WO2012079220A1

WO2012079220A1 - Method and apparatus for providing under voltage protection for lifepo4 battery

Info

Publication number: WO2012079220A1
Application number: PCT/CN2010/079751
Authority: WO
Inventors: Allen Li; Changjiang Zhang; Tao Zhang; Qi. M Chen; Carol Wu
Original assignee: Nokia Corporation
Priority date: 2010-12-14
Filing date: 2010-12-14
Publication date: 2012-06-21

Abstract

A method and an apparatus for providing under voltage protection for a LiFePO₄ battery (320). The apparatus is coupled to the battery (320) at a first connection pin (Vbat) and a second connection pin (GND), and includes a voltage divider circuit configured to generate a variable voltage in response to the variation of the voltage of the battery (320), a diode (350) configured to shut down the voltage divider circuit when the voltage drops below a predefined minimum voltage, and a first transistor (370) configured to monitor the variable voltage of the voltage divider circuit and to set a second transistor (360) to be on or off. The second transistor (360) is configured to cut off the connection between the first connection pin (Vbat) and an outlet pin (345) to prevent further leakage current draining from the battery (320). An integrated circuit including the under voltage protection circuit coupled to the battery (320) at the first connection pin (Vbat) and the second connection pin (GND) is applied in a cell phone.

Description

METHOD AND APPARATUS FOR PROVIDING UNDER VOLTAGE

PROTECTION FOR LiFeP0₄ BATTERY

TECHNICAL FIELD

[0001] The present application relates generally to protection circuits and more specifically to methods and apparatuses for providing undervoltage protection for LiFeP04 batteries of portable devices.

BACKGROUND

[0002] Batteries can be classified in two categories according to their use.

Primary batteries are used until they are exhausted and they are discarded. Secondary batteries also called "rechargeable batteries" are used and can be recharged after a discharge by an electric current passed in the opposite direction of the discharge flow. These second batteries are commonly used in cars, domestic appliances and electronic devices.

[0003] Electronic devices such as computers, laptops, cameras, and cell phones often use lithium batteries. But lithium batteries are expensive, not completely safe and not enough powerful for high power tools. Structurely, the anode of a lithium battery is the source of lithium ions. Therefore, the cathode is the sink of the lithium ions. Since good ion conductors are not good often good electron conductors, there is a need to find other types of materials which can replace the lithium batteries.

[0004] In order to avoid damaging the battery during the use which can render it unusable, the battery needs to be protected from over-voltage as well as under-voltage. Without a protection from under voltage, the battery is drained exhausted after the phone or electronic device is powered off for a certain time due to the off state current leakage. Once the phone or electronic device is completely exhausted, if the user decides to power on or to charge the battery, the phone or electronic device remains idle for some time which can be 10 to 20 minutes with the charge plugged in. Therefore, there is a need for an under voltage protection of the battery in order to mitigate the under performance of the battery in addition to the cost and the degradation of the user experience due to the battery low voltage discharge.

SUMMARY [0005] Various aspects of examples of under voltage protection according to the present invention are set out in the claims.

[0006] According to a first aspect of the present invention, an under voltage protection apparatus is disclosed. The protection apparatus is coupled to a battery cell of LiFePC"4 type at a first pin (Vbat) and a second pin (GND) and it comprises: a voltage divider circuit configured to generate a varying voltage responsive to the variations of the voltage of the battery cell; a diode configured to shut down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and a first transistor configured to monitor the varying voltage of the voltage divider circuit and to set a second transistor to on or off; wherein the second transistor is configured to cut off the connection between the first pin (Vbat) and an outlet pin to prevent further leakage current drain from the battery cell.

[0007] According to a second aspect of the present invention, the voltage divider circuit comprises a set of resistors wherein a first resistor having a first end coupled to the diode and a second end coupled to a second resistor; and wherein the second resistor has a first end coupled to the second pin (GND) and a second end coupled to the first resistor.

[0008] According to a third aspect of the present invention, the voltage divider circuit further comprises a third resistor having a first end coupled to the first transistor and a second end coupled to the second transistor; and a fourth resistor having a first end coupled to the first pin (Vbat) and a second end coupled to the third resistor.

[0009] According to a fourth aspect of the present invention, the first transistor is coupled to a first junction pin between the first and second resistors for monitoring the varying voltage at a second junction pin between the third and the fourth resistors by changing the voltage at the first junction pin.

[0010] According to a fifth aspect of the present invention, the first transistor has a base pin coupled to the first junction pin; a collector pin coupled to third resistor; and an emit pin coupled to the second pin (GND).

