Abstract:
The present invention provides a battery charger and a method for preventing an overshoot charging current during a mode transition. The battery charger includes a charging regulation circuit for outputting a charging current whose amount is regulated based on a regulation signal at a control terminal, a first current sensing unit for sensing a current level and outputting an error signal based on both the current level and a first reference signal, an operational amplifier for generating the regulation signal, and a reference voltage generator which includes an ADC for converting the error signal outputted by the current sensing unit to a digital signal, and a DAC for converting the digital signal to a reference voltage.

Description:
BACKGROUND  
       [0001]     The present invention relates to a battery charger, and more particularly, to a battery charger and related method for preventing the charging current from overshooting during the charging mode transition.  
         [0002]     In a battery charging system for a lithium-ion (Li-ion) battery, a constant current (CC) mode is adopted to apply a high current to an exhausted battery to activate a rapid charging operation. When the battery is charged to a desired voltage level, the battery charging system switches to a constant voltage (CV) mode to maintain the battery at this desired voltage level. Since there exists an internal resistor in the battery, the battery is not fully-charged at the end of the CC mode. The voltage drop on the internal resistor makes the battery voltage be higher than it really is in the CC mode. After entering the CV mode, the battery voltage will be kept at the desired voltage level. In other words, the battery will be kept charging until the charging current becomes zero. When the charging current is zero, there is no voltage drop on the internal resistor and the battery is fully-charged to the desired voltage level.  
         [0003]     The CC charging mode cannot be applied to the battery once the battery reaches the desired voltage level because the energy storage capacity of the battery would exceed the nominal rating, leading to AC adaptor, battery and charging system damage. However, the CC mode needs to be used during the first part of the charging operation in order to minimize overall charging time, i.e., the time for charging the battery with the CC mode must be maximized. Therefore, a proper transition between two charging modes is crucial to the battery charging system&#39;s performance.  
         [0004]     Please refer to  FIG. 1 , which shows a related art battery charger  100 . The related art battery charger  100  is used to charge a battery  150 , and includes a charging regulation circuit  110 , a comparator  120 , a constant current (CC) mode controller  130 , and a constant voltage (CV) mode controller  140 , wherein the comparator  120  can be a hysteresis comparator for stabilizing the charging mode. As shown in  FIG. 1 , it is well-known that the battery  150  is equivalent to a series connection of a resistor R int  and a capacitor C int , and the charging regulation circuit  110  is connected to a power supplier (not shown). The comparator  120  is used to compare a battery voltage V bat  of the battery  150  with a reference signal V ref     —     1  to check whether the battery voltage V bat  is below a threshold. If the battery voltage V bat  is lower than the reference signal V ref     —     1  the comparator  120  sends out a non-enabling signal D′ to switch on switches SW 1 , SW 3 , and to switch off switches SW 2 , SW 4 , and then the battery  150  is charged in the CC mode.  
         [0005]     In the CC mode the charging regulation circuit  110  is controlled to provide the battery  150  with a constant charging current. As shown in  FIG. 1 , the charging regulation circuit  110  is configured by a PMOS transistor  111  to regulate the required charging current. The CC mode controller  130  includes a sensor  135 , which monitors the charging current flowing through a resistor R. After measuring the charging current flowing through the resistor R, the sensor  135  outputs a voltage V 1 , which corresponds to the voltage drop across the resistor R, into an operational amplifier  132 . For example, if the detected voltage drop is 160 mV, the sensor  135  converts the voltage drop into a voltage level of 160 mV. The operational amplifier  132  sends out a regulation signal S 1  to adjust the gate voltage of the PMOS  111  for regulating and stabilizing the charging current outputted from the power supplier, which is further explained as follows. Here, the charging regulation circuit  110 , the resistor R, the sensor  135 , and the operational amplifier  132  form a closed loop; the voltage V 1  acts as a feedback signal. As shown in  FIG. 1 , the operational amplifier  132  determines the voltage level of the regulation signal S 1  by comparing the incoming voltage V 1  with a reference signal V ref     —     2 . Assume the charging current during the CC mode is designed to be 10 mA, and the resistance of the resistor R is a known value 50 Ω. It is clear that if the charging regulation circuit  110  successfully outputs the desired charging current 10 mA, the voltage drop cross the resistor R will be 500 mV. Therefore, the reference signal V ref     —     2  is set to 500 mV for checking whether the current flowing through the resistor R has the desired current value. If the voltage V 1  is greater than the reference signal V ref     —     2 , the regulation signal S 1 , which has a higher voltage level amplified by the operational amplifier  132 , controls the charging regulation circuit  110  to reduce the charging current; however, if the voltage V 1  is less than the reference signal V ref     —     2 , the regulation signal S 1 , which has a lower voltage level amplified by the operational amplifier  132 , controls the charging regulation circuit  110  to increase the charging current. As a result, the battery  150  receives a stable charging current.  
