Power Management DesignLine | How to Charge Li-Ion Batteries for Portable Devices More Efficiently

August 04, 2005

How to Charge Li-Ion Batteries for Portable Devices More Efficiently

By Jinrong Qian, Senior Member of Technical Staff, Texas Instruments

Driven by integrated functionality and shrinking form-factors, the demand for portable devices such as cellular phones, PDAs and portable DVD players has grown significantly during the last few years. The power source has quickly become a bottleneck for advancing technology, and improvement of battery-power density cannot keep-up with demand. The lithium-ion (Li-Ion) battery is widely-adopted because of its high energy density on both a gravimetric and volumetric basis. To achieve longer system run-time and smaller size, more and more system designers are finding that improving a system’s power conversion efficiency with advanced circuit topologies is no longer sufficient. Battery charging has become another area to focus on for maximizing battery capacity and extending its service cycle-life. The linear charger is suitable for the low-capacity battery charge applications with its reasonable cost and small-size advantages. With the increasing power demand from portable devices, the linear battery charger no longer adequately meets charge requirements due to its high-power dissipation. This paper presents a MHz synchronous switching battery charger and design considerations to efficiently charge the battery and extend its cycle-life.

How to Charge a Li-Ion Battery
Figure 1 shows a popular charge profile recommended for a Li-Ion battery. Most dedicated Li-Ion-charge integrated circuits (ICs) are designed to charge the battery in this manner. The charging of a Li-Ion battery consists of three phases: pre-charge; fast-charge constant current (CC); and constant voltage (CV) termination. In the pre-charge phase, the battery is charged at a low-rate (typical of 1/10 the fast charge rate) when the battery cell voltage is below 3.0 V. This provides recovery of the passivating layer which might be dissolved after prolonged storage in deep discharge state. It also prevents overheating at 1C charge when partial copper decomposition appears on anode-shorted cells on over-discharge. When the battery cell voltage reaches 3.0 V, the charger enters to the CC phase.

Figure 1: Li-Ion battery charge profile

Fast-charge current should be limited to 1C rate (0.7°C rate) to prevent overheating and resulting accelerated degradation. However, cells designed for high power capability can allow higher charge rates. Rates should be selected so that the battery temperature does not exceed 50°C at the end of charge. The battery is charged at the fast-charge rate until the battery reaches a voltage regulation limit (typical of 4.2 V/cell, but 4.1 V for coke-based anodes Li-Ion battery). The charger starts to regulate the battery voltage and enters CV phase while the charge current exponentially drops to a defined termination level. However, the output voltage regulation accuracy is critical to maximizing battery capacity and improving its service life. Less battery voltage regulation accuracy means to undercharge the battery, which results in a large decrease in battery capacity. The battery loses about eight percent capacity if it is undercharged by one percent voltage. On the other hand, less battery voltage regulation accuracy also means the battery is overcharged, which reduces the battery service life-cycle. To safely charge the Li-Ion battery, it only allows initiating to charge the battery when the ambient temperature is between 0°C to 45°C. Charging the battery at lower temperatures promotes formation of metallic Lithium, which increases the battery impedance and causes cell degradation. On the other hand, charging the battery at higher temperatures causes accelerated degradation because of promoting Li-electrolyte reaction. This presents a market need for more accurate, efficient and safe battery charge for portable devices.

The charger simply drops the adapter’s DC voltage down to the battery voltage. The power dissipation across the pass element equals adapter voltage minus the battery voltage times the charge current, which is given by

If a 5-V adapter is used to charge a 1200-mAh or 2200-mAh single cell Li-Ion battery, Figure 3 shows its power dissipation with 0.7 C rate fast-charge current. It has maximum power dissipation of 1.68 W and 3.0 W when the battery transitions from pre-charge to fast-charge phase, respectively. Power dissipation at 3.0-watt results in 141°C temperature rise for a 3 mm X 3 mm QFN package with 47°C/W thermal impedance. This definitely exceeds maximum 125°C silicon junction operating temperature at 25°C ambient temperature. The tolerance on the fast- charge current regulation and the AC adapter voltage is also very important in a linear charger. If the regulation tolerance is loose, the pass transistor and package will need to be oversized, adding to size and cost. The fundamental issue for the linear charger is its high power dissipation. A trade-off must be made between the charge current, size, cost and thermal requirements of the charging system. Therefore, the linear charger is usually suitable for the low capacity (less than 1300 mAh) Li-Ion battery applications because of their superior size, cost considerations and thermal issue. How to solve the thermal issue for high capacity battery packs or high input-to-output voltage difference applications? The answer is the high-efficiency synchronous switching battery charger.

High Efficiency MHz synchronous Switching Battery Charger with Integrated Power MOSFETs
Synchronous switching mode charging solutions are generally used in the applications that have high input-to-output voltage difference or for high capacity battery packs. For a 2200 mAh Li-Ion battery pack, it is extremely difficult to use a linear charger to charge a single cell battery from a car adapter (12 V) at a fast charge rate of 0.5°C to 1°C. A linear charger with thermal regulation could be used, but the charge cycle time at the reduced charge rate will be extremely long.

Figure 4 shows a standalone high efficiency synchronous switching buck battery charger with charge current up to 2A for portable devices such as DVD players and smart phones. It uses 1.1-MHz switching frequency voltage mode control architecture with internal type III loop compensator to minimize the external components. To further minimize the battery charger size, it has integrated two power MOSFETs into the PWM controller in a small 4 mm 4 mm package. The power MOSFETs Q1 and Q2 are complementarily turned on and off with optimized dead time to optimize the efficiency at high switching frequency. Q1 is used as P-channel MOSFET to eliminate an external boost strap capacitor and a diode when used for high side N-MOSFET gate driver and it is also easy to achieve 100 percent duty cycle when the input voltage is very close to the battery voltage by completely turning on Q1. The turn-on and turn-off time are controlled to regulate the battery charge current (CC phase) or battery voltage (CV phase) depending on the feedback control loops. The charger has highly integrated functions to safely and healthily charge the Li-Ion battery. It is able to program the pre-charge current, fast charge current, charge voltage, charge timer, battery temperature monitoring, automatic recharging, short circuit and over temperature protection. The circuit parameters are designed for the following specifications in the following design example.

