Managing device Li-ion battery power with MCU-controlled pulse width modulation - Embedded.com

Managing device Li-ion battery power with MCU-controlled pulse width modulation

Portable CE devices are achieving improved performance and increasingfunctionality, thus requiring maximized runtimes out of each batterycharge cycle. The addition of a growing number of features is alsocreating a demand for higher capacity batteries.

Li-ion batteries are ideal for avariety of portable-electronic applications because of their high cellvoltage, high density, long shelf life and maintenance free nature. Inaddition to the popular 4.2V charge voltage regulation 1C maximumcharge/ discharge rate, new technology in Li-ion batteries may requiredifferent charge voltages and deliver higher C-rates.

This article discusses some new trends in Li-ion batteries and showshow portable product designers can design a flexible Li-ion batterycharge management system using an MCU-controlled pulse-width modulation (PWM)module or a stand-alone integrated battery charge-managementcontroller-based solution.

Figure1: Charge voltage regulation is important to maximize capacity aftereach charge cycle. An undercharged battery voltage of 0.6 percent canresult in a 5 percent capacity loss for Li-ion batteries.

Design difficulties
Challenges in portable power design with Li-ion batteries includesafety concerns, battery chemistries, available space and requiredfeatures. For rechargeable Li-ion batteries, one must also consider thecharge/discharge rate, life cycles, maintenance and charge algorithm.To maximize the capacity after each charge cycle, chargevoltage-regulation accuracy is important.

Figure 1 above shows thatan undercharged battery voltage of 0.6 percent can result in a 5percent capacity loss for Li-ion batteries. However, overcharging aLi-ion battery is not recommended and can be dangerous.

Battery manufacturers recommend undercharging a 4.2V regulatedLi-ion battery at 4.1V to extend its life for backup energyapplications.

Production challenges are often associated with time-to-market,total system cost and reliability. Time-to-market is significant forconsumer products that have a short product life cycle.

Fast response to market changes is important in today's fast-pacedworld. The short time permitted from concept to final product alsominimizes the resources used and reduces cost by saving design time.

Additional components may result in extra failure factor points and,in some cases, increase in costs. Although it is not true in allapplications, saving space by creating highly-integrated solutions maycost more than a system built from discrete components. Thus,reliability should always come first when designing a product, ifperformance is the trade-off.

Figure2: A typical SEPIC topology multi-cell, multi-chemistrycharge-management system using the MCP1631 high-voltage PWM and thePIC12F683 general-purpose MCU is shown.

Using an MCU
If flexibility is important for product development and the ability tomake changes during a project is necessary, then an MCU directed PWMcontroller battery charge-management system is a perfect fit.

Figure 2 above demonstratesa typical single ended primary inductor converter (SEPIC)topology multicell, multichemistry charge-management system using a highvoltage PWM  and ageneral purpose 8-bitMCU..

Advanced MCUs are available for GPIOs and ADCs for additionalsensing and output status. SEPIC is a switching topology that deliversbetter efficiency and less power dissipations when I/O voltagedifference is wide and current flow is significant.

For example, operating a 9V input voltage when VBAT is 4V andICHARGE is 1A, the power dissipation for popular linear solutions is(9V – 4V) * 1A = 5W and the same condition for a switching solutionwith 90 percent efficiency is only 4W * (0.1/0.9) = 0.44W.

Cooling a 1/2W solution is much easier than cooling a 5W system. Thefollowing equations show the calculations for linear and switchingpower dissipations that are applied in the above example:

Figure 3 below depicts atypical charge profile of an MCU-directed PWM controller for asingle-cell 1,700mAh Li-ion battery with constant current/constantvoltage (CC-CV) algorithm at 1A charge rate. The algorithm starts withprecondition if the battery voltage is below the pre-conditioningthreshold.

Once it passes the pre-conditioning stage, the system goes intoconstant-current stage until a regulated voltage is detected. Thecharge termination value in this example is 200mA.

The system continues monitoring the battery voltage and rechargeswhen it falls below the recharge threshold voltage, to limit the numberof charge/discharge cycles and prolong the battery's life, whilekeeping its voltage at a safe level.

Figure3: A typical charge profile of an MCU-directed PWM controller for asingle-cell 1700mAh Li-ion battery with CC-CV algorithm at 1A chargerate is shown.

