Research and design of new solar chargers

1 Introduction
　
　　At present, in various photovoltaic power stations, solar cells are commonly used to collect solar energy and store it in batteries so that it can be inverted into 220V/50Hz AC power for users when needed. However, in the process of using solar cells to charge batteries, due to the nonlinearity of the solar cell output characteristics, the operating point of the solar cell is not always near the maximum power point, resulting in a waste of solar cell energy. The new solar charger developed in this project adopts the constant voltage tracking (CVT) method to achieve the maximum power tracking of the solar cell based on the working characteristics of the solar cell - the voltage value at the maximum output power point is basically unchanged under different sunlight. , effectively improve the working efficiency of solar cells, and also improve the working performance of the entire system.

2 System main circuit

　　The main circuit of the system is shown in Figure 1.

　　As can be seen from Figure 1, the main circuit topology is a Buck-type converter. The output pulse of the pulse width control chip TL494 is used to control the duty cycle of the main circuit power device (IGBT) to change the charging current of the battery, thereby realizing solar energy. The constant voltage tracking of the battery makes the output power of the solar cell close to the maximum power. At the same time, the main circuit is used to collect the battery voltage, charging current and solar cell voltage, so that the control circuit can realize various tracking and protection functions.

3 Working characteristics of solar cells

　　Figure 2 is the operating characteristic curve of the solar cell. It can be seen from the figure that the operating characteristics of the solar cell are a set of nonlinear curves. Points A, B, C, D, and E are the maximum output power points under different sunlight, and the voltage values ​​at the corresponding maximum output power points are different under different sunlight. Basically unchanged, according to this characteristic, using constant voltage tracking method and using simple hardware circuits, the output power of the solar cell can basically be maximized; at the same time, as can be seen from Figure 2, when the battery is overcharged, as long as the solar cell is Overcharge protection can be achieved by working in an open circuit state.

4. System control principle

　　4.1 System control block diagram

　　The control block diagram of the system is shown in Figure 3.

　　As can be seen from Figure 3, this system adopts the double closed-loop control method in classical control theory, in which the current loop is the inner loop, the voltage loop is the outer loop, and the output of the voltage loop is the given of the current loop; and the voltage loop also contains The circuit composed of the battery voltage and the circuit composed of the solar cell voltage play corresponding regulatory roles in each stage of the circuit operation.

　　4.2 Analysis of the working process of the system

　　During the charging stage, the circuit composed of the battery voltage does not work. The voltage loop is only composed of the circuit composed of the solar cell voltage. At this time, the output of the voltage loop is the given value of the current loop. By detecting the charging current of the battery in the main circuit and The given current is compared to change the output pulse width of TL494, so that the solar cell voltage closely tracks the given voltage. The specific performance is: when the solar cell voltage is greater than the given voltage, the deviation signal changes the given current after being adjusted by PI to make the The current input signal to TL494 becomes larger, and the output pulse width of TL494 increases. It is amplified and shaped by the drive circuit to drive the power device, so that its conduction duty cycle increases, and the battery charging current becomes larger. As can be seen from Figure 2, the solar cell voltage decreases. , when the circuit reaches a steady state, the solar cell voltage is equal to the given voltage, the given current loop is also a stable value, and the charging current of the battery is equal to the given current; conversely, when the solar cell voltage is less than the given voltage, TL494 outputs pulses The width decreases, and is amplified and shaped by the drive circuit to drive the power device, so that its conduction duty cycle decreases, the battery charging current becomes smaller, the solar cell operating voltage increases, and the solar cell voltage is equal to the given voltage when the circuit reaches a steady state.

　　During the overcharge stage, both circuits work, and the voltage loop is composed of a circuit composed of the solar cell voltage and a circuit composed of the battery voltage. At this time, the sum of the battery voltage and the given solar cell operating voltage is greater than the actual operating voltage of the solar cell. The deviation signal is added to the current input terminal of TL494 after being adjusted by PI, so that the output pulse width of TL494 is reduced, and the battery charging current becomes smaller. As can be seen from Figure 2, the actual operating voltage of the solar cell gradually increases until the steady state, when the solar cell operates In the open circuit state, the battery charging current is zero, thus realizing overcharge protection.

5 Pulse width modulation chip TL494 and its applications

　　5.1 Structure of pulse width modulation chip TL494

　　TL494 is a product of Texas Instruments in the United States. It is cheap and easy to purchase. It also solves the current regulator, pulse width modulation and maximum current limit internally. The chip also has some additional monitoring and protection functions, making the chip It has strong anti-interference ability and high reliability. The control system composed of this chip has fewer external components and a simple structure. Figure 4 is the internal structure diagram of the chip.

　　As can be seen from Figure 4, TL494 consists of an oscillator, two comparators, two error amplifiers, a flip-flop, dual AND gates and dual NOR gates, a +5V reference voltage source, and two NPN output transistors. The external resistors Rt and Ct connected to pin 6 and pin 5 determine the frequency fosc of the sawtooth wave generated by the oscillator fosc
=1/(RtCt)

　　The width of the output modulation pulse is determined by a comprehensive comparison of the forward sawtooth waveform at the capacitor Ct terminal and the two control signals input from pins 3 and 4. Pin 13 is used to control the output mode. Pin 4 is the dead time control terminal. Pins 1 and 16 and pins 2 and 15 are the non-inverting and non-inverting input terminals of the two error amplifiers respectively. They can be connected to the given signal and the feedback signal respectively and used as voltage and current regulators to complete the closed-loop control of the system. Or it can be used as a comparator for overcurrent, overvoltage, undervoltage and overheating to achieve protection functions. Pin 14 is the reference voltage terminal, which can provide a reference for the above-mentioned regulator and comparator.

　　5.2 Peripheral circuit composition of TL494

　　The peripheral circuit composition of TL494 is shown in Figure 5.

6 Conclusion

　　The charger developed based on the above control idea has complete protection functions such as overcharge, overcurrent, and overheating; after long-term operation, the system has shown good results, not only improving the working efficiency of the solar cells, but also protecting the The battery has great social benefits in utilizing green energy.

References
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