Method and apparatus for protection of batteries

The protection circuit (30) comprises at least one switch (FET1, FET2) comprising at least one control means (G1, G2) for adjusting the conductivity of said at least one switch (FET1, FET2). The conductivity is arranged to be adjustable by means of an electrical control conducted to the control means (G1, G2). The protection circuit (30) comprises means (22, 25, 26) for forming the electrical control, means (27, 28) for measuring at least one physical quantity affecting at least one switch (FET1, FET2), means (10) for providing information about the dependence of the conductivity properties of said at least one switch (FET1, FET2) on said at least one physical quantity, means (29) for determining the conductivity of said at least one switch (FET1, FET2) on the basis of said at least one physical quantity and the conductivity properties of said at least one switch (FET1, FET2) and means (29, 27) for determining the current (ITOT) conducted through said at least one switch (FET1, FET2) at least partly on the basis of said conductivity. Thus, said electrical control is arranged to be formed at least partly on the basis of the determined current.

The present invention relates to a protection circuit according to the preamble of the appended claim1. The invention also relates to an integrated circuit according to the preamble of the appended claim21. Furthermore, the invention relates to a host device according to the preamble of the appended claim22. In addition, the invention relates to a battery according to the preamble of the appended claim28. Moreover, the invention relates to a method according to the preamble of the appended claim30.

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 Rds(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 unit2(FIG. 1), a charging control block3, an over-current protection block4, a discharging-side overheating protection block5, a charging-side overheating protection block6and two field-effect transistors FET1, FET2.

The purpose of the over- and under-voltage detection unit2is to detect when the voltage of the cells1a,1b,1bof the battery is too high or too low. When a load (not shown), for example an electronic device, is connected across connectors P1, P2, in other words the battery is discharged, the over-voltage or under-voltage detection unit2monitors each cell1a,1b,1cof 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 FET1becoming non-conductive, whereupon discharging of the battery is terminated.

When a charging device (not shown) is connected across connectors P1, P2, i.e. the battery is charged, the over- and under-voltage detection unit2monitors each cell1a,1b,1cof 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 FET2becoming non-conductive, whereupon charging of the battery is terminated.

The purpose of the charging control block3is to control the second field-effect transistor FET2in such a way that when line L is in the second logical state, the second field-effect transistor FET2does not pass a charging current, i.e. the battery is not charged. Correspondingly, when line L is in the first logical state, the second field-effect transistor passes a charging current, i.e. the battery is charged.

The purpose of the over-current protection block4is to interrupt discharging of the battery when the current supplied to the electronic device is too high. The over-current protection block comprises two symmetrical circuits with substantially equal properties. The circuits are connected to the drain and source of the first field-effect transistor. As the current increases, the voltage difference between the drain and the source of the first field-effect transistor also increases. When this voltage difference reaches a certain value, it causes the over-current protection block to set the first field-effect transistor into a non-conductive state. Thus the current supply to the electronic device is interrupted.

Let us assume that a load (not shown), for example an electronic device, is connected between connectors P1, P2, i.e. the battery is discharged, and the cells1a,1b,1cof the battery are not in an under-voltage condition. In this situation, an over-current causes the temperature of the first field-effect transistor FET1to rise above normal. When the first field-effect transistor reaches a certain temperature, the discharging-side overheating protection block5switches the first field-effect transistor FET1into a non-conductive state, whereupon discharging of the battery is terminated.

Correspondingly, let us assume that a charging device is connected between connectors P1. P2, i.e. the battery is charged, and the cells1a,1b,1cof the battery are not in an over-voltage condition. In this situation, an over-current causes the temperature of the second field-effect transistor to rise above normal. When the second field-effect transistor reaches a certain temperature, the charging-side overheating protection block6switches the second field-effect transistor FET2into a non-conductive state, whereupon charging of the battery is terminated.

However, this solution has the disadvantage that it does not take into account changes in the drain-to-source resistance of the field-effect transistor as the temperature changes. As mentioned earlier in this description, a change in temperature changes the drain-to-source resistance between the source and the drain. Thus, the shut-off actually takes place at different current values at different temperatures.

