Introduction
Battery-powered portable devices such as cellular phones have become an important part of people's daily lives in the past few years. Many types of adapters are available to charge the lithium-ion (Li-ion) battery and power the system, and their electrical specifications usually differ from one manufacturer to another. This challenges system designers to build portable devices that will meet safety and reliability requirements when used with different adapters. This article describes a new battery car charger front-end (CFE) IC, the Texas Instruments (TI) bq243xx, which is optimized to improve the safety of charging Li-ionpowered systems. Together with the battery car charger IC and the protection module in a battery pack, a charging system using the bq243xx CFE provides more robust system-level protection.
Main safety concerns in charging systems
Damage to the charging system can occur due to input overvoltage, input overcurrent, battery overvoltage, or reverse input voltage. Input overvoltage can be caused by hot-plugging an adapter or using the wrong adapter; or by a transient or steady-state overvoltage condition. The most common occurrences are from hot plugging a charged, unregulated, or incorrect adapter; or from load transients. The unregulated adapter under no load will charge the adapter's output capacitor to the peak rectified AC voltage, about 1.4 times the rated DC voltage. This is often an issue with 'low-voltage-process' (7-V-process) ICs. Figure 1 shows the output voltage of a typical regulated adapter versus an unregulated adapter. Input-overcurrent challenges are not an issue with stand-alone car chargers, since their constant-current mode limits the amount of current delivered to the output or battery. However, with power-path-management parts, which have a direct connection from the input to the system bus voltage, there is often no protection from excessive current draw. Lately there has been some concern over the safety of operating adapters in their current-limit mode and a desire for a programmable inputcurrent- limit circuit to ensure that the adapter does not get into this mode. Li-ion and Li-polymer battery packs are known for the potentially dangerous 'flaming' condition that can occur if they are overcharged under high temperature. The key indication of overcharging is excessive cell voltage. To improve battery safety, many manufacturers are adding second-level overvoltage protection to remove the input power source when battery overvoltage is detected. With universal connectors, it is a concern that an adapter with reverse polarity will be connected to the input. Without input reverse-polarity protection, the parasitic diode between the substrate and the IC will become forward-biased, causing a malfunction or damage to the IC. The two basic solutions to achieving input reversepolarity protection are shown in Figure 2. The first solution is to add a diode in series with the input to block any reverse current. However, this will increase the power dissipation. The second solution is to use a low-rDS(on) MOSFET in series with the input to minimize the power dissipation.
Problems With Li-Ion Termination
The typical problem with applying a system load to the output of a Li-ion battery car charger, while charging the battery, is the loss of properly terminating the charge cycle. This often results in a maximum timeout fault condition which prohibits future charging without cycling the input power. Li-ion battery car chargers designed solely for battery charging make charging and termination decisions based on the current and voltage out of the car charger. Load currents cause three common problems.
If the load current is greater than the taper threshold, then the taper timer is not set, and normal termination does not occur. The taper timer is set once the battery is considered full, typically at one-tenth the fast charge current, to allow 30 minutes of additional charging to insure approximately 100% charge capacity.
If the average load is small, such that the taper timer is set, but has frequent load pulses above the taper threshold, this also continually resets the taper timer and prevents taper termination.
If the car charger is in the precharge mode, typically < 3 V (per cell), then the battery is charged at one-tenth the fast charge rate to determine if the battery will take a charge. Typically after 30 minutes, if the battery voltage is not above 3 V, the car charger declares a dead battery and enters a fault mode. Applying a system load, while the car charger is in precharge conditioning mode, reduces or eliminates the precharge current potentially keeping the cell below 3 V, causing the car charger to enter fault mode.
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by Shuoguan Car Charger