A charging solution based on MPPT technology to achieve efficient solar energy

Solar energy is recognized by the world as the new energy with the highest technical content and the most promising development. As a new energy system, solar power generation system (photovoltaic system) has attracted the attention and research of many countries. It will occupy an important position in the future energy structure and has important significance for energy consumption and the environment.

Since the main problem of photovoltaic systems at present is the low conversion efficiency and high price of batteries, how to further improve the conversion efficiency of solar cells and how to make full use of the energy converted by photovoltaic arrays have always been important research directions for photovoltaic systems. The output characteristics of photovoltaic arrays have nonlinear characteristics and are affected by light intensity and ambient temperature. With the difference in light intensity and ambient temperature, the terminal voltage of photovoltaic cells will change, causing a large change in output power. Photovoltaic cells themselves are an extremely unstable power source. Therefore, how to increase the output power of power supply under different light and ambient temperatures and improve system efficiency becomes a key challenge, which raises the problem of maximum power point tracking ( MPPT ) of photovoltaic cells in theory and practice.

Improving photovoltaic cell output power

The output of photovoltaic cells is affected by factors such as sunlight intensity and cell junction temperature. Figures 1 and 2 show the nonlinear function relationship of photovoltaic cells under different illumination, same junction temperature and same illumination, different junction temperature.

Case 1: The battery junction temperature remains unchanged, but the light intensity changes

From Figure 1 we can draw the following conclusions:

① The short-circuit current of a photovoltaic cell increases with the increase of light intensity, and the two are approximately proportional; the open-circuit voltage of a photovoltaic cell does not change much under various light conditions;

② The maximum output power of photovoltaic cells increases with the increase of light intensity, and there is a unique maximum output power point under the same lighting environment. On the left side of the maximum power point, the output power increases approximately linearly with the increase of battery terminal voltage; after reaching the maximum power point, the output power begins to decrease rapidly, and the rate of decrease is much greater than the rate of increase;

③ As shown in Figure 1(a): On the left side of the dotted line A, the characteristics of the photovoltaic cell are approximately a current source, and on the right side are approximately a voltage source. The dotted line A corresponds to the working current of the photovoltaic cell at the maximum power point, which is about 90% of the short-circuit current of the battery;

④ As shown in Figure 1(b): When the junction temperature is constant, the output voltage value corresponding to the maximum power point of the photovoltaic cell remains basically unchanged. This value is about 76% of the open circuit voltage.

Case 2: Battery junction temperature changes, but light intensity remains unchanged

From Figure 2 we can draw the following conclusions:

① As shown in Figure 2(a): the junction temperature of the photovoltaic cell has little effect on the short-circuit current of the photovoltaic cell. As the temperature rises, the output short-circuit current only increases slightly; the open-circuit voltage of the photovoltaic cell decreases as the junction temperature of the cell rises, and the range of change is relatively large;

② As shown in Figure 2(b): The overall change trend of photovoltaic cell output power is similar to the power change under different lighting conditions. However, under the same lighting conditions, its maximum output power decreases with the increase of cell temperature, and the operating voltage corresponding to the maximum power point decreases with the increase of temperature.

In summary, the output power of photovoltaic cells is closely related to the light intensity and ambient temperature. Under different external environments, the output power of photovoltaic cells will vary greatly. Therefore, the photovoltaic power generation system must use relevant circuits and control methods to control the output power to maximize its output power.

In photovoltaic systems, it is usually required that the output power of solar cells is always at the maximum, that is, the system must be able to track the maximum power point of the solar cell output. Since the operating point of the load does not fall exactly at the maximum power point provided by the battery, the maximum power that the battery can provide under current conditions cannot be fully utilized. Therefore, an impedance converter must be added between the solar cell and the load so that the converted operating point coincides with the maximum power point of the solar cell, so that the solar cell outputs at maximum power. This is the so-called maximum power tracking of solar cells. The traditional method is to design the maximum power point voltage of the solar cell under normal conditions to be close to the standard operating voltage of the load. This method is called constant voltage tracking (CVT).

The CVT method ignores the effect of temperature on the open circuit voltage of solar cells. Due to temperature changes and load changes, the CVT method usually has a large error. Taking a single-crystal silicon solar cell as an example, when the ambient temperature rises by 1°C, its open circuit voltage drops by 0.35% to 0.45%. This shows that the voltage corresponding to the maximum power point of the solar cell also changes with the change of ambient temperature. For areas with large temperature differences between the four seasons or the daily temperature difference, the CVT method cannot fully track the maximum power under all temperature environments.

The CVT method has the advantages of simple control, high reliability, good stability, and easy implementation. The traditional method can obtain 20% more electricity than the general photovoltaic system, and the improved method can obtain 20% more electricity than CVT, which is much more advantageous than direct coupling without CVT. The IV0300 chip launched by Innova uses the improved ACVT method to detect temperature changes regularly. Considering the impact of different temperatures on the open-circuit voltage of solar cells, the maximum power tracking point is adjusted in time to keep the solar cell output at maximum power.

IV0300 Technical Features

IV0300 is a solar charging control chip, which has improved constant voltage tracking method maximum power tracking technology (ACVT-MPPT) function and battery boost charging protection function. It is suitable for charging two to four NiMH or NiCd batteries, single lithium battery, and can withstand 1.5A peak input current. The main functions and technical features of IV0300 are as follows:

1. Adopt FPWM boost technology, with low EMI. The operating frequency of PWM fluctuates within a certain range, and the radiation energy on a single spectrum component is low, so the EMI is low.

2. Voltage-limited overcharge protection, overcharge voltage can be set by external resistor. The way to end charging can be selected by setting the Float pin level. When Float is grounded, the chip works in overvoltage protection mode and stops charging. When Float is connected to the positive pole of the battery and the battery voltage reaches the protection voltage, the system works in pulse charging (or floating charging) state.

3. Working status and charging end status indication. The output of the CHEND pin has three states: charging state, charging end state, and high impedance state.

4. The system automatically turns on and off through the solar panel output voltage.

5. Low static operating current. In order to protect the battery power, the static operating current is not more than 75μA when not charging.

6. The input voltage of the battery is 2V, and the minimum operating voltage of the solar panel input is 1/8 of the battery voltage, that is, 0.25V.

7. Up to 95% energy conversion efficiency.

Typical application circuit design and precautions

1. Two LEDs connected in series are used to display different states. Different colors can be selected, but the voltage at both ends must be higher than the battery voltage, otherwise the battery will discharge through the indicator light.

2. To improve energy conversion efficiency in the future, try to choose a battery panel whose operating voltage is close to the minimum voltage of the battery. For example, the open circuit voltage of a single lithium battery should be around 3.5V, and the maximum power output voltage should be around 2.8V. This can ensure that the working efficiency of the boost circuit is above 90%.

3. Charge cut-off voltage V(Max_Battery), set R1 and R2 to determine the charge cut-off voltage. R2 can be obtained by the following formula: R2=R1*(VOC/1.257V) /(1-(VOC/1.257V)), where 1.257V is the Vref voltage, and the charge cut-off voltage is VOC=(V(Max_Battery)-0.06)/5.

4. The SIN pin is used to measure the input voltage, so the voltage at this point is required to be stable to the ground. If the ripple is large, it will affect the normal operation of the chip, and the capacitance of capacitor C needs to be increased.

5. Usually the inductor current should be twice the maximum current of the circuit to ensure efficiency.