25-08-2017, 09:32 PM
micro chips
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INTRODUCTION
Using a solar panel or an array of panels without a
controller that can perform Maximum Power Point
Tracking (MPPT) will often result in wasted power,
which ultimately results in the need to install more
panels for the same power requirement. For
smaller/cheaper devices that have the battery
connected directly to the panel, this will also result in
premature battery failure or capacity loss, due to the
lack of a proper end-of-charge procedure and higher
voltage. In the short term, not using an MPPT controller
will result in a higher installation cost and, in time, the
costs will escalate due to eventual equipment failure.
Even with a proper charge controller, the prospect of
having to pay 30-50% more up front for additional solar
panels makes the MPPT controller very attractive.
This application note describes how to implement
MPPT using the most popular switching power supply
topologies. There are many published works on this
topic, but only a tiny portion of them show how to
actually implement the algorithms in hardware, as well
as state common problems and pitfalls. Even when
using the simplest MPPT algorithm with a
well-designed synchronous switching power supply, it
can be expected that at least 90% of the panel’s
available power will end up in the battery, so the
benefits are obvious.
SOLAR PANEL MPPT
The main problem solved by the MPPT algorithms is to
automatically find the panel operating voltage that
allows maximum power output. In a larger system,
connecting a single MPPT controller to multiple panels
will yield good results, but, in the case of partial
shading, the combined power output graph will have
multiple peaks and valleys (local maxima). This will
confuse most MPPT algorithms and make them track
incorrectly. Some techniques to solve problems related
to partial shading have been proposed, but they either
need to use additional equipment (like extra monitoring
cells, extra switches and current sensors for sweeping
panel current), or complicated models based on the
panel characteristics (panel array dependent). These
techniques only make sense in large solar panel
installations, and are not within the scope of this
application note.
Ideally, each panel or small cluster of panels should
have their own MPPT controller. This way the risk of
partial shading is minimized, each panel is allowed to
function at peak efficiency, and the design problems
related to converters handling more than 20-30A are
eliminated
FRACTIONAL OPEN CIRCUIT
VOLTAGE
The maximum power point voltage has a linear
dependency on the open circuit voltage VOC under
different irradiance and temperature conditions.
Computing the MPP (Maximum Power Point) comes
down to:
CONCLUSIONS
Using MPPT with solar panel installations has clear
advantages. The initial investment is smaller because
smaller panel wattage is required (very little potential
power is wasted), and adding correct battery-charging
algorithms will also decrease operating costs (batteries
are protected and last longer).
MPPT algorithms are simple enough, but implementing
a working MPPT controller is not a simple task,
because it is required to know the particularities of the
underlying switching converter. Many of the so-called
scientific papers published on this topic simply use
computer simulations instead of real hardware, and the
readers find themselves lacking vital information.