Design and implementation of intelligent solar charging circuit

With the development of wireless technology, wireless network technology is increasingly being put into practical applications. Wireless sensor networks are generally distributed over a wide range, and the construction of power supply lines requires large investments and high maintenance costs. If dry batteries are used for power supply, the power supply capacity of each node is limited. Replacing batteries for each node is not only time-consuming and labor-intensive, but also increases costs and affects work efficiency. Whether the power supply can be stable and continuous has become an important factor restricting the development of oilfield wireless dynamometers and their wireless networks. The development of solar energy technology has led to a leap in the development of power supply methods, and has become the development direction of power supply methods for oilfield wireless dynamometers and their relay network nodes. This paper intends to design an intelligent, maintenance-free solar charging circuit for oilfield monitoring dynamometers and relay network nodes to power wireless network nodes. The design circuit has the following characteristics: ① The charging network designed based on switching power supply technology has the function of automatically adjusting the duty cycle and has a wide input voltage range. ② A linear power management chip is used to complete the entire charging process using a pre-charge 2 constant current 2 constant voltage charging method. ③ It uses a low-noise, high-speed CMOS voltage regulator with high-precision constant voltage and constant current output. ④ Charging overvoltage protection and lithium battery over-discharge protection functions make lithium battery charging and discharging safe and reliable. ⑤ Automatic sun tracking function, the solar energy collection panel always keeps aiming at the sun to make full use of solar energy.

1 System Design

The output voltage of the existing photovoltaic cells is very low (below 1V). In this design, multiple photovoltaic cells are connected in series to form a solar panel. The power supply network that can automatically adjust the duty cycle ensures that the output voltage is basically stable when the light intensity and load change, providing a stable voltage input for the charging management chip. By monitoring the secondary voltage of the power supply network, the charging management chip is protected from damage due to excessive voltage. By monitoring the voltage at both ends of the battery, it is ensured that the lithium battery will not be damaged due to over-discharge. Since the power supply requirement of the wireless dynamometer and its relay network nodes is 313V, a low-noise, high-speed CMOS voltage regulator is used. Under the action of the automatic tracking controller, it always keeps tracking the sun all day long. In order to prevent insufficient solar power supply due to continuous rainy days, an emergency charging circuit is designed. During charging, the wireless dynamometer and its nodes operate normally. The specific system design module is shown in Figure 1.

Figure 1 System design diagram

2 Hardware Circuit Design

2.1 Solar panels and charging circuit design

In this design, 16 photovoltaic cells are connected in series to form a solar panel with a voltage of about 1218V. By collecting more light energy, it is ensured that the lithium battery can be fully charged by sunlight. The power supply network design circuit adopts the forward topology structure [1]. The specific circuit is shown in Figure 2.

Figure 2 Main circuit of intelligent solar charging circuit design

The electric energy generated by the solar panel is added to the collector (c) of the switch tube Q1 through the 122 winding of the switch transformer T1, and the other way is through R1 to provide the base voltage for Q1. When the voltage of the base (b) is high, Q1 starts to conduct, and 1 positive and 2 negative electromotive forces are generated in the 122 winding of the transformer T1. After coupling with T1, 3 positive and 4 negative induced electromotive forces are generated in the 324 winding of T1. This electromotive force is superimposed on the base (b) of Q1 through R5 and C2, causing Q1 to saturate and conduct quickly. Since the current between 122 of the transformer T1 cannot change suddenly, 1 negative and 2 positive electromotive forces will be generated in this process. 3 negative and 4 positive electromotive forces are induced in the 324 winding of the transformer T1, and Q1 is quickly turned off through R5 and C2. After R1 continuously charges C2, Q1 starts to conduct again and enters the next round of switching oscillation. During the conduction period, the secondary winding 526 of the T1 transformer transmits energy to the outside through the rectifier diode D4.

The voltage stabilizing circuit is composed of voltage stabilizing tube D0, triode Q2 and other components . When the load is reduced or the output voltage of the solar panel increases, the voltage at point A rises. When the voltage is greater than 511V, D0 breaks down, Q2 is quickly turned on due to the forward bias of the b2e junction, Q1 is turned off early, and the output voltage tends to decrease; conversely, the control process is opposite, so that the output voltage of the secondary side of transformer T1 is basically stable. When the load is too heavy, the c2e current of Q1 increases, and the voltage drop on R4 also increases accordingly. When the voltage is greater than 0.17V, Q2 is turned on and Q1 is turned off, achieving the purpose of overcurrent protection. In order to prevent the peak pulse induced by the 122 winding of transformer T1 from breaking through the switch tube Q1 during the cut-off period, a peak pulse absorption circuit is connected in parallel.

