Abstract:
A batter charger for charging a secondary batter using a power supply circuit which converts an AC input into a DC output, includes a first resistor for detecting constant-current control and a second resistor for detecting end of charging. The first resister and the second register are inserted in series in a current path of the charging current. The power supply circuit has output characteristics of a constant-current control characteristic and a constant-voltage control characteristic. The constant-current control is performed using a first detection voltage generated at the first resistor, and the constant-voltage control is performed by comparing a second detection voltage generated at a series resistor composed of the first resistor and the second resistor with a reference voltage using a comparator, and detecting an end of charging indicated by the second detection voltage fallen below the reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese patent Application No. 2007-193326 filed in the Japanese Patent Office on Jul. 25, 2007, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present application relates to a batter, charger for a charging battery pack of secondary batteries. 
     Battery chargers for charging secondary, batteries using commercial power sources have been known. The present inventors have already proposed a battery charger described in Japanese Patent No. 3430264 (Japanese Unexamined Patent Application Publication (KOKAI) No. H06-14473: Patent Document 1). 
       FIG. 1  shows a configuration similar to that shown in the Patent Document 1. Commercial alternating current (referred to as “AC” for convenience&#39; sake, hereinafter) power source is converted into a DC power source by an input filter  1  and a rectifier circuit  2 . A switching power source includes a pulse width modulation control circuit  3 , a transistor Q 1 , and a transformer T 1 . The transistor Q 1  as a switching element performs switching operation, for example, at 100 kHz, by output pulses from the pulse width modulation control circuit  3 . Rectified output of a diode D 1  and a capacitor C 1 , connected to a tertiary winding N 3  of the transformer T 1 , is supplied as a power source of the pulse width modulation control circuit  3 . 
     The transistor Q 1  regulates current flowing through a primary winding N 1 , and correspondent electric power is induced on a secondary winding N 2  and the tertiary winding N 3 . A voltage induced on the secondary winding N 2  is rectified by a diode D 2  and a capacitor C 2  to obtain a rectified output Vo. The rectified output Vo is extracted through a switching unit  4  composed of an FET F 1 , an FET F 2 , and a transistor Tr 1  and the like, between output terminals  5   a  [positive(+)side] and  5   b  [negative(−)side]. 
     A secondary battery BAT such as a lithium ion secondary battery, is connected between the output terminals  5   a  and  5   b . The secondary, battery BAT is connected in attachable/detachable manner to/from the battery charger. The battery charger includes a switch SW for detecting attachment/detachment of the secondary battery BAT. Upon attachment of the secondary battery BAT, the switch SW turns on, and a detection signal Batt at L (which means LOW level, the same applies hereinafter), indicating that the secondary battery BAT is attached, is supplied to a controller  11  composed of a microcomputer. 
     The rectified output Vo is divided by a resistor R 7  and a resistor R 8  to input to the negative(−)terminal of an operation amplifier AMP 1 . On the other hand, the positive(+)terminal of the operation amplifier AMP 1  is supplied with a reference voltage REF 1 . The output voltage Vo is compared with the reference voltage REF 1 , and an error signal indicating difference from the reference voltage is supplied to a photocoupler PH 1  via a diode D 3 . 
     The error signal transmitted from the secondary side to the primary side of the photocoupler PH 1  is supplied to the pulse width modulation control circuit  3 . The pulse width modulation control circuit  3  controls an ON period of output pulses from the transistor Q 1 , so as to control electric power to be supplied to the secondary side, whereby an output voltage set by the reference voltage on the secondary side is extracted. 
     An output (charge) current Io is detected by a resistor R 2 . The load-side (output-side) terminal of the resistor R 2  is connected to the negative terminal of an operation amplifier AMP 2  via a resistor R 5 . A voltage obtained by dividing the reference voltage REF 1  by resistors R 4  and R 6  is supplied to the positive terminal of the operation amplifier AMP 2 , to thereby raise voltage level at the positive terminal of the operation amplifier AMP 2 . 
