Load switch for PV charging circuit

[juicedry]

I am designing a circuit to charge 4V battery cells from PV solar panel. The peak current will be about 300mA. The charging cycle is being controlled by a dedicated uC. My question is what is a good load switch (MOSFET, BJT, dedicated load switch) for this circuit?

When the battery cells are discharged, the load switch needs to pass current through to the PV solar panel and start charging the battery until it is above the uC supply voltage to turn-on. Then the uC can control the charging cycle. Also if the battery is charged to greater potential than the PV solar panel, then the load switch must block the reverse leakage current.

The PV solar panel will be manually connected and disconnected by the user and it will be operating in hot climates, so potentially +50degC ambient temperature.

Overall, it needs to be a cheap solution.

 

[BradtheRad]

This may be difficult to do. You want a switching device that turns on by default when the supply voltage is low. The microcontroller cannot be in the circuit at such a time (I think).

Your switching device could be a transistor (or mosfet), with a pullup/pulldown resistor at the bias.

Then when the supply voltage rises, the microcontroller comes to life. Eventually it sends a signal to the transistor, overriding the pullup/down resistor, in order to shut off charging current.

There is a question which is more convenient to use, a P-device or N-device. It will depend on whether your microcontroller needs to sense voltage referenced to 0V ground, or whether it can do so in reference to the positive supply.

In fact you may not necessarily need a microcontroller, but just a comparator, op amp, etc.