[0011] According to a sixth aspect of the present invention, the voltage at the first junction pin is configured to be adjusted by varying values of the first and second resistors so as to set a voltage for the base voltage of the first transistor.

[0012] According to a seventh aspect of the present invention, the second transistor is coupled to the second junction pin between the third and fourth resistors, and between the first pin (Vbat) and the outlet pin. [0013] According to an eighth aspect of the present invention, the second transistor has a base pin coupled to the second junction pin; a collector pin coupled to the outlet pin; and an emit pin coupled to the first pin (Vbat).

[0014] According to a ninth aspect of the present invention, an apparatus is disclosed comprising: a voltage source circuit comprising a battery cell of LiFeP0₄ type for generating a voltage between a first pin (Vbat) and a second pin (GND); and a protection circuit coupled to the first and second pins of the voltage source circuit, comprising: a voltage divider circuit configured to generate a varying voltage responsive to the variations of the voltage of the battery cell; a diode configured to shut down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and a first transistor configured to monitor the varying voltage of the voltage divider circuit and to set a second transistor to on or off; and wherein the second transistor is configured to cut off the connection between the first pin (Vbat) and an outlet pin to prevent further leakage current drain from the battery cell.

[0015] According to a further aspect of the present invention, an integrated circuit including a protection circuit coupled to a battery cell of LiFeP0₄ type at a first pin (Vbat) and a second pin (GND) is disclosed. The integrated circuit comprises:

means for generating a varying voltage responsive to the variations of the voltage of the battery cell; means for shutting down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and means for monitoring the varying voltage of the generating means and to set a switching means to on or off; and wherein the switching means is configured to cut off the connection between the first pin (Vbat) and an outlet pin to prevent further leakage current drain from the battery cell.

BRIEF DESCRIPTON OF THE DRAWINGS

[0016] Fig.1 shows a block diagram of the implementation of the under voltage protection in the phone or electronic device circuit according to the present invention.

[0017] Fig.2 shows in detail an example of embodiment of the circuitry of the under voltage protection in connection to the battery circuitry.

DESCRIPTION OF THE DRAWINGS

[0018] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings.

[0019] For traditional battery, the under voltage protection (UVP) is embedded into the battery which makes the overall cost high due to a dedicated printed wiring board (PWB) or a dedicated printed circuit board (PCB). However, according to the present invention, the under voltage protection circuit is shifted to the phone or the electronic device PWB or PCB in order to reduce the cost and the complexity of the integration of the protection circuit into the battery circuit. Accordingly, it is possible to implement the under voltate protection in a single circuit in the phone or the electronic device PWB or PCB which can be made for instance in an application specific integrated circuit (ASIC).

[0020] FIGURE 1 illustrates a schematic block diagram of an example of implementation of the under voltage protection in the phone or electronic device circuit. According to the present invention, the size of the under voltage protection circuitry is limited in order to fit in the PWB or PCB of the phone or the electronic device. As a matter of fact, a handheld portable device such a mobile phone contains more and more features such that the electronic circuitry embedded therein has to be as tiny as possible.

[0021] Furthermore, the under voltage protection circuitry has to be as stable as possible at very low voltage below 3V and preferably between 2V and 3V. This stability characteristic needs to be operational in connection with the type of battery. The LiFeP0₄ battery appears to present the stability characteristics required for tiny circuitry as well as the cost and safety characteristics required for portable electronic devices such as cell phones or mobile phones.

[0022] As previously mentioned, there is a need to use other types of batteries or types of materials which are cheap and which can be good electron conductors. The cathode material is another class of materials which has the advantage of lowering the cost. More particularly, LiFeP0₄ is an iron-based cathode material which is not only

environmental safe but also cheap because of its abundance. Futhermore, while the discharge potential of the lithium is around 4V, the discharge potential of the LiFeP0₄ is around 3V or even lower than 3V with great stability upon charge/discharge.

[0023] The phosphate used in the LiFeP0₄ battery improves the stability characteristics required for the electronic devices and for the cell phones in particular. Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing to avoid explosion or fire. Furthermore, when abuse does occur, the phosphate based cathode material will not burn and is not prone to thermal runaway. For normal Li-ion battery, if the protection circuit from the battery, the safety risk is very high and there is no guarantee that an explosion, a fire or a leakage will not occur in extreme conditions.

[0024] Moreover, phosphate chemistry also offers a longer life cycle and they significantly reduce the drawbacks of the Cobalt chemistry, particularly the cost, safety and environmental characteristics. Once more the trade off is a reduction of at least 14% in energy density, but higher energy variants are being explored.

