Secondary battery cell protection circuit

Details Secondary battery cell protection circuit Secondary battery cell protection circuit

2000-07-20

2001-10-30

Wong, Peter S. (Department: 2838)

Electricity: battery or capacitor charging or discharging

Battery or cell discharging

With charging

Reexamination Certificate

active

06310463

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary battery cell protection circuit. More particularly, the invention relates to a circuit for protecting a secondary battery cell such as a lithium-ion battery that suspends charging of the battery when overcharge of the battery is detected.
2. Description of the Related Art
Compared to a nickel-cadmium and a nickel-hydrogen battery, a lithium-ion battery has an approximately three times higher operating voltage, and double the energy density. Volume energy density of a lithium-ion battery is higher than that of a nickel-cadmium and a nickel-hydrogen battery. Therefore, the size of a lithium-ion battery is smaller, and the weight of the battery is lighter than other batteries having the same energy. Thus, a lithium-ion battery is used for a portable electronic device such as a video camera, a cellular phone, a PHS, and a laptop computer.
For a lithium-ion battery, a protection circuit is used for safety and in order to maximize the performance of the battery. The protection circuit detects overcharge of the battery and suspends charging of the battery.
FIG. 2
shows an example of conventional protection circuits for secondary batteries. In
FIG. 2
, a lithium-ion battery
14
is connected between a positive power-supply terminal
10
and a negative power-supply terminal
12
. The positive terminal
10
is connected to an inverting input terminal of a comparator
18
located in an integrated circuit
16
. A standard voltage VTH
1
is provided for detecting overcharge of the battery, and is supplied to a non-inverting input terminal of the comparator
18
from a constant voltage source
20
. An output signal of the comparator
18
is high-level when the voltage of the terminal
10
is lower than the standard voltage VTH
1
, and is low-level when the voltage of the terminal
10
is higher than VTH
1
.
The output signal of the comparator
18
is provided to a base of an npn-transistor Q
1
. The transistor Q
1
has its emitter grounded and its collector connected to a collector of a pnp-transistor Q
2
. The transistor Q
2
is diode-connected by connecting its collector and its base together. The pnp-transistor Q
2
also forms a current-mirror circuit with its base connected to a base of a pnp-transistor Q
3
. Each emitter of the pnp-transistor Q
2
and the pnp-transistor Q
3
is connected to the power-supply terminal
10
. A collector of the pnp-transistor Q
3
is connected to an output terminal
22
, and the output terminal
22
emits an overcharge detection signal. The collector of the transistor Q
3
is also connected to one end of a resistor R
1
and to a gate of an overcharge preventing MOS transistor Q
5
. The other end of the resistor R
1
is connected to the negative power-supply terminal
12
. The overcharge detection signal is supplied to the gate of the overcharge preventing MOS transistor Q
5
.
In addition, the power-supply terminal
10
is connected to an over-discharge detection unit
24
located in the integrated circuit
16
. The over-discharge detection unit
24
outputs an over-discharge detection signal. When the voltage of the terminal
10
is higher than or equal to a standard voltage VTH
2
, the over-discharge detection signal is high-level. When the voltage of the terminal
10
is lower than the standard voltage VTH
2
, the signal is low-level. This over-discharge detection signal is then supplied to a gate of a discharge preventing MOS transistor Q
4
outside the integrated circuit
16
.
A negative pole of the lithium-ion battery
14
is connected to a source of the discharge preventing MOS transistor Q
4
. A drain of the MOS transistor Q
4
is connected to a drain of the overcharge preventing MOS transistor Q
5
, and a source of the MOS transistor Q
5
is connected to the negative power-supply terminal
12
. Further, the potential at the negative pole of the battery
14
is set to the ground level. Since the gate is connected to a substrate at each of the MOS transistors Q
4
and Q
5
, body-diodes D
4
and D
5
are formed between the drain and the source of the respective MOS transistors Q
4
and Q
5
.
The over-discharge detection signal is high-level when the voltage of the power-supply terminal
10
is higher than or equal to the standard voltage VTH
2
, so that the over-discharge prevention MOS transistor Q
4
is activated unless the lithium-ion battery
14
is over discharged. Further, the overcharge detection signal is high-level when the voltage of the terminal
10
is lower than or equal to the standard voltage VTH
1
. The overcharge preventing MOS transistor Q
5
is activated unless the battery
14
is overcharged.
In addition, if the battery
14
is overcharged while charging the battery through a charger circuit that is connected between the terminals
10
and
12
, the output signal of the comparator
18
becomes low-level, and the transistor Q
1
is deactivated. Consequently, the transistors Q
2
and Q
3
are deactivated, and the output terminal
22
outputs a low-level overcharge detection signal. As a result, the overcharge preventing MOS transistor Q
5
is deactivated, and the battery
14
is no longer charged.
In the above-described prior circuit, after the lithium battery
14
is overcharged and the charging process is stopped, a load such as a video camera, a cellular phone, a PHS, a laptop computer may be connected to the power-supply terminals
10
and
12
. When the load is connected, the over-discharge prevention MOS transistor Q
4
is active, and the body-diode D
5
of the overcharge detection MOS transistor Q
5
turns on itself to allow an electrical current to flow from the battery
14
through the body-diode D
5
to the load.
Though it is not a problem that the discharged current from the battery
14
flows through the loads, the temperature of the overcharge prevention MOS transistor Q
5
increases because of the current flowing not through the overcharge prevention MOS transistor Q
5
but through the body-diode D
5
. Thus, the MOS transistor Q
5
deteriorates due to the increase in temperature thereof.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a protection circuit for a secondary battery that eliminates one or more of the disadvantages of the related art.
The above-described object of the present invention is achieved by a secondary battery cell protection circuit including:
an overcharge detection circuit detecting overcharge of a secondary battery cell;
an overcharge prevention MOS transistor which is deactivated by a signal supplied from the overcharge detection circuit when the secondary battery cell is overcharged so as to suspend charging of the secondary battery cell, the overcharge prevention MOS transistor including a body-diode;
a discharge detection circuit detecting discharge of the secondary battery cell through the body-diode; and
a circuit that activates the overcharge prevention MOS transistor when the discharge detection circuit detects the discharge of the secondary battery cell.
In the present invention, the discharge detection circuit detects the discharge of the secondary battery cell through the body-diode. When such discharge of the secondary battery cell is detected, the overcharge prevention MOS transistor is activated so as to discharge the secondary battery cell without having a current passing through the body-diode. Consequently, the substrate temperature of the overcharge prevention MOS transistor does not increase due to the discharged current flowing through the body-diode of the overcharge prevention MOS transistor when the load is connected to the overcharged secondary battery cell. This prevents the overcharge prevention MOS transistor from being deteriorated.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.


REFERENCES:
patent: 5705913 (1998-01-01), Takeuchi et al.
patent: 5808446 (1998-09-01), Eguchi
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