Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2000-12-29
2004-10-12
Huynh, Kim (Department: 2182)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S093800, C361S018000, C361S090000, C320S134000
Reexamination Certificate
active
06804100
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a protection circuit, integrated circuit and host device for the protection of batteries.
BRIEF DESCRIPTION OF RELATED DEVELOPMENTS
At present, it is very important for the users of different electronic devices that the electronic device can be used as long as possible before it is necessary to charge the battery. Furthermore, especially in portable devices, the size of the battery is significant, and thus it is not necessarily reasonable to reduce the need to charge the battery by increasing the capacity of the battery. Therefore, especially in wireless communication devices and in portable computers, the use of Lithium-based batteries, such as Li-ion (Lithium ion), Li-poly (Lithium polymer) or Li-metal (Lithium metal) batteries has become increasingly common.
A Li-ion battery is considerably lighter and has a somewhat larger capacity than NiCd and NiMH batteries, and thus considerably longer operating times are attained without increasing the size of the battery. On the other hand, the manufacture of a Li-ion battery is far more expensive than the manufacture of NiCD and NiMH batteries. Recharging of a Li-ion battery does not require that the battery is (fully) discharged. On the other hand, the longest possible service life is obtained from NiCD batteries if the battery is discharged completely before recharging. In Li-ion batteries self-discharging is less than e.g. in NiCD batteries (approximately 1 to 2% per month), and thus an unused Li-ion battery may retain its charge for a comparatively long time. In subzero temperatures, the operation of a Li-ion battery is similar to that of NiMH batteries, in other words it is not particularly good.
An advantage of the Li-poly battery is that it is easier to manufacture and it is possible to make the battery smaller and lighter than the Li-ion battery. A Li-poly battery can be shaped quite freely. The self-discharge rate of a Li-poly battery is even smaller than that of a Li-ion battery.
Li-ion and Li-poly batteries should be protected from over-voltage and under-voltage by means of a rather complex protection circuit, because otherwise the cells of the battery can be damaged so that they become unusable. The most important rule when charging Li-ion and Li-poly batteries is to keep the charging voltage as constant as possible during the entire charging process. Normally, the charging voltage is either approximately 4.1 V or approximately 4.2 V. The purpose of the protection circuit is to interrupt the charging process when a particular voltage is attained, for example 0.15 V over the charging voltage. After the operation of the over-voltage protection circuit, the battery can nevertheless be discharged. When the battery has been discharged, it can be charged again. In addition to too high a voltage (over-voltage), Li-ion and Li-poly batteries are particularly sensitive to too low a voltage (under-voltage) and to over-current when they are charged or discharged. In these cases, the purpose of the protection circuit is to interrupt the discharging or charging of the battery.
In order to implement the functionality of the protection circuit, the protection circuit should advantageously contain at least a control block and two switch means such as two field-effect transistors (FET), connected in series. One field-effect transistor protects the battery from over-voltage and the other from under-voltage. By means of this arrangement of two field-effect transistors it is possible to enable the battery to be discharged after an over-voltage condition and to be charged after an under-voltage condition.
Because of parasitic diodes internal to the field-effect transistor, current can be passed in the opposite direction through the field-effect transistor from the drain to the source when the field-effect transistor is in a high impedance state. This enables a battery protected by the protection circuit to be discharged after an over-voltage condition and to be recharged after an under-voltage condition.
In a particular prior art solution, a low impedance resistance is connected in series in the voltage supply line of the battery. The voltage across this resistance is measured, wherein an over-current condition can be detected when the voltage exceeds a predetermined limit. The use of components that increase the impedance is not desirable, because they reduce the voltage supplied to the electronic device and unnecessarily increase power consumption. Thus, the operating time of the device using the battery is shortened.
In another prior art solution, an over-current condition is detected in such a way that the voltage across the drain and source of the field-effect transistor is measured. Additionally, the value of the resistance between the drain and source, the so-called conducting state drain-to-source resistance R
ds(on)
, is estimated. In prior art solutions, this drain-to-source resistance is presumed constant. Thus, an estimate of the current is obtained by dividing the voltage across the drain and the source of the field-effect transistor by the drain-to-source resistance. One disadvantage of this solution is that the drain-to-source resistance is not constant, but changes as the gate voltage of the field-effect transistor changes. Moreover, the drain-to-source resistance depends to a considerable degree on the temperature of the field-effect transistor.
In prior art solutions over-current conditions that occur during charging are not monitored, but the battery is protected only e.g. by a fuse. Charging currents are usually smaller and easier to predict than currents that occur when the battery is being discharged, and consequently, over-current during charging has not been considered a problem. However, it is not impossible that an over-current condition can also arise during charging, for example due to a defective charging device. Thus, it is also advantageous to protect the battery from over-current during charging.
Patent application JP 10223260 discloses a protection circuit for a battery, in which the aim is to compensate the effect of temperature when measuring the current, so that more reliable measurement results are obtained. The protection circuit of the invention according to JP 10223260 comprises an over-voltage and under-voltage detection unit
2
(FIG.
1
), a charging control block
3
, an over-current protection block
4
, a discharging-side overheating protection block
5
, a charging-side overheating protection block
6
and two field-effect transistors FET
1
, FET
2
.
The purpose of the over- and under-voltage detection unit
2
is to detect when the voltage of the cells
1
a
,
1
b,
1
b
of the battery is too high or too low. When a load (not shown), for example an electronic device, is connected across connectors P
1
, P
2
, in other words the battery is discharged, the over-voltage or under-voltage detection unit
2
monitors each cell
1
a
,
1
b
,
1
c
of the battery separately to detect an under-voltage state. If the voltage of any cell is lower than a certain first threshold value, the over-voltage and under-voltage detection unit sets line P into a first logical state, which results in the first field-effect transistor FET
1
becoming non-conductive, whereupon discharging of the battery is terminated.
When a charging device (not shown) is connected across connectors P
1
, P
2
, i.e. the battery is charged, the over- and under-voltage detection unit
2
monitors each cell
1
a
,
1
b
,
1
c
of the battery separately to detect an over-voltage state. If the voltage of any cell exceeds a certain second threshold value, the over-voltage and under-voltage detection unit sets line L into a second logical state, which results in the second field-effect transistor FET
2
becoming non-conductive, whereupon charging of the battery is terminated.
The purpose of the charging control block
3
is to control the second field-effect transistor FET
2
in such a way that when line L is in the second logical state, the second field-effect transistor FET
2
does not
Huynh Kim
Nokia Mobile Phones Ltd.
Perman & Green LLP
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