Sub-miniature high efficiency battery charger exploiting leakage inductance of wall transformer power supply, and method therefor

• US 6,100,664 A
• Filed: 03/31/1999
• Issued: 08/08/2000
• Est. Priority Date: 03/31/1999
• Status: Expired due to Term

• Current Assignee: Amperex Technology Limited (TDK Corporation)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Geren, Michael D., Oglesbee, John Wendell
• Primary Examiner(s): Riley, Shawn
• Assistant Examiner(s): LUK, LAWRENCE W
• Application Number:
  US09/282,726
• Time in Patent Office: 496 Days
• Field of Search: 320/125, 320/139, 320/140, 320/129, 323/222
• US Class Current: 320/125
• CPC Class Codes: H02J 2207/20 Charging or discharging characterised by the power electronics converter
• CPC Class Codes: H02J 7/02 for charging batteries from ac mains by converters
• CPC Class Codes: Y02B 40/00 Technologies aiming at improving the efficiency of home appliances, e.g. induction cooking or efficient technologies for refrigerators, freezers or dish washers

Abstract

A battery charger (100) suitable for sub-miniaturization and connection to a wall transformer power supply (20) to charge a battery (30). The battery charger (100) features a switch (130) that controls flow of current from the transformer (20) either to output terminals for charging the battery (30) or to ground, a voltage regulator (120), a microprocessor (110), a current sensing resistor (150) and a Schottky diode (140). The microprocessor (110) is coupled to the switch (130) to control whether the switch is open or closed. The secondary leakage inductance of the wall transformer (20) is exploited to control charging of the battery. The microprocessor (110) is programmed to initiate a charging mode comprising oscillation between a conduction interval and a flyback interval. A charging pulse is delivered to the battery (30) during the flyback interval.

Claims

1. A battery charger comprising:

input terminals for a connection to a wall transformer power supply to receive a supply of current;

output terminals for connection to a battery to be charged;

a switch coupled to the input terminals that controls flow of current from the transformer either to the output terminals for charging the battery or to ground;

a resistor connected in series with the switch and the input terminals;

a diode connected between the input terminals and output terminals so as to permit current flow to the input terminals when the voltage at the input terminals is greater than the voltage at the output terminals;

a controller coupled to the switch to control whether the switch is open or closed, coupled to the resistor to monitor current flow through the resistor and coupled to the output terminals to monitor voltage in the battery, the controller determining when there is sufficient residual charge in the battery to initiate a charging mode comprising oscillation between first and second intervals, during the first interval the controller closing the switch and creating a short-circuit current path through the switch wherein current flows through the switch and increases at a rate dependent on a secondary leakage inductance of the transformer until the current through the resistor reaches a first threshold, and during the second interval the controller opening the switch so that the current from the transformer flows through the diode into the battery and through the resistor and decreases at rate dependent on the secondary leakage inductance of the transformer until it reaches a second threshold which is less than the first threshold.

2. The battery charger of claim 1, wherein the controller sets the first threshold to be approximately four times a desired average charging current for the battery and sets the second threshold to be approximately three times the desired average charging current for the battery.

3. The battery charger of claim 1, wherein the controller continuously and periodically determines an average charging current delivered to the battery by measuring voltage across the resistor.

4. The battery charger of claim 3, wherein the controller adjusts the first and second threshold to control the average charging current to a desired value.

5. The battery charger of claim 1, wherein the controller determines whether voltage is available at the input terminals, and when it is determined that insufficient voltage is available at the input terminals, the controller turns and maintains the switch off and enters a low power standby mode.

6. The battery charger of claim 1, wherein when voltage at the output terminals representative of the charge on the battery is less than a predetermined value, the battery charger enters a backup mode of operation wherein the diode is biased to conduct current from the transformer to the output terminals in order to charge the battery until the controller determines that the charge on the battery is greater than a predetermined value.