Stand-alone system
The main reasons that designers select fully integrated, single-chipbattery charge-management systems are compact size, low cost andminimum design time/effort/ resources.

The stand-alone Li-ion battery charger IC, especially for the lineartopology, may require only SMD capacitors to maintain AC stability andprovide compensation when a battery load is not present. Thus, therequired PCB space and associated components are minimized when usingan integrated solution.

Figure 4 below shows atypical application circuit when a fully integrated battery managementcontroller is applied as a stand-alone battery charger. Since thecharge algorithm and housekeeping circuits are built into the IC, nofirmware is required and the design is straightforward.

Figure4: The required PCB space and associated components are minimized whenusing an integrated battery-management controller.

Semiconductor companies typically deliver good product support inthe form of detailed datasheets and application notes to help designersimplement the battery charger IC into the system.

This saves time-to-market and reduces cost by shortening developmenttime and eliminating software development. On the flip side,inflexibility is a major barrier to standalone charge management ICs intoday's rapidly-changing battery world.

Overcome challenges
The nominal voltage and charge voltages of rechargeable batteriesdepend on chemistry. The differences between chemicals that are usedfor anode and cathode potentials determine battery voltage and otherassociated characteristics such as energy density, internal resistanceetc.

For example, the recommended charge voltage from batterymanufacturers for cobalt and manganese Li-ion batteries is 4.2V, whilethe phosphate Li-ion batteries are recommended to be charged at 3.6V.Although phosphate- based Li-ion batteries can be charged at a higherregulated voltage to maximize the capacity after each cycle, batterylife will decrease as a trade-off.

Table1. A comparison of MCU + PWM controller vs. stand-alone charger IC ispresented.

The MCU-managed system can easily modify voltage regulation,preconditioning threshold voltage, maximum charge current and otherparameters, without changing hardware. The system can be easilymodified for Ni-MH, Ni-Cd sealed lead acid and other secondary batterychemistries, with the proper firmware and some minor hardware updates.

The MCU enables other system intelligence beneficial to portabledevices, such as system monitoring and providing output signals,authentication and communication to avoid end-users accidentally usinglow quality or counterfeit batteries.

Lack of flexibility makes it difficult for integrated systems tocompete against MCU- and PWM-based charge management systems. IC designhouses and semiconductor manufacturers often try to overcome theseissues by offering different preset voltages, selectable orprogrammable currents – preconditoning current, charge current andtermination current – and using external resistors and capacitors toprogram certain parameters.

Often, charge management ICs employ the battery manufacturer'srecommend CC-CV charge algorithm. Safety timers can either beprogrammable or selectable. The system raises a fault flag or shutsdown when the safety timer expires before termination. The safety timeris available to prevent hazards from overcharging Li-ion batteries andto identify a dead battery.

Figure5: A typical stand-alone linear Li-ion battery charge managementcontroller's complete charge profile is shown.

For example, healthy Li-ion batteries move into constant currentstage in a short period of time when proper voltages are applied. Ifsafety timers expire during precondition, the battery may need to bereplaced.

A typical stand-alone linear Li-ion battery charge managementcontroller's complete charge profile is shown in Figure 5 above. Total charge timerequired can vary based on different termination options. At thebeginning of each charge profile, the thermal foldback regulates thedevice temperature when internal power dissipation is high.

The constant current resumes to its maximum programmed value whenthe device temperature is below the maximum value, which improves thecharger's reliability and safety.

The trade-off for this feature is that the full-charge periodincreases slightly. In comparing Figures3 and 5 , thethermal-regulation feature actually delays the full profile byapproximately 7mins, which is not significant in most applications whenthe completed charge cycle is about 3hrs.

Conclusion
Fully-integrated ICs can help designers implement the battery- chargingfunction quickly and at low costs. However, these standard devices donot meet the needs of all portable device designs and designers. Aproduct designer may often have difficulties finding a battery-chargingsolution that meets all of the design requirements. A batterychargemanagement controller IC is usually designed for generalapplications – it is not application specific. Some manufacturersattempt to provide single-chip multi-chemistry solutions.

However, the built-in algorithms associated with these solutions areeither too expensive or not user-friendly. An MCU plus PWMcontroller-based system is ideal for high-end battery charge managementsystems, or designs where the battery chemistry may change with futurerevisions of the product.

Brian Chu is an ApplicationsEngineer in the Analog & Interface Products Division at MicrochipTechnology Inc.

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