Solar MPPT using IC LM5118

The need for renewable energy sources is on the rise because of the acute energy crisis in the world today. The main limitation for the saturation and reach of solar PV systems is their low efficiency and high capital cost. In this thesis, we propose a schematic to extract maximum obtainable solar power from a PV module and use the energy for a DC application. This circuit works with the concept of Maximum Power Point Tracking (MPPT) to increase the efficiency of the solar photovoltaic system.

The output power of a PV panel is a function of temperature, radiation and the position of the panel. It is also the function of the product of voltage and current. By varying these parameters, the power can be maximized. To maximize the output power generally, MPPT is used. There are several MPPT method exists in order to maximize the output power. This circuit is based on input voltage-based peak power tracking.

MPPT

A Maximum Power Point Tracker, MPPT, is a high-frequency DC to DC converter. It takes the DC input, from the solar panels in our case, and changes it to high-frequency AC, and then rectifies it back down to a different DC voltage and current to exactly match the panels to the batteries. An MPPT controller “looks” for the point where the sharp peak occurs (below), and then performs a voltage/current conversion to change it to exact values that the battery requires. The peak will always vary due to changes in light conditions and weather.

The application of an MPPT, in the real world, is dependent on the array, climate, and seasonal load pattern. According to Maximum Power Transfer theorem, the power output of a circuit is maximum when the Thevenin impedance of the circuit (source impedance) matches with the load impedance. Hence our problem of tracking the maximum power point reduces to an impedance matching problem. By changing the duty cycle of the boost /buck converter appropriately we can match the source impedance with that of the load impedance.

Project implementation

We are going to implement LM5118 IC for maximum power point tracking which features all the functions necessary to implement an efficient high voltage buck or buck-boost regulator using a minimum of external components. The regulator switches smoothly from buck to buck-boost operation as the input voltage approaches the output voltage, allowing operation with the input greater than or less than the output voltage.

This IC integrates high side and low-side MOSFET drivers capable of supplying peak currents of 3 A. The regulator control method is based on output current mode control utilizing an emulated current ramp. The operating frequency is user programmable from 50 kHz to 500 kHz. Fault protection features include current limiting, thermal shutdown and remote shutdown capability.

An under-voltage lockout input allows regulator shutdown when the input voltage is below a user selected threshold, and a low state at the enable pin will put the regulator into an extremely low current shutdown state. These two features can be utilized to make the system in standby mode during night times.

Since the buck-boost power converters are not as efficient as buck regulators, the IC LM5118 has been designed as a dual mode controller whereby the power converter acts as a buck regulator while the input voltage is above the output. As the input voltage approaches the output voltage, a gradual transition to the buck-boost mode occurs. The dual mode approach maintains regulation over a wide range of input voltages, while maintaining the optimal conversion efficiency in the normal buck mode. The gradual transition between modes eliminates disturbances at the output during transitions.

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