Battery protection circuit employing active regulation of charge and discharge devices

Abstract

A battery protection circuit includes back-to-back connected metal-oxide-semiconductor field-effect transistors (MOSFETs). Detection circuitry detects whether the battery is in a normal charge condition, an overcharged condition, or an over-discharged condition, and for the overcharged and over-discharged conditions the circuitry asserts a corresponding enable signal. For each MOSFET, a corresponding gate voltage regulating circuit controls the gate voltage such that (i) when the corresponding enable signal is de-asserted, the gate voltage is sufficient to enable the MOSFET to strongly conduct current in either direction, and (ii) when the corresponding enable signal is asserted, the gate voltage is a function of the polarity of drain-to-source voltage of the MOSFET. For each MOSFET, the corresponding gate voltage regulating circuit prevents the MOSFET from conducting when the drain-to-source voltage has an undesired polarity, and allows the MOSFET to conduct when the drain-to-source voltage has a desired polarity. One MOSFET prevents the flow of charge current, and the other prevents the flow of discharge current. When either MOSFET is conducting, its drain-to-source voltage is prevented from achieving a value sufficient to forward bias a parasitic diode associated with the source and drain terminals of the MOSFET. Current of correct polarity flows through the source-to-drain channel of a MOSFET rather than through the parasitic diode during the overcharged and over-discharged conditions.