It is an objective of the present invention to provide a protection circuit for batteries, such as Li-ion and Li-poly batteries, which is capable of more accurately protecting a battery from over-current, over-voltage and under-voltage when the battery is charged or discharged by taking into account the dependence of the properties of a field-effect transistor on at least one physical quantity, such as temperature and/or gate voltage. Because in the solution according to the invention, the value of the current can be determined more accurately compared with present solutions, the battery's charge can also be measured considerably more accurately when compared with present solutions. Another objective of the invention is to avoid the introduction of additional impedance into the protection circuit.

According to the invention, the first objective can be attained by using a value of drain-to-source resistance, compensated using at least one physical quantity, such as temperature and/or gate voltage, to detect over-current. The compensation takes place in such a way that information concerning the behaviour of the field-effect transistors at different temperatures and/or different gate voltages, as well as measured temperature and/or voltage values is stored in a parameter memory of the protection circuit. This information is used to obtain a value of drain-to-source resistance which is as accurate as possible, whereupon the actual value of the current can be determined more accurately than in prior art methods. Furthermore, monitoring can take place in connection with both charging and discharging of the battery. Because the temperature and/or the gate voltage of the field-effect transistor is taken into account when the current is determined, a more precise value is also obtained for the charge of the battery compared with prior art solutions. Because the current is measured in a more precise manner when compared to prior art, it is also possible to increase the operating time of the host device. According to the invention, the second objective can be attained in such a way that an over-current is detected using the drain-to-source resistances and/or drain-to-source voltage of the field-effect transistors, whereupon additional resistors are not necessary. The protection circuit according to the invention can be advantageously implemented in an application specific integrated circuit (ASIC), wherein the battery protection circuit becomes considerably smaller and less expensive when compared to circuits where separate components are used.

More precisely, the protection circuit according to the invention is characterized in what is presented in the characterizing part of claim1. Furthermore, an integrated circuit according to the invention is characterized in what is presented in the characterizing part of claim21. Also, the host device according to the invention is characterized in what is presented in the characterizing part of claim22. In addition, a battery according to the invention is characterized in what is presented in the characterizing part of claim28. Moreover, a method according to the invention is characterized in what is presented in the characterizing part of claim30.

By means of the present invention, considerable advantages are achieved compared with solutions according to prior art. Because the protection circuit according to the invention protects a battery from over-current considerably better than prior art solutions, the operating life of the battery is extended because, as a result of more accurate over-current protection, the probability of damage to the battery is reduced. By means of the solution according to the invention, it is possible to protect a battery from over-current both when discharging and charging the battery, and thus the battery is also protected, for example, from a faulty charging device. Because over-current protection is implemented in such a way that the protection circuit does not contain unnecessary resistive components which cause power dissipation, the operating time of the device using the battery is increased. Furthermore, the protection circuit according to the invention is less expensive and smaller compared with prior art protection circuits, because it can be implemented in a single application specific integrated circuit. Because the charge of the battery can be measured considerably more accurately using the protection circuit according to the invention than in prior art solutions, it is possible to estimate e.g. the shutdown time of the device that is being used.

An object of the method for protection of a battery according to the invention is to determine the actual charging and discharging current as accurately as possible. To achieve this, the first step is to determine the value of at least one physical quantity, preferably temperature and/or gate voltage related to at least one field-effect transistor used as a switch, after which a compensation is performed in which at least one said quantity is taken into account when determining the drain-to-source resistance.

FIG. 3shows a protection circuit30according to a preferred embodiment of the invention, implemented in an integrated circuit, as well as its various functional blocks. The purpose of field-effect transistors FET1and FET2is to protect the battery from over- or under-voltage in a manner substantially similar to prior art solutions, but considerably more accurately. The protection circuit employs two field-effect transistors, because there should also be a possibility to discharge the battery31in a situation where the first field-effect transistor FET1prevents charging of the battery31in an over-voltage state. Similarly, there should be a possibility to charge the battery when the second field-effect transistor FET2prevents discharge of the battery in an under-voltage state.

The field-effect transistors are connected in series in such a way that the drain D1of the first field-effect transistor is connected to the drain D2of the second field-effect transistor. The source S1of the first field-effect transistor FET1is advantageously connected to the ground potential GND, and the source S2of the second field-effect transistor FET2, in turn, is connected to the negative pole P4of the battery31. The gate G1of the first field-effect transistor is connected to a voltage measuring block27and to an over-voltage prevention block26. Correspondingly, the gate G2of the second field-effect transistor is connected to voltage measuring block27and to an under-voltage prevention block25.

