Abstract:
A two-step charger includes an AC/DC power supply, a transformer, a compensator, a controller, a comparator and an isolating controller. The comparator detects whether a battery is connected to the transformer. If so, a command voltage of the charger is set at a high level. Since the battery voltage is lower than the command voltage, the controller orders the compensator to send out a compensating current. The current entering the primary side of the transformer is increased to promote the output current from the secondary side of the transformer. Once the comparator detects that the battery reaches as high as the charger, the command voltage is adjusted to a low level. The controller orders the compensator to stop outputting the compensating current. Therefore, a larger current can speed up battery charging. Once the battery is fully charged, the charging voltage is lowered to avoid high temperature.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a charger and, in particular, to a two-step charger that can adjust the level of its charging command voltage depending upon the charging status of the battery. 
   2. Description of Related Art 
   In view of environmental protection, many objects that cannot be decomposed by nature or cause pollution after they are decomposed are phasing out. For example, non-rechargeable batteries contain materials harmful to the environment. Therefore, rechargeable batteries become more popular nowadays. With an appropriate charger, the rechargeable batteries can be repeatedly used. 
   Most of the chargers on the market have the function of two-step charging. That is, they allow the user to decide whether to charge the batteries at a normal speed or a faster speed. The so-called quick charging refers to the charging with a larger current. Although this method can finish charging in a shorter time, people normally do not immediately remove the batteries from the charger once it is done. Therefore, the larger current usually produces a high temperature on the batteries. If the batteries are exposed to the high temperature for a long time, their lifetime may be greatly shortened. 
   Charging at a normal speed refers to the charging with a smaller current. As a result, the charging time is longer. It may take several hours to charge a battery in this scheme. Therefore, it is inconvenient. 
   From the above description, it is seen that current chargers are unable to charge batteries at a quicker speed without sacrificing their lifetime. 
   SUMMARY OF THE INVENTION 
   To achieve a balance between shortening the charging time and maintaining the battery lifetime, the invention provides a two-step charger that adjusts the magnitude of charging current according to the charging status of the battery. 
   To achieve the above objective, the two-step charger includes: an AC/DC power supply, a transformer, a compensator, a controller, a comparator, and an isolating controller. 
   The AC/DC power supply is connected to an AC source and converts the AC current into a first DC current for output. 
   The transformer has a primary side and a secondary side. The primary side is connected to the AC/DC power supply. The secondary side detects the first DC current and outputs a second DC current. The secondary side is connected to batteries in a removable way. 
   The compensator is connected to both ends of the primary side of the transformer and outputs a compensating current to increase the DC current entering the primary side of the transformer. 
   The controller is connected to the transformer and the compensator. 
   The comparator is connected between the secondary side of the transformer and the battery to determine the charging status of the battery. 
   The isolating controller has an emitting end and a receiving end. The emitting end is connected to the comparator. The receiving end is connected to the controller. The isolating controller operates the controller according to the charging status of the battery, so that the compensator sends out a compensating current to change the magnitude of the second DC current. 
   Using the above-mentioned techniques, the disclosed charger uses the comparator to detect whether a battery is connected to the transformer. If so, the controller orders the compensator to send out a compensating current, thereby increasing the current entering the primary side of the transformer and thus the current output by the secondary side of the transformer. Once the comparator detects that the battery is fully charged, the controller stops the compensator from sending out the compensating current. Therefore, the invention can set the charging voltage at a high level to provide a larger current for charging the battery, thus shortening the charging time. After the battery is fully charged, the voltage is lowered to reduce the current entering the battery. This helps reducing the temperature on the battery. Therefore, the battery lifetime is not sacrificed due to quick charging. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram of the preferred embodiment of the invention; and 
       FIG. 2  is a circuit diagram of the preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIG. 1 , a preferred embodiment of the disclosed two-step charger includes: an AC/DC power supply  10 , a transformer  20 , a compensator  30 , a controller  40 , a comparator  50 , an isolating controller  60 , and a charging indicator  70 . 
