FEATURE POWER ELECTRONICS
TAMING LITHIUM-ION BATTERIES: Making operation simpler and safer
Khagendra Thapa, sensor and power management business unit director at Diodes Incorporated explores how and why lithium-ion technology has become the technology of choice in satisfying the growing requirement for high power-density in today’s electronic equipment
the market, which are specifically for this purpose. Aside from current regulation, these devices also ensure that batteries aren’t damaged by over- or under-voltage conditions.
T
oday’s electronic equipment designs, especially for portable applications
have become increasingly dependent on rechargeable batteries. Products like smart phones and wearable devices present the conflicting demands of long battery life and small size. Consequently, lithium-ion technology has become the technology of choice in satisfying this requirement for high power-density. However, as has been evident from a number of
high-profile news stories, the use of lithium-ion has not been without its problems, as witnessed by incidents of batteries overheating in aircraft systems, hoverboards, smart phones and fitness watches. Such potential problems, while relatively rare, are understandable because of the inherent ability of a battery to deliver energy. Good battery management, including the ability to detect fault conditions, is clearly a vital obligation that all equipment manufacturers should acknowledge.
THE POWER-DENSITY CHALLENGE The ever-shrinking geometries of semiconductor processes have allowed us to put more processing power into the phones in our pocket than we had in our desktop computers just a few years ago. Another consequence of this technology trend is that supply voltages have reduced, especially the core voltages required for processor and memory chips. This has the benefit that battery-powered equipment can now operate from single-cell batteries but can also mean that these batteries need to be capable of supplying higher currents. Consequently, battery technology has had to
evolve to meet this demand for high-current, high capacity, small form-factor solutions. This has led to innovations in the chemistries, materials and manufacturing processes used, which now allow
22 MARCH 2017 | ELECTRONICS Figure 1:
The AP9234L integrated circuit from Diodes operates with an
extremely low quiescent current of just 3.0µA
batteries to be flexible or to be shaped to fit the available space or contours of an end-product design. Despite these advances, the operation of a battery remains fundamentally unchanged and relies on a chemical process for storing and supplying energy. In turn this requires an appropriate solution to manage the charge and discharge processes to ensure that the currents and voltages involved are within the limits specified for the particular battery type. Other key operating conditions must also be taken into account, particularly to avoid thermal extremes, which are one of the biggest enemies of battery technology.
DEPLOYING BATTERY MANAGEMENT Compared to some battery chemistries, the principle of charging a lithium cell is fairly simple, requiring a constant current at a relatively low voltage, moving to a constant voltage and a trickle current as it nears full charge. In practice this needs to align to parameters set by the battery manufacturer, which may impose quite tight current and voltage tolerances. Clearly the characteristics of a charger need to meet the given requirements of a battery, so it is not just for commercial reasons that equipment manufacturers instruct users to only use the recommended charger. Typically, a DC/DC converter can be configured as a power source for charging rechargeable batteries. While their controllers often include some protection features, it can be highly desirable to use a separate functional circuit block to monitor the charging voltage and current at the point it is applied to the battery terminals. Such a circuit needs to be able to detect and isolate the battery in the event of over- voltage or over-current conditions, respond rapidly to dangerous short-circuits, and protect a battery from the damage that over-discharge can cause. To implement this additional circuitry with discrete
Figure 2:
Functional block diagram of Diodes AP9234L battery protection IC
High temperatures accelerate the chemical processes and degrade a battery’s performance. Where this is uncontrolled, overheating can escalate, sometimes resulting in fire or explosion. Unregulated charge or discharge currents, including short-circuiting a battery’s terminals are the most common cause of increased battery temperature. At the very least, a battery should be protected from such conditions with thermal or electrical fuses. Even better would be the use of a one of the many battery management ICs now available in
components can be costly and space-consuming. Fortunately, there are now single-chip solutions (see figure 1) that integrate the detection and control logic functionality with the power MOSFETs that connect the battery to the charger or load under normal conditions but isolate it when a fault occurs. Designed to protect single-cell lithium-ion rechargeable battery packs in smart phone, wearable and similar applications, the AP9234L integrated circuit from Diodes operates with an extremely low quiescent current of just 3.0μA, while the ultra-low Rss(on) of its dual MOSFETs minimises losses during normal charge/discharge operation. On detecting over-charge or over- discharge voltages or currents it will take protective action after a predetermined delay time. All this is accomplished within a package measuring just 2.5mm by 3.5mm and 0.5mm high. The benefits of lithium-ion battery technology
are well-known but so are the risks if this technology if it isn’t tamed through the inclusion of inexpensive battery protection circuits.
Diodes Incorporated
www.diodes.com T: +49 89 45 49 49 0
/ ELECTRONICS
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