When a load, for example an electronic device33(FIG. 5) is connected between connectors P3and GND, i.e. the battery31is discharged, the under-voltage prevention block25monitors the state of the battery. If the voltage of the battery falls below a particular predetermined threshold value, the under-voltage prevention block25transmits information about this situation to control block22. As a result, the control block transmits information about the under-voltage state via an interface bus BUS to the electronic device33(FIG. 5). The control block22also transmits a signal concerning the under-voltage state to the under-voltage prevention block25. When the under-voltage prevention block25receives this signal, it connects a voltage (advantageously approximately 0 V in the case of an N-type field-effect transistor) to the gate of the second field-effect transistor FET2, by means of which the drain-to-source resistance of the second field-effect transistor FET2is put into a high impedance state. This also results in the current supply to the electronic device33being interrupted, but current can still flow through the second field-effect transistor FET2in the opposite direction (through a parasitic diode), i.e. it is still possible to charge the battery.

Correspondingly, when a charging device34(FIG. 5) is connected between connectors P3and GND, i.e. the battery is charged, the over-voltage prevention block26monitors the state of the battery31. If the voltage of the battery exceeds a particular predetermined threshold value, the over-voltage prevention block26transmits information about this situation to control block22. As a result of this, control block22transmits information on the over-voltage state via the interface bus BUS to the host device33(FIG. 5). The control block22also transmits a signal concerning the over-voltage state to the over-voltage prevention block26. In this case, the over-voltage prevention block26connects a voltage to the gate of the first field-effect transistor FET1, by means of which the drain-to-source resistance of the first field-effect transistor FET1is put into a high impedance state. This results in the supply of charging current to the battery31being interrupted, but current can still be conducted through the first field-effect transistor FET1in the opposite direction (through a parasitic diode), i.e. it is still possible to discharge the battery.

FIG. 3shows an example in which the battery31comprises only one cell. Naturally, it is possible that the battery31may have several cells, in which case the under-voltage prevention block25and the over-voltage prevention block26advantageously monitor the voltage of each cell separately. If the voltage of any cell is lower than a particular predetermined under-voltage threshold value, or if it exceeds a particular predetermined over-voltage threshold value, actions similar to those presented above are performed.

The power supply block24of the circuit is connected to the positive voltage P3of the battery31and to the ground potential GND of the protection circuit30. When the battery voltage is within permissible limits or in an over-voltage state, the circuit's power supply block supplies the protection circuit with the current it requires via the control block22, i.e. the protection circuit acts as part of the load. When the battery voltage falls below the threshold value, i.e. the battery is in an under-voltage state, the under-voltage prevention block25puts the second field-effect transistor FET2into a non-conductive state. Thus, the power supply from the battery to the protection circuit, as well as the load, is interrupted, whereupon the protection circuit does not consume the charge of the battery. If the power supply to the protection circuit were not interrupted in the under-voltage state, the power consumed by the protection circuit could cause the battery voltage to fall too low, whereupon the battery could be damaged and become unusable. When charging of the battery is initiated after an under-voltage condition, the protection circuit again receives the necessary operating current to protect the battery.

In the under-voltage state presented above, the control block22transmits a signal relating to the under-voltage to the under-voltage prevention block25, which interrupts power supply to the host device33. However, before this under-voltage state occurs, the control block22advantageously transmits information on the forthcoming under-voltage state to the host device33, preferably at a stage when the voltage of the battery31falls below a certain threshold value. This threshold value is advantageously somewhat greater than the under-voltage threshold value. This arrangement enables the electronic device33to formulate a notification to the user indicating that the battery31has become empty before the power supply to the electronic device is interrupted. The notification is given, for example, using a sound signal, and/or a message presented on the display device, in a manner known as such.

In compensation block29, the aim is to obtain the most accurate possible estimate of the actual current passing through the field-effect transistors FET1, FET2, irrespective of their temperatures and gate voltages. The estimate of the actual magnitude of the current provided by compensation block29is used to protect the battery from over-current during discharging and charging, and to determine the charge of the battery more accurately. In order for the compensation block29to calculate a compensated value for the current, it requires information relating to the gate voltages of the field-effect transistors FET1, FET2and on the voltage across the field-effect transistors from a voltage measurement block27, information on the temperature of the integrated circuit from a temperature sensor28, as well as information on the properties of the field-effect transistors from a parameter memory10.

