U.S. patent number RE41,915 [Application Number 10/854,781] was granted by the patent office on 2010-11-09 for charge/discharge control circuit and secondary battery.
This patent grant is currently assigned to Fujitsu Semiconductor Limited. Invention is credited to Akira Haraguchi, Takashi Matsumoto.
United States Patent |
RE41,915 |
Haraguchi , et al. |
November 9, 2010 |
Charge/discharge control circuit and secondary battery
Abstract
A charge/discharge control circuit for controlling charging and
discharging of a secondary battery that includes a cell. The
charge/discharge control circuit includes an overdischarge
detection circuit for detecting an overdischarged state of the
battery, and an overcharge detection circuit for detecting an
overcharged state of the battery. A discharge control switch is
deactivated in the overdischarged state. A first charge control
switch is deactivated in the overdischarged state. A second charge
control switch is activated in the overdischarged state. A
current-limiting circuit is connected in series with the second
charge control switch for limiting the charging current when
charging is performed.
Inventors: |
Haraguchi; Akira (Aichi,
JP), Matsumoto; Takashi (Aichi, JP) |
Assignee: |
Fujitsu Semiconductor Limited
(Yokohama, JP)
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Family
ID: |
18687527 |
Appl.
No.: |
10/854,781 |
Filed: |
May 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09816108 |
Mar 26, 2001 |
06396246 |
May 28, 2002 |
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Foreign Application Priority Data
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Jun 22, 2000 [JP] |
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2000-187567 |
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Current U.S.
Class: |
320/134;
320/136 |
Current CPC
Class: |
H02J
7/0031 (20130101); H02J 7/0021 (20130101); H02J
7/0048 (20200101); H02J 7/0016 (20130101); H02J
7/00306 (20200101); H02J 7/00304 (20200101); H02J
7/00302 (20200101) |
Current International
Class: |
H01M
10/46 (20060101) |
Field of
Search: |
;320/116,127,128,134,135,136 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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05-064377 |
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Mar 1993 |
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JP |
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6-105458 |
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Apr 1994 |
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JP |
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7-326389 |
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Dec 1995 |
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JP |
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10-014123 |
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Jan 1998 |
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JP |
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11-164489 |
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Jun 1999 |
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JP |
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11-215716 |
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Aug 1999 |
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JP |
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2000-106220 |
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Apr 2000 |
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JP |
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2001-275271 |
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Oct 2001 |
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JP |
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Primary Examiner: Tso; Edward
Attorney, Agent or Firm: Arent Fox LLP
Claims
What is claimed is:
1. A charge/discharge control circuit for controlling charging and
discharging of a secondary battery, wherein the secondary battery
includes a cell, the control circuit comprising: an overdischarge
detection circuit for comparing a voltage of the cell with a
predetermined discharge reference voltage, determining whether the
secondary battery is in an overdischarged state, and generating an
overdischarge determination signal; an overcharge detection circuit
for comparing the voltage of the cell with a predetermined charge
reference voltage, determining whether the secondary battery is in
an overcharged state, and generating an overcharge determination
signal; a discharge control switch connected to the overdischarge
detection circuit and deactivated in the overdischarged state based
on the overdischarge determination signal; a first charge control
switch deactivated in the overdischarged state based on the
overdischarge determination signal and the overcharge determination
signal; a second charge control switch activated in the
overdischarged state based on the overdischarge determination
signal and the overcharge determination signal; and a
current-limiting circuit connected in series with the second charge
control switch for limiting a charging current when charging is
performed.
2. The charge/discharge control circuit according to claim 1,
further comprising: an output circuit connected to the
overdischarge detection circuit and the overcharge detection
circuit for receiving the overdischarge determination signal and
the overcharge determination signal and generating first and second
control signals to control activation and deactivation of the first
and second control switches.
3. The charge/discharge control circuit according to claim 2,
wherein the output circuit generates the first and second control
signals so as to deactivate the first and second charge control
switches when the cell is overcharged, wherein the output circuit
generates the first and second control signals so as to activate at
least the second charge control switch when the cell is
overdischarged, and wherein the output circuit generates the first
and second control signals so as to activate only the first charge
control switch when the cell is neither overcharged nor
overdischarged.
4. The charge/discharge control circuit according to claim 1,
wherein the second charge control switch is connected in parallel
with the first charge control switch.
5. The charge/discharge control circuit according to claim 1,
wherein the current-limiting circuit includes a resistor having a
resistance set such that a voltage drop across the resistor is
greater than a predetermined target charging voltage when
performing charging in the overdischarged state.
6. The charge/discharge control circuit according to claim 1,
wherein a constant voltage charging is performed by the
current-limiting circuit and the second charge control switch when
the second charge control switch is activated, and a target
charging voltage for the constant voltage charging is substantially
equal to the voltage between a source and gate of the second charge
control switch.