         [0006]     The battery charger  100  turns into the CV mode from the CC mode when the battery voltage V bat  is at a termination voltage level, that is, a reference voltage V ref     —     3 . When the battery charger  100  is charging in the CC mode, the comparator  120  keeps comparing the battery voltage V bat  with the reference voltage V ref     —     3 . When the battery voltage V bat  is not less than the reference voltage V ref     —     3  the comparator  120  sends out the enabling signal D to change the on/off states of the switches. Therefore, switches SW 1 , SW 3  are switched off, and switches SW 2 , SW 4  are switched on. As a result, the battery charger  100  enters the CV mode. In the CV mode, the comparator  120  keeps comparing the battery voltage V bat  with the reference voltage V ref     —     1 . When the battery voltage V bat  is lower than the reference voltage V ref     —     1 , the battery charger  100  returns to the CC mode.  
         [0007]     In the CV mode the charging regulation circuit  110  charges the battery  150  to the termination voltage level and the battery charger  100  maintains the battery voltage V bat  at the termination voltage level. In the CV mode, the CV mode controller  140  acts as a regulator to regulate the charging current. An operational amplifier  142  within the CV mode controller  140  compares the battery voltage V bat  with a reference voltage V ref     —     3 , and sends out a regulation signal S 2  to control the gate voltage of the PMOS  111  for further tuning the charging current. Similar to the CC mode, the CV mode also forms a closed loop including the charging regulation circuit  110 , the resistor R, and the operational amplifier  142 . In order to stabilize the battery voltage V bat  at the reference voltage V ref     —     3 , the operational amplifier  142  compares the reference voltage V ref     —     3  with the battery voltage V bat  to decide how to regulate the charging current. In other words, the gate voltage of the PMOS transistor  111  is precisely adjusted by the regulation signal S 2  when the battery voltage V bat  deviates from the reference voltage V ref     —     3 . As a result, the battery  150  is steadily charged at the constant battery voltage V bat .  
         [0008]     The battery charger  100  will enter the CC mode again when the battery voltage V bat  is lower than the reference voltage V ref     —     1 , for example, fully-charged battery  150  is removed and a new exhausted battery is connected, which is explained as follows. During charging in the CV mode, the operational amplifier  142  controls the charging regulation circuit  110 , while the comparator  120  keeps comparing the battery voltage V bat  with the reference signal V ref     —     1 , which is lower than the reference voltage V ref     —     3 . When a fully-charged battery is taken away and an exhausted battery is connected to the battery charger  100 , the battery voltage V bat  becomes low. If the battery voltage V bat  is lower than the reference signal V ref     —     1 , the comparator  120  sends out a non-enabling signal D′ to initialize the battery charger  100 , setting it into the CC mode wherein switches SW 1 , SW 3  are on and switches SW 2 , SW 4  are off. Therefore, the battery charger  100  again provides the exhausted battery with a constant charging current.  
         [0009]     As mentioned above, there are two modes, the CC mode and the CV mode, selectively used by the battery charger  100 ; hence there is a transition between these two modes. However, because the comparators  120  and the operational amplifier  142  are not perfectly matched due to well-known manufacturing variations, the transition between these two modes may not be very smooth, and this situation could cause an overshoot charging current to damage an AC adaptor, the battery charger  100  or the battery  150 . For example, assume that the constant charging current during the CC mode is 800 mA, the reference signal V ref     —     1  is 4.1 V, and the reference voltage V ref     —     3  is 4.2V, which is the same as the termination voltage. Ideally, the transition occurs when the battery voltage V bat  is equal to 4.2V, and then the CV mode closed loop is enabled to maintain the battery voltage V bat  at 4.2V. As a result, the charging current would still be kept at 800 mA at the moment of entering the CV mode, and the transition between two modes would therefore be very smooth. However, practically, because the comparator  120  and the operational amplifier  142  are not matched in their characteristics, the voltage at which the transition occurs and the voltage at which the CV mode loop tries to maintain are likely to be different. That is, if the comparator  120  abnormally sends out an enabling signal D when V bat  is still less than 4.2V, for example 4.1 V, and then the operational amplifier  142  will sends out a regulation signal S 2  to maintain the battery voltage V bat  at 4.2V by further increasing the charging current by controlling the PMOS  111 . The charging current at the situation is larger than the current in the CC mode. Therefore, an overshoot charging current capable of damaging the AC adaptor, the battery charger  100  or the battery  150  is likely to be induced.  
