Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2000-08-21
2001-11-06
Toatley, Jr., Gregory J (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C361S082000, C361S084000, C361S093500
Reexamination Certificate
active
06313610
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention is related to the field of battery protection circuits.
It is common to incorporate protection circuitry in battery-based power supplies. Protection circuitry detects certain battery conditions, and in response the circuitry controls the charging and discharging of the battery to prevent unsafe operation or operation that can damage the battery. Certain types of batteries, such as lithium-ion batteries, require protection from conditions such as overheating, overcharging, and over-discharging to prevent potentially explosive failure.
Generally, battery protection circuits include detection circuitry that monitors battery voltage, current, or other operating parameters, and one or more controlled elements such as transistors that interrupt or otherwise control battery current when an unsafe condition is detected.
One protection circuit in common use today includes two metal-oxide-semiconductor field-effect transistors (MOSFETs) arranged in a back-to-back configuration in series between the battery and a connection point for the load and charger. The respective gate voltages of the MOSFET are controlled by signals from detection circuitry that detects overcharged and over-discharged conditions of the battery. When both transistors are conducting, either charge or discharge current can flow. When either transistor is off, current is prevented from flowing in one direction. A parasitic element associated with each transistor, which is referred to as a “body diode”, allows current to flow in the other direction. As a result, when the battery is in an overcharged condition, for example, the body diode of the non-conducting transistor permits the flow of discharge current but prevents the flow of charge current. Similar operation obtains when the battery is in an over-discharged condition.
The above approach suffers from the problem of excessive power dissipation under both overcharged and over-discharged operating conditions. Because current is flowing through a forward-biased diode with a non-negligible voltage drop, the power dissipated in the transistor is considerable. For example, if the charge or discharge current is 1 ampere, dissipated power is on the order of 1 watt. For many applications, such wasteful operation is very undesirable.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a battery protection circuit is disclosed that provides overcharge and over-discharge protection while substantially reducing power dissipation caused by the flow of recovery current to/from the battery.
The disclosed battery protection circuit includes a pair of 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 magnitude and polarity of drain-to-source voltage of the MOSFET. For one MOSFET, the corresponding gate voltage regulating circuit prevents the MOSFET from conducting when the drain-to-source voltage has a charging polarity, and allows the MOSFET to conduct when the drain-to-source voltage has a discharging polarity. For the other MOSFET, the corresponding gate voltage regulating circuit prevents the MOSFET from conducting when the drain-to-source voltage has a discharging polarity, and allows the MOSFET to conduct when the drain-to-source voltage has a charging polarity. 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. As a result, current of correct polarity flows through the source-to-drain channel of a MOSFET during the overcharged and over-discharged conditions, rather than flowing through a MOSFET body diode, resulting in reduced power consumption in these operating states.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.
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Texas Instruments Incorporated
Toatley Jr. Gregory J
Weingarten, Schurgin Gagnebin & Hayes LLP
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