7. A charge/discharge control circuit for controlling charging and
discharging of a secondary battery, wherein the secondary battery
includes a cell, the control circuit comprising: an overdischarge
detection circuit for comparing a voltage of the cell with a
predetermined discharge reference voltage, determining whether the
secondary battery is in an overdischarged state, and generating an
overdischarge determination signal; an overcharge detection circuit
for comparing the voltage of the cell with a predetermined charge
reference voltage, determining whether the secondary battery is in
an overcharged state, and generating an overcharge determination
signal; a discharge control switch connected to the overdischarge
detection circuit and deactivated in the overdischarged state based
on the overdischarge determination signal; a first charge control
switch deactivated in the overdischarged state based on the
overdischarge determination signal and the overcharge determination
signal; a second charge control switch activated in the
overdischarged state based on the overdischarge determination
signal and the overcharge determination signal; a current-limiting
circuit connected in series with the second charge control switch
for limiting a charging current when charging is performed; and an
erroneous functioning prevention circuit for controlling activation
and deactivation of the first charge control switch in the
overdischarged state based on a level of a voltage supplied by the
secondary battery.
8. The charge/discharge control circuit according to claim 7,
wherein the erroneous functioning prevention circuit has a
threshold voltage, and wherein the erroneous functioning prevention
circuit compares the voltage of the second battery to the threshold
voltage, generates a comparison signal, and controls the activation
and deactivation of the first charge control switch based on the
comparison signal.
9. The charge/discharge control circuit according to claim 8,
wherein the threshold voltage of the erroneous functioning circuit
is variable.
10. The charge/discharge control circuit according to claim 7,
further comprising: an output circuit connected to the
overdischarge detection circuit, the overcharge detection circuit
and the erroneous functioning prevention circuit, for receiving the
overdischarge determination signal, the overcharge determination
signal, and the comparison signal and generating first and second
control signals to control the activation and deactivation of the
first and second control switches.
11. The charge/discharge control circuit according to claim 10,
wherein the output circuit generates the first and second control
signals so as to deactivate the first and second charge control
switches when the cell is overcharged, wherein the output circuit
generates the first and second control signals so as to activate at
least the second charge control switch when the cell is
overdischarged, and wherein the output circuit generates the first
and second control signals so as to activate only the first charge
control switch when the cell is neither overcharged nor
overdischarged.
12. The charge/discharge control circuit according to claim 7,
wherein the second charge control switch is connected in parallel
with the first charge control switch.
13. The charge/discharge control circuit according to claim 7,
wherein the current-limiting circuit includes a resistor having a
resistance set such that a voltage drop across the resistor is
greater than a predetermined target charging voltage when
performing charging in the overdischarged stage.
14. The charge/discharge control circuit according to claim 7,
wherein a constant voltage charging is performed by the current
limiting circuit and the second charge control switch when the
second control switch is activated, and a target charging voltage
for the constant voltage charging is substantially equal to the
voltage between a source and gate of the second charge control
switch.
15. A secondary battery including: a cell; and a charge/discharge
control circuit connected to the cell for controlling charging and
discharging of the cell, the charge/discharge control circuit
comprising: an overdischarge detection circuit for comparing a
voltage of the cell with a predetermined discharge reference
voltage, determining whether the secondary battery is in an
overdischarged state, and generating an overdischarge determination
signal; an overcharge detection circuit for comparing the voltage
of the cell with a predetermined charge reference voltage,
determining whether the secondary battery is in an overcharged
state, and generating an overcharge determination signal; a
discharge control switch connected to the overdischarge detection
circuit and deactivated in the overdischarged state based on the
overdischarge determination signal; a first charge control switch
deactivated in the overdischarged state based on the overdischarge
determination signal and the overcharge determination signal; a
second charge control switch activated in the overdischarged state
based on the overdischarge determination signal and the overcharge
determination signal; and a current-limiting circuit connected in
series with the second charge control switch for limiting a
charging current when charging is performed.
16. The secondary battery according to claim 15, wherein the
charge/discharge control circuit further includes: an erroneous
functioning prevention circuit for controlling activation and
deactivation of the first charge control switch in the
overdischarged state based on a level of a voltage supplied by the
secondary battery.
.Iadd.17. A charge/discharge control circuit for charging a
secondary battery via first and second charge paths connected in
parallel and for discharging the secondary battery, the first
charge path comprising a first charge control switch, and the
second charge path comprising a second charge control switch and
bypassing the first charge path, wherein the conductance of the
first charge path is higher than that of the second charge path,
and wherein the secondary battery includes a cell, the control
circuit comprising: an overdischarge detection circuit for
determining that the secondary battery is in an overdischarged
state by comparing a voltage of the cell with a predetermined
discharge reference voltage to generate an overdischarge
determination signal which deactivates the first charge control
switch and activates the second charge control switch..Iaddend.
.Iadd.18. The charge/discharge control circuit according to claim
17, further comprising an output circuit connected to the
overdischarge detection circuit for receiving the overdischarge
determination signal and generating first and second control
signals to control the activation and deactivation of the first and
second control switches..Iaddend.
.Iadd.19. The charge/discharge control circuit according to claim
18, wherein the output circuit generates the first and second
control signals to activate at least the second charge control
switch when the cell is overdischarged..Iaddend.
.Iadd.20. The charge/discharge control circuit according to claim
17, further comprising a current-limiting circuit connected in
series to the second charge control switch for limiting charging
current when charging is performed, wherein the current-limiting
circuit includes a resistor having a resistance set so that a
voltage drop at the resistor is greater than a predetermined target
charging voltage when performing charging in the overdischarged
state..Iaddend.
