Tjyylli

im having some trouble with DC converters in a solar system.
I have a solar panel of 21V (Voc) and 1.33 (Isc).
I have studied that with a DC buck converter with input regulation (Feedback loop taken from the input) would result in a fixed output and equal to the MPP.

The thing is that i have a LM2596 adjustable regulator , and i was thinking about soldering the resistor divider taken from the output of the original topology, and take the input as a source.
Could I get the MPP voltage and current?
If yes, how could i calculate?

I attach the datasheet from Texas.
http://www.ti.com/lit/ds/symlink/lm2596.pdf

Original Topology from Texas

Edit: The objetive is to charge a battery pack of 3 Li-Ion cells efficiently, firstly mppt step is where my doubts are.

Although MPPT trackers are based around typically a buck converter, there's additional actions from a control loop, in hardware or programming, that actively tracks the point where maximum power is being drawn from the cell - this varies a little with solar intensity, so you might find a fixed voltage that is good enough for most conditions, but it also varies a lot with cell temperature, so the controller does need to be able to compensate.

Since the solar intensity and cell temperature can vary, an MPPT tracker constantly adjusts the set point up and down, and compares the power generated, and aims to settle at the peak.To be able to vary the input, the controller changes the duty cycle of the buck converter, but obviously this assumes that the load (the battery pack) is capable of taking the full output, and once the battery reaches full charge the controller needs to limit the output voltage too, and the power available at the solar cell goes unused. Regulating only the input voltage would risk overcharging.

Increasing the duty cycle increases the load and decreases the voltage at the solar cell, and a simple regulator wired with the feedback at the input would do the opposite.

You're far batter off getting a dedicated MPPT controller.

I doubt that would work, the output will start to oscillate trying to compensate the error.

If you want to do MPPT on your solar panel, perhaps the easier way is to use devices that are made for that.

One way of doing so is to use a buck topology to which the switching is controlled by a MCU and to write some simple MPPT algo.

The MPP is not a fixed point but is highly depending on the amount of light shining on the panel.
A MPP-Tracker is always checking the input power, slightly changing the point on the IU-curve and checking the resulting power again. By comparison of the power values the Tracker is able to determin, if the MPP is reached or if the point on the curve has to be shifted a bit.

This means, that you are not able do set a fixed value for your converter.

If you put no load on a solar panel at all, the voltage is maximum but the current is zero. If you short out a solar panel, the current is maximum but the voltage is zero. The power is the product of the current and the voltage.

The point of MPP tracking is to figure out the amount of load on the solar panel that maximizes the power produced by the solar panel. Since different MPPs will result in different voltages produced by the solar panel, this requires some kind of voltage converter that can take whatever voltage the MPP happens to be and make it useful.

If, for example, you are using 12V batteries and a solar panel whose MPP tends to be around 26 volts, a buck converter will do. But you need something to adjust the load the converter places on the panel to find the MPP. MPP trackers typically have software that varies the converter parameters and measures the voltage across the panel and the current it is supplying to discover the configuration that maximizes the power delivered. That's what MPPT is.

This requires discovery as the light on the panel changes. Sometimes it is brighter, sometimes it is dimmer. This results in changes to the maximum power point that must be discovered by varying the load the converter places on the panel.

Without any device to actually do the MPP tracking, you will not track the MPP.