Neil Mula

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  • Solar Energy Boost with Charge Controller

  • Created: Aug 29, 2014

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Summary

The design demonstrates Maximum Power Point Tracking Capability which increases the efficiency of Solar Energy Charging. It boost charging basically by performing successive increase in repetition rate of PWM pulses that results to a higher current output.


The system work in this way, the inductor L1 charges when Q1 turns on and L1 discharges into the battery via D1 when Q1 turns off. Performing this simple operation thousands of times per second results in appreciable output current. It is also called inductive discharge. For this to function, the input voltage must be lower than the output voltage. Also, with a solar panel source, energy storage in the form of a capacitor (C1) is required so that the solar panel may continue to output current between cycles. The circuit consists of essentially three sections including a 555 MOSFET gate driver, 555 PWM modulator and op amp voltage limiter. The 555 with its totem pole output can source as well as sink roughly 200mA and makes a great low power gate driver. The 555 PWM modulator is the classic 555 oscillator circuit. To regulate the C3 discharge time (inductor charge time), pin 5 is held at a regulated 5V. The Op amps integrates the battery voltage signal when the divided set point voltage is compared with the 5V reference. When the voltage exceeds the setting, the output integrates in the negative direction thus reducing the repetition rate of the PWM generator and limiting any subsequent charging. This effectively prevents overcharging. Since the circuit must operate at low voltages (this one works down to about 4V input) a logic level MOSFET is required. It turns on at fully at 4.5V. The device I actually used was the MTP3055. In this circuit, the battery may not be disconnected or the MOSFET will self-destruct when it turns off. Since this is too much to be expected, 24V zener D2 performs a safety clamp function. Without this, I, myself would have destroyed many MOSFETS. As the solar panel voltage /current increases, the PWM generator increases its repetition rate thus resulting in increased output current. At the same time, additional voltage is applied to the inductor thus increasing its charge current. As a result, the boost regulator really digs in as the voltage increases, or lets up as the voltage diminishes. To achieve maximum transfer of power with full sunlight, potentiometer R8 is adjusted so that the battery charging current is maximized –this is the maximum power point. If the circuit is operating properly, there will be a very shallow peak as R5 is rotated. Diode D3 makes the automatic MPPT adjustment function more sensitive by subtracting a fixed voltage from the voltage difference between the battery and the average voltage across C3. Under lower light conditions, you will find that R3 is not exactly at optimum, but it will not be significantly off. Note that intelligent MPPT controllers can do a better job across the full range, but such improvement is very marginal.


This circuit is an excellent development for solar energy. It is also suitable for different types of charging source but it is perfect for renewable energy where its capacity factor is lower than the fueled power generator.

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