Due to the pressing global energy crisis, there is an increasing demand for renewable energy sources. Solar photovoltaic (PV) systems represent a promising solution; however, they are hampered by their relatively low efficiency and high capital costs. In this thesis, we propose a schematic designed to extract the maximum available solar power from a PV module, harnessing this energy for a DC application. The core principle employed in this circuit is Maximum Power Point Tracking (MPPT), which enhances the efficiency of the solar photovoltaic system.
The output power generated by a PV panel is contingent on factors such as temperature, radiation, and the orientation of the panel. It is also influenced by the product of voltage and current. By manipulating these parameters, it becomes possible to maximize power output. Generally, MPPT techniques are employed to achieve this optimization. Various MPPT methods have been developed to maximize output power. This particular circuit operates on the principle of input voltage-based peak power tracking.
MPPT
A Maximum Power Point Tracker (MPPT) serves as a high-frequency DC-to-DC converter. In our context, it takes the DC input from the solar panels, converts it into high-frequency AC, and subsequently rectifies it to produce a different DC voltage and current precisely tailored to match the requirements of the batteries. An MPPT controller essentially “searches” for the point at which the sharp peak in power output occurs (as depicted below), and then carries out a voltage/current conversion to transform it into the exact values needed by the battery. It’s important to note that this peak will continually shift due to variations in lighting conditions and weather.
The practical application of an MPPT system is contingent upon factors such as the configuration of the solar panel array, the local climate, and the seasonal load patterns. Adhering to the Maximum Power Transfer theorem, which stipulates that the power output of a circuit reaches its maximum when the Thevenin impedance of the circuit (source impedance) aligns with the load impedance, our task of tracking the maximum power point essentially boils down to an impedance matching problem. By judiciously adjusting the duty cycle of the boost/buck converter, we can achieve the desired impedance matching between the source and load.
Project implementation
We will be employing the LM5118 IC for maximum power point tracking, which encompasses all the essential functionalities needed to implement a highly efficient high-voltage buck or buck-boost regulator, with the use of minimal external components. This regulator smoothly transitions between buck and buck-boost operation modes as the input voltage converges with the output voltage, allowing for operation when the input voltage is either greater than or less than the output voltage. The LM5118 IC integrates both high-side and low-side MOSFET drivers with the capacity to deliver peak currents of up to 3 A. The control method employed by this regulator relies on output current mode control, utilizing an emulated current ramp. Users can program the operating frequency within the range of 50 kHz to 500 kHz. Fault protection mechanisms are also included, featuring current limiting, thermal shutdown, and the ability for remote shutdown.
Furthermore, the IC includes an under-voltage lockout input, which enables the shutdown of the regulator when the input voltage falls below a user-defined threshold. Applying a low state to the enable pin will place the regulator in an extremely low-current shutdown state. These two features can be effectively employed to place the system in standby mode during nighttime hours.
Recognizing that buck-boost power converters are not as efficient as buck regulators, the LM5118 IC has been engineered as a dual-mode controller. In this configuration, the power converter functions as a buck regulator when the input voltage exceeds the output voltage. As the input voltage progressively approaches the output voltage, a seamless transition to buck-boost mode takes place. This dual-mode approach ensures regulation across a wide spectrum of input voltages while maintaining optimal conversion efficiency in the typical buck mode. The gradual mode transition mitigates disruptions at the output during these shifts.
Circuit operation
The following figure shows the basic operation of the LM5118 regulator in the buck mode. In buck mode, transistor Q1 is active and Q2 is disabled. The inductor current ramps in proportion to the VIN – VOUT voltage difference when Q1 is active and ramps down through the recirculating diode D1 when Q1 is off. The first order buck mode transfer function is VOUT/VIN = D, where D is the duty cycle of the buck switch, Q1.
The following figure shows the basic operation of buck-boost mode. In buck-boost mode both Q1 and Q2 are active for the same time interval each cycle. The inductor current ramps up, (proportional to VIN) when Q1 and Q2 are active, and ramps down, through the recirculating diode during the off time. The first order buck-boost transfer function is VOUT/VIN = D/(1-D), where D is the duty cycle of Q1 and Q2.
VIN
The solar panel output is coupled to the V IN pin directly. The capacitors C1 & C2 are used as input filters.