   The AC/DC power supply  10  is connected to an AC source  11  and converts the AC current into a first DC current I 1  for output. 
   The transformer has a primary side and a secondary side (both not shown). The primary side is connected to the AC/DC power supply  10 . The secondary side detects the first DC current I 1  and outputs a second DC current I 2 . The secondary side is connected to a battery  23  in a removable way. 
   The compensator  30  is connected to the primary side of the transformer  20  and outputs a compensating current I C  to increase the DC current entering the primary side of the transformer  20 . 
   The controller  40  is connected to the transformer  20  and the compensator  30  to control the compensator  30  in outputting the compensating current I C . 
   The comparator  50  is connected between the secondary side of the transformer  20  and the battery  23  to determine the charging status thereof. 
   The isolating controller  60  has an emitting end  61  and a receiving end  62 . The emitting end  61  is connected to the comparator  50 . The receiving end  62  is connected to the controller  40 . The isolating controller  60  operates the controller  40  according to the charging status of the battery  23 , so that the compensator  30  sends out a compensating current I C  to change the magnitude of the second DC current I 2 . Once the comparator  50  detects that the battery  23  is fully charged, the controller  40  stops the compensator  30  from sending out the compensating current I C . A smaller current I 2  is then used to charge the battery  21 . 
   The charging indictor  70  is connected to the battery  23  and the comparator  50  to display the charging status of the battery  23 . 
   A detailed circuit of the above-mentioned embodiment is shown in FIG.  2 . 
   The AC/DC power supply  10  comprises primarily a full-wave rectifier (D 1 ˜D 4 ). 
   The compensator  30  comprises a resistor Ra 1 , a capacitor Ca 1 , and a diode Da 1 . The capacitor Ca 1  and the resistor Ra 1  are connected in parallel before connecting to the diode Da 1  in series. The other ends of the capacitor Ca 1  and the resistor Ra 1  are connected to one end on the primary side  21  of the transformer  20 . The other end of the diode Da 1  is connected to the other end of the primary side  21  of the transformer  20 . 
   The controller  40  mainly includes a control IC U 1  and a metal oxide semiconductor (MOS) transistor Q 1 . The control IC U 1  obtains its work power from the AC/DC power supply  10 . The gate of the MOS transistor Q 1  is connected to the control IC U 1 . The drain is connected to the primary side  21  of the transformer  20  and the diode Da 1 . The source is connected to the ground. 
   The comparator  50  includes a voltage divider  51 , a reference voltage circuit  52 , a first comparator US 1 A, and a second comparator US 1 B. The voltage divider  51  is composed of resistors RS 12 ˜RS 14  connected in series. The resistor RS 12  is connected to the secondary side  22  of the transformer  20  and the battery  23 . The resistor RS 14  is connected to the ground. The reference voltage circuit  52  includes a reference voltage IC  521 . In this embodiment, the reference voltage IC  521  is the TL 431  IC. Its anode A is connected to the ground, while its cathode K is connected to the secondary side  22  of the transformer  20  and the battery  23 . The positive input of the first comparator US 1 A is connected to the reference electrode R of the reference voltage IC  521  to obtain a reference voltage. The negative input is connected between the resistors RS 12  and RS 13  to obtain the partial voltage drop between the resistors RS 13  and RS 14 . The positive input of the second comparator US 1 B is connected to the output of the first comparator US 1 A via a diode DS 2 , and to the reference electrode R of the reference voltage IC  521  to obtain the reference voltage. The negative input is connected between the resistors RS 13  and RS 14  to obtain the voltage drop across the resistor RS 14 . 