The voltage measurement block27measures the gate voltage UGS1, UGS2of both field-effect transistors FET1, FET2and the voltage across the field effect transistors, UTOT. Advantageously, the measurement is performed by means of one or more AD converters. Preferably, the measurement is effected by means of three separate AD converters, wherein the voltages do not have to be measured consecutively. The voltage values obtained are transmitted to the compensation block29.

The field-effect transistors FET1, FET2and the temperature sensor28are preferably located in the same integrated circuit, because in that case the protection circuit30does not have to comprise a separate temperature sensor for both field-effect transistors. Furthermore, the temperature sensor28is preferably located inside the integrated circuit with the field-effect transistors FET1, FET2, because the temperature on the surface of the integrated circuit can be substantially different from that inside the circuit, and the temperature changes considerably more slowly on the surface of the circuit than inside the circuit.

Preferably, the behaviour of the drain-to-source resistance Rds(on), measured across the field-effect transistor at different temperatures (e.g.FIG. 4a) and at different gate voltages (e.g.FIG. 4b) with respect to reference values T0and V0over a sufficiently large range, is stored in the parameter memory10of the protection circuit. Advantageously, the parameter memory takes the form of an EEPROM (Electrically Erasable Programmable Read Only Memory) and the aforementioned behaviour information is stored during the manufacture of the protection circuit. In the example situations shown inFIGS. 4aand4b, the reference temperature T0is 23° C. and the reference voltage V0is 3.5 V. Furthermore, the value of the drain-to-source resistance Rds(on)at temperature T0and gate voltage V0is stored in the parameter memory. If the behaviour of the drain-to-source resistance Rds(on)at different temperatures can be assumed to be approximately linear over the particular range of interest (as is the case inFIG. 4a), it is only necessary to store information about the behaviour (e.g. the value Rds(on)) at two different temperatures which are sufficiently far apart (e.g. T0+/−20° C.). Using these points, the value of the drain-to-source resistance Rds(on)at other temperatures can be obtained e.g. by means of interpolation, which is known as such. As is shown inFIG. 4b, the behaviour of the drain-to-source resistance Rds(on)at different gate voltages is not exactly linear, and thus it is preferable to store the behaviour (e.g. the value of Rds(on)) for at least three points (e.g. V0, VMINand VMAX) in the parameter memory10, by means of which the behaviour/value of the drain-to-source resistance Rds(on)at other gate voltages can be calculated. This can be done, for example, using a mathematical function that passes through or approximates the points V0, Vminand Vmaxand models the behaviour of the drain-to-source resistance Rds(on)with respect to variations in gate voltage.

Preferably, a value for the drain-to-source resistance Rds(on)at a particular temperature T and gate voltage V is determined by means of correction coefficients, which are used to modify (correct) the value of the drain-to-source resistance Rds(on)defined at the reference temperature T0and the reference gate voltage V0. Referring to the value of the drain-to-source resistance Rds(on)defined in the reference conditions as Rds(on)0, a value for the drain-to-source resistance Rds(on)at temperature T1, different from temperature T0, is advantageously determined by deriving a temperature correction coefficient from the temperature-related behaviour information stored in the parameter memory10and multiplying the drain-to-source resistance Rds(on)by the temperature correction coefficient so defined. In a similar manner, a value for the drain-to-source resistance Rds(on)at a gate voltage V1, different from V0, can be obtained by deriving a gate voltage correction coefficient from the gate-voltage related behaviour information stored in the parameter memory and multiplying the drain-to-source resistance Rds(on)by the gate voltage correction coefficient thus obtained. Advantageously, a value for the drain-to-source resistance Rds(on)at a temperature T1and a gate voltage V1is obtained by deriving both a temperature correction coefficient and a gate voltage correction coefficient and performing appropriate multiplications of the drain-to-source resistance Rds(on). Examples of how this may be done are presented later in the text. Preferably the correction coefficients take the form of numerical factors representing a ratio between the value of the drain-to-source resistance Rds(on)at a given temperature (or gate voltage) divided by the value of the drain-to-source resistance Rds(on)in the reference conditions (Rds(on)0). For example, a temperature correction coefficient intended to compensate for a change in the drain-to-source resistance Rds(on)occurring between the reference temperature T0and a temperature T1could be determined from the following relationship: Kt(T1)=Rds(on)T1/Rds(on)0. In other words, the temperature correction coefficient effectively represents a ratio between the value of the drain-to-source resistance Rds(on)at temperature T1and the value of the drain-to-source resistance Rds(on)at the reference temperature T0. The gate voltage correction coefficient can be viewed in an analogous manner. It should be noted that if the temperature correction coefficient is dependent on the gate voltage and/or the gate voltage correction coefficient is dependent on the temperature, it is necessary to use several correction tables for the gate voltage and/or temperature.

