NOTE: The discussed MPPT circuits in this post do not employ the conventional control methods like "Perturb and observe", "Incremental conductance, "Current sweep", "Constant voltage"......etc etc...
Rather here we concentrate and try implementing a couple of basic things:
1) To make sure that the input "wattage" from the solar panel is always equal to the output "wattage" reaching the load.
2) The "knee voltage" is never disturbed by the load and the panel's MPPT zone is efficiently maintained.
What's Knee Voltage and Current of a Panel:
Simply put, the knee voltage is the "open circuit voltage" level of the panel, while the knee current is the "short circuit current" measure of the panel at any given instant.
If the above two are maintained as far as possible, the load could be assumed to be getting the MPPT power throughout its operation.
Before we Delve into the Proposed Designs, let's first get acquainted with some of the basic facts regarding solar battery charging
We know that the output from a solar panel is directly proportional to the degree of the incident sunlight, and also the ambient temperature. When the sun rays are perpendicular to the solar panel, it generates the maximum amount of voltage, and deteriorates as the angle shifts away from 90 degrees The atmospheric temperature around the panel also affects the efficiency of the panel, which falls with increase in the temperature.
Therefore we may conclude that when the sun rays are near to 90 degrees over the panel and when the temperature is around 30 degrees, the efficiency of the panel is toward maximum, the rate decreases as the above two parameters drift away from their rated values.
The above voltage is generally used for charging a battery, a lead acid battery, which in turn is used for operating an inverter. However just as the solar panel has its own operating criteria, the battery too is no less and offers some strict conditions for getting optimally charged.
The conditions are, the battery must be charged at relatively higher current initially which must be gradually decreased to almost zero when the battery attains a voltage 15% higher than its normal rating.
Assuming a fully discharged 12V battery, with a voltage anywhere around 11.5V, may be charged at around C/2 rate initially (C=AH of the battery), this will stat filling the battery relatively quickly and will pull its voltage to may be around 13V within a couple of hours.
At this point the current should be automatically reduced to say C/5 rate, this will again help to keep the fast charging pace without damaging the battery and raise its voltage to around 13.5V within the next 1 hour.
Following the above steps, now the current may be further reduced to C/10 rate which makes sure the charging rate and the pace does not slow down.
Finally when the battery voltage reaches around 14.3V, the process may be reduced to a C/50 rate which almost stops the charging process yet restricts the charge from falling to lower levels.
The entire process charges a deep discharged battery within a span of 6 hours without affecting the life of the battery.
An MPPT is employed exactly for ensuring that the above procedure is extracted optimally from a particular solar panel.
A solar panel may be unable to provide high current outputs but it definitely is able to provide with higher voltages.
The trick would be to convert the higher voltage levels to higher current levels through appropriate optimization of the solar panel output.
Now since the conversions of a higher voltage to higher current and vice versa can be implemented only through buck boost converters, an innovative method (although a bit bulky) would be to use a variable inductor circuit wherein the inductor would have many switchable taps, these taps may be toggled by a switching circuit in response to the varying sunlight so that the output to the load always remains constant regardless of the sun sunshine.
The concept may be understood by referring to the following diagram:
The main processor in the above diagram is the IC LM3915 which switches its output pinout sequentially from the top to the bottom in response to the diminishing sun light
These outputs can be seen configured with switching power transistors which are in turn connected with the various taps of a ferrite single long inductor coil.
The lower most end of the inductor can be seen attached with a NPN power transistor which is switched at around 100kHz frequency from an externally configured oscillator circuit.
The power transistors connected with the outputs of the IC switch in response to the sequencing IC outputs, connecting the appropriate taps of the inductor with the panel voltage and the 100kHz frequency.
This inductor turns are appropriately calculated such that its various taps become compatible with the panel voltage as these are switched by the IC output driver stages.
Thus the proceedings make sure that while the sun intensity and the voltage drops, it's appropriately linked with the relevant tap of the inductor maintaining almost a constant voltage across all the given taps, as per their calculated ratings.
Let's understand the functioning with the help of the following scenario:
Suppose the coil is selected to be compatible with a 30V solar panel, therefore at peak sunshine let's assume that the upper most power transistor is switched ON by the IC which subjects the entire coil to oscillate, this allows the entire 30V to be available across the extreme ends of the coil.
Now suppose the sunlight drops by 3V and reduces its output to 27V, this is quickly sensed by the IC such that the first transistor from the top now switches OFF and the second transistor in the sequence switches ON.
The above action selects the second tap (27V tap) of the inductor from top executing a matching inductor tap to voltage response making sure that the coil oscillates optimally with the reduced voltage...similarly, now as the sunlight voltage drops further the respective transistors "shake hands" with the relevant inductor taps ensuring a perfect matching and efficient switching of the inductor, corresponding to the available solar voltages.
Due to the above matched response between the solar panel and the switching buck/boost inductor...the tap voltages over the relevant points can be assumed to maintain a constant voltage through out the day regardless of the sunlight situation....
For example suppose if the inductor is designed to produce 30V at the topmost tap followed by 27V, 24V, 21V, 18V, 15V, 12V, 9V, 6V, 3V, 0V across the subsequent taps, then all these voltages could be assumed to be constant over these taps regardless of the sunlight levels.
Also please remember that these voltage can be altered as per user specs for achieving higher or lower voltages than the panel voltage.
The above circuit can also be configured in the flyback topoogy as shown below:
In both the above configurations, the output is supposed to remain constant and stable in terms of voltage and wattage regardless of the solar output.
Using I/V Tracking Method
The following circuit concept ensures that the MPPT level of the panel is never disturbed drastically by the load.
The circuit tracks the MPPT "knee" level of the panel and makes sure that the load is not allowed to consume anything more which might cause a dropping in this knee level of the panel.
Let's learn how this can can done using a simple single opamp I/V tracking circuit.
Please note that the designs which are without a buck converter will never be able to optimize the excess voltage into equivalent current for the load, and might fail in this regard, which is considered as the crucial aspect of any MPPT design.
A very simple yet effective MPPT type device can be made by employing a LM338 IC and an opamps.
The IC 741 stage is the solar tracker section and forms the heart of the entire design.
The solar panel voltage is fed to the inverting pin2 of the IC, while the the same is applied to the non-inverting pin3 with a drop of around 2 V using three 1N4148 diodes in series.
The above situation consistently keeps the pin3 of the IC a shade lower than pin2 ensuring a zero voltage across the output pin6 of the IC.
However in an event of an inefficient overload, such as a mismatched battery or a high current battery, the solar panel voltage tends to get pulled down by the load, when this happens pin2 voltage also begins dropping, however due to the presence of the 10uF capacitor at pin3, the potential stays solid and does not respond to the above drop.
When the above situation is triggered, instantly causes pin3 to go high than pin2, which in turn toggles pin6 high, switching ON the BJT BC547.
BC547 now immediately disables LM338 cutting off the voltage to the battery, the cycle keeps switching at a rapid pace depending upon the IC's rated speed.
The above operations make sure that the solar panel voltage never drops or gets pulled down by the load, maintaining an MPPT like condition throughout.
Since a linear IC LM338 is used, the circuit could be yet again a bit inefficient....the remedy is to replace the LM338 stage with a buck converter...that would make the design extremely versatile and comparable to a true MPPT.
Below shown is an MPPT circuit using a buck converter topology, now the design makes a lot of sense and looks much closer to a true MPPT
48V MPPT Circuit
The above simple MPPT circuits can be also modified for implementing high voltage battery charging, such as the following 48V battery MPPT charger circuit.
The ideas are all exclusively developed by me.