     Flow of output current Io induces voltage drop over the resistor R 2  ascribable to the output current Io. As a consequence, a voltage divided be the resistors R 4  and R 6  decreases. Increase in the output current Io causes further voltage drop at the positive terminal of the operation amplifier AMP 2 . When the potential at the positive terminal of the operation amplifier AMP 2  falls down to the potential at the negative terminal or therebelow, the output signal from the operation amplifier AMP 2  shifts from H (which means HIGH level, the same applies hereinafter) to L. 
     The output signal from the operation amplifier AMP 2  is supplied to the pulse width modulation control circuit  3  via a diode D 4  and a photocoupler PH 1 , so that the power control is performed similarly to voltage control. More specifically, voltage drop occurs at the positive terminal of the operation amplifier AMP 2  depending on the amount of current flowing through the resistor R 2 , the potential of the positive terminal is compared with that of the negative terminal, and the amount of output current is controlled to keep voltage generated at the resistor R 2  constant. The output current is regulated at a constant level in this way. 
     A predetermined voltage stabilized from an output voltage V o  by the regulator  12  is supplied to the controller  11  as a source voltage. An LED (light emitting diode)  13  as a display element, indicating the state of charging operation, is connected to the controller  11 . 
     The switching unit  4  is operated by drive pulse signals DR 1 , DR 2 , and DR 3  outputted from the controller  11 . When the controller  11  detects the attachment of the secondary battery BAT by receiving the detection signal Batt, charging operation starts to perform a predetermined charging operation under monitoring of battery voltage Vbatt. 
     The above-described battery charger charges the secondary battery BAT based on a CC-CV (constant current-constant voltage) charging system, which is a combined system of constant-current charging and constant-voltage charging.  FIG. 2  shows output characteristics of the above-described battery charger. The abscissa represents charging current, and the ordinate represents charging voltage. The battery charger first operates in the constant-current control mode, for example, at 1.0 A, and then operates in the constant-voltage control mode, for example, at 4.2 V. In the initial charging mode in the early stage of charging, the charging at initial charging current I f  is proceeded. When the voltage reaches a rapid switching voltage, for example, at 2.7 V, the charging mode switches to a rapid charging mode. 
       FIG. 3  shows time-dependent changes(charging curve) in the charging voltage and charging current during charging. For example, the constant-current control proceeds in a region where the battery voltage is as high as the constant-voltage control voltage (4.2 V, for example) or below, whereby the constant-current charging is performed under a constant current (1.0 A, for example). When the battery voltage (internal electromotive force) elevates to reach 4.2 V as a result of charging, the battery charger switches the operation into those under the constant-voltage control, whereby the charging current gradually decreases. When the charging current is detected to reach the end-of-charging detection value I s , the end-of-charging is detected. From this point in time, a float timer activates, and the battery is charged until the time-out to terminate the charging of the battery. The charging adopts the floating timer, because the capacity may slightly be increased even after the point in time when the end-of-charging is detected. 
     In the configuration shown in  FIG. 1 , during the constant-current charging, the output of the operation amplifier AMP 2  is supplied to the photocoupler PH 1  via the diode D 4 , and the power source is regulated to give constant output current. During the constant-voltage charging, the output of the operation amplifier AMP 1  is supplied to the photocoupler PH 1  via the diode D 3 , and the power source is regulated by the output voltage of the operation amplifier AMP 1  so as to give constant output voltage Vo. In the configuration shown in  FIG. 1 , one end of the load-side of the current detecting resistor R 2  is connected to the negative terminal of the comparator  6 , the other end of the load-side is connected to the negative side of a reference voltage REF 2 , and the positive side of the reference voltage REF 2  is connected to the positive terminal of the comparator  6 . The charging current is converted into voltage by the resistor R 2 , and the voltage is compared with the reference voltage REF 2 . When the charging current decreases, the reference voltage at the positive terminal of the comparator  6  becomes larger than the detection voltage at the negative terminal thereof, whereby an output Cs of the comparator  6  inverts. The output Cs of the comparator  6  is supplied to the controller  11 , and the controller  11  detects the end of charging. 
