An Ultra-Compact and Efficient Li-ion Battery Charger

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An Ultra-Compact and Efficient Li-ion Battery Charger

  1. 1. An Ultra-Compact and Efficient Li-ion Battery Charger Circuit for Biomedical Applications TOPIC :
  2. 2. INTRODUCTION Analog Li-ion battery charging circuit intended for operation in a wirelessly rechargeable medical implant. <ul><li>Why Li-ion batteries are preferred in medical implants? </li></ul><ul><li>High performance in both energy and power densities. </li></ul><ul><li>Wide variety of shapes and sizes efficiently fitting the devices they power. </li></ul><ul><li>Much lighter than other energy-equivalent secondary batteries. </li></ul><ul><li>No memory effect. </li></ul><ul><li>Components are environmentally safe as there is no free lithium metal. </li></ul>
  3. 3. <ul><li>Battery longevity is a primary concern in implanted medical devices . </li></ul><ul><li>Battery longevity, in turn, is highly sensitive to the accuracy of the final charging voltage on the battery . </li></ul><ul><li>If the Li-ion battery is overcharged, dangerous thermal runaway can occur. </li></ul><ul><li>Deeply discharging the Li-ion battery below 3 V can permanently reduce the cell’s capacity . </li></ul>BACKGROUND
  4. 4. Problems encountered in previous charger designs <ul><li>Unnecessarily complex control circuitry needed. </li></ul><ul><li>This requires more circuit area and power consumption . </li></ul><ul><li>Requires a sense resistor in order to detect end-of-charge. </li></ul><ul><li>Needs precision on-chip resistor fabrication. </li></ul>
  5. 5. Theoretical Li-ion charging profile <ul><li>trickle-charging </li></ul><ul><li>constant current charging </li></ul><ul><li>constant voltage charging </li></ul><ul><li>end-of-charge </li></ul>
  6. 6. Theoretical Li-ion charging profile <ul><li>Trickle-charge region:- During trickle-charge, the battery is charged with a small amount of current. </li></ul><ul><li>Constant current region:- Above 3.0 V, the battery may be charged at higher currents. </li></ul><ul><li>Constant voltage region:- As the battery voltage approaches 4.2 V, the charging profile enters the constant voltage region. </li></ul><ul><li>End-of-charge region:- Charging current should be decreased until a certain threshold is met. </li></ul>
  7. 7. Simplified battery charger block diagram <ul><li>The OTA compares the battery voltage to the 4.2 V band gap reference. </li></ul><ul><li>As the battery voltage reaches 4.1 V the OTA enters the linear region. </li></ul><ul><li>The current gain stage is simply composed of current mirrors. </li></ul><ul><li>All current mirrors in this design including those in the OTA are of the Wilson Current Mirror type in order to reduce channel length modulation error. </li></ul>BLOCK DIAGRAM
  8. 8. Wilson Current Mirror <ul><li>A current mirror is a circuit designed to copy a current through one active device. </li></ul><ul><li>The wilson current mirror circuit eliminates the base current mis-match of the conventional current mirror. </li></ul>Wilson Fudh current source
  9. 9. simple current mirror Wilson current mirror <ul><li>The simple current mirror has two main imperfections: </li></ul><ul><li>The output current differs from the input one because of the two base currents that the transistors Q 1 and Q 2 &quot;suck&quot; from the input current. </li></ul><ul><li>The output current varies when the output (load) voltage changes because of the Early effect in BJT or channel length modulation error in MOSFETS. </li></ul>Disadvantages of simple current mirror
  10. 10. <ul><li>The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current . </li></ul><ul><li>It uses the differential input voltage to produce a gain in current as the output signal. </li></ul><ul><li>The operational transconductance amplifier produce current, while the standard op amp would produce voltage. </li></ul>Operational Transconductance Amplifier Schematic symbol for the OTA . Different types of OTA .
