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Posted on 01 January 2019

Battery Charge Management

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New and Emerging Trends

The new and emerging generation of handheld devices such as cellular phones, portable media and navigation equipment, Bluetooth headsets continue to shrink in size and form factor while improving performance and features. When it comes to charge management, these improvements force the design engineer to strike a balance between user convenience and experience, total charge time and cost.

By Masoud Beheshti, Texas Instruments Incorporated

 

This is, however, a very challenging task. More advanced features typically translate into the need for higher battery capacity in order to meet more power demands. At the same time, end users are demanding faster charge rates to minimize the charge time, and a more convenient method of charging their devices.

As a result, designers are increasingly focusing their attention on new charge management techniques in order to optimize overall system performance. This article examines some of these trends and applications.

Switchmode Charging Topology

Traditionally, handheld devices use a linear charging topology. This method offers the designer several advantages such as low implementation cost, design simplicity and “quiet” operation due to the absence of high frequency switching. Linear topology, however, introduces power dissipation in the system. Especially as the charge rates increase due to higher battery cell capacity and high difference voltages between the input and battery voltage. This is a major drawback if the designer has no means to manage the thermal issues in the design.

Another major drawback comes into play when the USB port on a PC is used as the power source. The USB charging option is offered on many portable devices today and can provide charging rates of up to 500mA. With a linear solution, due to its low efficiency, the amount of “power” from the PC’s USB used for charging the battery is greatly reduced. This translates into extensive long battery charge time. This is where a switchmode topology come

s to the rescue. The main advantage of a switchmode topology is increased efficiency. Unlike linear regulators, the power switch (or switches) is operated in the saturation region, which substantially reduces the overall losses. The main sources of power loss in a buck converter include both switching and conduction losses (in power switches) and DC and AC losses in the filter inductor. Depending on the design parameters, it is not uncommon to see efficiencies well over 95% in these applications.

Most people picture large ICs, big Power- FETs and supersized inductors when they hear the term switchmode! Although in fact that may be a realistic picture for applications handling tens of amperes of current, it certainly is not true for the new generation of solutions for handheld devices. New generations of single-cell Lithium-Ion (Li-Ion) switchmode chargers provide the highest level of silicon integration and use frequencies above 1MHz to minimize the size of the inductor and capacitors (see Figure 1).[1] The silicon size, which has both high- and low-side PowerFETs integrated, is less than 4 mm2 in area. With a 3MHz switching frequency, this solution requires a small 1uH chip inductor with 2mm x 2.5mm x 1.2mm (WxLxH) in dimensions.

A handleld 3MHz switchmode charger

Wireless (contactless) Charging

Wireless power has been around for a long time and overtime has found applications in many areas. In the industrial area induction heating, for instance, has enabled a practical and efficient way to melt large amounts of metals in a manufacturing environment. In the consumer area it has been used successfully for many years to charge toothbrushes and other small personal care products. However, when it comes to charging the new generation of portable appliances such as cellular phones, portable media players, Bluetooth headsets and so on, it is at its infancy. There are several reasons for this:

• The technology previously used in the consumer market (toothbrushes for instance) was not optimized for efficiency or speed. These chargers “trickle” charged at a low rate overnight, and the form factor was customized to accept only the intended end equipment.
• Most consumers did not own a multitude of portable devices, each with its own power cable and, in many cases, proprietary connectors.

But times are changing. Most consumers are beginning to expect the same type of convenience offered by wireless data transfer when it comes to charging their portable devices. While this concept is simple, it has a number of barriers for design solution and acceptance:

• Unlike a toothbrush, new portable devices need to be charged at a standard fast charge rate, meaning reaching 70% of capacity in about an hour. As a result, the solution needs to be very power efficient.
• Each portable device uses a different size battery and charge rate (i.e., power rating). Therefore, the “one size fits all” rule does not apply. The wireless charger needs to have the intelligence to recognize this and adjust itself accordingly.
• Consumer safety is very important. So the wireless charger not only needs to differentiate between a coin and a cell phone, but it also needs to make certain that no hazardous situations are created under any operating condition.
• Ultimately, consumers will be paying for the “convenience.” Therefore, the wireless charger needs to prove that it is substantially easier to use and operate than the easiest corded charger around!