[0025] A large range of new environmentally friendly cathode active materials based on Lithiated transition metal phosphates for Lithium-ion applications have been produced during the recent developments.

[0026] Furthermore, the operating performance of the cell can also be "tuned" by changing the identity of the transition metal. This allows the voltage as well as the specific capacity of these active materials to be regulated. According to an examplary embodiment of the present invention, cell voltages in the range 2.1 to 5 Volts are used. This

characteristic particularly fits to the type of use for the under voltage protection of electronic devices such as a cell phone according to the present invention. [0027] FIGURE 1 shows a schematic block diagram of an example of implementation of the under voltage protection circuit in the phone or electronic device 300-P. According to the present invention, the under voltage protection circuit is located in the phone or electronic device PWB or PCB and is connected to the battery pack 300-B with two connection points. The under voltage protection works when the battery pack 300-B outlet voltage drops too low and shut down the leakage current. After the under voltage protection resume working, the battery pack 300-B will be active again when it is coupled to the charging device 200.

[0028] FIGURE 2 shows a detailed examplary implementation of the under voltage protection according to the present invention. The reference numeral 300-B and 300-P refer to a circuit for detecting a low level of voltage produced by a battery cell 320 which may be a LiFeP04 battery wherein it is desired to produce a signal or alarm when the battery begins to wear out and voltage lower than 2.50V at cell output. Battery pack 300-B is connected with Phone pack via 3 connection pins: a Vbat pin 311 , a BSI pin 333 and a GND pin 332.

[0029] Inside the Battery pack 300-B, battery cell 320 is connected between the GND pin 332 and a PTC resistor 310. Also a BSI resistor 330 is connected between the BSI pin 333 and junction point 331 for battery type identification.

[0030] Inside Phone pack 300-P, a voltage divider network consisting of a resistor 340-1 or Rl which has one end connected to the junction point 348 and the other end connected to the diode 350. Another resistor 340-2 or R2 has one end connected to junction point 348 and the other end connected to the source of reference potential junction point 341. The cathode of diode 350 is shown connected to the junction point 347 which is the output of Vbat pin. Diode 350 is placed in the circuit for purpose of shutting down the voltage divider network when voltage at junction point drops to below 2.35 V. The voltage divider formed by resistors 340-2 and 340-1 produces a voltage at point 348 which will vary with the variations of the voltage from battery pack 300-B and the working mode of diode 350. The level of voltage at point 348 can be adjusted by changing the value of resistors 340-2 and 340-1 so as to set the desired voltage for the base voltage of a first transistor 370. In the figure, the first transistor 370 has a base terminal connected to junction point 348, a collect terminal connected to resistor 340-3 and a emit terminal connected to GND 342. The purpose of the first transistor 370 is used for controlling the voltage on junction point 344 by changing the voltage on point 348 and furtherly set a second transistor 360 working mode switch on or off. As shown the second transistor 360 has a base terminal connected to junction point 344, a collect terminal connected to point 345 and a emit terminal connected to junction point 346. Also a resistor 340-4 or R4 is used between junction points 344 and 346.

[0031] During the operations, when the battery cell output voltage decreases to 2.50V, the voltage at junction point 347 will be around 2.35 V due to the voltage drop on battery connector, the PTC resistor 310 and the PWB trace. Diode 350 is selected to guarantee that the voltage on junction point 348 is low enough when voltage on junction point 347 is around 2.35V. This further impacts the voltage on the collect terminal of the first transistor 370 and will terminate the second transistor 360 to cut off the connection between junction points 346 and 345. When the second transistor 360 is off, the battery pack 300-B discharge to phone will be stopped and no leakage current drain from battery cell 320.

[0032] According to an exemplary embodiment of the present invention, the PTC 310 (Positive Temperature Coefficient Resistor) is implemented in the battery pack 300-B between the battery cell 320 and the first connection pin Vbat 311, and acts as a type of circuit breaker. In general, the resistance increases dramatically as the PTC threshold temperature is exceeded due to the load current passing through it. This presents a high resistance, which in effect opens the circuit in order to protect the device in question in the event of an under voltage condition or an over current condition. Basically, the PTC is well suited for battery protection and more particularly for under voltage protection for handheld electronic devices and cell phones.

[0033] In a particular implementation, the values of the resistors are: Rl = 5.1 K; R2 = 4.7 K; R3 = 10 K; R4 = 50 K. Any other values may also be used. It should be kept in mind that these values are giving as examples and it should not be used to limit the scope of protection of the claimed invention.