It is obvious that in addition to temperature and voltage, mentioned in this description, the invention can also be applied for the compensation of other physical quantities which affect the properties, especially the conductivity, of the switches FET1, FET2. One such quantity is aging, wherein by means of the method according to the invention it is possible to take into account the operating age of the protection circuit and/or the battery.

Naturally, it is possible to store complete reference tables indicating the behaviour of the drain-to-source resistance Rds(on)at different temperatures and at different gate voltages. Thus, temperature correction coefficients for the drain-to-source resistance Rds(on), or actual values, corrected (multiplied) using the correction coefficients, are stored in the first table over an appropriate temperature range (e.g. −50° C. to +150° C.) advantageously at intervals corresponding to the resolution with which the temperature can be measured. Correspondingly, gate voltage correction coefficients for the drain-to-source resistance Rds(on), or actual values, are stored in a second table over an appropriate voltage range (e.g. 2.5 V to 5.0 V) advantageously at intervals corresponding to the resolution with which the gate voltage can be measured. Furthermore, it is possible that these tables are combined to form a single, two-dimensional table from which it is possible to find a correction coefficient or an actual value for each temperature-gate voltage combination.

If actual values are stored in the parameter memory instead of correction coefficients, the operation of the protection circuit can be accelerated to some extent, because it is necessary to perform a smaller number of calculations than in a situation where correction coefficients are used.

Preferably, the properties of the field-effect transistors FET1and FET2are substantially alike, wherein it is only necessary to store information about one field-effect transistor in parameter memory10. Thus, it is possible to use the same information contained in the parameter memory10for both field-effect transistors. If the field-effect transistors have different properties, the properties of both field-effect transistors are stored separately in the parameter memory. To ensure that the temperature and voltage values stored in the parameter memory10are sufficiently accurate, the information is calibrated, advantageously in connection with the manufacture of the protection circuit30.

FIG. 2is a block diagram showing the manner in which a protection circuit according to a preferred embodiment of the invention defines a temperature and gate voltage compensated current when the properties of both field-effect transistors FET1, FET2are substantially alike. It is possible to use the same temperature correction coefficient for both field-effect transistors, because both field-effect transistors are located in the same integrated circuit, and thus the temperatures of both field-effect transistors are substantially the same. Furthermore, it is also possible to use the same value of drain-to-source resistance Rds(on)0for both field-effect transistors, because their properties are substantially the same.

Naturally, it is possible that the field-effect transistors FET1, FET2are of different types, in which case it is not possible to use the same value of drain-to-source resistance Rds(on)0for them both. Consequently in this case, if is necessary to make a separate behaviour model for the drain-to-source resistances Rds(on)0of each field-effect transistor FET1, FET2in the parameter memory10. Furthermore, it is possible that the temperatures of the field-effect transistors are not substantially the same, especially if the field-effect transistors are of different types. In this case, it is preferable to use separate temperature sensors28for both field-effect transistors FET1, FET2and to store the temperature behaviour of both field-effect transistors separately.

To start with the value of the drain-to-source resistance Rds(on)0in the reference states T0and V0is retrieved from the parameter memory. Next, a temperature11is measured, on the basis of which it is possible to determine a particular temperature correction coefficient12from the temperature compensation values stored in the parameter memory10(FIG. 3). In this case, it is presumed that both field-effect transistors FET1, FET2are at the same temperature, in which case it is possible to use the same temperature correction coefficient for both field-effect transistors. Next, the gate voltages14a,14bof both field-effect transistors are measured, on the basis of which it is possible to determine particular gate-voltage correction coefficients15a,15bfrom the gate voltage compensation values stored in the parameter memory10. The gate voltage correction coefficients15a,15bcan be added together because the drain-to-source resistances of the field-effect transistors FET1, FET2are connected in series. As a result of the addition16a, a combined gate voltage correction coefficient is obtained. A drain-to-source resistance17compensated with respect to both the temperature and gate voltage is obtained by multiplying the drain-to-source resistance defined in the reference conditions T0and V0(retrieved from the parameter memory10), with the temperature correction coefficient12and the combined gate voltage correction coefficient16b. In this way, an estimate of the actual value20of the current is obtained by dividing19the voltage18measured across the field-effect transistors by the drain-to-source resistance17compensated according to the temperature and gate voltages. In other words, the actual current is obtained according to the following formula:ITOT=UTOTRds⁡(on)⁢0·KT·(KU1+KU2)

