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MPPT or Maximum Power Point Tracking is a crucial element in any solar powered satellite. We all remember that power = voltage x current, so it makes sense that if you want more power, you increase voltage or current, or both. However this is not the case with solar panels: in fact the maximum output power will DECREASE if you draw more current or voltage, past a certain point. There is a bit of a ‘sweet spot’ where voltage and current peak, this is called the maximum power point (MPP).

Power output and maximum power point of a solar panel.
As can be seen from the graph, if this particular system starts drawing over about 33V at 7A, the net power output starts decreasing.

 

On the ground, the MPP of a solar panel can be calculated and the system can be optimised to draw maximum power. The problem is that the MPP is highly dependent on the temperature of the panel, and in the vacuum of space where temperature differentials from shade to sun can be as high as 300°, this becomes a huge problem.

Enter MPPT, or the ability to track exactly where the maximum power point is, and adjust the system accordingly so we are squeezing every watt of power we can get out of the panels. The way this is achieved is through Pulse Width Modulation. By adjusting the duty cycle of the output, we can effectively control how much current and voltage we draw, and ensure that we are drawing the correct amount in order to be at or sufficiently near the Maximum Power Point.

Block Diagram of the Maximum Power Point Tracking (MPPT) system.

This is a block diagram of the MPPT Implementation we’re currently testing. At the moment, our panels will supply us with about 4.5V. The grey SM72445 box is the brains of the MPPT system, and provided with current and voltage output information, it adjusts the duty cycle of the square wave in our DC/DC converter. After being boosted to 9V, the system can charge all the batteries onboard the satellite and ensure that it never goes without power!