Examination Committee

Prof Philip K T MOK, ECE/HKUST (Chairperson)
Prof Wing Hung KI, ECE/HKUST (Thesis Supervisor)
Prof Jiang XU, ECE/HKUST
 

Abstract

The properties of the present generation Lithium-ion batteries, such as high energy density, light weight, longevity and low memory effects, have made it an obvious choice amid all the energy storing devices. While Lithium-ion battery technology is flourishing every day such as developing cells with higher power and density, improving the performance of its battery management system is equally necessary for an optimal performance. One of the key functions of a battery management system is to provide an accurate state-of-charge (SOC) of the battery which basically conveys the amount of charge stored in the battery during charging or discharging operation.
 
In this thesis, a fully integrated low-side sensing Coulomb counter based on a voltage-tofrequency conversion algorithm with a gain of 1.2MHz/V is designed and fabricated with a standard 0.35μm CMOS process. The proposed Coulomb counter is capable of monitoring the charging and discharging current through the battery, and indicating the SOC of the battery by incrementing and decrementing the count of an accumulating charge register (ACR). Low-side sensing eliminates the problem of using high-voltage devices. A ground-sensing polarity detector is proposed to (1) identify the charging and discharging condition of the battery by detecting the voltage in the order of a few ±mV across the sensing resistor; (2) define the initial switching status of the system based on battery’s condition; (3) govern the ACR for incrementing and decrementing count. For a commercial Lithium-ion 18650 battery, the voltage range is between 2.5V to 4.2V. To accommodate the voltage range, a low-dropout regulator (LDR) with a dropout voltage of 200mV is designed to provide a constant VDD = 2.3V to all the analog blocks. A reference voltage of 1.25V to the LDR is supplied through a bandgap reference (BGR) circuit. A self-biased BGR with a startup circuit is designed to provide a constant VH = 1.25V and VL = 0.85V to the comparator and constant current source for biasing the overall circuit. Except for the sensing resistor, no other external bulky and expensive components are used. The proposed Coulomb counter system is first simulated with extracted layout over all the corners (process, temperature, and supply variations) and then evaluated in the lab over several sample that demonstrates a small quiescent current is consumed without compromising the accuracy and linearity of measuring the SOC of the battery at different corners.