> As noted before - an opamp or comparator and FET
> will give you close to zero volt drop if it matters enough to you.

I see further comment had been made on that (I don't have the emails
accessible as I write this).

As suggested, the offset voltage of the comparator or opamp used will
tend to set a limit to how close to zero drop you can easily get. You
could trim the offset if you wanted minimum drop, while leaving enough
for variations with temperature and time. You can probably get under
10 mV with work.

For minimum offset voltage with very low cost, the very old LM393 /
LM339 comparator has a lower offset voltage than the almost as old
LM324/LM358 op amp. Offset voltage is 2 mV typical and 9mV worst case
across temperature range. You can probably safely design for say 20 mV
of differential drop in an unadjusted circuit. The comparator is open
collector, which is not a problem as long as you remember :-). Maximum
common mode range is an issue if you are trying to operate your
"FET-as-a-diode" on the battery high side (ie the maximum voltages
which can be dealt with at the input are smaller than supply voltage -
typically 1.5V below Vdd and worst case 2V. (The comparator will
operate from a  2V supply, but then has zero 'legal' input voltage range
:-( ).

You can instead operate it on the low side and this allows use of a
better performance per $ N channel FET switch.  As with many circuits,
the basic concept is simple but it will take a little playing to
achieve a good result.

Note that whether a high or low side switch is used, if you are
switching a MOSFET it needs to be "upside down" in polarity compared
to the way it is more normally used (ie drain and source need to be
swapped). This is because a MOSFET has an unavoidable internal
parasitic "body diode" which conducts with the opposite polarity to
the way the FET is USUALLY used. If you use the FET in the "normal"
way the body diode turns on in the undesired direction when PV panel
voltage falls below battery voltage. Reversing the FET's polarity to the
opposite of normal means that the body diode now conducts when the PV
panel is above the battery voltage, and you then turn on the FET to
lower the voltage drop.

This polarity reversal of the FET was covered in the very clever 2
transistor plus FET circuit cited in this thread. Note that the 2N7000
MOSFET that they used is a very low spec device - it has highish gate
turn on voltage and undesirably high on resistance. As a demonstrator
of the concept it may be OK but, if you are serious about using that
circuit to get a near zero voltage PV panel switch, then there are
many many better FETs available.


        Russell McMahon
        Applied Technology ltd
        New Zealand
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