.Iadd.21. The charge/discharge control circuit according to claim
17, wherein constant voltage charging is performed by the second
charge control switch when the second control switch is activated,
and the target charging voltage for the constant voltage charging
is substantially equal to the voltage between a source and gate of
the second charge control switch..Iaddend.
.Iadd.22. A charge/discharge control circuit for charging a
secondary battery via first and second charge paths connected in
parallel and for discharging the secondary battery, the first
charge path comprising a first charge control switch, and the
second charge path comprising a second charge control switch and
bypassing the first charge path, wherein the conductance of the
first charge path is higher than that of the second charge path,
and wherein the secondary battery includes a plurality of cells,
the control circuit comprising: an overdischarge detection circuit
for receiving voltages of the plurality of cells and determining
that the secondary battery is in an overdischarged state when one
of the voltages of the plurality of cells is lower than a
predetermined discharge reference voltage to generate an
overdischarge determination signal, in the overdischarge
determination signal, in the overdischarged state, which
deactivates the first charge control switch and activates the
second charge control switch..Iaddend.
.Iadd.23. The charge/discharge control circuit according to claim
22, further comprising an output circuit connected to the
overdischarged detection circuit for receiving the overdischarge
determination signal and generating first and second control
signals to control the activation and deactivation of the first and
second control switches..Iaddend.
.Iadd.24. The charge/discharge control circuit according to claim
23, wherein the output circuit generates the first and second
control signals to activate at least the second charge control
switch when the cell is overdischarged..Iaddend.
.Iadd.25. The charge/discharge control circuit according to claim
22, further comprising a current-limiting circuit connected in
series to the second charge control switch for limiting charging
current when charging is performed, wherein the current-limiting
circuit includes a resistor having a resistance set so that a
voltage drop at the resistor is greater than a predetermined target
charging voltage when performing charging in the overdischarged
state..Iaddend.
.Iadd.26. The charge/discharge control circuit according to claim
22, wherein constant voltage charging is performed by the second
charge control switch when the second control switch is activated,
and a target charging voltage for the constant voltage charging is
substantially equal to the voltage between a source and gate of the
second charge control switch..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a charge/discharge control
circuit, and more particularly, to a charge/discharge control
circuit of a secondary battery used in a portable electronic
device.
Enhanced performance of contemporary portable electronic devices
(e.g., personal computers) has created a demand for batteries
having prolonged lifetime. Lithium ion batteries, which are
commonly used as secondary batteries, are widely used in recent
portable electronic devices. To prolong the lifetime of a lithium
ion battery, charging and discharging of the battery has to be
controlled. During discharge control, discharging is prohibited
when the battery is overdischarged. During charge control, charging
is prohibited when the battery is overcharged.
FIG. 1 is a schematic circuit diagram of a prior art
charge/discharge control circuit 50. The charge/discharge control
circuit 50 includes a control unit 3 and two control switches that
are externally connected to the control unit 3. The two control
switches are a discharge control switch 4 and a charge control
switch 5a. The charge/discharge control circuit 50 controls the
charge/discharge control current of a battery 1. The battery 1 is a
lithium ion battery that can be used as a secondary battery, and
includes three series-connected cells 2a, 2b, 2c. The battery 1
provides power to a portable electronic device, for instance.
The discharge control switch 4 and the charge control switch 5a
each include a p-channel MOS transistor. Each p-channel MOS
transistor includes a parasitic diode formed between its source and
drain. The drain of the discharge control switch 4 is connected to
the drain of the charge control switch 5a.
The positive terminal of the battery 1 is connected to an output
terminal t1 via the control switches 4, 5a. The negative terminal
of the battery 1 is connected to the ground GND and an output
terminal t2.
The charge control switch 5a is controlled based on a charge
control signal Cout of the control unit 3. The discharge control
switch 4 is controlled based on a discharge control signal Dout of
the control unit 3.
The cells 2a, 2b, 2c of the battery 1 are each connected to a cell
voltage detection circuit 6 incorporated in the control unit 3. The
cell voltage detection circuit 6 includes three comparators 7a, 7b,
7c. The comparator 7a detects voltage V2a between terminal BH and
terminal BM. The comparator 7b detects voltage V2b between terminal
BM and terminal BL. The comparator 7c detects voltage V2c between
terminal BL and the GND terminal.
The output signals of the comparators 7a, 7b, 7c are each provided
to positive input terminals of an overcharge detection circuit 8
and to negative input terminals of an overdischarge detection
circuit 9. A charge reference voltage VTH is provided to a negative
input terminal of the overcharge detection circuit 8. A discharge
reference voltage VTL is provided to a positive input terminal of
the overdischarge detection circuit 9.
The overdischarge detection circuit 9 provides the discharge
control signal Dout to the gate of the discharge control switch 4.
The overcharge detection circuit 8 provides the charge control
signal Cout to the gate of the charge control switch 5a.
The control unit 3 includes a bias generation circuit 10. When the
battery 1 supplies the bias generation circuit 10 with power supply
voltage Vcc, the control unit 3 is activated.