2.2 Overvoltage protection control

Overvoltage protection control, the specific circuit is shown in Figure 3: Rectifier diode D4 is connected to the overvoltage protection relay JDQ1 output. The maximum input voltage of the charging control management chip MCP73831 is 6V. Although the basic output voltage of the power supply network is 5V, when the light intensity changes drastically or the load changes greatly, the output voltage will still fluctuate to a certain extent. In order to protect the MCP73831 from being damaged by short-term voltage fluctuations, an overvoltage protection controller is designed. When the voltage of W1 exceeds 6V, JDQ 1 will disconnect the output circuit, and the MCP73831 will be protected by power failure. The specific analysis is as follows: This part of the circuit design mainly uses the LM 2903 voltage comparator and peripheral circuit expansion. LM 2903 contains two comparators, 1, 2, 3 pins for one, 1 pin for OU TPU TA, 2, 3 pins for IN PU TA. 5, 6, 7 pins for the other, 7 pin for OU TPU TB, 5, 6 pins for IN PU TB. The overvoltage protection controller uses the comparator at pins 5, 6, 7. Resistors R11 and R13 divide the voltage and connect it to pin 5 of the comparator. When the voltage is greater than 6V, that is, the voltage division value is greater than 214V. The output level of pin 7 of the comparator changes from low to high. Q3 is saturated and turned on, then Q5 is turned off, the safety working indicator light goes out, and the contact J1 is at a high level. At this time, JDQ 1 starts to work, the power supply circuit is disconnected from the subsequent circuit, and the overvoltage red warning light is on.

Figure 3 Overvoltage and overdischarge protection control circuit

2.3 Over-discharge protection control

When the lithium battery voltage is lower than 3.15V, that is, when the battery power is released by more than 92%, it is considered that the discharge cannot continue, otherwise the internal medium of the lithium battery will change, causing the charging characteristics to deteriorate and the capacity to decrease. For this reason, an over-discharge protection control circuit is designed. The specific design of this circuit is shown in Figure 3, and the analysis is as follows: a comparator composed of 1, 2, and 3 pins of LM 2903 is used to form an over-discharge voltage comparator with peripheral devices. R12 and R14 divide the voltage and connect to the 3rd pin of LM 2093. When the voltage value is less than 3.15V, the voltage division value is less than 2.14V, the 1st pin of LM 2903 changes from high level to low level, Q4 changes from on to off state, Q6 is saturated and turned on, JDQ2 works, and the over-discharge red indicator light is on.

2.4 Automatic tracking controller

At the input end of the controller, the photosensitive sensor is composed of two photoresistors connected in series and cross-combined. One of the two photoresistors in each group is the upper bias resistor of the comparator, and the other is the lower bias resistor. One detects sunlight, and the other detects ambient light. The comparison level sent to the input end of the comparator is always the difference between the two. The specific circuit is shown in Figure 4: photoresistors RT1, RT2 and potentiometer R27 and photoresistors RT3, RT4 and potentiometer R28 constitute photosensitive sensing circuits respectively. RT1 and RT3 are installed on one side of the vertical sunshade, and RT4 and RT2 are installed on the other side. When RT1, RT2, RT3 and RT4 are simultaneously affected by ambient natural light, the center point voltage of R27 and R28 remains unchanged. When only RT1 and RT3 are exposed to sunlight, the internal resistance of RT1 decreases, the potential of pin 5 of LM 2903 increases, pin 7 outputs a high level, transistor Q7 turns on, JDQ 4 works, and its contacts 3 and 5 are closed. At the same time, the internal resistance of RT3 decreases, the potential of pin 3 of LM 2903 decreases, JDQ 5 does not work, and motor M rotates forward; when only RT2 and RT4 are exposed to sunlight, similarly, motor M rotates in the opposite direction. When the light illuminance on both sides of the vertical sunshade is the same, JDQ 4 and JDQ 5 are both turned on, and motor M stops. During the continuous shift of the sun, the intensity of the light illumination on both sides of the vertical sunshade changes alternately, and the motor keeps moving, so that the solar receiving device always faces the sun.

Figure 4 Automatic tracking controller

2.5 Charging Management Circuit Design

The charging process of lithium batteries is generally divided into three stages: ① Trickle charging stage. ② Constant current charging stage. Generally, the battery can be charged to about 85% of its capacity. ③ Constant voltage charging stage. Overcharging of lithium batteries can reduce battery life and deteriorate performance, or even cause leakage. In the design of this article, the linear charging management chip MCP73831 is used, as shown in Figure 1. The chip has the functions and features of accurate output voltage, arbitrary setting of charging current, automatic conversion of charging mode, extremely low current consumption (25uA), overcharge monitoring protection, etc. The functions of each pin of MCP73831 are:

VDD is the input voltage terminal; VSS is the reference zero voltage terminal; VBA T is the charge control output terminal; STA T is the charge status output terminal. PROG is the current setting and charge control enable terminal. When charging a lithium battery, the PROG interface of the charge management chip MCP73831 must be connected to VSS with an external resistor. The specific calculation formula is: IREG = 1000 (V) /RPROG, where RPROG is in kΩ and IREG is in mA. In this design, RPROG = 2kΩ.

