Electronic – Can we use n-channel mosfet in the place of p-channel

Note that this is not a power converter but effectively a chip that uses a PFET to implement active rectification. As such, the current is intended to flow from the battery to the load when the switch is on. The FET is being used like a diode, making use of the inherent body diode of the part. The chip then turns on the FET to make the diode look more ideal when it should already be on. The chip will turn off the FET quickly when is sees SENSE go higher than Vin. If it didn't, the "diode" would conduct backwards.

The advantage of such a design is that the effective diode has very low forward drop, which is useful when it's in series with a battery since less of the battery power will be wasted. The drawback is that the overall diode has slow reverse recovery time, since the chip has to sense the reverse voltage and then actively shut off the FET. In this case the diode is used for power ORing, so a few µs reverse on time won't matter much.

Just use some low-voltage-drop diodes, such as Schottky diodes, to ensure that the battery will not try to energize the solar cell.

^{simulate this circuit – Schematic created using CircuitLab}

Here, the solar cell is represented using the photodiode symbol D1, which is the closest thing available in the schematic editor. Current can flow from the photocell only if it develops a voltage which exceeds that of the battery by at least about 0.25V, a difference that is needed to forward-bias the Schottky diode D2. When this condition does not exist (for instance, the light is too low), then the Schottky is not forward biased and prevents the reverse current flow from battery to D1.

Here is a version of the above schematic which incorporates a more detailed discrete component model of the cell:

^{simulate this circuit}