   The isolating controller  60  in this embodiment is a photo coupler. The emitting end  61  of the isolating controller  60  is an equivalent light-emitting diode (LED) inside the photo coupler, and is connected between the secondary side  22  of the transformer  20  and the output terminal of the second comparator US 1 B. The receiving end  62  of the isolating controller  60  is the equivalent light-emitting transistor (LET) inside the photo coupler, and is connected to one input terminal of the control IC U 1 . 
   The charging indictor  70  includes an LED LEDS 1  and a transistor QS 1  in this embodiment. The LED LEDS 1  is connected to the secondary side  22  of the transformer  20  and the battery  23 . The base of the transistor QS 1  is connected to the output terminal of the first comparator US 1 A. The collector of the transistor QS 1  is connected to the LED LEDS 1 . 
   The action of the above-mentioned two-step charger is explained with further reference to  FIG. 2 . 
   When the battery  23  is not yet connected to the invention, the second DC current I 2  completely flows into the comparator  50  so that the partial voltage drops between the resistors RS 13  and RS 14  and the resistor RS 14  are both higher than the reference voltage. Therefore, both the first comparator US 1 A and the second comparator US 1 B output a low-voltage signal. The transistor QS 1  cuts off, and therefore the LED LEDS 1  does not emit light. On the other hand, the equivalent LED of the photo coupler emits light, making the equivalent LET of the photo coupler conductive. At this moment, the control IC U 1  sends out a work signal to make the MOS transistor Q 1  conductive. 
   When the invention is connected with a battery  23  without sufficient energy, some of the second DC current I 2  flows into the battery  23 . The partial voltage between the resistor RS 13  and the resistor RS 14  and the partial voltage on the resistor RS 14  are both lower than the reference voltage. Therefore, both the first comparator US 1 A and the second comparator US 1 B output a high-level signal. The transistor QS 1  is thus conductive so that the LED LEDS 1  emits light. The equivalent LED of the photo coupler, on the other hand, does not emit light. Consequently, the equivalent LET of the photo coupler is not conductive, either. In this case, the control IC U 1  detects that the equivalent LET is not conductive and stops sending out the work signal. The MOS transistor Q 1  is off. As a result, the current flowing through the primary side  21  of the transformer  20  cannot pass through the MOS transistor Q 1  and enter the ground. It flows instead into the compensator  30 . The compensator  30  further sends out a compensating current I C , so that the DC current on the primary side entering the transformer  20  is I 1 +I C . Therefore, the second DC current I 2  also increases to charge the battery  23  at a larger current. 
   Once the battery  23  is fully charged, the current entering the battery  23  decreases, raising the current sending into the comparator  50 . The partial voltages between the resistors RS 13  and RS 14  and across the resistor RS 14  are both higher than the reference voltage. Therefore, both the first comparator US 1 A and the second comparator US 1 B output a low-level signal. Therefore, the transistor QS 1  cuts off, and the LED LEDS 1  does not emit light. The equivalent LED of the photo coupler is conductive and thus emits light. As a result, the equivalent LET of the photo coupler is conductive. The control IC U 1  detects the conduction of the equivalent LET and sends out a work signal so that the MOS transistor Q 1  is conductive. Therefore, the current flowing through the primary side  21  of the transformer  20  can pass through the MOS transistor Q 1  and enter the ground, instead of flowing into the compensator  30 . The compensator  30  no longer sends out the compensating current I C . Therefore, the current on the primary side  21  of the transformer  20  is only I 1 , greatly reducing the charging current on the battery  23 . 
   According to the above description, the disclosed two-step charger uses its comparator to detect whether a battery is attached and whether the battery is fully charged. When the battery needs to be charged, the isolating controller orders the controller to increase the output current from the secondary side of the transformer, thereby speeding up the battery charging. Once the battery is fully charged, the isolating controller orders the controller to reduce the output current from the secondary side of the transformer. Therefore, a larger current is used to charge a battery when it requires charging. The current is lowered to the normal range once the battery is fully charged. This avoids high temperature on a fully charged battery. The lifetime of battery therefore does not get shorter. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.