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Brief Introduction Of MPPT Solar Controller
- May 08, 2018 -


A brief introduction to MPPT


The full name of the MPPT controller, "Maximum Power Point Tracking" solar controller, is an upgrading product of the traditional solar charge and discharge controller.


The MPPT controller can detect the generating voltage of the solar panel in real time and track the maximum voltage and current value (VI), so that the system can charge the battery with maximum power output. It is the brain of photovoltaic system to coordinate solar panels, batteries and loads in solar photovoltaic systems.


Current voltage characteristic curve


There is a complex relationship between the maximum power output of solar cells and the environment. The shape factor is defined as the maximum power of a solar cell, divided by the ratio of the open circuit voltage Voc and the short-circuit current Isc product. In the calculation, shape factor is often used to estimate the maximum power P=FF*Voc*Isc that photovoltaic cells can generate under certain conditions. In most applications, FF, Voc and Isc have been able to simulate the electrical characteristics of photovoltaic cells under general conditions.


The current voltage curve of a photovoltaic cell has a line passing through the turning point of each section, the turning point, that is, the maximum power point.


Under certain operating conditions, the battery will have a working point, and its current (I) and voltage (V) product will be the maximum. This value corresponds to a specific resistance, which is equal to V / I according to Ohm's law. And the power can be calculated by P=V*I. In the curve range of its main application, photovoltaic cells approximate the current source. However, in the range of the maximum power point of photovoltaic cells, there is an exponential function between the voltage and current. According to the basic circuit and the calculus theory, if the slope dI/dV (differential quantity) of the I-V curve is equal to the I/V ratio, the symbol is opposite, dP/dV=0, and the output power is the maximum. This location is the turning point of the maximum power point (MPP), corresponding to the curve.


If the load impedance R=V/I of the solar cell is equal to the reciprocal of the above value, the maximum power can be output from the solar cell at this time. Sometimes this value is called the "characteristic impedance" of a solar cell, which is a dynamic amount, which is related to the sunshine degree, temperature and the life of the solar cell. If the resistance is less than or greater than this value, the extracted power will be less than the maximum power, so the solar cell does not operate in the most ideal and most efficient conditions. The maximum power point tracking will use several different control resistors or logic to find the maximum power point, so that the converter can extract the maximum power from the solar cell.


The principle of MPPT


To charge the battery, the output voltage of the solar panel must be higher than the current voltage of the battery. If the voltage of the solar panel is lower than the voltage of the battery, the output current will be close to 0. So, to be safe, the peak voltage of the solar panel (Vpp) is about 17V when the solar panel is made out of the factory, which is set by the standard when the ambient temperature is 25 C. When the weather is very hot, the peak voltage of the solar panel will drop to about 15V, but in cold weather, the peak voltage of the solar energy can reach Vpp of 18V, Vpp.


Now let's go back to compare the difference between the MPPT solar controller and the traditional solar controller. The traditional solar charge and discharge controller is a bit like a manual gearbox. When the engine speed is higher, the speed can be affected if the gear position of the gearbox does not increase accordingly. But for the MPPT solar controller, the charging parameters are set before the factory is set up. That is, the MPPT controller will track the maximum power point in the solar panel to maximize the efficiency of the solar panel. The higher the voltage, the more power can be generated by maximum power point tracking, thereby improving the charging efficiency. In theory, the solar power system using the MPPT controller will be 50% higher than the traditional efficiency, but according to our actual tests, the final efficiency can also be increased by 20%-30% due to the environmental impact and energy loss.


In this sense, the MPPT solar charge and discharge controller will eventually replace the traditional solar controller.


The function of MPPT


The main function of the MPPT controller is to detect the DC voltage and output current of the main circuit, calculate the output power of the solar array and track the maximum power point. The disturbance resistance R and the MOSFET are connected together, and the average current passing through the resistance is changed by changing the duty ratio of the MOSFET, so the disturbance of the current is generated. At the same time, the output current and voltage of the photovoltaic cell will also change. By measuring the change of the output power and voltage of the photovoltaic cell before and after the disturbance, the disturbance direction of the next cycle is determined. When the direction of the disturbance is correct, the output power of the solar energy board is increased, the next period continues to disturb in the same direction, and conversely, the negative direction is disturbed. In this way, disturbance and observation are repeated to make the output of solar photovoltaic panels reach the maximum power point.