Two-step battery charger
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.
1. A two-step charger comprising:
an AC/DC power supply connected to an AC source and converting an AC current to a first DC current for output;
a transformer having a primary side and a secondary side, the primary side connected to the AC/DC power supply, the secondary side outputting a second DC current based on the first DC current and connected to a battery in a removable way;
a compensator connected to both ends of the primary side of the transformer and sending a compensating current to increase a DC current entering the primary side of the transformer;
a controller connected to the transformer and the compensator;
a comparator unit connected between the secondary side of the transformer and the battery to determine a charging status of the battery, the comparator unit comprising;
a voltage divider comprising first, second, and third resistors connected in series, the first resistor connected to the secondary side of the transformer and the battery, and the third resistor connected to the ground;
a reference voltage circuit comprising a reference voltage IC to provide a reference voltage;
a first comparator having a positive input connected to the reference voltage IC to obtain the reference voltage, and having a negative input connected between the first resistor and the second resistor to obtain the voltage drop across the second and third resistors; and
a second comparator having a positive input connected to the reference voltage IC and output terminal of the first comparator via a diode, and having a negative input connected between the second resistor and the third resistor to obtain the voltage drop across the third resistor; and
an isolating controller having an emitting end and a receiving end, the emitting end connected to the comparator unit, and the receiving end connected to the controller;
wherein the isolating controller operates the controller according to the charging status of the battery, so that the compensator sends the compensating current to change the magnitude of the second DC current.
2. The two-step charger as claimed in claim 1 , the compensator comprising a resistor, a capacitor, and a diode, wherein the capacitor and the resistor are connected in parallel and then connected to the diode in series, the other ends of the capacitor and the resistor are connected to one end of the primary side of the transformer, and the other end of the diode is connected to the other end of the primary side of the transformer.
3. The two-step charger as claimed in claim 1 , the controller including a control IC and a MOS transistor having a gate, a drain and a source, wherein the control IC obtains a work power from the AC/DC power supply, the gate of the MOS transistor is connected to the control IC, the drain is connected to the primary side of the transformer and the compensator, and the source is connected to the ground.
4. The two-step charger as claimed in claim 1 further comprising a charging indicator connected to the compensator for displaying the charging status of the battery.
5. The two-step charger as claimed in claim 4 , the charging indicator including a light-emitting diode (LED) and a transistor, wherein
a positive terminal of the LED is connected to the secondary side of the transformer and the battery; and
a base of the transistor is connected to the output terminal of the first comparator, and a collector of the transistor is connected to a negative terminal of the LED.
6. The two-step charger as claimed in claim 1 , wherein the isolating controller is a photo coupler, wherein
the emitting end of the isolating controller is an equivalent LED inside the photo coupler and connected between the secondary side of the transformer and the output terminal of the second comparator; and
the receiving end of the isolating controller is an equivalent light-emitting transistor inside the photo coupler and connected to the controller.
7. The two-step charger as claimed in claim 2 , the controller including a control IC and a MOS transistor having a gate, a drain and a source, wherein the control IC obtains a work power from the AC/DC power supply, the gate of the MOS transistor is connected to the control IC, the drain is connected to the primary side of the transformer and the compensator, and the source is connected to the ground.
8. The two-step charger as claimed in claim 7 further comprising a charging indicator connected to the compensator for displaying the charging status of the battery.
9. The two-step charger as claimed in claim 8 , the charging indicator including a light-emitting diode (LED) and a transistor, wherein
a positive terminal of the LED is connected to the secondary side of the transformer and the battery; and
a base of the transistor is connected to the output terminal of the first comparator, and a collector of the transistor is connected to a negative terminal of the LED.
10. The two-step charger as claimed in claim 9 , the isolating controller being a photo coupler, wherein
the emitting end of the isolating controller is an equivalent LED inside the photo coupler connected between the secondary side of the transformer and the output terminal of the second comparator; and
the receiving end of the isolating controller is an equivalent light-emitting transistor inside the photo coupler and connected to the controller.