Adapter DC voltage: 12 V
Two-cell Li-Ion battery pack: 4.2 V/cell, 1900 mAh/Cell
Pre-charge current: IPRE-CHG=133 mA
Fast charge current: ICHG=1.33 A
Charge time limit: tCHG = 5-hour
Temperature range for initiating charge: T= 0°C-45°C.

Since the size of the battery charger is very critical for portable devices, the output inductor is required to be as small as possible. For a given inductor ripple current, the required inductance is given by

Where f_s and ΔI_ripple,L are the switching frequency and inductor ripple current, respectively. Substituting V_IN=12 V, V_BAT=6.0 V (3.0 V/cell), ΔI_ripple,L=30 percent I_CHG, I_CHG=1.33 A and f_s=1 MHz into the above equation yields L=7.5 μH. Selecting a 10-μH shielded inductor. Note that the shielded inductor has better capability to restrain the magnetic flux within the inductor and minimize the radiated electromagnetic interference (EMI). It is shown that the required inductance is inversely proportional to the switching frequency. The inductance can be 10 times smaller and it has a smaller size at 1 MHz than at 100 kHz On the other hand, the higher the switching frequency, the higher the switching loss across Q1 and Q2 and the higher the core loss in the inductor as well. Therefore, 1MHz switching frequency is a good compromise for a practical design between inductor size and power conversion efficiency.

Selecting inductor current rating is also critical for achieving desired efficiency. The peak inductor current I_Peak is calculated by:

The inductor has maximum ripple current when the battery voltage is half of the input voltage. Therefore, the inductor saturation current rating should be always higher than the maximum peak inductor current at all operating conditions.

It is important to select a small ESR ceramic output capacitor with good temperature characteristics such as X7R and X5R ceramic capacitor. The ripple current flowing into the battery is given by

Where ESR, R_SNS and R_BAT are the equivalent series resistance of the output capacitor, current-sensing resistor and battery internal impedance including the Rdson of the protection MOSFETs in the battery pack, respectively. The smaller the ESR of the output capacitor, the smaller the ripple current going to the battery. The ripple current to the battery should be designed less than one-tenth of the inductor ripple current and a 10 μF/10 mΩ ESR ceramic capacitor is usually enough to meet this requirement.

* Select the current sensing resistor R_SNS
Choose R_SNS based on the regulation threshold V_IREG across the sensing resistor. Let V_IREG=133mV for achieving standard sensing resistor value. Solve for R_SNS

* Select fast charge current set resistor R_SET1. R_SET1 is used to set the fast charge current, R_SET1 is given by:

* Select pre-charge current set resistor R_SET2. R_SET2 is used to set the pre-charge current and is determined by:

* Select the maximum charge time set capacitor CTTC The charge timer detects the “bad” battery pack if the battery has not been fully-charged when the charge timer is expired. CTTC is used to program the charge timer and it provides 2.6minute per nF.

* Select minimum and maximum battery charge temperature set resistors R_T1 and _RT2 R_T1 and R_T2 are used to program the battery charge temperature range between 0°C and 45°C for initiating the battery charger. For a commonly used 103AT-2 thermistor in the battery pack, R_T(0°C)=R_TL=27.28kΩ, R_T(45°C)=RTH=4.911 kΩ, the R_T1 and R_T2 are determined by:

Substituting RTL and RTH into the above equations yields RT1=9.31 kΩ and RT2=442 kΩ.

Figure 5 shows the efficiency curves with various input voltages. It is shown that it has over 90 percent efficiency even at 16-V input voltage. Compared with the linear charger, the power dissipation is much smaller and it is possible to design the synchronous-switching charger in the battery pack’s side to minimize the use of mother board’s real estate since the inductor size is small due to MHz frequency operation. One important thing to remember is that the battery’s service life is strongly dependent on its temperature. Charging the Li-Ion battery with synchronous switching charger basically generates less heat. As a result, it has longer service cycle-life compared with the linear charger.

Conclusion
The linear charger is suitable for the low-capacity battery charge applications with low cost and small size advantages. Continuing power demand increases from portable devices such as portable DVD players and smart phones, making the linear battery charger no longer efficiently adequate to charge the Li-Ion battery due to its inherent high power dissipation limitation. A high-efficiency synchronous switching battery charger with integrated MOSFETs provides an efficient charging solution for those advanced portable devices with less heat and longer battery service life.

Author Info
Jinrong Qian is an applications manager and senior member of technical staff for the portable power battery management group at Texas Instruments. He has published more than 30 peer-reviewed power electronics journal and conference papers and holds 19 U.S. patents. Dr. Qian’s primary research interests include high-efficiency power converter topologies, portable power supply design and battery power management ICs and their applications. Dr. Qian was an Associate Editor of the IEEE Transactions on Power Electronics from 1999 to 2001 and a senior member of IEEE in 1999. He earned a bachelors of science degree in Electrical Engineering from Zhejiang University and a Ph. D. from Virginia Polytechnic Institute and State University.
Jinrong Qian, Senior Member of Technical Staff, Portable Power Battery Management Applications Manager Texas Instruments, 12500 TI Blvd, MS8709, Dallas, TX75243