When any one of the cell voltages V2a, V2b, V2c is higher than the
charge reference voltage VTH, that is, in an overcharged state, the
charge control signal Cout is high and the discharge control signal
Dout is low. Thus, the discharge control switch 4 is activated and
the charge control switch 5a is deactivated. Accordingly, charging
is prohibited.
In this state, a discharge route, which includes the parasitic
diode of the discharge control switch 5a, the discharge control
switch 4, and the battery 1, is formed between the output terminals
t1, t2. Accordingly, if a portable electronic device is connected
between the output terminals t1, t2, the battery 1 provides a
current to the portable electronic device. This lowers each cell
voltage.
When all of the cell voltages V2a, V2b, V2c are included between
the charge reference voltage VTH and the discharge reference
voltage VTL, that is, in a normal state, the charge control signal
Cout and the discharge control signal Dout are both low. This
activates both of the control switches 4, 5a and enables charging
and discharging of each cell.
When charging the battery 1, constant current charging is
performed. Since the charging voltage is significantly greater than
the threshold voltage of the charge control switch 5a, the ON
resistance of the charge control switch 5a is small. In contrast to
constant voltage charging, the current value in constant current
charging is greater. However, the ON resistance of the discharge
control switch 5a is smaller. Thus, the voltage between the source
and drain of the switch 5a is lower. As a result, the power
consumption in the charge control switch 5a decreases and the
charge control switch 5a is not heated.
When any one of the cell voltages V2a, V2b, V2c is lower than the
discharge reference voltage VTL, that is, in an overdischarged
state, the charge control signal Cout is low and the discharge
control signal Dout is high. This activates the charge control
switch 5a and deactivates the discharge control switch 4.
Accordingly, discharging is prohibited.
In this state, the parasitic diode of the discharge control switch
4 forms a charge route between the output terminals t1, t2. This
enables charging. If a charger is then connected between the output
terminals t1, t2 and charges the battery 1, which is in an
overdischarged state, the cell voltages increase. This provides
power to the portable electronic device.
If the battery 1 is charged when any one of the cells 2a, 2b, 2c is
in an overdischarged state, the charge/discharge control circuit 50
performs constant current charging. In the prior art, when a
lithium ion battery is charged, constant current charging is
performed if the power supply voltage Vcc is low. When the power
supply voltage Vcc becomes equal to a predetermined voltage (e.g.,
12.6V), constant current charging is switched to constant voltage
charging. This is because constant current charging charges the
battery more quickly, since the charging current in constant
current charging is greater than that in constant voltage
charging.
When constant current charging is performed in an overdischarged
state, such as when the level of the power supply voltage Vcc is
extremely low (e.g., Vcc.apprxeq.0), the charging voltage becomes
low in comparison to normal constant current charging. During
constant voltage charging, the charging voltage of a typical
charger is set at 12.6V. However, during constant current charging,
the charging voltage is controlled in accordance with the level of
the power supply voltage Vcc.
Thus, when the discharge reference voltage VTL of the overdischarge
detection circuit 9 is 2.5V and the cell voltages V2a, V2b, V2c are
each 3V (normal state), the power supply voltage Vcc is 9V. In this
state, the charging voltage is significantly greater than the
threshold voltage of the charge control switch 5a, which is
typically 4V.
When the level of the power supply voltage Vcc decreases to a value
close to 0V (overdischarged state), the charging voltage decreases
to 4V and becomes equal to the minimum voltage between the source
and gate of the charge control switch 5a that enables activation of
the charge control switch 5a. In this state, the ON resistance of
the charge control switch 5a is large. Thus, if the voltage drop at
the parasitic diode of the discharge control switch 4 is 1V, the
voltage between the source and drain of the charge control switch
5a is 3V.
When the charging current is 1 A, the power consumption of the
charge control switch 5a is 3 W and thus large. Hence, in the
charge/discharge control circuit 50, the charge control switch 5a
is heated when the battery 1 is charged in an overdischarged state,
due to a large power consumption in the charge control switch
5a.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
charge/discharge control circuit that decreases the power consumed
by a charge control switch when a secondary battery is charged in
an overdischarged state.
To achieve the above object, the present invention provides a
charge/discharge control circuit for controlling charging and
discharging of a secondary battery. The secondary battery includes
a cell. The charge/discharge control circuit includes an
overdischarge detection circuit for comparing a voltage of the cell
with a predetermined discharge reference voltage, determining
whether the secondary battery is in an overdischarged state, and
generating an overdischarge determination signal. There is also an
overcharge detection circuit for comparing the voltage of the cell
with a predetermined charge reference voltage, determining whether
the secondary battery is in an overcharged state, and generating an
overcharge determination signal. A discharge control switch is
connected to the overdischarge detection circuit and deactivated in
the overdischarged state based on the overdischarge determination
signal. A first charge control switch is deactivated in the
overdischarged state based on the overdischarge determination
signal and the overcharge determination signal. A second charge
control switch is activated in the overdischarged state based on
the overdischarge determination signal and the overcharge
determination signal. A current-limiting circuit is connected in
series with the second charge control switch for limiting a
charging current when charging is performed.
