table 1. China, which uses solar cells produced by single-crystal micro-solar panels, its maximum output power of 15 W, the open circuit voltage 17. 4 V; lithium batteries 18650 with 4 series lithium battery, charging cut-off voltage 16. 8 V, the battery pack capacity of 10. 4 Ah.

Table 1 traditional MPPT charger charger and experimental data comparison

experimental results show that the efficiency of traditional solar charger is about 66%, while the MPPT charger utilization of this program is about 97%, a significant increase in output power. By SPCE061 MCU solar charger with MPPT function not only significantly improve the efficiency of solar cells, and contains a three-stage charging intelligent charging strategy, the software module added to prevent overcharging of the security policy, and in the light intensity dropped below the system overhead of the system automatically sleep. By improving the algorithm, more accurate set of parameters, the charging efficiency can be further improved.

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