     The known battery charger which detects the end of charging in this way needs to provide the reference voltage REF 2  for detecting end of charging, and needs to use a precision-offset comparator having a small offset voltage as the comparator  6  for detecting end of charging, which is an expensive component. 
     A similar battery charger is described also in Japanese Unexamined Patent Application Publication (KOKAI) No. 2007-20299 (Patent Document 2). 
     The Patent Document 2 proposes a method of improving sensitivity of current detection. The method is performed by switching the resistance value for detecting charging current to a larger value when the charging current decreases to fall below a set value. In this case, a change-over switch for changing the resistance value is necessary. In any attempt of using, for example, an FET element for the switch, it is necessary to select an FET considerably small in the resistance value, which leads to require an expensive FET. 
     SUMMARY 
     Accordingly, it is desirable to provide a battery charger, which may be configured at low cost, without using any expensive elements or change-over switch in a configuration for detecting the end of charging. 
     In accordance with an embodiment, a battery charger for charging a secondary battery using a power supply circuit which converts an AC input into a DC output, is provided which includes a first resistor for detecting constant-current control and a second resistor for detecting end of charging. The first register and the second register are inserted in series in a current path of the charging current. The power supply circuit has output characteristics of a constant-current control characteristic and a constant-voltage characteristic. The constant-current control is performed by using a first detection voltage generated at the first resistor. The constant-voltage control is performed by comparing a second detection voltage generated at a series resistor composed of the first resistor and the second resistor with a reference voltage using a comparator, and detect an end of charging indicated by the second detection voltage fallen below the reference voltage. 
     According to an embodiment, it is possible to increase detection level of current when the charging completes, so that the use of an expensive small-offset comparator may become unnecessary. Consequently, a low-cost battery, charger can be implemented, and the reference voltage REF 2  may be set more easily. 
     Furthermore, according to an embodiment, the detecting resistor for detecting constant-current control and the resistor for detecting end of charging are connected in series, whereby providing advantages in that current value for constant-current control and current value for end-of-charging detection may independently be set by setting a resistance value of the individual resistors, without altering the internal reference voltage level, and in that the degree of freedom in designing current setting may be expanded, making the setting more easier. 
     The battery charger may include a forward-biased diode to a charging current, which is connected in parallel with the resistor for end-of-charging detection. According to such construction, if voltage generated at the resistor becomes equal to or larger than the forward-biased voltage drop of the diode, the current may be bypassed via the diode. As a result, the loss of resistance value may be reduced. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a connection diagram of an example of a known battery charger; 
         FIG. 2  is a chart showing the output characteristics of the known battery charger; 
         FIG. 3  is a chart showing voltage and current changes during the charge operation of the known battery charger; 
         FIG. 4  is a connection diagram of a battery charger according to an embodiment; 
         FIG. 5  is a flowchart illustrating processes according to an embodiment; 
         FIGS. 6A and 6B  are graphs showing voltage and current changes at the individual components according to an embodiment; and 
         FIG. 7  is a graph showing voltage and current changes at the individual components, obtained when a Schottky diode connected in one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Paragraphs below will explain an embodiment, referring to  FIG. 4 . A battery charger shown in  FIG. 4  has a configuration improved from the known battery charger shown in  FIG. 1 , and charges the secondary battery BAT based on a CC-CV (constant current-constant voltage) charging system, which is a combined system of constant-current charging and constant-voltage charging. An output voltage Vo rectified by the diode D 2  and the capacitor C 2  is applied to both ends of the secondary battery BAT via the switching unit  4 . 
     The change of the output voltage Vo is detected by the resistor R 7 , the resistor R 8 , the operation amplifier AMP 1 , and the reference voltage REF 1 , and is regulated to keep a desired voltage by the pulse width modulation control circuit  3 . 
     In an embodiment, a resistor R 22  for detecting end of charging is connected in series to a resistor R 21  for detecting charging current. In parallel with the resistor R 22 , a forward-biased diode with respect to the charging current Io, which is a Schottky diode D 5 , for example, is connected. 