ITOT=Estimated value of the current conducted via the field-effect transistors

Rds(on)0=Drain-to-source resistance in reference conditions

KU1=Gate voltage correction coefficient for the first field-effect transistor

KU2=Gate voltage correction coefficient for the second field-effect transistor

UTOT=Voltage measured across the field-effect transistors

If the field-effect transistors FET1, FET2are not at substantially the same temperature, but their properties are the same, it is not possible to use the same value KTfor the temperature correction coefficients for both field-effect transistors, but separate values should be used for both field-effect transistors.ITOT=UTOTRds⁡(on)⁢0·(KT1·KU1+KT2·KU2)

ITOT=Estimated value of the current conducted via the field-effect transistors

Rds(on)0=Drain-to-source resistance in reference conditions

KT1=Temperature correction coefficient for the first field-effect transistor

KT2=Temperature correction coefficient for the second field-effect transistor

KU1=Gate voltage correction coefficient for the first field-effect transistor

KU2=Gate voltage correction coefficient for the second field-effect transistor

UTOT=Voltage measured across the field-effect transistors

If, on the other hand, the properties of the field-effect transistors FET1, FET2are not substantially alike, but they are at substantially the same temperature, it is not possible to use the same value of drain-to-source resistance Rds(on)0for both field-effect transistors in the calculation, but separate values should be used for both field-effect transistors.ITOT=UTOTKT·(Rds⁡(on)⁢01·KU1+Rds⁡(on)⁢02·KU2)

ITOT=Estimated value of the current conducted via the field-effect transistors

Rds(on)01=Drain-to-source resistance of the first field-effect transistor in reference conditions

Rds(on)02=Drain-to-source resistance of the second field-effect transistor in reference conditions

KU1=Gate voltage correction coefficient for the first field-effect transistor

KU2=Gate voltage correction coefficient for the second field-effect transistor

UTOT=Voltage measured across the field-effect transistors

If the properties of the field-effect transistors FET1, FET2are not substantially alike and they are not at substantially the same temperature, temperature and gate voltage compensation is slightly more complex than in the preceding cases. Neither the same drain-to-source resistance value Rds(on)0nor the same temperature correction coefficient can be used, but separate values should be used for both field-effect transistors.ITOT=UTOTRds⁡(on)⁢01·KT1·KU1+Rds⁡(on)⁢02·KT2·KU2)ITOT=Estimated value of the current conducted via the field-effect transistors

Rds(on)01=Drain-to-source resistance of the first field-effect transistor in reference conditions

Rds(on)02=Drain-to-source resistance of the second field-effect transistor in reference conditions

KT1=Temperature correction coefficient for the first field-effect transistor

KT2=Temperature correction coefficient for the second field-effect transistor

KU1=Gate voltage correction coefficient for the first field-effect transistor

KU2=Gate voltage correction coefficient for the second field-effect transistor

UTOT=Voltage measured across the field-effect transistors

The function of the control block22is to control the under-voltage prevention block25and the over-voltage prevention block26and to transmit information on the battery's charge, over-current conditions and possible over- or under-voltage states to the host device, for example a mobile phone, via the interface bus BUS. In order for the control block22to realise all its functions, it is provided with a memory35. If the control block22detects that the battery's voltage is too low, it transmits a signal to the under-voltage prevention block25. However, in advance of the under-voltage condition the control block22transmits information on the forthcoming under-voltage state to the electronic device33, advantageously when the voltage of the battery31falls below a certain threshold value. This threshold value is advantageously slightly higher than the under-voltage threshold value. Because the invention enables the battery's charge to be determined more accurately, it is possible to increase the operating time of the electronic device33, because it is not necessary to switch off the electronic device before it is absolutely essential. In practice, the electronic device is switched off earlier because the value of under-voltage harmful to the battery is typically significantly lower than the voltage at which the electronic device ceases to function. In an over-voltage condition, the control block22advantageously transmits information on the over-voltage to the over-voltage prevention block26and to the host device33. It is not necessary to transmit information on a forthcoming over-voltage state to the host device33in advance, because in this case power to the electronic device is not switched off.

