power supply with Battery Backup -MOSFET reverse polarity does not work as intended

Question

^{simulate this circuit – Schematic created using CircuitLab}

This is my battery backup system for a laptop pc running 24/7.

So normally the Load is powered from primary supply and the battery is being charged.When the primary power is lost its sensed by the switching circuit turns on the boost converter.

I don't mind a momentary power loss when the switching happens.

Current design is using a diode (schottky?) to block the power going back so it doesn't trigger the power sense circuit or charge the battery when under battery power.

What i need to know

• Is there a way to make this system work without adding a diode so there wont be a voltage drop from primary power to load (MOSFET?)
• what type of circuit is suitable for the switching circuit?

some points to consider

• I cant change the input voltage or Load
• Boost Converter output voltage is adjustable

Also i would prefer not to use any mechanical switching such as relays.

Edit 1:

First Attempt

by adding a MOSFET on the high side of the main power connection

Using MOSFET reverse polarity protection doesn't work as intended because when boost converter is active current is fed back in to the charger

^{simulate this circuit}

Answer 1

If you don't mind soldering an exposed pad TSSOP package -- try wiring up a LTC4358 as per the datasheet. If you mind having a package that's at least somewhat easier to solder, but don't mind a few external parts, you can use a LTC4357 controller with an external N-FET instead, as depicted below:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Yes, you can do this with a MOSFET. It is done by operating the MOSFET in the third quadrant. Basically the MOSFET is run backwards so the body diode conducts when the circuit is powered. Once current is flowing the MOSFET channel conducts reducing the voltage drop across the body diode. You'll find example circuits if you search MOSFET third quadrant operation.