     The resistor R 21  corresponds to the resistor R 1  in the configuration of  FIG. 1 . More specifically, the load-side (output-side) terminal of the resistor R 21  is connected to the negative terminal of the operation amplifier AMP 2  via the resistor R 5 , whereas the positive terminal of the operation amplifier AMP 2  is supplied with the voltage obtained by dividing the reference voltage REF 1  by the resistors R 4  and R 6 , whereby the potential at the positive terminal of the operation amplifier AMP 2  is elevated. 
     The voltage drop ascribable to the output current occurs at the resistor R 21  by flowing the output (charge) current Io. As a consequence, the voltage divided by the resistors R 4  and R 6  is lowered. Furthermore, the increase in the output current Io further lowers voltage at the positive terminal of the operation amplifier AMP 2 . When the positive terminal of the operation amplifier AMP 2  is brought dose to the same potential as the negative terminal thereof or below, the output signal of the operation amplifier AMP 2  shifts from H to L. 
     The output signal of the operation amplifier AMP 2  is supplied to the pulse width modulation control circuit  3  via the diode D 4  and the photocoupler PH 1 , whereby the power control is performed by the pulse width modulation circuit  3  on the primary side, similarly to voltage control. More specifically, the positive terminal of the operation amplifier AMP 2  causes voltage drop depending on the amount of current flowing through the resistor R 21 , the resultant potential is compared with that of the negative terminal, and the amount of output current is then controlled so as to keep voltage generated at the resistor R 21  constant. Thus, the output current is regulated at a constant value. 
     One end of the power source side (input side) of the resistor R 22  is connected to the negative side of the reference voltage REF 2 , and the positive side of the reference voltage REF 2  is connected to the positive terminal of the comparator  6 . The negative terminal of the comparator  6  is connected to one end of the load-side (output-side) of the resistor R 21 . The comparative output Cs of the comparator  6  is supplied to the controller  11 . 
     When the output current Io is large, the comparative output Cs of the comparator  6  is L, whereas if the output current Io falls below a predetermined current value, which is 0.1 A, for example, the output of the comparator  6  goes H. As a result, the device enters the end-of-charging detection mode. 
     A transistor Tr 1 , the FET F 1 , and the FET F 2  contained in the switching unit  4  are switched respectively by the drive signals DR 1 , DR 2 , and DR 3  outputted from the controller  11 . The controller  11  is supplied with the detection signal Batt generated by the switch SW which indicates whether the secondary battery BAT is attached. The battery voltage Vbatt is extracted from the connection point of the FET F 1  and the FET F 2  connected in series, and the battery voltage Vbatt is then supplied to the controller  11 . The LED  13  indicating the state of charging is connected to the controller  11 . 
     The charging control by the controller  11  will be explained, referring to a flowchart shown in  FIG. 5 . 
     Upon detecting attachment of the secondary battery BAT by the switch SW, the detection signal Batt goes L, whereby the charging operation starts. 
     In step S 1 , the drive signal DR 1  goes H to thereby turn FET F 1  off, the drive signal DR 2  goes L to thereby turn the FET F 2  on, and the drive signal DR 3  goes L to thereby turn the transistor Tr 1  on. As a consequence, the secondary battery BAT is initially charged through the transistor Tr 1 , a resistor R 10 , and the FET F 2 . In the initial charging mode, the LED  13  kept turned off in the standby mode illuminates. 
     The initial charging current If is expressed by the equation (1) below. In the equation (1), Vtr represents emitter-collector voltage of the transistor Tr 1 .
 
 If =( Vo−Vtr )/  R 10   (1)
 
     In step S 2 , whether the battery voltage Vbatt exceeds a predetermined voltage, which is 2.7 V, for example, is judged. When the battery voltage Vbatt is judged as exceeding the predetermined voltage, which is 2.7 V, for example, a rapid timer activates in step S 3 , whereby the device enters a rapid charging mode. 