There are a variety of solutions being developed to address these concerns. A good example is eCoupledTM technology developed by Fulton Innovation (see Figure 2). This technology includes an inductively-coupled power circuit that dynamically seeks resonance. This allows the primary supply circuit to adapt its operation to match the device’s needs, maximizing the power transferring and conversion efficiency. It does so by communicating with each device individually in real time. This allows the technology to determine the power needs as well as factors such as the age of a battery or device and its charging lifecycles in order to supply the optimal amount of power to keep a device at peak efficiency.

Cellular phone being charged wirelessly

USB Charging

As mentioned earlier, the USB power port on personal computers and laptops are increasingly used for transferring data as well as recharging handheld devices. Although this provides another convenient method of charging for end users, it also brings new challenges to the design engineers.

Most USB hubs are capable of providing up to 500mA of current, which can be used to charge the battery in the portable device. However, the portable device needs to have a reliable method of detecting the USB port, the available power level and proper control of the charge circuit to meet the USB specification. It also needs to address startup of the system when the battery pack is completely discharged.

“USB-friendly” chargers need to provide, at a minimum, the following features:

• Selectable Current Level A USB port may be able to provide only 100mA of current for low-power mode. The charger should be configured to support either the 100 or 500mA of current for all applications.

• System Start Up with Dead or Deeply Discharged Battery When the battery pack is completely discharged, it will take minutes before the pack voltage is charged up high enough for the system to start up. To enable an instantaneous turn on of the system, the charger needs to provide an intelligent power path management to route as much of the available current as needed to start up the system. Any “remaining” current is used to charge the battery. The power path management battery charger allows the system to operate while charging the deeply discharged battery simultaneously.

• Inrush Current Control The USB specification allows up to 10uF to be hard-started, which establishes 50uC as the maximum inrush charge value when exceeding 100mA. In order to meet this spec, the charger device needs to provide input current control.

• Protection Against Poor USB Source The charger device needs to prevent against a poorly designed or incorrectly configured USB port. A feature referred to as “input dynamic power management” throttles back on the charge current, if the input voltage (i.e., the output of the USB port) begins to drop or collapse.

Figure 3 shows an example of a USB-compliant charging solution.[2] The latest version of the battery charging standard can be found at: http://www.usb.org/developers/devclass_docs

USB complaint and power path management battery charger

Battery Charge Management Remains Critical

Regardless of the type of topology or connector used, proper battery management techniques are required. These techniques ensure that the batteries are charged to their maximum capacity each and every time without compromising consumer safety or the battery’s cycle life.

Similarly, other types of rechargeable batteries such as Li-Ion batteries must be qualified and possibly conditioned before fast charge. Fast charge is prohibited if the battery voltage or temperature is outside the allowed limits. For safety, any charging of a HOT battery (typically above 45°C) is suspended until the battery cools to the normal operating temperature range. To condition a COLD (typically below 10°C) or over-discharged battery (typically below 3V per cell), a gentle pre-charge current is applied.

Following qualification and pre-conditioning (see Figure 4), Li-Ion batteries are first charged with a current of 1C or less, until the battery reaches its charge voltage limit. This stage of charge typically replenishes up to 70% of the capacity. The battery is then charged with a constant voltage of 4.1 or 4.2V. The constant-voltage limits vary depending on the cell manufacturer and the anode material (coke or graphite). To maximize safety and available capacity and improve the cycle life, the charge voltage must be regulated to ?1%. The higher the voltage regulation accuracy the longer the battery cycle life, and the more battery capacity. During this stage of charge, the charging current drawn by the battery tapers down. Charge is typically terminated once the current level falls below 10–15% of the initial charging current for a 1C-charging rate. In addition to battery temperature monitoring, a safety timer is needed throughout the cycle.

Charge Profile for Lithium-Ion battery

 

References:

1) To download the bq24150 datasheet, visit: http://focus.ti.com/docs/prod/folders/print/bq24150.html
2) To download the bq24073 datasheet, visit: http://focus.ti.com/docs/prod/folders/print/bq24073.html
3) The latest version of the battery charging standard can be found at: http://www.usb.org/developers/devclass_docs
 

For more information about battery management solutions, visit: http://focus.ti.com/en/download/aap/selectiontools/battery-chargers/tool.htm

    

 

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