[0034] In case an Si transistor is implemented, the relation between the resistors Rl, R2, R3 and R4 in the voltage divider network or circuit will be to keep:

[0035] R4 > 5*R3, to guarantee the Vb of the second transistor 360 is less than 0.6V even when the voltage at the junction point 347 reaches 3.6V.

[0036] R2 > 0.75*R1, to guarantee Vb of the first transistor 370 is higher than

0.7V even when the voltage at the junction point 347 reaches 2.35V.

[0037] The sum of resistors R4+R3 is enough significant to prevent leakage current when both transistors work. [0038] The sum of resistors R1+R2 is enough significant to prevent leakage current when both transistors work.

[0039] Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer, with one example of a computer described and depicted in FIGURE 1 or 2. A computer- readable medium may comprise a computer-readable storage medium that may be any media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0040] If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

[0041] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

[0042] It is also noted herein that while the above describes example

embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.

Claims

A protection apparatus coupled to a battery cell of LiFeP04 type at a first pin and a second pin, comprising: a voltage divider circuit configured to generate a varying voltage responsive to the variations of the voltage of the battery cell; a diode configured to shut down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and a first transistor configured to monitor the varying voltage of the voltage divider circuit and to set a second transistor to on or off; wherein the second transistor is configured to cut off the connection between the first pin and an outlet pin to prevent further leakage current drain from the battery cell.

The apparatus of claim 1 wherein the voltage divider circuit comprises a set of resistors comprising at least: a first resistor having a first end coupled to the diode and a second end coupled to a second resistor; and wherein the second resistor has a first end coupled to the second pin (GND) and a second end coupled to the first resistor.

The apparatus of claims 2 wherein the voltage divider circuit further comprises: a third resistor having a first end coupled to the first transistor and a second end coupled to the second transistor; and a fourth resistor having a first end coupled to the first pin (Vbat) and a second end coupled to the third resistor.

4. The apparatus of claim 3 wherein the first transistor is coupled to a first junction pin between the first and second resistors for monitoring the varying voltage at a second junction pin between the third and the fourth resistors by changing the voltage at the first junction pin.

5. The apparatus of claim 4 wherein the first transistor has: a base pin coupled to the first junction pin; a collector pin coupled to third resistor; and an emit pin coupled to the second pin (GR ).

6. The apparatus of claim 5 wherein the voltage at the first junction pin is configured to be adjusted by varying values of the first and second resistors so as to set a voltage for the base voltage of the first transistor.

7. The apparatus of claims 4 wherein the second transistor is coupled to the second junction pin between the third and fourth resistors, and between the first pin (Vbat) and the outlet pin.

8. The apparatus of claim 7 wherein the second transistor has: a base pin coupled to the second junction pin; a collector pin coupled to the outlet pin; and an emit pin coupled to the first pin (Vbat).

9. The apparatus of any of the preceding claims wherein the predefined minimum voltage is 2.35V and is determined at the first pin (Vbat).

10. The apparatus of any of the preceding claims wherein the diode has a cathode pin coupled to the first pin (Vbat).

11. An apparatus comprising: a voltage source circuit comprising a battery cell of LiFeP04 type for generating a voltage between a first pin and a second pin; and a protection circuit coupled to the first and second pins of the voltage source circuit, comprising: o a voltage divider circuit configured to generate a varying voltage responsive to the variations of the voltage of the battery cell; o a diode configured to shut down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and o a first transistor configured to monitor the varying voltage of the voltage divider circuit and to set a second transistor to on or off; o wherein the second transistor is configured to cut off the connection between the first pin and an outlet pin to prevent further leakage current drain from the battery cell.

12. The apparatus of claim 11 to be used in a portable battery powered device.

13. The apparatus of claim 12 wherein the LiFeP04 battery cell has the predefined minimum voltage of 2.50V below which the second transistor is cut off.

14. The apparatus of any of claims 11 to 13 wherein the voltage source circuit including a PTC resistor and the LiFeP04 battery cell are serially coupled to the first pin (Vbat) and the second pin (GND).

15. An integrated circuit including a protection circuit coupled to a battery cell of LiFeP04 type at a first pin and a second pin, comprising: means for generating a varying voltage responsive to the variations of the voltage of the battery cell; means for shutting down the voltage divider circuit when the voltage drops below a predefined minimum voltage; and means for monitoring the varying voltage of the generating means and to set a switching means to on or off; wherein the switching means is configured to cut off the connection between the first pin and an outlet pin to prevent further leakage current drain from the battery cell.