Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – Having variable number of cells or batteries in series
Reexamination Certificate
2004-03-04
2004-11-09
Tibbits, Pia (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
Having variable number of cells or batteries in series
C320S146000
Reexamination Certificate
active
06815929
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of battery charging circuits, and particularly to methods of blocking current flow from a charged battery when the charging circuit is not in use.
2. Description of the Related Art
A Conventional battery charging circuit is shown in
FIG. 1. A
charger supply voltage V
supply
is provided to the charging circuit at a charger voltage input terminal
8
. The charging circuit provides a charging current to a rechargeable battery
10
connected to a battery connection terminal
12
; the voltage across the battery is designated V
bat
. A pass device, typically a PNP transistor Q
1
, is connected between V
supply
and battery connection terminal
12
, and conducts the charging current in response to a control signal applied at Q
1
's base. The charging circuit typically includes an isolation diode
14
connected in series between V
supply
and Q
1
(with the voltage on the cathode side of diode
14
being “V
chg
”), and a blocking diode
16
connected in series between Q
1
and battery connection terminal
12
. Control circuits such as a charge current sense circuit
18
, a battery voltage sense circuit
20
, and a pass device drive circuit
22
complete the charging circuit.
Isolation diode
14
is necessary to prevent damage to the charging circuit in case a voltage having a reverse polarity is inadvertently applied to charger voltage input terminal
8
, and to prevent V
bat
from being present on the input terminal when V
supply
is not present.
Blocking diode
16
is needed to prevent reverse conduction of the pass device. Reverse conduction could occur, for example, if the charging voltage never exceeds the battery voltage. If this occurs, battery voltage V
bat
, through the parasitic base-collector diode of a PNP pass device (or body diode of a MOSFET pass device), could bias up the charger control circuits; thereby gradually draining the battery.
A conventional battery charging circuit also typically includes a means of holding the pass device off when the charger is not in use. This may be accomplished, for example, by connecting a resistor
24
or a switch between Q
1
's base and emitter. There are several situations in which it is important to hold Q
1
off when the charger is not in use. For example, in some applications—charging a lithium ion (Li+) battery, for example—when the charger must not continue to provide current to the battery after charging is complete. The pass device is held off to effect this. Also, the charger must not attempt to charge a battery when V
chg
<V
bat
. Isolation diode
14
would prevent V
bat
from appearing on charger voltage input terminal
8
, but the system should also hold Q
1
off to prevent control circuits on the V
supply
side of the charging circuit from draining the battery. This is also true if V
supply
is not present at all.
The configuration shown in
FIG. 1
suffers from several drawbacks, however. The charging circuit requires two diodes, which can be costly. In addition, V
supply
must be at least two diode drops above V
bat
to maintain charging. This increases power consumption, and forces V
supply
to be at a higher voltage than might be desirable.
SUMMARY OF THE INVENTION
A system and method for battery isolation in a charging system is presented, which overcomes the problems noted above. The invention enables the charger circuit's current consumption to be zero when not in use, eliminates the need for a blocking diode, and reduces power consumption and charger supply voltage required to maintain charging.
The present battery charging system and method include a charger voltage input terminal for connection to a charger supply voltage V
supply
, an isolation diode connected to the charger voltage input terminal, and a PNP pass transistor connected in series between the isolation diode and a battery connection terminal and which conducts a charging current in response to a drive signal applied to its base; the pass transistor side of the diode is at a voltage V
chg
. The current charges a battery connected to the battery connection terminal; the voltage across the battery is designated V
bat
.
The system also includes a first switch arranged to couple the pass transistor's base to V
chg
when closed in response to a first control signal, and a second switch arranged to couple the base to V
bat
when closed in response to a second control signal. A controller provides the first and second control signals to the switches. The first switch is closed and the second switch is opened when V
chg
>V
bat
, such that the pass transistor's base-collector junction blocks current from a charger supply voltage from flowing through the pass transistor when the charger is not in use. The second switch is closed and the first switch is opened when V
bat
>V
chg
such that the pass transistor's base-emitter junction blocks current from a battery connected to the battery connection terminal from flowing through the pass transistor when the charger is not in use. Thus, when the system is not charging or the charging supply is low or not present, the PNP acts as a blocking diode to prevent the battery voltage from appearing on the charger supply side of the pass transistor and providing bias to the charger controller circuitry.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 5136231 (1992-08-01), Faulk
patent: 5315253 (1994-05-01), Alexandres et al.
Analog Devices “High Frequency Switch Mode Dual Li-Ion Battery Chargers, ADP3801/ADP3802”, pp. 1-20 (1998).
Analog Devices “GSM Power Management System, ADP3408”, pp. 1-20, (2002).
Dagan Marc E.
Slavnov Sergei
Analog Devices Inc.
Koppel, Jacobs Patrick & Heybl
Tibbits Pia
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