THESIS
2017
13 unnumbered pages, 88 pages : illustrations ; 30 cm
Abstract
The present generation lithium-ion batteries with high energy density, light weight, long lifetime and low memory effects have made them an obvious choice amid all the energy storing devices. While the lithium-ion battery technology is advancing in developing cells with higher power and density, improving the performance of the battery management system is equally important to attain 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 and discharging operations.
In this thesis, a fully-integrated low-side sensing Coulomb Counter based on a voltage-to-frequency conversion algorithm with a gain of 1.2MHz/V is designe...[
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The present generation lithium-ion batteries with high energy density, light weight, long lifetime and low memory effects have made them an obvious choice amid all the energy storing devices. While the lithium-ion battery technology is advancing in developing cells with higher power and density, improving the performance of the battery management system is equally important to attain 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 and discharging operations.
In this thesis, a fully-integrated low-side sensing Coulomb Counter based on a voltage-to-frequency conversion algorithm with a gain of 1.2MHz/V is designed and fabricated with a standard 0.35μm CMOS process. It occupies an active chip area of 450 × 225 μm
2. The proposed coulomb counter is capable of monitoring the charging and discharging current through the battery, and indicating the state-of-charge 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 the battery's condition; (3) govern the ACR for incrementing and decrementing the count. For a commercial Lithium-ion 18650 battery, the voltage range is between 2.5V (end voltage of a depleted battery) to 4.2V (maximum voltage of a fully-charged battery). An on-chip low-dropout regulator (LDR) with a dropout voltage of 200mV is designed to provide a constant power supply to all the analog blocks. A self-biased bandgap reference circuit (BGR) with a startup circuit is designed to provide the reference voltages to the appropriate analog blocks, and constant current source to bias the Coulomb Counter system. Except for the sensing resistor of 50mΩ, no other external components are used. Measurement results show that, for a bidirectional currents ranging from −5A to +5A and over a temperature range of 0℃ to +125℃, the maximum gain error is ±0.03%/℃. For a battery voltage range of 2.5V to 4.2 V, the gain variation is 0.04%/V. A small quiescent current is consumed without compromising the accuracy and linearity of measuring the SOC of the battery.
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