  11. 11. OTA and trickle-charge circuit schematic <ul><li>If the battery voltage is less than 3 V, the Trickle Charge Flag is low enabling M1, thus transistor M2 conducts some current. </li></ul><ul><li>The reduction in charging current during trickle-charge is </li></ul><ul><li>proportional to the ratio of W/L of M2 to the W/L of M6. </li></ul><ul><li>Once the battery voltage crosses the 3 V threshold, the Trickle Charge Flag goes high disabling the current path through M1 and M2. </li></ul>
  12. 12. End-of-charge current comparator <ul><li>The end-of-charge is detected by comparing the output of the OTA to a reference current; </li></ul><ul><li>The End-of-charge Output signal goes low when the OTA output is higher than IREF. </li></ul><ul><li>When the End-of-charge Output signal is high, the last stage of current mirrors in the current gain block is disabled. </li></ul>
  13. 13. Trickle charge threshold detector <ul><li>This low-power detector circuit is to determine when the battery reaches the 3 V threshold. </li></ul><ul><li>This circuit is used to detect critically low battery voltage. </li></ul><ul><li>when critical threshold is reached, the detector circuit cuts off power to the load. </li></ul><ul><li>The designed threshold detector consumes only 3 µW. </li></ul>
  14. 14. Advantages over other charger designs <ul><li>The circuit naturally transitions between constant current (CC) and constant voltage (CV) charging regions. </li></ul><ul><li>Achieving an efficiency of greater than 75%. </li></ul><ul><li>This design does not require sense resistors to determine end-of-charge. </li></ul><ul><li>This design represents a simple, analog, power- and area-efficient version of previous, more complicated and power-hungry designs . </li></ul>
  15. 15. System Performance during trickle-charge <ul><li>The battery was charged with 1.5 mA and 2.2 mA during trickle-charge and constant current, respectively. </li></ul><ul><li>the design can easily be modified if a higher charging current is required. </li></ul>
  16. 16. System Performance during constant current, constant voltage, and end-of-charge. <ul><li>The transition between constant </li></ul><ul><li>current and constant voltage is </li></ul><ul><li>Continuous. </li></ul><ul><li>The end of charge is reached when the current is approximately 0.26 mA. </li></ul><ul><li>Has an accuracy of 99.8%. </li></ul>
  17. 17. Comparison of this design with previous Li-ion charger circuits in the literature . <ul><li>Most of the literature uses the maximum power efficiency </li></ul><ul><li>during charging as a figure of merit for battery chargers. </li></ul>Design Power Efficiency Layout Area Charge-Pump Technique 67.9% 1.96 mm2 Hysteresis-current-controlled buck converter 82% 2.6 mm2 A Multi- Mode LDO-Based Li-ion Battery Charger 72.3% Not Specified This Work 75% 0.15 mm2
  18. 18. CONCLUSION <ul><li>A novel design for a Li-ion battery charger that simplifies the control circuit by using the tanh output current profile of an OTA has been presented and experimentally verified. </li></ul><ul><li>This design does not require the use of sense resistors to determine the end-of-charge point, reducing layout area and charging errors due to resistor variability. </li></ul><ul><li>efficiency can be further improved if one designs the circuit to operate with a lower supply voltage or with an adaptive supply that varies with battery voltage. </li></ul>
  19. 19. REFERENCES <ul><li>D. Linden and T. B. Reddy, Handbook of Batteries. New York: Mc-Graw Hill, 2002, ch. 35. </li></ul><ul><li>Y. S. Hwang, S. C. Wang, F. C. Yang and J. J. Chen, “New Compact CMOS Li-ion Battery Charger Using Charge-Pump Technique for Portable Applications,” IEEE Trans. on Circuits and Systems - part I: Regular papers, Vol. 54, No. 4, pp. 705-712, Apr. 2007. </li></ul><ul><li>F. C. Yang, C. C. Chen, J. J. Chen, Y. S. Hwang and W. T. Lee,” Hysteresis-current-controlled buck converter suitable for Li-ion battery charger,” Proc. of IEEE International Conference on Communications, Circuits and Systems (ICCCAS), pp. 2723-2726, Guilin, China, June 2006 </li></ul><ul><li>P. Li and R. Bashirullah, “A Wireless Power Interface for Rechargeable </li></ul><ul><li>Battery Operated Medical Implants,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 54, no. 10, pp. 912–916, Oct. 2007. </li></ul>
  20. 20. THANK YOU

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