The present invention also provides an alternative charge/discharge
control circuit for controlling charging and discharging of a
secondary battery. The secondary battery includes a cell. The
charge/discharge control circuit includes an overdischarge
detection circuit for comparing a voltage of the cell with a
predetermined discharge reference voltage, determining whether the
secondary battery is in an overdischarged state, and generating an
overdischarge determination signal. There is also an overcharge
detection circuit for comparing the voltage of the cell with a
predetermined charge reference voltage, determining whether the
secondary battery is in an overcharged state, and generating an
overcharge determination signal. A discharge control switch is
connected to the overdischarge detection circuit and deactivated in
the overdischarged state based on the overdischarge determination
signal. A first charge control switch is deactivated in the
overdischarged state based on the overdischarge determination
signal and the overcharge determination signal. A second charge
control switch is activated in the overdischarged state based on
the overdischarge determination signal and the overcharge
determination signal. A current-limiting circuit is connected in
series with the second charge control switch for limiting a
charging current when charging is performed. Furthermore, an
erroneous functioning prevention circuit controls activation and
deactivation of the first charge control switch in the
overdischarged state based on a level of a voltage supplied by the
secondary battery.
The present invention further provides a secondary battery
including a cell and a charge/discharge control circuit connected
to the cell for controlling charging and discharging of the cell.
The charge/discharge control circuit includes an overdischarge
detection circuit for comparing a voltage of the cell with a
predetermined discharge reference voltage, determining whether the
secondary battery is in an overdischarged state, and generating an
overdischarge determination signal. There is also an overdischarge
detection circuit for comparing the voltage of the cell with a
predetermined charge reference voltage, determining whether the
secondary battery is in an overcharged state, and generating an
overcharge determination signal. A discharge control switch is
connected to the overdischarge detection circuit and deactivated in
the overdischarged state based on the overdischarge determination
signal. A first charge control switch is deactivated in the
overdischarged state based on the overdischarge determination
signal and the overcharge determination signal. A second charge
control switch is activated in the overdischarged state based on
the overdischarge determination signal and the overcharge
determination signal. A current-limiting circuit is connected in
series with the second charge control switch for limiting a
charging current when charging is performed.
Other aspects and advantages of the present invention will become
apparent from the following description taken in conjunction with
the accompanying drawings, illustrating by way of example the
principle of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with objects and advantages thereof, may
best be understood by reference to the description of the following
exemplary embodiments together with the accompanying drawings in
which:
FIG. 1 is a schematic circuit diagram of a prior art
charge/discharge control circuit;
FIG. 2 is a schematic circuit diagram of a charge/discharge control
circuit according to a first embodiment of the present
invention;
FIG. 3 is a schematic circuit diagram of a charge/discharge control
circuit according to a second embodiment of the present invention;
and
FIG. 4 is a schematic circuit diagram of a charge/discharge control
circuit according to a third embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[First Embodiment]
FIG. 2 is a schematic circuit diagram of a charge/discharge control
circuit 100 according to a first embodiment of the present
invention. The charge/discharge control circuit 100 includes a
control unit 30, a discharge control switch 4, and a first charge
control switch 5a. The discharge and charge control switches 4, 5a
are connected to the control unit 30. The control unit 30 further
includes a cell voltage detection circuit 6, an overcharge
detection circuit 8, and an overdischarge detection circuit 9.
The charge/discharge control circuit 100 further includes a second
charge control switch 5b, a resistor R, and an output circuit 20.
The output circuit 20 includes a first OR circuit 11a, a second OR
circuit 11b, and an inverter 12.
The second charge control switch 5b and the resistor R are
connected in series. The first charge control switch 5a is
connected in parallel with the series-connected switch 5b and
resistor R. The resistor R is a current-limiting resistor. The
second charge control switch 5b is preferably a p-channel MOS
transistor.
A parasitic diode is formed between the source and drain of the
second charge control switch 5b. The cathode of the parasitic diode
is connected to the source of the switch 5b.
Constant voltage charging is performed by the second discharge
control switch 5b. When a charging current flows through the second
charge control switch 5b, the voltage drop across the resistance R
increases the charging voltage to a predetermined target charging
voltage.
The resistance of the resistor R is set such that the voltage drop
across the resistor R is greater than the target charging
voltage.
The output signal of the overcharge detection circuit 8 is provided
to first input terminals of the first and second OR circuits 11a,
11b respectively. The output signal (discharge control signal) Dout
of the overdischarge detection circuit 9 is provided to the gate of
the discharge control switch 4 and a second input terminal of the
first OR circuit 11a. The discharge control signal Dout is also
provided to a second input terminal of the second OR circuit 11b
via the inverter 12.
The output signal (first charge control signal) Cout of the first
OR circuit 11a is provided to the gate of the first charge control
switch 5a. The output signal (second charge control signal) PreCout
of the second OR circuit 11b is provided to the gate of the second
charge control switch 5b. Accordingly, when the first charge
control signal Dout is high and the output signal of the overcharge
detection circuit 8 is low, the second charge control signal
PreCout is low. In this state, the second charge control switch 5b
is activated.
