Lithium battery charger and its system optimization

Small, low-cost netbooks, ultra-mobile PCs (UMPCs), and mobile Internet devices (MIDs) are becoming increasingly popular and widely accepted by users. The lithium-ion (Li-Ion) battery charging systems used in these portable devices are much more complex than those used in mobile phones. Understanding the many requirements for their battery chargers is key to improving system safety. This article will discuss many requirements for lithium-ion battery charging, such as charging system safety and performance optimization between the charger and the system. It also introduces a synchronous switching stand-alone battery charger IC controller design example with dynamic power management capabilities to optimize adapter power ratings and fast battery charging for netbook applications.

Netbooks are a fast-growing class of small, lightweight and low-cost laptops used primarily for general computing and accessing network applications. Most netbooks use an Intel Atom microprocessor and two or three lithium-ion battery packs connected in series.

An ultra-mobile PC (UMPC) has a powerful processor, a 4-7 inch display with touch function, and can run software compatible with Windows Vista. An ultra-mobile PC also has a global positioning system (GPS) device, a fingerprint reader, a TV tuner, and a memory card reader. It is powered by 2-3 lithium-ion battery packs connected in series.

A mobile Internet device (MID) is a multimedia-enabled handheld computer that provides wireless network access, two-way communication and real-time sharing capabilities, and is designed to provide entertainment, information and location-based services to individuals (non-enterprise users). MIDs are larger than smartphones but smaller than UMPCs, and are typically powered by a lithium-ion battery pack.

Dynamic Power Management

Battery charge voltage and current are critical to battery life and battery capacity. The higher the battery charge voltage, the higher the battery capacity. These portable devices require less total power than notebook computers due to their low power microprocessors, and the power adapters used are generally less than 40 watts, while notebook computers typically use 60 watt and 90 watt adapters. However, such adapters are still required to power the system while charging the battery, thereby minimizing the adapter power rating. Due to the high pulsating power characteristics of the microprocessor, the total power required to charge the battery and provide maximum power to the microprocessor easily exceeds the maximum available power of the adapter.

To optimize the system and battery charger, we use dynamic power management (DPM) by introducing a maximum adapter current regulation loop. If the input adapter current reaches the regulation threshold, the battery charger automatically reduces the active charge current while giving priority to powering the system so that it does not exceed the maximum power limit of the adapter. After powering the system, the remaining power is used to charge the battery. Once the pulse power ends, the charger automatically resumes the fastest charging mode to shorten the charging time (Figure 1). An important specification is the input current regulation accuracy. The higher the input current regulation accuracy, the more power the adapter can provide and the faster the battery can be charged.

Figure 1. Battery charging structure of dynamic power management

Computing System and Battery Charger Safety

a. Adapter input and battery overvoltage protection (OVP)

For laptops, 19-V and 16-V adapters are commonly used, while 5-V adapters are popular for smartphones. Netbooks, UMPCs, and MIDs often use these adapters to save development costs, but they often do not require a 19V adapter to charge a 1 to 3-series battery pack. In addition, IEEE P1725 requires the system to include input adapter and battery OVP. If these portable devices encounter excessive input voltage, it will prevent you from turning on the system. If the battery is overcharged, the battery charger is immediately turned off. If a reverse adapter voltage occurs, the system cannot be turned on.

b. Battery charging safety

It is dangerous to charge Li-ion batteries under extremely low or high battery temperature conditions. Li-ion batteries with LiCoO2 negative electrode materials may explode when the battery temperature reaches 175oC at 4.3V. Many industrial battery charger safety regulations (such as Japan Electronics and Information Technology Industries Association (JEITA)) have been issued to achieve the purpose of battery safe charging by reducing the battery charging current and voltage at low or high battery temperatures.

To start the charging process, the battery charging temperature range is generally between 0oC and 40oC. Therefore, the battery temperature must be monitored by a fuel gauge or charger. The safety timer is another layer of protection when the battery charging system fails. When the safety timer expires, the battery stops charging.

c. Battery charger output short circuit and overcharge current protection

For some computing applications, the most commonly used lithium-ion battery is the 18650 lithium-ion battery with a capacity of 2200-2600mAh. The charging current is about 2-4A at a 0.7oC charging rate from a 12-V or 19-V adapter. Efficient charging requires a synchronous switching buck topology. It also requires a smoke-free charging system in the event of component failure or abnormal operating conditions (for example: charger output short circuit or inductor short circuit, etc.). The charger needs such protection mechanisms to prevent fire or smoke.

Battery Charger Solutions for Netbooks, UMPCs and MIDs

Based on the system optimization and safety requirements, Figure 2 shows a standalone high-efficiency synchronous switching Li-Ion battery charger with dynamic power management for netbook applications. This design example uses 200mA pre-charge current and 2A fast charge current with a 3-hour safety timer to charge 2 Li-Ion batteries. The DPM function is implemented by monitoring the voltage drop across the input current sensing resistor R1. The synchronous switching charger operates at a switching frequency of 600kHz to optimize efficiency and solution size. The external resistor divider R11 and R12 are used to set the ideal battery charging voltage. To obtain the maximum battery capacity, set the external resistor divider to a charging voltage of 4.2V per battery.

To get the longest battery life, set the battery charging voltage to 4.1V per battery. This can charge 1 to 6 Li-ion batteries in series up to 10A by making the external power MOSFET suitable for many different battery charging applications without a main controller. It also has other protection functions, such as input overvoltage, battery charging overvoltage, battery short circuit, overcharge current protection, etc., and automatically monitors the battery temperature to achieve safe charging.

Figure 2 Netbook charger design example

As portable computing devices evolve and gain more features, battery charging and system design become the most important design factors to achieve high safety and high performance systems.