In the solar off-grid system, the role of the photovoltaic controller is to convert the electricity from the photovoltaic modules and store it in the battery. In addition, it also protects the battery and prevents the battery from overcharging. At present, there are two main technical routes for the controller: Pulse Width Tuning (PWM) and Maximum Power Point Tracking (MPPT), each of which has its own advantages and disadvantages, and can be selected according to different scenarios.

1. PWM controller

The early photovoltaic controllers are all PWM. This kind of electrical structure is simple. The controller consists of a power main switch, a capacitor, and a drive and protection circuit. The output voltage is controlled by the PWM duty cycle of the switch.

PWM controller, there is only one switch between the solar array and the solar panel. As the battery is gradually charged and the battery voltage increases, the PWM controller will gradually reduce the power supplied to the battery, and the photovoltaic output will be output according to the maximum power. The PWM controller has the function of battery charge and discharge management, which can prevent the battery from overcharging and over discharging.

Since there is only one switch between the PWM controller solar module and the battery, and there is no voltage divider such as an inductance in the middle, the voltage of the module is about 1.2-2.0 times the voltage of the battery during design, such as a 24V battery, the module input The voltage is between 30-50V, and each string can only be equipped with one module. For a 48V battery, the input voltage of the module is between 60-80V, and each string can only be equipped with two modules.

2. MPPT controller

The MPPT controller is the second generation solar controller. Compared with the PWM controller, it has an additional inductor and a power diode, so it is more powerful.

One is that it has a maximum power tracking function. During the charging period of the battery, the solar module can output the maximum power unless the battery reaches a saturation state; the other is that the voltage range of the photovoltaic module is wide, and there is a power switch tube and an inductance circuit in the middle of the controller. The voltage of the module is between 1.2-3.5 times the voltage of the battery. If it is a 24V battery, the input voltage of the module is between 30-80V. Each string can be equipped with one or two modules. If it is a 48V battery, the input voltage of the module is between 60-110V, each string can be equipped with two to three components.

How to choose a PWM and MPPT controller?

Both PWM and MPPT controllers have their own unique advantages and disadvantages, and which one to choose depends on the design characteristics of the solar photovoltaic array, cost, and conditions such as the external environment. When we choose to focus on the following factors:

The PWM method has mature technology, simple and reliable circuit, and low price, but the utilization rate of components is low. The utilization rate of the module is about 80% or more; MPPT solar controller refers to the solar controller with the maximum power point tracking function. There is a buck step-down circuit between the module and the battery, and the utilization rate of the module is about 90% or more.

The main users of small off-grid systems below 2kW are poor areas without electricity, such as remote mountainous areas and some poor countries in Africa, mainly to meet the needs of lighting, and users are very sensitive to prices, so it is recommended to use PWM controllers to correct the wave Inverter, the controller, inverter and battery are integrated. This method is simple in structure, high in efficiency, convenient for user wiring, and cheap in price, and there is no problem in driving light bulbs, small TVs, and small fans.

For off-grid systems above 2kW, it is recommended to use the MPPT controller, which has high component utilization, high overall machine efficiency, and flexible component configuration. In general, no single technology is the best, so users should take the advice of the experts to make sure they choose the right controller for their needs.