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Implement safe battery charging between gadgets

Posted: 20 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:generic battery charging? safe battery charging between gadgets?

By Shadi Hawawini, George Paparrizos
Summit Microelectronics

One of the challenges consumers, and especially business travelers, face today is finding a common yet practical technique to charge the wide variety of portable devices such as cell phones, wireless headsets, and MP3 players now at our disposal. It's a hassle to carry a number of different (AC/DC) wall adapters, and so is finding a wall plug to access the necessary power. Addressing this challenge requires safe charging methods that allow portable devices with higher energy storage (smartphones, laptops) to charge portable devices with lower power requirements. Several industry initiatives worldwide support this trend to standardize the physical interfaces among devices and wall adapters, allowing for more flexibility and universal charging.

Generic battery charging specs
First let's establish the nomenclature for charging Li-ion cells. The battery pack consists of the battery cell, protection circuitry (i.e., provides overvoltage, undervoltage and overcurrent protection), and connection terminals, all enclosed in a protective pack. One of the most important parameters is the pack voltage, which refers to the voltage at the pack of the battery, and differs from the cell voltage or open circuit voltage, as these are separated by an equivalent series resistor (ESR) that is a function of various chemical and mechanical factors.

The traditional, and most widely-used method for charging Li-ion and lithium-polymer batteries is the constant-current/constant-voltage (CC-CV) technique. Here, we charge the battery with a constant current, usually at 1C (for a 1,000mA-h battery), where 1C is a charge current of 1A, until the pack voltage reaches its float voltage (4.2V to within 1 percent accuracy). Once we reach the float voltage, we apply a constant voltage (CV), in which the voltage is now fixed at float voltage and the current is allowed to taper down towards the so-called termination current level.

System design considerations
Charging timeThe fall-off of current in taper charging is exponential, so there are diminishing returns to setting the termination current too low (Figure 1). In addition, while a longer charge time allows more charge to enter the battery and thus increase the percentage of available battery capacity, the time to secure the incremental increase in battery capacity becomes significantly greater as the battery approaches full charge. Take, for example, a 150mA-h battery. If we set the termination current to 75mA, it will greatly decrease the battery's perceived charge time, but the battery may reach just 85 percent of full capacity. If we set the termination current to 50mA, the battery will charge to perhaps 90 percent, but the charge time will be significantly greater, and so on for smaller termination currents.

Figure 1: Termination current tradeoffs.

Another issue that arises during the CC charging state is the need for high charge-current levels in order to reduce charge time, which generally makes the use of a linear battery charger impractical for thermal reasons. Consider, for instance, an application where a battery discharges to 3.3V as we use a mobile device up to system shutdown, whereupon the charge cycle commences. If the battery is rated at 1,000mA-h battery and we desire a 1C charge rate from a 5V input, the power dissipation in a linear battery charger will be (Equation 1):

PDiss,Linear = IBQ (Vin - Vbatt)

where IBQ is the battery charge current. In this case, the dissipation is 1.7W, which represents an overall charging efficiency of 66 percent. Clearly, a linear charger is not very practical for solutions that require a charger current greater than 500mA. For these other applications, the buck switchmode charging topology is superior for many reasons beyond thermal considerations. From a thermal perspective the buck switch-mode topology is far better because, rather than regulating the output by dissipating extra energy as heat, it regulates by providing energy only when the output requires it and uses energy storage devices to aid in this. Using the same example from above, and assuming the efficiency of the buck regulator is 90 percent, the power dissipation in a buck regulator is (Equation 2),

or just 360 mW.

Another benefit of the buck switch-mode topology is in the use of current limited sources such as a USB port. The buck topology maximizes the effectiveness of the USB port and any current limiting power source using the concept of current multiplication (Summit's TurboCharge technology). By holding the input current constant and using Equation 3,

we can calculate the battery charge current that a buck regulator would produce for a given efficiency and battery voltage.

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