True MPPT Solar Controller Circuit using IC 555

Are you wondering if it was possible to build a very simple MPPT circuit using ordinary parts like IC 555 and IC 741, which would work most efficiently imitating a true MPPT? The answer is yes, it is possible, and we will learn about it details, in the following article.

In this design I used the IC 555 as a variable PWM generator. That means the IC 555 here is continuously creating a train of pulses at its pin#3. These pulses decide how fast or slow the MOSFET IRF540 is switching, which means how much power is allowed to reach the battery.

But now the trick part—this PWM pulse width is not fixed, it is controlled from solar panel voltage. That is done using pin#5 of the 555 which is connected to solar panel output through a preset.

If you don't want to read the whole article below, you can watch the following video instead:

How we used ic 555 like pwm generator

So in this idea what we tried is we used that ic 555 as one variable pwm signal producer. That means this ic 555 it keeps giving out one train of pulse signals from its pin 3 continuously.

Then those pulse signals they go and tell the mosfet irf9540 how fast it should do the switching. So now that mosfet it also handles how much current or power is going to battery side.

How pwm width changes with solar voltage

But the special thing here is that width of those pwm pulses it does not stay fixed. It keeps changing depending on how much voltage we are getting from solar panel. So we used that trick by connecting the pin 5 of ic 555 to the output of solar panel through one preset.

Now it is like this, when we are getting more sunlight then that solar panel also starts giving more voltage. Then that higher voltage reaches the pin 5 of ic 555 and pin 5 voltage also goes up. Inside this ic 555 there is that internal modulation part.

When pin 5 voltage increases, then the pwm duty cycle at pin 3 also increases. So this makes the pulse width to become more wide.

That will make us feel like mosfet will stay on for longer time. But that is not true because that mosfet is p-channel type.

So actually the MOSFET conduction actually gets narrow when duty cycle becomes high.

What happens due to all this

When we get full sunlight, that time the duty cycle of the mosfet becomes smaller. That means the on time becomes less.

So now the buck converter takes less energy from the solar panel. So because of this the panel voltage does not fall too much and it remains near that mpp level which is maximum power point.

Then when sunlight becomes weak, the solar voltage also starts coming down. That reduced voltage goes to pin 5 of ic 555 and pin 5 voltage drops.

So again the pwm duty cycle becomes narrow at pin 3. But since it is p channel mosfet, this narrow pulse actually makes the mosfet turn on for longer time.

So now the mosfet gets more on time and because of that the buck converter starts pulling more current from the panel. This helps to keep output power stable.

But we know that panel is already weak in performance, so the circuit automatically decreases pwm and controls the panel voltage so that it does not fall much or get overloaded.

How the circuit balances itself always

So this whole thing works like one balancing system which always adjusts depending on sunlight strength.

How mppt function happens in second part

Now second part of this system makes our circuit work like mppt tracker. That means it tries to follow maximum power point logic.

For this we used one opamp ic 741. It is made in such way that it keeps watching how panel voltage is changing and keeps one past voltage value in memory. That memory is held using some diodes and one capacitor.

So how it is wired and how it works

So pin 2 which is inverting input of opamp gets live voltage from solar panel directly. Then pin 3 which is non-inverting input also gets same panel voltage but it has to go through 3 diodes.

These 3 diodes make voltage drop of around 1.6V. So normally pin 3 stays lower than pin 2 by that 1.6V as long as panel voltage is same or increasing.

There is one 10uF capacitor at pin 3 side which makes voltage drop happen slowly. This helps to hold previous level.

But what happens if voltage drops suddenly

Now suppose the load like battery pulls heavy current because battery is very low. So panel voltage drops quickly. Then this low voltage reaches pin 2 and it falls fast.

But pin 3 cannot fall that fast because that capacitor holds it. So now pin 3 becomes more than pin 2. Then because of this the output of opamp goes high.

What this high output does

This high signal is sent to pin 5 of ic 555. This increases pwm duty cycle at pin 3 of ic 555. So pulse becomes wider. But this again makes gate of p channel mosfet get narrow pwm. So on time becomes short. This reduces output power of buck converter. Then load current becomes less. Because of that, the panel voltage starts rising again.

Final outcome of all this

So this opamp 741 part works like feedback control system. It checks when panel is getting too much load and tries to bring it down fast. Then it lets the panel recover voltage and slowly tries to increase again. This is how the circuit works like intelligent mppt controller.

Buck Converter Inductor Formula

L = (Vin - Vout) × D / (f × ΔI)

Where:

• L = Inductance in Henry (H)
• Vin = Input voltage from solar panel (V)
• Vout = Output voltage to battery (V)
• D = Duty cycle = Vout / Vin
• f = Switching frequency (Hz)
• ΔI = Ripple current through inductor (A), usually 30% to 40% of load current

Example: Based on Your Circuit

Let us assume:

• Vin = 18V (solar panel under full sun)
• Vout = 13V (for charging 12V battery)
• Load current = 4A (so ripple 30% of 4A = 1.2A)
• f = 50kHz (PWM frequency of 555 IC)

Example:

Let us assume:

• Vin = 18V (solar panel under full sun)
• Vout = 13V (for charging 12V battery)
• Load current = 4A (so ripple 30% of 4A = 1.2A)
• f = 50kHz (PWM frequency of 555 IC)

Step 1: Calculate Duty Cycle

D = Vout / Vin = 13 / 18 = 0.72

Step 2: Plug in Formula

L = (Vin - Vout) × D / (f × ΔI)
= (18 - 13) × 0.72 / (50000 × 1.2)
= 5 × 0.72 / 60000
= 3.6 / 60000
= 0.00006 H
= 60 µH

Final Answer:

L = 60 µH inductor

That means, for best performance of your buck converter with 18V solar input and 12V battery output at 4A load, you should use around 60 microhenry inductor.

You can use toroidal or iron core inductor rated at least 5A current with low resistance for best results.

Pro Tip:

If switching frequency is lower like 20kHz then L will go up:

L = 3.6 / (20000 × 1.2) = 150 µH

So lower frequency = bigger inductor and higher frequency = smaller inductor, but more EMI.

Summarizing:

• 555 makes PWM, controlled from solar voltage.
• 2N2222 flips it, feeds it to MOSFET gate.
• When sun is strong, MOSFET gets less ON time (voltage maintained).
• When sun is weak, MOSFET gets more ON time (current increased).
• 741 compares present and previous solar voltage.
• If voltage falls too fast, it makes 555 give max PWM.
• After inverter, this cuts down MOSFET PWM.
• Load current reduces, panel voltage goes up again.
• Then again PWM increases slowly... cycle repeats.