Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2001-12-18
2003-05-13
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S136000
Reexamination Certificate
active
06563292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a charge/discharge protection circuit for a secondary battery and, more particularly, to a charge/discharge protection circuit protecting a charge/discharge control field effect transistor (FET) from overheating due to repetition of on and off. The present invention also relates to a battery pack incorporating such a charge/discharge control field effect transistor, and an electronic device using such a battery pack.
2. Description of Related Art
Many portable electronic devices use a lithium-ion secondary battery. The lithium-ion secondary battery has a problem in that an accident may occur when being overcharged. Moreover, the lithium-ion secondary battery has a problem in that a number of charge/discharge cycles within a service life is reduced if the lithium-ion secondary battery is over-discharged.
In order to solve such a problem, a protection switch is provided to a charge/discharge path between a lithium secondary battery and a main part of a device. The protection switch is turned off so as to prevent the lithium-ion secondary battery being further overcharged or over-discharged when a state (overcharged state) where the lithium-ion secondary battery is charged to have a voltage more than a predetermined voltage or a state (over-discharged state) where the lithium-ion secondary battery is discharged to have a voltage below a predetermined voltage is detected.
The charge/discharge protection circuit, which protects a secondary battery by detecting an overcharged state, an over-discharged state or an over-current so as to cut off a charge/discharge path to the secondary battery, is disclosed in, for example, Japanese Laid Open Patent Applications No. 11-103528, No. 10-285810 and No. 9-182283 and Japanese Patent Publication No. 2872365.
In the above-mentioned conventional charge/discharge protection circuit, an overcharge detection circuit has a hysteresis. That is, if an overcharge of a secondary battery is detected, a charge control field effect transistor (FET) is turned off so as to establish a charge unable state, and, thereafter, the charge control FET is turned on so as to establish a charge enable state when a battery voltage returns to a charge return voltage which is lower than an overcharge detection voltage.
However, in a semiconductor device of the conventional protection circuit, if a deteriorated battery having a high internal-impedance caused by repeated charge and discharge is charged by a charger having a normal current value but a large voltage value, or if a battery having a normal internal-impedance is charged by a charger having a large voltage value and a large current value, the battery voltage drops when a charge current path is cut off (the charge control FET is turned off) due to an overcharge state being detected by an overcharge detection circuit. Under such circumstances, the battery-voltage drop may exceed the hysteresis of the over-discharge detection circuit, and the battery voltage may reach the level of the over-discharge return voltage.
In such a case, a charge operation is started again by turning the charge control FET on, and, thereafter, the same operation is repeated. That is, an oscillating operation occurs in that detection of overcharge→cut off of a charge current (turn off the charge control FET)→drop of the battery voltage→start of charge (turn on the charge control FET)→detection of overcharge are repeated. In such a case, there is a risk in that the charge control FET generates heat, which causes an ignition in the worst case.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an improved charge/discharge protection circuit in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a charge/discharge protection circuit which prevents a switching FET from being overheated or ignited so as to improve safety by preventing an oscillating operation of the switching FET due to repeated detection of overcharge and a returning operation in a case where a battery having a high internal impedance is charged by a charger having a normal current value but a large voltage value and also a case where a battery having a normal internal impedance by a charger having both a large voltage value and a large current value.
Another object of the present invention is to provide a battery pack having the above-mentioned charge/discharge protection circuit and an electronic device, such as a cellular phone, which uses such a battery pack.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a charge/discharge protection circuit comprising: an overcharge detection circuit which detects an overcharge of a secondary battery connected to the charge/discharge protection circuit and outputs an overcharge detection signal; a discharge over-current detection circuit which detects a discharge over-current of the secondary battery and outputs a discharge over-current detection signal; a charge control field effect transistor (FET) connected to a charge path of the secondary battery in series so as to cut off a charge current supplied to the secondary battery; a discharge control field effect transistor (FET) connected to the charge path of the secondary battery in series so as to cut off a discharge current supplied from the secondary battery; a latch circuit which latches the overcharge detection signal output from the overcharge detection circuit and outputs a signal so as to control the charge control FET; a first delay circuit which delays the discharge over-current detection signal output from the discharge over-current detection circuit and supplies the delayed discharge over-current detection signal to the discharge control FET; and a reset circuit which resets the latch circuit so as to turn on the charge control FET when the discharge over-current is detected by the discharge over-current detection circuit and when the overcharge is not detected by the overcharge detection circuit.
In the above-mentioned invention, the reset circuit may cause the latch circuit to continuously output the signal to turn on the charge control FET when the over-current detected by the discharge over-current detection circuit disappears during a delay time of the delay circuit. Additionally, the reset circuit may includes: a second delay circuit which delays the charge over-current detection signal for a predetermined time; and a logic circuit which logically operates the overcharge detection signal after being delayed by the second delay circuit and the discharge over-current detection signal output by the discharge over-current detection circuit. Further, the logic circuit may includes: an inverter inverting the overcharge detection signal after being delayed by the second delay circuit; and an AND circuit performing an AND operation on the inverted overcharge detection signal and the discharge over-current detection signal output by the discharge over-current detection circuit.
Additionally, the charge/discharge protection circuit according to the present invention may further comprise a logic circuit between the first delay circuit and the discharge control FET so as to maintain the discharge control FET being turned on when the latch circuit is reset during a delay time of the first delay circuit. The logic circuit may include: a first inverter inverting the discharge over-current detection signal output from the discharge over-current detection circuit; a second inverter inverting the discharge over-current detection signal inverting the discharge over-current detection signal after being delayed by the first delay circuit; and an OR circuit which performs a summing operation on outputs of the first and second inverters and supplies a result of the summing operation to discharge control FET.
Additionally, according to other aspects of the present invention, there are provided a battery pack using t
Dickstein , Shapiro, Morin & Oshinsky, LLP
Ricoh & Company, Ltd.
Tso Edward H.
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