An MPPT or Maximum Power Point Tracker for solar panels is a method which enables deriving maximum available current from a solar panel throughout the day without disturbing its specified voltage, thus allowing greatest efficiency from the panel.
As we all know, acquiring highest efficiency from any form of power supply becomes feasible if the procedure doesn’t involve shunting the power supply voltage, meaning we want to acquire the particular required lower level of voltage, and maximum current for the load which is being operated without disturbing the source voltage level, and without generating heat.
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Briefly, a concerned MPPT should allow its output with maximum required current, any lower level of required voltage yet making sure the voltage level across the panel stays unaffected.
One method which is discussed here involves PWM technique which may be considered one of the optimal methods to date.
We should be thankful to this little genius called the IC 555 which makes all difficult concepts look so easy.
In this concept too we incorporate, and heavily depend on a couple of IC 555s for implementing the MPPT effect.
Looking at the given solar mppt circuit using IC555 we see that the entire design is basically divided into two stages.
The upper voltage regulator stage and the lower PWM generator stage.
The upper stage consists of a p-channel mosfet which is positioned as a switch and responds to the applied PWM info at its gate.
The lower stage is a PWM generator stage. A couple of 555 ICs are configured for the proposed actions.
IC1 is responsible for producing the required square waves which is processed by the constant current triangle wave generator comprising T1 and the associated components.
This triangular wave is applied to IC2 for processing into the required PWMs.
However the PWM spacing from IC2 depends on the voltage level at its pin#5, which is derived from a resistive network across the panel via the 1K resistor and the 10K preset.
The voltage between this network is directly proportional to the varying panel volts.
During peak voltages the PWMs become wider and vice versa.
The above PWMs are applied to the mosfet gate which conducts and provides the required voltage to the connected battery.
As discussed previously, during peak sunshine the panel generates higher level of voltage, higher voltage means IC2 generating wider PWMs, which in turn keeps the mosfe switched OFF for longer periods or switched ON for relatively shorter periods, corresponding to an average voltage value that might be just around 14.4V across the battery terminals.
When the sun shine deteriorates, the PWMs get proportionately narrowly spaced allowing the mosfet to conduct more so that the average current and voltage across the battery tends to remain at the optimal values.
The 10K preset should be adjusted for getting around 14.4V across the output terminals under bright sunshine.
The results may be monitored under different sun light conditions.
The proposed MPPT circuit ensures a stable charging of the battery, without affecting or shunting the panel voltage which also results in lower heat generation.
Note: The connected soar panel should be able to generate 50% more voltage than the connected battery at peak sunshine. The current should be 1/5th of the battery AH rating.
An enhanced and a technically more correct version of the above design using buck converter can be seen in this Simple Solar MPPT Circuit – Part 2
How to Set up the Circuit
It may be done in the following manner:
Initially keep S1 switched OFF.
Expose the panel to peak sunshine, and adjust the preset to get the required optimal charging voltage across the mosfet drain diode output and ground.
The circuit is all set now.
Once this is done, switch ON S1, the battery will start getting charged in the MPPT mode.
Adding a Current Control Feature
A careful investigation of the above circuit shows that as the mosfet tries to compensate the falling panel voltage level, it allows the battery to draw more current from the panel, which affects the panel voltage dropping it further down inducing a run-away situation, this may be completely against the MPPT law.
A current control feature as shown in the following diagram takes care of this problem and prohibits the battery from drawing excessive current beyond the specified limits. This in turn helps to keep the panel voltage unaffected.
RX which is the current limiting resistor can be calculated with the help of the following formula:
RX = 0.6/I, where I is the specified minimum charging current for the connected battery