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solar controller ICs:




datasheets on ICs used for solar control (and often battery charge or tending)

Unfortunately, the cheap ones often claim to be MPPT (Maximum Power Point Tracking) when they are
merely a voltage regulator with the sense on the panel instead of the output. These chips use a voltage
divider between the solar panel output and ground, with the resistor values selected to scale the panel's
(known) Vmpp to some fixed voltage the IC is designed for. While the IC does a good job of keeping the
panels at their MPP, the resistors are fixed and no actual power tracking is happening. I'm sure they would
argue that tracking panel voltage counts, but that's not what anyone expects from something claiming it has
power tracking. "Tracking" also usually means adjusting X to keep Y where it belongs. This isn't adjusting
a separate variable, it's just keeping Y where it belongs. (it's just a voltage regulator with extra features)

This design means specific resistors must be selected for the Vmpp of the panel being used. If the panel is
changed for one with a different Vmpp (or if conditions change, like partial shading of the panel) there will
be no adjustment by the controller. Then the panel will leave its MPP, possibly by a wide margin, which
completely defeats the purpose of MPPT.

Calling these controllers "MPPT" is, IMO, fraud. I've also seen at least one vendor try to squirm around
the semantics by saying MPPT stands for "Maximum Power Point Trimming", where the resistor network
works as a trimmer to program the IC for the panel's Vmpp. For clarity I will refer to these ICs as "FPVR"
("Fixed Panel Voltage Regulator") since they only regulate panel voltage at a fixed point.


Check the datasheet on the CN3791:

Pin.6: Photovoltaic Cell Maximum Power Point Tracking Pin. Connect this pin to
the external resistor divider for maximum power point tracking. In maximum
power point tracking status, the MPPT pin's voltage is regulated to 1.205V.

In other words, you must manally set a two-resistor network between the panel and ground, and feed the
center to this pin. You need to know the panel's Vmpp in advance, and set the resistors so that pin 6 gets
1.205v when the panel is at Vmpp. There's NO TRACKING, it's just a dumb voltage regulator with the
voltage sense moved from the output to the input.



BQ24650 FPVR solar charge controller.pdf (2 MB)


CN3791 FPVR 1-cell controller.pdf


CN3795 FPVR low voltage LiPo controller.pdf



last updated 11/29/2025 at 17:47:08 by make_www_index.command version 2025.11.05.A