The operation of the charge/discharge control circuit 100 will now
be discussed. The cell voltage detection circuit 6 detects cell
voltages V2a, V2b, V2c and provides the respective detection
signals to the overcharge detection circuit 8 and the overdischarge
detection circuit 9.
When any one of the cell voltages V2a, V2b, V2c exceeds the charge
reference voltage VTH of the overcharge detection circuit 8
(overcharged state), the output signal of the overcharge detection
circuit 8 goes high. The overcharge detection circuit 8 provides
the high output signal to the first and second OR circuits 11a,
11b. In this state, the discharge control signal Dout is low and
the discharge control switch 4 is activated.
The low discharge control signal Dout is also provided to first OR
circuit 11a and the inverter 12. The inverter 12 inverts the
discharge control signal Dout and provides a high signal Dout to
the second OR circuit 11b.
Since the first and second charge control signals Cout, PreCout are
both high in this state, the first and second charge control
switches 5a, 5b are both deactivated. This prohibits charging.
In this state, if a portable electronic device is connected between
output terminals t1, t2, the battery 1 supplies the portable
electronic device with a discharging current via the discharge
control switch 4 and the parasitic diode of the charge control
switch 5a.
As discharging proceeds and the power supply voltage Vcc decreases,
all of the cell voltages V2a, V2b, V2c become included between the
charge reference voltage VTH and the discharge reference voltage
VTL (normal state). In this state, the output signals of the
overcharge detection circuit 8 and the overdischarge detection
circuit 9 go low. Accordingly, the discharge control signal Dout
goes low and the discharge control switch 4 is activated.
The first charge control signal Cout generated by the first OR
circuit 11a also goes low and hence the first charge control switch
5a is activated. The second charge control signal PreCout generated
by the second OR circuit 11b goes high and hence the second charge
control switch 5b is deactivated.
In this state, if a charger is connected between the output
terminals t1, t2, the charger supplies the battery 1 with a
charging current via the first charge control switch 5a and the
discharge control switch 4 so to perform constant current charging.
If, for example, the discharge reference voltage VTL is 2.5V and
each of the cell voltages V2a, V2b, V2c is 3V, the power supply
voltage Vcc is 9V. Further, the charging voltage for performing
constant current charging in the normal state is about 10V, which
is slightly higher than the power supply voltage. The threshold
voltage of the first charge control switch 5a is about 4V.
Since the voltage between the source and gate of the first charge
control switch 5a is 10V and significantly greater than the
threshold voltage of the first charge control switch 5a (which is
4V), the ON resistance of the first charge control switch 5a is
small. This low voltage between the source and drain of the first
charge control switch 5a reduces the power consumption of the first
charge control switch 5a. Thus, charging is performed without
heating the first charge control switch 5a.
If the target charging voltage is, for example, 12.6V, and constant
current charging increases the battery voltage Vcc to the target
charging voltage of 12.6V, the constant current charging is
switched to constant voltage charging.
When any one of the cell voltages V2a, V2b, V2c becomes lower than
the discharge reference voltage VTL (overdischarged state), the
discharge control signal Dout goes high. This high discharge
control signal Dout deactivates the discharge control switch 4, and
prohibits discharging. Further, the first charge control signal
Cout generated by the first OR circuit 11a goes high and the first
charge control switch 5a is deactivated.
Additionally, the second charge control signal PreCout generated by
the second OR circuit 11a goes low and the second charge control
switch 5b is activated. In this state, a conductive route is formed
between the output terminals t1, t2 by way of the second charge
control switch 5b, the resistor R, the parasitic diode of the
discharge control switch 4, and the battery 1. The battery 1 is
charged when the charger is connected between the output terminals
t1, t2.
In the charge/discharge circuit 100, if the battery voltage Vcc in
the overdischarged state decreases to a value close to 0V and the
charging voltage is low, a voltage drop occurs across the resistor
R when a charging current flows through the second charge control
switch 5b. This immediately increases the charging voltage to the
target charging voltage of 12.6V.
Accordingly, the charge/discharge control circuit 100 performs
constant voltage charging (12.6V) in the overdischarged state.
During the constant voltage charging, the voltage between the
source and gate of the second charge control switch 5b is
substantially equal to the target charging voltage of 12.6V and
significantly higher than the threshold voltage of the second
charge control switch 5b, which is 4V.
Therefore, the ON resistance of the second charge control switch 5b
is small, and the voltage between the source and drain of the
second charge control switch 5b is small. This reduces the power
consumption in the second charge control switch 5b and enables
charging to be performed without heating the second charge control
switch 5b.
The charging increases the voltages of the cells from being in the
overdischarged state until each of cell voltage V2a, V2b, V2c
exceeds the discharge reference voltage VTL. This causes the output
signals of the overcharge detection circuit 8 and the overdischarge
detection circuit 9 to go low. As a result, the discharge control
switch 4 and the first charge control switch 5a are both activated
and the second charge control switch 5b is deactivated, so to
perform constant current charging.
