Electricity: battery or capacitor charging or discharging – Battery or cell charging
Reexamination Certificate
2000-02-10
2001-10-09
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
Reexamination Certificate
active
06300744
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to battery chargers for portable electronic devices such as laptop computers and, in particular, to a battery charger that is relatively inexpensive to construct and yet substantially maximizes the power delivered to the battery when the electronic device is operating.
BACKGROUND OF THE INVENTION
Portable electronic devices such as cellular phones and laptop computers are in widespread use. A key feature of these devices is that they are battery operated, which allows them to be used in locations where AC power is not available. It is also important that the batteries be capable of powering the device for long periods of time and of being recharged in minimal periods, to maximize the time during which the device is available for use.
A particular problem occurs when the device is operated while its battery is being recharged. The AC/DC converter, commonly called an “AC adapter”, that is used to supply DC power to the device is capable of delivering a certain amount of power. The battery charger, typically a DC/DC stepdown converter, is designed to deliver a fixed constant current to the battery, the magnitude of the current being determined by such factors as the battery characteristics, the AC adapter power level, and the charge time requirement. Considerations of size, weight and cost prevent the AC adapter from being made large enough to supply all of the power necessary to both operate the device and charge the battery simultaneously. Thus, some technique must be employed to assure that the device has sufficient power to operate in this situation. Ideally, the device would receive whatever power is necessary to operate it and all of the remaining power that the AC adapter is capable of delivering would be used to recharge the battery.
Several techniques have been used to address this problem. The simplest is illustrated in FIG.
1
. Shown in
FIG. 1
are an AC adapter
10
, which could be any of a variety of AC/DC converters that are available on the market. The input terminal of AC adapter
10
is connected to a power main. The output terminal of AC adapter
10
is connected to a load
11
, which could be, for example, a laptop computer, and through a battery charger
12
(a DC/DC stepdown converter) to a rechargeable battery
13
, which powers load
11
when AC adapter
10
is not being used.
Since the voltage of rechargeable battery
13
varies from, for example, 2.7 volts when it is fully discharged to 4.2 volts when it is fully charged, battery charger
12
in the form of a DC/DC stepdown converter is necessary to ensure that the voltage supplied to the input terminal of battery
13
is at the proper level. The output voltage of AC adapter
10
is fixed at, for example, 5 volts ±5%. The structure of battery charger
12
is well known to those skilled in the art and will not be described in detail here. See, e.g., P. T. Krein,
Elements of Power Electronics,
Oxford University Press (1998). A driver
120
supplies signals over the lines designated DH and DL to the gate terminals, respectively of a high-side MOSFET
121
and a low-side MOSFET
122
, which are connected in series between the input terminal of battery charger
12
and ground. The signals from driver
120
turn on MOSFETs
121
and
122
on and off in sequence, MOSFET
121
being on when MOSFET
122
is off, and vice-versa. To prevent current shoot-through from AC adapter
10
to ground, there is a “break-before-make” interval between the instant that one of MOSFETs
121
or
122
is turned off and the other MOSFET is turned on. The current delivered by battery charger
12
is directly related to the percentage of the time that MOSFET
121
is turned on (sometimes referred to as the duty cycle).
The voltage at the common node
123
between MOSFETs
121
and
122
therefore alternates between the output voltage of AC adapter
10
and ground. This voltage is fed to a series LC circuit containing inductor
124
and capacitor
125
, which alternately store and deliver energy such that a generally constant current is generated on line
126
to battery
13
.
Battery charger
12
also contains circuitry which allows the size of the current delivered to battery
13
to be set. A current detector
127
connected to line
126
generates a signal indicating the size of the current in line
126
. This signal is delivered over a feedback loop
128
to a charge current control
129
. Charge current control
129
has a charge current set input which is set to the desired current to be delivered to battery
13
. Charge current set
126
compares the set current against the current detected by detector
127
and delivers an error signal which causes driver to adjust the duty cycle of the pulses delivered to MOSFETs
121
and
122
in such a way that the current on line
126
equals the set current.
In the circuitry shown in
FIG. 1
the condition of the load
11
being operative is detected and the driver
120
and hence battery charger
12
is simply disabled in this situation. This solves the problem, but the battery is not charged at all whenever the load is on, thereby forfeiting valuable charge time that might be available.
An alternative solution is illustrated in FIG.
2
. Here, instead of disabling the driver
120
, the current setting in charge current control
129
is adjusted such that battery charger
12
delivers a reduced fixed current to battery
13
when the load
11
is operating. While this allows some charging to take place while the load is operative, the reduced fixed current delivered to the battery must be set based on the assumption that the load is drawing full power. Thus, when the load is not drawing full power, the power delivered to the battery is lower than AC adapter
10
is capable of delivering.
A more sophisticated solution is illustrated in
FIG. 3. A
current sense resistor
30
is connected to the output terminal of AC adapter
10
. A current sense unit
32
reads the voltage drop across current sense resistor
30
and sends a signal representing the magnitude of the current to a compare unit
33
, where the signal is compared against a signal representing the rated current of AC adapter
10
. If the current through resistor
30
is greater than the rated current, less a safety margin, charge current control
129
reduces the current delivered by battery charger
12
to battery
13
. With this arrangement a considerable safety margin must be applied and thus not all the available power is actually used. Moreover, the circuitry must be set for the rated output current of each AC adapter with which it is used; and sensing the size of a current, particularly at the high output voltages of many AC adapters, tends to be expensive. U.S. Pat. No. 5,939,862 describes an arrangement somewhat similar to the one shown in FIG.
3
.
SUMMARY OF THE INVENTION
According to this invention, an output terminal of a DC power source such as an AC/DC converter is connected both to an electronic device and to a battery charger. The battery charger contains a current control unit for regulating an magnitude of a current at an output terminal of the battery charger. A rechargeable battery is connected to the output terminal of the battery charger. A voltage detector is connected to the output terminal of the DC power source, and an output terminal of the voltage detector is connected to the current control unit. Thus, the voltage detector supplies to the current control unit a signal representing the magnitude of a voltage at the output terminal of the DC power source.
So long as the DC power source is operating within its rated power capability, the voltage at its output is at or within a predetermined margin of a predetermined output voltage. As the power output of the DC power source exceeds its rated power, the voltage at its output terminal begins to fall. This drop in voltage is sensed by the voltage detector and is used to reduce the current output by the battery charger to the battery until the output voltage of the DC power source is again within th
Siliconix incorporated
Skjerven Morrill & MacPherson LLP
Steuber David E.
Tso Edward H.
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