When the control block22receives too high a current value from the compensation block29, the control block22transmits a signal either to the under-voltage prevention block25or to the over-voltage prevention block26depending on whether the battery is being discharged or charged, as a result of which the corresponding field-effect transistor FET1, FET2is switched into a high impedance state.

In the charge determination block23, it is possible to determine the charge of the battery at a given time. This can be performed in a manner known as such, for example in such a way that the charging current fed to the battery during charging is measured. More accurate determination of the current according to the invention enables the accumulated/remaining charge to be determined more accurately than in prior art solutions. There are numerous known methods for determining charge on the basis of a current, and thus it is not necessary to discuss them in further detail here. In the protection circuit according to the present invention, charge determination can be performed more accurately than in prior art solutions, because a temperature and voltage compensated drain-to-source resistance is used to determine the current. Furthermore, determination of the current is performed without components that introduce additional resistance, and thus power consumption is reduced compared with prior art solutions.

Integration of the entire protection circuit into a single application specific integrated circuit (ASIC) requires some special knowledge concerning manufacturing techniques used to produce integrated circuits. On the other hand, the protection circuit30is less expensive and smaller than prior art solutions, because all the components required by the protection circuit can be placed in the same application specific integrated circuit.

Battery packs32, even those intended to be connected to the same device, e.g. a wireless terminal33(FIG. 5), can have different properties. For example, both Li-ion and Li-poly batteries can be used with the same device, and they both require a protection circuit to prevent damage caused by charging and discharging. However, the properties of these batteries are different. Therefore, a separate protection circuit for each different battery pack should be provided and advantageously it should be possible to select the correct settings for the battery pack being used at a particular time in the protection circuit. If the protection circuit30were located in the wireless communication device33, a problem would arise as to how the protection circuit could recognize the type of the battery pack and select the correct protection values. In this case, the manufacturing costs of the wireless communication device would be increased to some extent.

Preferably, the protection circuit30is located in the battery pack32with the battery31, in which case it is unnecessary to provide any kind of protection for the battery in the wireless communication device itself. In this case, it is possible to reduce costs, because it is possible to implement the most optimal and most advantageous protection circuit30for each battery31. Furthermore, it is not necessary to provide the wireless communication device and the battery pack with equipment for recognizing the type of the battery, which would increase the costs and occupy space. Moreover, provision of a protection circuit30implemented in a single application specific integrated circuit35in the battery pack does not significantly increase the size of the battery pack. On the other hand, it is possible that the application specific integrated circuit35has similar properties and operates in a substantially identical fashion, irrespective of the type of the battery31to be protected. Advantageously parameters suitable for each battery type can be stored in the memory10of the application specific integrated circuit in connection with its manufacture. In other words, the same integrated circuit can be adapted to protect different batteries/battery types by storing appropriate parameters describing the behaviour of the battery in question in the parameter memory10. Thus, the protection circuit may be provided with parameters describing the behaviour of a particular battery/battery type or parameters describing the behaviour of more than one battery/battery type may be stored in the parameter memory. InFIG. 5the application specific integrated circuit is indicated with reference numeral35.

When the protection circuit30is located in the battery pack32, advantageously only a voltage line P3, a ground potential line GND and an interface bus BUS are provided as outputs. The wireless communication device33obtains its operating voltage from voltage line P3, and from ground potential line GND. The wireless communication device obtains information about the charge of the battery as well as on exceptional states via the interface bus BUS.

It is, of course possible that the protection circuit30is not located in the battery pack32with the battery31. Thus, the protection circuit can be installed e.g. in the host device33. In this case, the type of the battery31is preferably identified separately, so that the protection circuit can function properly. Thus, the battery type is identified separately, advantageously via the interface bus BUS. When the battery type has been identified, it is possible to select the correct protection parameters in the protection circuit for precisely this battery type. This situation may also arise if, for example, the host device33does not contain a separate battery pack32, but the battery is located inside the host device. In this case, the protection circuit30is preferably also positioned inside the host device.

The present invention is not restricted solely to the embodiments presented above but can be modified within the scope of the accompanying claims. Although the examples used in the description relate to Lithium-based batteries, the invention can also be applied to other types of accumulators or batteries.