Then IREG = 500mA. The logical relationship of each interface state of STA T and the indicator light in the circuit design is shown in Table 1. The charging management chip MCP73831 determines the various states of the battery by detecting the BAT pin of the lithium battery, thereby managing the battery charging. When there is no overvoltage protection, the power supply network provides 5V voltage to MCP73831 on the one hand. On the other hand, it is transmitted to JDQ2 through D5 to power the subsequent circuit. During emergency charging, an external 5V power supply is connected, one way through D5 to relay JDQ 2. The other way reaches MCP73831 to charge the lithium battery. The output voltage of the cathode end of D5 is 5 (V) - 0.17 (V) = 4.13 (V). Since the voltage of the lithium battery is lower than the cathode output voltage of D6 when it is fully charged or not fully charged (D5, D6 are common cathodes), RT9193 works normally during emergency charging. Connecting a 22nF capacitor between the BP terminal and ground of the CMOS (compact metal oxide semiconductor ) voltage regulator RT9193 can greatly reduce the output noise of the regulator. Under normal temperature, when the lithium battery with a voltage of 412V is fully charged, the voltage will still remain at 315V when 90% of the power is consumed. The voltage regulator RT9193 is selected in this design. Even when it is 314V, the output voltage can still be stabilized at 313V.

Table 1 Logical relationship of indicator lights in MCP73831 circuit design

3 Experimental data and results analysis

In debugging, the modular test method is adopted, and finally joint debugging is carried out. The power supply network is tested, and an adjustable power supply is selected to adjust the input voltage, output voltage and test data as shown in Table 2. The standard 5V voltage is connected through the emergency charging interface, and the RT9193 is disconnected. When testing, the diodes D5 and D6 are not connected, and it is found that the indicator light of MCP73831 is incorrect. Analysis shows that without connecting diodes D5 and D6, it is equivalent to RT9193 directly connected to the BAT pin output. At the moment when MCP73831 is powered on, the state of BAT needs to be detected. The input pin and branch of RT9193 are connected to the positive pole of the lithium battery, which directly affects the detection state of MCP73831 on the BAT pin, causing the charging to enter the trickle charging stage. After adding D5 and D6, the test is carried out again, and the indicator light meets the logic requirements. The maximum output current of the test is 485mA. When the charging voltage reaches 412V, the green indicator light goes out and the red indicator light comes on, completing the charging of the lithium battery. W1 is connected to a 0~10V adjustable voltage source (initial value is set to 5V), M1 is connected to a 0~5V adjustable voltage source (initial value is set to 4V), and the sliding rheostat R13 and R14 are adjusted. When the input voltage of W1 is 6V, the 7th pin of LM 2903 changes from low level to high level. Measure At this time, the sliding resistor R13 = 3115kΩ, and this resistance value is fixed. When the input voltage of M1 is 315V, the 1st pin of LM 2903 changes from high level to low level. At this time, the sliding rheostat R14 = 1kΩ, and this resistance value is fixed. At this time, it is found that the output of pin 1 of LM 2903 is at a critical value, constantly changing between high and low levels, and relay JDQ2 is constantly on and off, which reduces the service life of JDQ2, easily damages the wireless dynamometer and wireless network equipment, and has a great impact on the life of the wireless equipment. Analysis found that: in the over-discharge protection process, if the detection value and the comparison value reach a basically consistent state, critical protection will occur. For this reason, an electrolytic capacitor C13 is connected between resistors R20 and R′20. By charging and discharging the capacitor, the shutdown time of Q4 is delayed, and the time interval between opening and closing is increased. The size of the capacitor determines the length of the time interval. This time is the protection delay time of the over-discharge protection controller. The design uses a 212μF capacitor, and the test finds that the delay is about 15s.

Automatic tracking controller debugging, W1 is connected to 5V power supply during debugging, a 100W bulb is used to illuminate RT1 and RT3 and move the light, it can be found that the solar energy collection panel moves with the light. However, the motor keeps vibrating in the stable state. At this time, a 417uF capacitor is added between resistors R31 and R32 to delay the start and stop time of the motor. After testing, it is found that the delay time is about 40s, which is negligible relative to the sun exposure time and does not affect the tracking function. Similarly, a 417μF capacitor is added between resistors R34 and R35. After testing, it is found that the motor vibration phenomenon can be completely eliminated and the tracking effect is good. After the independent debugging of each part is completed, the power supply network and the charging management chip MCP73831 are jointly debugged, and then RT9193 is added for debugging, and finally the debugging of the entire system is realized. It has been proved by testing that the design goals and functional requirements have been achieved.

4 Conclusion

This intelligent solar charging circuit has the advantages of stable working performance, safe and reliable operation, low loss, high efficiency, simple structure, and high output voltage accuracy. The combination of the power supply network that automatically adjusts the duty cycle and the power management chip, overvoltage and over-discharge protection, automatic tracking of the sun and other functions are relatively creative design methods, especially applying these designs to oilfield wireless dynamometers and wireless network nodes, which is a new attempt and a breakthrough in application. At present, the solar charging and automatic tracking circuit designed and developed in this article has been successfully applied to the wireless dynamometer and its wireless communication network in Jiangsu Oilfield. Practice has proved that the system has fast charging speed, high efficiency, can track the sun in real time, stable operation, and low maintenance. It has high practical and promotion value.