Abstract:
A charger circuit comprising: a charging path coupled between an input voltage and a battery; a power switch on the charging path; a switch control circuit controlling the power switch; a timer counting a charging period; and a low current control circuit issuing a signal to the switch control circuit to control the power switch such that a charging current is maintained to be a predetermined low current when the timer counts to a predetermined maximum charging period.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a charger circuit, and particularly to a charger circuit capable of shifting to a low charging current after a predetermined maximum charging period is reached. 
         [0003]    2. Description of Related Art 
         [0004]    When a battery is being charged, whether to stop charging or not is usually determined by detecting if the battery voltage reaches a predetermined voltage. Yet, if the battery is dead, the battery voltage will never reach the predetermined voltage and the charger will be charging forever. Therefore, the prior art further provides a timer, such that the charging current is stopped when a predetermined maximum charging period is reached. The battery may be charged by a constant current or a variable current. U.S. Pat. No. 6,922,039 discloses a method for counting a charging time of a variable charging current, wherein the charging current is stopped when a predetermined maximum charging period is reached. 
         [0005]    However, if the battery is a poor battery but not dead, that is, it is charged by a low rate for certain reasons but is not completely damaged, the poor battery which is still usable will be determined dead and is no more charged by the foregoing prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the foregoing drawback, the present invention provides a charger circuit, which maintains a predetermined low charging current after a predetermined maximum charging period is reached, lest a usable battery is determined dead. The low current for example can be a current below a lower limit of a normal charging current. 
         [0007]    According to the foregoing objective, in one perspective of the present invention, it provides a charger circuit comprising: a charging path coupled between an input voltage and a battery; a power switch on the charging path; a switch control circuit controlling the power switch; a timer counting a charging period; and a low current control circuit issuing a signal to the switch control circuit to control the power switch such that a charging current to the battery becomes a predetermined low current after the timer counts to a predetermined maximum charging period. 
         [0008]    The foregoing charger circuit preferably further comprises an oscillator whose frequency is changed according to the charging current, and wherein the timer counts the charging period according to the frequency of the oscillator. 
         [0009]    In the foregoing charger circuit, the power switch can be a power switch in one of the following power converter circuits: synchronous buck power converter, synchronous boost power converter, synchronous buck-boost power converter, synchronous inverter power converter, fly-back power converter, asynchronous buck power converter, asynchronous boost power converter, asynchronous buck-boost power converter, and asynchronous inverter power converter. 
         [0010]    In the foregoing charger circuit, the switch control circuit can turn off the power switch after the battery is charged by the predetermined low current for a predetermined period. 
         [0011]    The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic circuit diagram showing a first embodiment of the present invention. 
           [0013]      FIG. 2  is a schematic circuit diagram showing a second embodiment of the present invention. 
           [0014]      FIG. 3  is a schematic circuit diagram showing a third embodiment of the present invention. 
           [0015]      FIG. 4  is a schematic circuit diagram showing a fourth embodiment of the present invention. 
           [0016]      FIGS. 5A-5G  show several embodiments of the power stage circuit  20 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Please refer to  FIG. 1 , which illustrates a first embodiment of a charger circuit according to the present invention. As shown in the drawing, an input voltage Vin charges a battery via a power transistor switch Q 1  on a charging path, wherein the battery voltage is Vb. The power switch Q 1  is controlled by a switch control circuit  12 . In this embodiment, the charging current can be a non-constant value; the switch control circuit  12  determines not only the on/off state of the power switch Q 1  but also how much it is turned on. The charger circuit detects the battery voltage Vb and the charging current Ic, and such information are fed back to the switch control circuit  12 , so that the switch control circuit  12  controls the power switch Q 1  thereby. 
         [0018]    There are many ways to detect the charging current Ic. In this embodiment, a transistor Q 2  is employed to copy the charging current Ic so as to generate a current Ic′ flowing through a resistor R; hence, a voltage Ic′×R is generated at the left side of the resistor R, wherein this voltage is proportional to the charging current Ic. Notably, the transistor Q 2  is not required to completely match the power switch Q 1 . The current Ic′ is not required to be equal to Ic, but is only required to be a function of Ic. If the battery is charged by the charger circuit with a constant current, the circuit for detecting the charging current Ic can be omitted. 
