Hello,
Hopefully someone can help me understand it.
I read that MPPT uses a DC-DC swicthed converter and vary the frequency using PWM to vary the duty cycle and therefore controlling the charge.
What I don't understand are the advantages over a simple, say LM22676, DC-DC converter which will maintain the output at a desired value.
Thanks.
Wikipedia has a good explanation for MPPT (Maximum Power Point Tracking). The nut of it is that you can get about 15% more power out of the solar cell with MPPT.
I read it, couldn't understand it.
The "knee" is basically where the V * I produces the maximum power. This stays valid for any converter.
I still don't see how this is 15% better than a quality DC-DC converter.
The only situation when probing panel voltage and amps could be benefic is when the output is below the minimum input required by an ordinary DC -DC converter, situation when the later will most likely shut down.
Anyone who can enlight me please? 
The Wikipedia page has a graph of the I V curve for a solar panel under various levels of sunlight. Pick any one level of irradiance to work with, and draw a rectangle with one corner at 0,0 and the opposite corner somewhere along that curve. The coordinates where that corner touches the curve represents your amperage and voltage output and the area of the rectangle represents the power output (Power = Amps * Volts).
If you try to pull a lot of current from your panel then your rectangle is going to be very tall and narrow -- not much area and thus not much power. If you pull very little current from it then again your rectangle is going to be very flat and wide. As you've seen, mathematically, the largest rectangle you can fit under that curve is going to be where the corner hits the curve right at the "knee".
So my question to you is, if you connect your DC-DC converter to your panel, how many amps is it going to pull and what will your rectangle look like?
Thanks for the explanation, I got it the IV curve, the intersection point where the product of voltage times current is maximum.
I'm looking at it from a practical perspective.
Assuming solar panel output 9V and 0.2A. The load, resistive, requires 0.5A at 5V
A classic DC-DC converter:
Temperature and sunlight affect the panel output, lets take few possible outputs:
Not sure if they are following a typical panel specs, just some values.
Assuming buck/step-down topology, what I don't understand is why using MPPT will give me a nice 15% efficiency in (2.) case over a classical DC converter considering that:
a. The load always requires 5V voltage
b. The load alwats requires 0.5A current
I think I have missunderstood their scope in the first place, MPPT was not designed for a fixed output V and I but rather for a range of I and V values to produce the maximum power.
Assuming solar panel output 9V and 0.2A. The load, resistive, requires 0.5A at 5V
Isn't that a show stopper right there? Load requires 2.5 watts, panel assumption is 1.8 watts, 2.5 > 1.8 ?
No "converter" can solve that delima unless it also has access to another power reserve like a battery or alternate voltage/current source.
Lefty
Yes, but it was just some random values, wrong choosed I agree. The purpose of the example was different anyway, how a MPPT can improve the efficiency.
Say 9V 0.35A, would make it fit to scope.
You arnt understanding how a MPPT converter works.
The 15% improvement in effiency only occurrs when all the power from the Solar Panel can be used, ie you need a load
that consumes exactly the same amount of power that the Solar Panel is producing at any given time.
MPPT converters always have a battery between the converter and the load to absorb any differance in power between
whats produced and whats consumed.
They are of no value if you want a constant regulated output voltage.
Very good explanation, I got it now, thanks.