         [0010]     With this in mind, it is desirable to provide a battery charger which can minimize the overshoot charging current to prevent the AC adaptor, the battery charger and the battery from being damaged.  
       SUMMARY  
       [0011]     One objective of the claimed invention is therefore to provide a battery charger for preventing an overshoot charging current to solve the above problem.  
         [0012]     According to an embodiment of the present invention, a battery charger is disclosed. The battery charger includes: a charging regulation circuit which has an input terminal, a control terminal, and an output terminal, wherein the input terminal is for being coupled to a power supplier, the output terminal is coupled to a node N 1 , the control terminal is coupled to a node N 5 , and the charging regulation circuit outputs a charging current whose amount is regulated based on a regulation signal at the control terminal; a current sensing unit having a first and a second input terminals and a first and a second output terminals, wherein the first input terminal is coupled to the node N 1 , the first output terminal is coupled to a node N 2  that is for being coupled to the battery, the second input terminal is inputted by a first reference signal, the second output terminal is coupled to a node N 3 , and the current sensing unit senses a current level on a path from the node N 1  to the node N 2  and outputs an error signal at the node N 3  based on the current level and the first reference signal; an operational amplifier having a positive input terminal coupled to the node N 2 , a negative input terminal coupled to a node N 4 , and an output terminal coupled to the node N 5  for generating the regulation signal; and a reference voltage generator having an input terminal coupled to the node N 3  and an output terminal coupled to the node N 4 , including: an ADC for converting the error signal outputted by the current sensing unit to a digital signal; and a DAC for converting the digital signal to a reference voltage.  
         [0013]     By adding a voltage generator to the related art battery charger, the overshoot charging current will be reduced and the mode transition will be smooth.  
         [0014]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a diagram illustrating a battery charger according to the related art.  
         [0016]      FIG. 2  is a diagram illustrating a battery charger according to an embodiment of the present invention.  
         [0017]      FIG. 3  is a circuit diagram of a Digital-to-Analog Converter (DAC) shown in  FIG. 2 .  
         [0018]      FIG. 4  is a diagram illustrating a battery charger according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     In order to reduce the overshoot charging current, an adjusting mechanism can be applied to control the battery voltage V bat . Please refer to  FIG. 2 , which shows a battery charger  200  according to the present invention. In this embodiment, the battery charger  200  includes the charging regulation circuit  110  formed by the PMOS transistor  111  the comparator  120 , the CC mode controller  130 , the CV mode controller  140 , and a reference voltage generator  210  to charge a battery  150 , wherein the comparator  120  can be a hysteresis comparator for stabilizing the charging mode. Compared with the related art battery charger  100  shown in  FIG. 1 , the battery charger  200  is very similar to the battery charger  100 , and the only difference lies in that the battery charger  200  contains an extra reference voltage generator  210 , which is electrically connected to the operational amplifier  142 , to properly tune the reference voltage V ref     —     3 . Further, the reference voltage generator  210 , as shown in  FIG. 2 , is electrically connected to an output node of the operational amplifier  132  through a switch SW 5 , and is electrically connected to the comparator  120  through the switch SW 3 . Please note that the functionality and operation of each of the charging regulation circuit  110 , the comparator  120 , the CC mode controller  130 , and the CV mode controller  140  has been described above, and the lengthy description is omitted for brevity.  