7. The battery charger of claim 6, and further comprising a non-linear temperature sensitive current limiting resistor connected between the input terminals and the diode, wherein the non-linear temperature sensitive current limiting resistor regulates current flow to the battery during the backup mode of charging.

8. The battery charger of claim 1, and further comprising a super capacitor connected in parallel with the output terminals to reduce voltage ripple.

9. A method for charging a battery with current from a wall transformer power supply, comprising steps of:

(a) coupling a switch to receive current from a wall transformer power supply;

(b) closing the switch so that current from the wall transformer power supply is short-circuited through the switch and permitted to increase at a rate dependent on a secondary leakage inductance of the wall transformer;

(c) monitoring the current through the switch and determining when it reaches a first threshold;

(d) opening the switch when the current through it reaches the first threshold, thereby coupling current from the wall transformer power supply to the battery and allowing the current to decrease at a rate dependent on the secondary leakage inductance of the wall transformer;

(e) monitoring the current through the switch and determining when it reaches a second threshold; and

(f) repeating steps (b)-(e).

10. The method of claim 9, wherein the first threshold is approximately four times a desired average charging current for the battery and the second threshold is approximately three times the desired average charging current for the battery.

11. The method of claim 9, and further comprising the step of adjusting the first and second thresholds so as to control the average charging current delivered to the battery.

12. A battery charger comprising:

input terminals for connection to a wall transformer power supply to receive a supply of current;

output terminals for connection to a battery to be charged;

a switch coupled to the input terminals that controls flow of current from the transformer either to the output terminals f or charging the battery or to ground;

a current sensing resistor connected in series with the switch and the input terminals;

a diode connected between the input terminals and output terminals so as to permit current flow to the input terminals when the voltage at the input terminals is greater than the voltage at the output terminals;

a microprocessor coupled to the switch to control whether the switch is open or closed, coupled to the current sensing resistor to monitor current flow therethrough and coupled to the output terminals to monitor voltage in the battery, the microprocessor being programmed to initiate a charging mode comprising oscillation between a conduction interval and a flyback interval when the microprocessor determines there is sufficient residual charge in the battery, wherein during the conduction interval the microprocessor closes the switch and creates a short-circuit current path through the switch wherein current flows through the switch and increases at a rate dependent on a secondary leakage inductance of the transformer until the microprocessor determines that current through the current sensing resistor reaches the upper threshold, and during the flyback interval the microprocessor opens the switch so that current from the transformer flows through the diode into the battery and through the current sensing resistor and decreases at a rate dependent on the secondary leakage inductance of the transformer until it reaches the lower threshold which is less than the upper threshold; and

a voltage regulator coupled to output terminals and to the microprocessor to control power to the microprocessor.

13. The battery charger of claim 12, wherein the microprocessor is programmed to set the upper threshold to be approximately four times a desired average charging current for the battery and sets the lower threshold to be approximately three times the desired average charging current for the battery.

14. The battery charger of claim 12, wherein the microprocessor is programmed to continuously and periodically determine an average charging current delivered to the battery by measuring current through the current sensing resistor.

15. The battery charger of claim 14, wherein the microprocessor is programmed to adjust the upper and lower thresholds based on the average charging current to control the average charging current to a desired value.

16. The battery charger of claim 12, wherein the microprocessor is programmed to determine when there is insufficient voltage available at the input terminals to turn and maintain the switch off and enter a lower power standby mode.

17. The battery charger of claim 12, wherein when voltage at the output terminals representative of the charge on the battery is less than a predetermined value, the battery charger enters a backup mode of operation wherein the diode is biased to conduct current from the transformer to the output terminals in order to charge the battery until the microprocessor determines that the charge on the battery is greater than a predetermined value.

18. The battery charger of claim 17, and further comprising a non-linear temperature sensitive current limiting resistor connected between the input terminals and the diode, wherein the non-linear temperature sensitive current limiting resistor regulates current flow to the battery during the backup mode.

19. The battery charger of claim 12, and further comprising a super capacitor connected in parallel with the output terminals to reduce voltage ripple.