120V AC power path for solar charge controller

[davegravy]

I have a 65W MPPT solar charge controller module from Genasun:

https://sunforgellc.com/product/gv-5-mod/

I want to be able to use this charger from a 120V AC source when it's present, solar panel otherwise. I'm planning on using an off-the-shelf AC-to-DC converter followed by a buck converter to provide 3.5A constant current.

I'm looking at the LTC4412 as a means to provide the OR logic between solar panel and the AC source.

I found this circuit which I figure is mostly what I want:

https://circuitdigest.com/fullimage?i=circuitdiagram/LTC4412-Power-Path-Controller-Circuit-Diagram.png

Can this work?

The "load" in this circuit would be the "panel" terminals of the Genasun. Primary would be the AC source and Aux would be the solar panels. I'm not sure that any capacitors are necessary since it's not a problem if the voltage drops when switching between sources (the Genasun always has a battery connected).

I guess I'll want back-to-back MOSFETS rather than the single configuration or else I could have AC source power flowing back into the solar panel.

Anything else I should consider?

[Peabody]

I haven't used the LTC4412, but it would need to switch to the AC source if it is present, regardless of the panel voltage.  If there's any light, the panel voltage will rise to its maximum open-circuit voltage at the current illumination level.  If that's higher than the AC source voltage,  a switchover based on relative voltage levels wouldn't work because the panels may not be able to provide any current.  So you need the 4412 to give absolute priority to the AC source if it is present, even if it is below panel voltage.

As for the mosfets, your example circuit has mosfets in both lines, both oriented to prevent backflow when they are off.  I don't think you need more than that.

[davegravy]

I haven't used the LTC4412, but it would need to switch to the AC source if it is present, regardless of the panel voltage.  If there's any light, the panel voltage will rise to its maximum open-circuit voltage at the current illumination level.  If that's higher than the AC source voltage,  a switchover based on relative voltage levels wouldn't work because the panels may not be able to provide any current.  So you need the 4412 to give absolute priority to the AC source if it is present, even if it is below panel voltage.

You're right, I hadn't considered that.

I thought a while about it and don't see a way to make the 4412 (or a pair of them) work to assign priority (see attached).

I found the 4418 which may be a better fit, about to study it.

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4418.pdf

[Peabody]

These power mux chips can be very complicated, and often come in very hobbyist-unfriendly packages.  But I wonder if we can back up and rethink the problem.  Does it really matter if the solar panels take over providing charge current even if the AC source is plugged in?  In other words, what would be wrong with a two-diode setup?  The source with the higher voltage would power the load.  The diode voltage drop presumably wouldn't matter much in the AC line, but if it's a problem in the panel line, then that diode could be replaced by a mosfet.

In the circuit below, the panels could still take over via the body diode if the sun is strong enough, but I'm not sure that would be a problem.

[davegravy]

I like the simplicity but doesn't this have the same issue due to the fact the panel can be higher voltage than than the AC source even when it's not actually sunny, (ie can't provide meaningful current)?

I measured my panel Voc face down on the floor (almost no light condition) and it's higher than I expect to get from the AC source. I expect voltage drops sharply as soon as any current is drawn in this condition, but does that then lead to oscillation where the MOSFET turns on, voltage drops, MOSFET turns off, etc?

[Peabody]

The panel voltage will only go above the main source voltage if the sun is strong enough for the panel to provide *ALL* of the load current. Up to that point, the panel will provide as much current as it can at the voltage set by the AC source, which functions as a low-impedance "sink" at its voltage.

Solar panels behave strangely, and you have to keep in mind that they are basically current sources, not voltage sources. 

Below is a modified circuit which adds a schottky diode across the mosfet.  In effect, this diode takes the place of the body diode, and should be more able to handle the charge current.  When the AC source is not present, the mosfet turns on, which bypasses the extra diode as well as the body diode, and there is no voltage drop.  But even when AC is present, some current will flow through the extra diode depending on how bright the sun is.

Both schottky diodes should be 3A or whatever your maximum charge current is.  The SB220 shown in the previous reply is a 2A diode, so it wouldn't be enough.

[davegravy]

Thanks, ok I'm going to try this.

I thought of another possible issue, since it looks like in this scheme both sources can be pushing current simultaneously.  The panels are rated to 50W, the AC brick is 60W, and the Genasun is 65W. Should I implement some form of limiting to prevent overloading the Genasun?

[Peabody]

I don't think either source will be "pushing" current.  I think the Genasun will draw what it needs.  But the Gensun should be able to handle the maximum voltage the panel can reach.