The charge/discharge circuit 100 of the first embodiment has the
advantages described as follows. (1) If the battery 1 is charged
when any of the cells 2a, 2b, 2c is in an overdischarged state, the
charging current flows through the resistor R via the second charge
control switch 5b. Thus, even if the power supply (battery) voltage
Vcc is decreased to a value close to 0V, the voltage drop across
the resistor R immediately increases the charging voltage to the
target charging voltage of 12.6V. By performing the constant
voltage charging at 12.6V, the power consumption in the second
charge control switch 5b decreases and charging is performed
without heating the second charge control switch 5b. (2) The
voltage drop across the resistor R is set such that it is greater
than the target charging voltage of 12.6V. In other words, during
constant voltage charging, the charging current flowing through the
second charge control switch 5b is restricted to a value that is
smaller than the current value during constant current charging.
Accordingly, charging is performed without heating the second
charge control switch 5b. (3) Since the circuit including the
second charge control switch 5b and the resistor R is connected in
parallel with the first charge control switch 5a, an overdischarged
battery 1 and a normal-state battery 1 are charged by way of
different control switches. This prevents the control switches from
being heated during charging the battery 1 in either the
overdischarged or normal state. (4) When the voltages of the three
series-connected cells in the battery 1 are imbalanced, the first
and second charge control switches 5a, 5b are both deactivated so
to prohibit further charging. An imbalanced state refers to a state
in which one of the three cells is overdischarged and the remaining
one or two cells are overcharged, for instance. (5) The
charge/discharge circuit 100 allows the battery 1 to be charged
without heating the control switches. This simplifies the
configuration of an external charging circuit that is to be used
for charging the battery 1, and decreases the number of components
in the system. [Second Embodiment]
FIG. 3 is a schematic circuit diagram of a charge/discharge control
circuit 200 according to a second embodiment of the present
invention. The charge/discharge control circuit 200 makes use some
of the components in the charge/discharge control circuit 100 of
the first embodiment, as identified by those labeled with identical
numerals. The charge/discharge control circuit 200 further includes
a control unit 30A that differs from the control unit 30 in the
charge/discharge control circuit 100 of the first embodiment.
The control unit 30A in the charge/discharge control circuit 200
includes an erroneous functioning prevention circuit 13, which is
connected to a bias generation circuit 10, and an output circuit
20A. The output circuit 20A includes an OR circuit 11c and an NOR
circuit 15, and is connected to the erroneous functioning
prevention circuit 13 via an inverter 14.
The erroneous functioning prevention circuit 13 has a predetermined
threshold voltage Vth3. The erroneous functioning prevention
circuit 13 generates an erroneous functioning prevention signal
(comparison signal). The erroneous functioning prevention signal
goes high when any one of the cells 2a, 2b, 2c is overdischarged
and the power supply voltage (battery voltage) Vcc is higher than
the threshold voltage Vth3. The erroneous functioning prevention
signal goes low when the battery voltage Vcc is lower than the
threshold voltage Vth3.
The erroneous functioning prevention signal is provided to and
further inverted by the inverter 14. The inverted erroneous
functioning prevention signal is provided to first input terminals
of the OR circuit 11c and the NOR circuit 15 in the output circuit
20A. The first and second charge control switches 5a, 5b are
controlled based on the erroneous functioning prevention
signal.
The output signal of the overcharge detection circuit 8 is provided
to a second input terminal of the OR circuit 11c. The output signal
Dout of the overdischarge detection circuit 9 is provided to the
discharge control switch 4 and a second input terminal of the NOR
circuit 15. The output signal (first charge control signal) Cout of
the OR circuit 11c is provided to the first charge control switch
5a. The output signal (second charge control signal) PreCout of the
NOR circuit 15 is provided to the second charge control switch
5b.
The operation of the charge/discharge control circuit 200 of the
second embodiment will now be discussed.
In an overdischarged state, the erroneous functioning prevention
circuit 13 provides the inverter 14 with a high erroneous
functioning prevention signal. The inverter 14 inverts the high
erroneous functioning signal and thereby causes the erroneous
functioning signal to go low. The low erroneous functioning
prevention signal is then provided to the OR circuit 11c and the
NOR circuit 15. The output signal of the overcharge detection
circuit 8 goes high, and the output signal Dout from the
overdischarge detection circuit 9 goes low.
As a result, the first charge control signal Cout provided by the
OR circuit 11c to the first charge control switch 5a goes high, and
the second charge control signal PreCout provided by the NOR
circuit 15 to the second charge control switch 5b goes high. This
causes both of the first and second charge control switches 5a, 5b
to be deactivated. In this state, the discharge control switch 4 is
activated. Hence, charging is prohibited and discharging is
performed.
In a normal state, the erroneous functioning prevention signal
output by the erroneous functioning prevention circuit 13 is high.
The signals output by the overcharge detection circuit 8 and the
overdischarge detection circuit 9 are both low. In this state, the
discharge control switch 4 and the first discharge control switch
5a are activated. Thus, discharging or charging (constant current
charging) is performed.
In an overdischarged state, the signal output by the overcharge
detection circuit 8 is low, and the signal output by the
overdischarge detection circuit 9 is high. A high discharge control
signal Dout deactivates the discharge control switch 4. Hence,
discharging is prohibited.