     In the rapid charging mode (step S 4 ), the drive signal DR 1  goes L to thereby turn the FET F 1  on, the drive signal DR 2  goes L to thereby turn the FET F 2  on, and the drive signal DR 3  goes H to thereby turn the transistor Tr 1  off. As a consequence, the secondary battery BAT is charged through the FET F 1  and the FET FF 2 . In the rapid charging mode, the LED  13  is kept illuminated. 
     In the rapid charging mode, whether the rapid timer has expired is judged in step S 5 . If the rapid timer is judged as having not expired, the end-of-charging is judged in step S 6 . If the charging current falls below a predetermined value, which is 0.1 amperes, for example, and the end-of-charging is judged, the detection signal Cs of the comparator  6  goes H. If the rapid timer has not expired and the end-of-charging is not detected, the rapid charging mode in step S 4  continues. 
     If it is judged by the rapid timer as having expired in step S 5 , or if the end-of-charging is detected in step S 6 , the device enters the end-of-charging detection mode in step S 7 . In the end-of-charging detection mode, the drive signal DR 1  goes L to thereby turn the FET F 1  on, the drive signal DR 2  goes L to thereby turn the FET F 2  on, and the drive signal DR 3  goes H to thereby turn the transistor Tr 1  off, and a timer for float charging (float timer) activates. As a consequence, the secondary battery BAT is charged through the FET F 1  and the FET FF 2 . In the end-of-charging detection mode, the LED  13  is turned off. The end-of-charging is informed to the user by the lights-out of the LED  13 . 
     In step S 8 , whether the float timer has expired (time-out) is judged. If the float timer is judged as having expired, the device enters a charging stop mode in step S 9 . In the charging stop mode, the drive signal DR 1  goes H to thereby turn the FET F 1  off, the drive signal DR 2  goes H to thereby turn the FET F 2  off, and the drive signal DR 3  goes H to thereby turn the transistor Tr 1  off. By turning the switching unit  4  off, the charging current is interrupted, and the charging of the secondary battery BAT comes to the end. The LED  13  is still kept turned off. 
     As shown in  FIG. 6A , the constant-current charging proceeds in the region where the battery voltage is lower than the constant-voltage control voltage (4.2 V, for example), and thus the constant-current charging is performed under a constant charging current (1.0 A, for example). If the battery voltage V (internal electromotive force) elevates to reach 4.2 V as a result of charging, the battery charger switches its operations into those of the constant-voltage control, whereby the charging current gradually decreases. When the charging current is detected as having reached the end-of-charging detection value Is, the end-of-charging is detected. From this point in time, a float timer is activated, and the battery is charged until the time-out to terminate the charging of the battery. These charging operations are similar to those take place in the battery charger shown in  FIG. 1 . 
       FIG. 6B  shows voltage changes on both ends of the resistor R 21  for detecting charging current, and voltage changes on both ends of the series circuit (end-of-charging detecting circuit) composed of the resistor R 21  for detecting charging current and the resistor R 22  for detecting end of charging. Exemplary conditions include R 21 =0.1 Ω, R 22 =0.9 Ω, rapid charging current Ic=1.0 A, and end-of-charging current Is=0.1 A. 
     In the period of constant-current control in which the charging current is kept constant at 1.0 A, voltage drop (detection voltage V 21 =Ic×R 21 =1.0 A×0.1 Ω=0.1 V) occurs at the resistor R 21 . The resultant 0.1 V is set equal to the reference voltage to be inputted to the positive terminal of the operation amplifier AMP 2 , whereby a charging current of 1.0 A flows through the resistor R 21 . As a result, the potential at the positive terminal of the operation amplifier AMP 2  goes down to reach the same potential as the negative terminal of the operation amplifier AMP 2 , whereby the power control such as keeping the charging current 1.0 A unchanged is performed. 