         [0020]     When the battery charger  200  is in the CC mode, the switches SW 1 , SW 3  are on and switches SW 2 , SW 4 , SW 5  are off, and the comparator  120  compares the battery voltage V bat  with the reference voltage V ref     —     3 . In the CC mode, the reference voltage V ref     —     3  is set to a termination value controlled by the reference voltage generator  210 . When the battery voltage V bat  is not less than the reference voltage V ref     —     3 , 4.2V for Li-ion battery for example, the comparator  120  sends out the enabling signal D to switch off SW 1 , SW 3  and switch on SW 2 , SW 4 , SW 5 , and the battery charger  200  hence enters the CV mode. The operational amplifier  142  tries to maintain the battery voltage V bat  at the reference voltage V ref     —     3 , i.e., 4.2V, and meanwhile the operational amplifier  132  serves as a pure comparator since the switch SW 1  is now off and the operational amplifier  132  is not included in the closed loop mentioned above. The operational amplifier  132  compares the voltage V 1  with the reference signal V ref     —     2  to determine how to adjust the charging current, wherein the voltage V 1  indicates a current level of the charging current flowing through the resistor R. For example, if the resistor R is 0.2 Ω, and the constant charging current of the CC mode is 800 mA, the reference signal V ref     —     2  should be 160 mV, which is equal to 0.2 Ω×800 mA. When the battery charger  200  transits form CC mode to CV mode, if the transient charging current is higher than 800 mA, the voltage V 1  is greater than 160 mV. As a result, the comparator  132  sends an error signal D 1  having a positive voltage level to the reference voltage generator  210 . The error signal D 1  is further received by an analog-to-digital converter (ADC)  211 . In the present invention, the ADC  211  contains a sampler  212 , which is activated by the enabling signal D and is triggered by an external clock signal CLK, for example, a 10 kHz clock, and a shift register  214 . After the sampler  212  samples the error signal D 1 , the error signal D 1  is converted from an analog signal to a digital shift signal R/L to right-shift/left-shift the bits stored in the shift register  214 .  
         [0021]     In the present invention the shift register  214  has 9 bits, where only one bit is set to “1” and other bits hold “0” for defining a control code (for example, “000000001”, “000000010”, . . . , or “100000000”). This type of control code is called a “1 of 9 code”. Then, the stored control code is transferred to a digital-to-analog converter (DAC)  216 .  
         [0022]     The DAC  216  converts the received digital control code into the reference voltage V ref     —     3 . In this embodiment, the DAC  216  can be simply implemented by a multiplexer (MUX). Please refer to  FIG. 3 , which shows the inner circuitry of the DAC  216 . The DAC  216  has nine inputs corresponding to nine reference levels ranging equally from 3.9V to 4.2V. The control code outputted from the shift register  214  determines which of the 9 inputs is selected and then outputted to be the third voltage reference V ref     —     3 . For example, as shown in  FIG. 3 , the control code is “001000000”, so the reference voltage V ref     —     3  will be “4.2−2ΔV”, where ΔV is equal to (4.2−3.9)/8.  
         [0023]     Generally speaking, the control code is initialized with “100000000”, so when the battery charger  200  works during the CC mode or in the beginning of the CV mode, the reference voltage V ref     —     3  is equal to 4.2V. During the CV mode, if the sampler  212  samples a high level from the incoming error signal D 1 , that is, if the current level of the charging current is higher than the desired 800 mA, the shift signal R/L right-shifts the bit “1” to form a new control word “010000000”. At this time, the reference voltage V ref     —     3  is decreased by ΔV, so the battery voltage V bat  is held at a lower voltage level and the charging current decreases accordingly. Moreover, if the sampler  212  later samples a low level from the incoming error signal D 1 , that is, if the current level of the charging current is lower than 800 mA, the shift signal R/L left-shifts the bit “1” bit, so the reference voltage V ref     —     3  is increased by ΔV, leading to an increase of the charging current.  
         [0024]     According to another embodiment of the present invention, please refer to  FIG. 4 , the battery charger  400 , compared with the battery charger  200 , has an additional current sensing unit  410  for taking charge of the work that the CC mode controller  130  does in the CV mode. The current sensing unit  410  has a sensor  415  for sensing the current flowing through the resistor R and an operational amplifier  412  for comparing the voltage V 1  with the reference signal V ref     —     2  to determine how to adjust the charging current. In this case, the operational amplifier  132  of the CC mode controller  130  is used purely for generating the regulation signal S 1  in the CC mode, while the operational amplifier  412  of the current sensing unit  410  is used purely for generating the error signal D 1  in the CV mode. Although there is a small difference between battery chargers  200  and  400  due to the additional current sensing unit  410 , however, the functions of other elements and the operating principle of the battery charger  400  are the same as those mentioned in  FIG. 2 .  
         [0025]     In summary, by gradually and adequately increasing or decreasing the reference voltage V ref     —     3 , the unwanted over-current charging originally affecting the battery charger during the mode transition can be greatly reduced. As a result, for a large range of mismatch that may be between the comparator  120  and the operational amplifier  142 , the overshoot of the charging current can be almost totally prevented by adding the adjusting mechanism to the related art battery charger  100 .  
         [0026]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.