         [0019]    The information related to the charging current Ic can be further provided to an oscillator (OSC)  14 , such that the frequency of the oscillator  14  is changed according to the charging current Ic, and a timer  16  counts the charging period according to the frequency of the oscillator  14 . Thus, the charging period can be traced dynamically. However, if the battery is charged by the charger circuit with a constant current, the oscillator  14  can be omitted, similarly as the above. 
         [0020]    When the timer  16  counts to a predetermined maximum charging period, it issues a signal to a low current control circuit  18 , and the low current control circuit  18  issues a signal to the switch control circuit  12  correspondingly. After the predetermined maximum charging period is reached, the switch control circuit  12  will not control the power switch Q 1  according to the feedback information related to the battery voltage Vb and the charging current Ic. Instead, it controls the power switch Q 1  to keep charging the battery with a predetermined low current Imin. In one embodiment, the low current Imin is a constant, such as a current below a lower limit of a normal charging current. In another embodiment, the low current Imin is a non-constant. 
         [0021]    Note that  FIG. 1  is drawn for illustrating the concept of the present invention. The low current control circuit  18  does not have to be a stand-alone circuit; it can be part of the switch control circuit  12 . 
         [0022]    The maximum charging period can be determined during circuit design, or set externally from outside of the circuit during circuit application. In addition, after the battery is charged by the low current Imin for a period of time, the power switch Q 1  can be turned off. 
         [0023]      FIG. 2  shows another embodiment of the present invention. In this embodiment, an operational amplifier (OP)  11  is provided, to match the current through the transistor Q 2  to the current through the power switch Q 1  more precisely. The other parts of this circuit are the same as those in the first embodiment, and the details are thus omitted here. 
         [0024]    In the foregoing embodiments, the battery is charged by the voltage Vin via the power switch Q 1 . However, the spirit of the present invention is not limited therein; the input voltage Vin can charge the battery via many types of power converters. Referring to  FIG. 3 , the battery is charged by the voltage Vin via a power stage circuit  20 . This power stage circuit  20  can be, for example but not limited to, one of the following circuits: synchronous buck power converter, synchronous boost power converter, synchronous buck-boost power converter, synchronous inverter power converter, fly-back power converter, asynchronous buck power converter, asynchronous boost power converter, asynchronous buck-boost power converter, and asynchronous inverter power converter, for example as shown in  FIGS. 5A-5G . Referring back to  FIG. 3 , the charger circuit detects the battery voltage Vb and the charging current Ic, and such information are fed back to the switch control circuit  12 . The switch control circuit  12  controls the power switch in the power stage circuit  20  according to such information. 
         [0025]    Similarly to the abovementioned embodiments, the information related to the charging current Ic can also be provided to the oscillator  14 , and the timer  16  counts the charging period according to the frequency of the oscillator  14 . When the timer  16  counts to the predetermined maximum charging period, the low current control circuit  18  issues a signal to the switch control circuit  12 . Accordingly, the switch control circuit  12  controls the power switch in the power stage circuit  20  to keep charging the battery by the low current Imin. 
         [0026]      FIG. 4  shows an example for detecting the charging current Ic, in which a resistor Rs is connected in series on the path of the charging current Ic. Hence, information of the charging current Ic can be indicated by voltage Ic×Rs; namely, the information related to the charging current Ic can be obtained by dividing a voltage difference across the resistor Rs by the resistance of the resistor Rs. 
         [0027]    The oscillator  14  can be voltage-controlled or current-controlled. In the embodiment in  FIG. 4 , as one example, the oscillator  14  is controlled by voltage and is changed in accordance with information related to the charging current Ic, that is, K(Ic×Rs). The information K(Ic×Rs) can be obtained by comparing the voltages at two ends of the resistor Rs, by an operational amplifier. This information can be acquired independently from the charging path. Yet, since the information related to the charging current Ic has been obtained by the switch control circuit  12 , the oscillator  14  can directly retrieve such information from the switch control circuit  12  to save cost. 
         [0028]    In the embodiments of  FIG. 3  and  FIG. 4 , similarly, if the charger circuit charges the battery by a constant current, the oscillator  14  can be omitted. The low current control circuit  18  does not have to be a stand-alone circuit; it can be part of the switch control circuit  12 . The low current Imin can be a constant current or non-constant current. After the low current Imin has charged the battery for a period of time, the power switch Q 1  can be turned off. 
         [0029]    The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.