     At this time, the voltage drop (detection voltage V 2122 =Ic×(R 21 +R 22 )=1.0 A×(0.1+0.9)Ω=1.0 V) occurs at the series circuit composed of the resistors R 21  and R 22 . This voltage is applied to the negative terminal side of the reference voltage REF 2 . The detection voltage V 2122 , which generates between the resistors R 21  and R 22  connected in series, is compared by the comparator  6  with the reference voltage REF 2 . Because the series resistor circuit has a larger resistance value than that of the single resistor R 21 , the detection voltage V 2122  becomes larger than the detection voltage V 21 . In the constant-current charging, the output Cs of the comparator  6  is L. 
     When the mode shifts from the constant-current charging to the constant-voltage charging, the charging current gradually decreases, whereby the detection voltage V 2122  decreases. When the charging current falls down to the end-of-charging current Is=0.1 A or below, V 2122  is given as (V 2122 =0.1 A×(0.1+0.9) Ω=0.1 V). The voltage having the same value as this voltage is given as the reference voltage REF 2  connected to the positive terminal side of the comparator  6 . In this case, the output level of the comparator  6  shifts from L to H. If judges as the shift of the output level of the controller  11  from L to H, the device enters the end-of-charging detection mode. 
     Referring now to the known battery charger shown in  FIG. 1 , the detection of the constant current Ic and the end-of-charging current Is using the resistor R 2  will be discussed under the conditions same as those described in the above, assuming resistors as R 2 =R 21 =0.1 Ω, and voltage drop V 2  occurs at the resistor R 2 . Since the current value at the end of charging is Is=V 2 /R 2 , (V 2 =Is×R 2 =0.1 A×0.1 Ω=0.01 V) is given. 
     This voltage value is only 1/10 as large as V 2122 =0.1 V which is the above-described voltage value in one embodiment. This means that it is necessary to set the reference voltage REF 2  for end-of-charging detection to an extremely small value, and to use a precision-offset comparator having a small offset voltage, and is therefore expensive, as the comparator  6 . An embodiment of the present application can solve this problem, and improves accuracy of the end-of-charging detection. 
     Moreover, an embodiment can reduce power loss at the resistor R 22 , because the Schottky diode D 5  is connected in parallel with the resistor R 22 . The detection voltage V 2122  shown in  FIG. 6B  is obtained in the absence of the Schottky diode D 5 , and is given as V 2122 =0.9 V under the constant-current control. The forward-biased voltage drop Vf caused by the Schottky diode D 5  is 0.4 V, for example. As shown in  FIG. 7 , the detection voltage V 2122  is suppressed to as low as 0.4 V or below by the Schottky diode D 5 . As a consequence, the loss at the resistor R 22  may be halved or below. Any diodes other than Schottky diode may be adoptable, wherein those causative of possibly minimum forward-biased voltage drop may be preferable in view suppressing the loss. 
     An embodiment of described in the above may take effects as below. 
     1) By virtue of the configuration in which the resistor R 22  is connected in series with the current-detecting resistor R 21  for constant-current control, and voltage generated at the series resistors (R 21 +R 22 ) is compared with the reference voltage REF 2  using the comparator, the detection level of at the end of charging may be elevated, whereby comparators of the general offset level may be adoptable. Accordingly, the reference voltage REF 2  may be set more easily. 
     2) The current of the constant-voltage control may be set by the resistance value of the resistor R 21 , and also the current value of the end-of-charging detection may be set by the resistance value of the resistor R 22  connected in series thereto, so that the degree of freedom in setting current may be expanded without altering the internal reference voltage value, which brings about advantageous of facilitating the circuit design. 
     3) By connecting the diode D 5  in parallel with the resistor R 22  added in series for the end-of-charging detection, the current may be bypassed through the diode if voltage generated at the resistor R 22  becomes equal to or larger than the forward-biased voltage drop of the diode, so that the loss occurred at the resistor R 22  may be reduced. Reduction of the loss occurred at the resistor R 22  may provide significant advantages, under relatively large charging current. 
     For example, a plurality of secondary batteries may be charged at a time, and the power supply circuit outputting the charging voltage and charging current may be anything other than those shown in one embodiment. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.