As a way of example, if the threshold voltage Vth3 is set at 4V,
the discharge reference voltage VTL is set at 2.5V, and each of the
cell voltages is 1V, the power supply voltage Vcc is 3V. Since this
power supply voltage (3V) is lower than the threshold voltage Vth3
(4V), the erroneous functioning prevention signal generated by the
erroneous functioning prevention circuit 13 is low. This low
erroneous functioning prevention signal is provided to the inverter
14.
The inverter 14 inverts the low erroneous functioning signal and
thereby causes the erroneous functioning prevention signal to go
high. The high erroneous functioning prevention signal is then
provided to the OR circuit 11c and the NOR circuit 15. As a result,
the first charge control signal Cout generated by the OR circuit
11c goes high, and the second charge control signal PreCout
generated by the NOR circuit 15 goes low. Thus, only the second
charge control switch 5b is activated. Accordingly, charging is
performed with a charging current flowing through the resistor R.
Thus, constant voltage charging is performed.
If the threshold voltage Vth3 is set at 4V, the discharge reference
voltage VTL is set at 2.5V, and each of the cell voltages is 2V,
the power supply voltage Vcc is 6V. Since this power supply voltage
(6V) is higher than the threshold voltage Vth3 (4V), the erroneous
functioning prevention signal generated by the erroneous
functioning prevention circuit 13 is high. The high erroneous
functioning prevention signal is then provided to the inverter 14.
In this state, the first charge control signal Cout generated by
the OR circuit 11c and the second charge control signal PreCout
generated by the NOR circuit 15 are both low. Hence, both of the
first and second charge control switches 5a, 5b are activated.
Charging in this state is performed with a charging current flowing
through the first charge control switch 5a that has a low
impedance. Thus, constant current charging is performed, regardless
of the overdischarged state.
The charge/discharge control circuit 120 of the second embodiment
has the advantages described as follows. (1) In an overdischarged
state, the erroneous functioning prevention circuit 13
simultaneously activates the first and second charge control
switches 5a, 5b. This prevents momentary deactivation of the first
and second charge control switches 5a, 5b when switching from
constant voltage charging to constant current charging. Thus, the
charging voltage does not increase. (2) In an overdischarged state,
unless the power supply voltage Vcc is extremely close to 0V,
constant current charging may be performed by the first charge
control switch 5a. In other words, even in an overdischarged state,
the first charge control switch 5a is allowed to perform constant
current charging so long as the charging voltage is high enough to
prevent heating of the first charge control switch 5a. Accordingly,
charging is performed within a short time period without heating
the control switches. (3) By changing the threshold voltage Vth3 of
the erroneous functioning prevention circuit 13, the switching of
the first and second charge control switches 5a, 5b may be
controlled as desired. [Third Embodiment]
FIG. 4 is a schematic circuit diagram of a charge/discharge control
circuit 300 according to a third embodiment of the present
invention. The charge/discharge control circuit 300 makes use some
of the components in the charge/discharge control circuit 200 of
the second embodiment, as identified by those labeled with
identical numerals. The charge/discharge control circuit 300
includes an output circuit 20B, which is configured by adding an OR
circuit 11d to the output circuit 20A incorporated in the
charge/discharge control circuit 200 of the second embodiment.
The OR circuit 11d is connected to the NOR circuit 15 and the
overcharge detection circuit 8. That is, the output signal of the
overcharge detection circuit 8 is provided to a first input
terminal of the OR circuit 11d, and the output signal of the NOR
circuit 15 is provided to a second input terminal of the OR circuit
11d. The OR circuit 11d generates and provides the second charge
control signal PreCout to the second charge control switch 5b.
The operation of the charge/discharge control circuit 300 of the
third embodiment will now be discussed.
In a normal state or an overdischarged state, the charge/discharge
control circuit 300 functions in the same manner as the
charge/discharge control circuit 200 of the second embodiment.
When any one of the cells 2a, 2b, 2c is in an overcharged state,
the output signal of the overcharge detection circuit 8 is high.
This high output signal is provided to the OR circuits 11c, 11d.
Consequently, the output signal Cout of the OR circuit 11c is high
and provided to the first charge control switch 5a. The output
signal PreCout of the OR circuit 11d is high and provided to the
second charge control switch 5b. Thus, the first and second charge
control switches 5a, 5b are both deactivated.
The charge/discharge control circuit 300 of the third embodiment
provides the same advantages as the charge/discharge control
circuit 100 of the first embodiment, or the charge/discharge
control circuit 200 of the second embodiment, as described
above.
It should be apparent to those skilled in the art that the present
invention may be embodied in many other alternative forms without
departing from the principle and the scope of the invention.
Particularly, it should be understood that the present invention
may be embodied in the following forms.
The output circuits 20, 20A, 20B in FIGS. 2-4 are not limited to
those exemplary configurations illustrated in the above
embodiments. In other words, these output circuits may be altered
as desired, so long as the predetermined first and second charge
control signals Cout, PreCout are provided.
The present examples and embodiments are to be considered as
illustrative and not restrictive, and the invention is not to be
limited to the details given herein, but may be modified within the
scope and equivalence of the appended claims.
* * * * *