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I am using a buck converter with output commanded by a micontroncontroller to charge a lithium ion battery from one to three cells (3.0 to 4.2V, 6.0 to 8.4V or 9.0 to 12.6V) I would like to protect the out from reverse polarity conection of battery and reverse current.

I could use a simple diode but I want to avoid the voltage drop and loss of energy, so I am trying a solution with MOSFET.

Schematic

I think this MOSFET works as protection when battery is connected wrongly with reverse polarity. But what happens when polarity is correct, but the battery voltage is upper than voltage generated by the buck converter?

For example, just at the beguining when battery is plugged the out of buck converter is zero volts until the battery is correctly identified.

This P-MOSFET would protect for reverse current?

Thanks enter image description here

D. Arius
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  • I have a last query about buck converter protection with PFET. For example if the converter is charging a battery at 4.2V, but suddenly the converter out drops to 4.0V or less and battery still remains at 4.2V. Does the PMOS protect for a reverse current from drain to source? – D. Arius Oct 01 '17 at 15:36

2 Answers2

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This circuit will work, but you have to be careful selecting the PFET to make sure that you choose one with a very low gate threshold voltage to make sure it is fully on at the lowest operating voltage. Remember that with a one cell battery, Vin might be much lower than 3V for a deeply discharged battery.

http://www.ti.com/lit/an/slva139/slva139.pdf

To answer your other question, when the battery is first connected and the buck converter Vout is 0V, the PFET will be off and the charger will not be able to sense the battery.

Dean Franks
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  • Thanks I would like to use the P-MOSFET Si2323DS, that can conduce with Vgs = -1.8V. You're right when buck out is 0V the charger will not be able to sense the battery, the problem is that IC I am using like voltmeter should be protected to reverse-polarity too. Maybe I could connect voltmeter directly to battery with a diode, and add voltage drop to the measurement. – D. Arius Oct 01 '17 at 00:18
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For the voltmeter protection, use a series resistor to limit current and a shorting diode to prevent reverse voltage. It will let a reverse voltage equal to the forward voltage drop of the diode through. Assuming the series resistance of the voltmeter is very high, the series resistor will not require any correction in the voltage measurement.

schematic

simulate this circuit – Schematic created using CircuitLab

When the diode is reverse biased (up to the breakdown voltage) it effectively be open.

Dean Franks
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  • This protection would work fine but voltmeter absolute maximum range is from -0.3 to 40V. With 1N4148 when battery is in reverse polarity the voltmeter could reach up to -0.6V. Maybe using a low drop diode like a Schottky will be ok. – D. Arius Oct 01 '17 at 13:20