Linear and switching charging solutions for CR1220 battery

　　Li-ion charging

　　Charge or discharge rates are often expressed in terms of battery capacity. This speed is called the C rate. C rate is equal to the charge or discharge current under specific conditions and is defined as follows:

　　I=M×Cn

　　in:

　　I=charge or discharge current, A

　　M=C multiples or fractions

　　C=value of rated capacity, Ah

　　N = number of hours (corresponds to C).

　　A battery discharged at 1x C rate will release its nominal rated capacity within an hour. For example, if the nominal capacity is 1000mAhr, then a discharge rate of 1C corresponds to a discharge current of 1000mA, and a rate of C/10 corresponds to a discharge current of 100mA.

　　Usually the battery capacity specified by the manufacturer refers to n=5, that is, the capacity of 5 hours of discharge. For example, the above battery can provide 5 hours of working time when discharged at 200mA constant current. Theoretically, the battery can provide 1 hour of working time when discharged at a constant current of 1000mA. However, in fact, due to the reduced performance of the large battery when it is discharged, the working time at this time will be less than 1 hour.

　　So how can you charge a lithium-ion battery correctly? The most suitable charging process for CR1220 battery can be divided into four stages: trickle charging, constant current charging, constant voltage charging and charge termination. Refer to Figure 1.

　　Phase 1: Trickle Charging - Trickle charging is used to first precharge (recovery charge) fully discharged battery cells. When the battery voltage is lower than about 3V, the battery is first charged with a maximum constant current of 0.1C.

　　Stage 2: Constant current charging - When the battery voltage rises above the trickle charging threshold, the charging current is increased for constant current charging. The current of constant current charging is between 0.2C and 1.0C. The current during constant current charging does not need to be very precise, quasi-constant current is also acceptable. In linear charger designs, current often rises as battery voltage rises to minimize thermal issues on the pass transistor.

　　Constant current charging greater than 1C will not shorten the entire charging cycle time, so this approach is not advisable. When charging at higher currents, the battery voltage rises more rapidly due to overvoltage from the electrode reactions and the voltage rise across the battery's internal impedance. The constant current charging phase will become shorter, but since the time of the following constant voltage charging phase will increase accordingly, the total charging cycle time will not be shortened.

　　Stage 3: Constant voltage charging - When the battery voltage rises to 4.2V, constant current charging ends and the constant voltage charging stage begins. For optimal performance, the regulation tolerance should be better than +1%.

　　Stage 4: Charge Termination – Unlike nickel batteries, continuous trickle charging of CR1220 battery is not recommended. Continuous trickle charging can cause plate plating effects on lithium metal. This can make the battery unstable and potentially cause sudden automatic rapid disintegration.

　　There are two typical charging termination methods: using minimum charging current judgment or using a timer (or a combination of both). The minimum current method monitors the charging current during the constant voltage charging stage and terminates charging when the charging current decreases to the range of 0.02C to 0.07C. The second method starts from the start of the constant voltage charging phase and terminates the charging process after two hours of continuous charging.

　　The above four-stage charging method takes about 2.5 to 3 hours to charge a fully discharged battery. Advanced chargers also incorporate additional safety measures. For example, if the battery temperature exceeds a specified window (usually 0°C to 45°C), charging will be suspended. Lithium-Ion Charging—System Considerations

　　To complete the charging process quickly and reliably requires a high-performance charging system. To achieve a reliable and cost-effective solution, the following system parameters should be considered during design:

　　input source

　　Many applications use extremely cheap wall adapters as input power. Its output voltage depends primarily on the AC input voltage and the load current drawn from the wall adapter.

　　The AC bus input voltage on standard wall sockets in the United States generally ranges from 90VRMS to 132VRMS. Assume the rated input voltage is 120VRMS and the tolerance is +10%, −25%. The charger must provide appropriate voltage regulation to the battery so that it is not affected by the input voltage. The input voltage of the charger is proportional to the AC bus voltage and charging current:

　　VO=2VIN×a-1O(REQ+RpTC)-2×VFD

　　REQ is the sum of the resistance of the secondary winding and the reflected resistance of the primary winding (Rp/a2). RpTC is the resistance of pTC and VFD is the forward voltage drop of the bridge rectifier. In addition, the loss of the transformer core will also slightly reduce the output voltage.

　　Applications that use car adapters for charging will encounter similar problems. The output voltage of a car adapter typically ranges from 9V to 18V.

　　Constant current charging rate and accuracy

　　The choice of topology for a specific application may be dictated by the charging current. Many large constant current charging applications or multi-cell charging applications use switching charging solutions to achieve higher efficiency and avoid excessive heat generation. Due to size and cost considerations, low- and mid-range fast charging applications tend to use linear solutions. However, linear solutions lose more energy in the form of heat. For linear charging systems, the tolerance of constant current charging becomes extremely important. If the voltage regulation tolerance is too large, the pass transistor and other components will need to be larger, increasing size and cost. In addition, if the constant current charging current is too small, the entire charging cycle will be prolonged.

　　Stable accuracy of output voltage

　　In order to fully utilize the battery capacity as much as possible, the output voltage regulation accuracy is very critical. A small decrease in output voltage accuracy can also result in a large reduction in battery capacity. However, due to safety and reliability considerations, the output voltage cannot be set too high arbitrarily. Figure 2 shows the importance of output voltage stabilization accuracy.

　　Charging termination method

　　There is no doubt that overcharging is always a major concern when charging CR1220 battery. Accurate charge termination method is critical to a safe and reliable charging system.

　　Battery temperature monitoring Generally, the temperature range of lithium-ion battery charging should be between 0℃ and 45℃. Charging the battery outside this temperature range can cause the battery to overheat. Rising pressure within the battery during a charge cycle can also cause the battery to swell. Temperature is directly related to pressure. As the temperature rises, the pressure will also become excessive, which may cause mechanical rupture or material leakage inside the battery, and in severe cases, may lead to explosion. Charging the battery outside this temperature range can also harm the battery's performance or shorten the battery's life expectancy.

　　Usually thermistors are used in lithium-ion battery packs to accurately measure battery temperature. The charger detects the resistance of the thermistor. When the resistance exceeds the specified operating range, that is, when the temperature exceeds the specified range, charging is prohibited.

　　Battery discharge current or reverse leakage current

　　In many applications, the charging system remains connected to the battery even when input power is not present. The charging system must ensure that minimal current is drawn from the battery when input power is not present. The maximum leakage current should be less than a few microamps, and usually less than one microampere.

　　Lithium-Ion Charging – Application Examples

　　By fully considering the above system considerations in advance, a suitable charging management system can be developed.

　　linear solution

　　Linear charging solutions are often used when a well-regulated input power supply is available. In such applications, the advantages of linear solutions include ease of use, small size, and low cost. Because linear charging solutions are inefficient, the most important factor affecting the design is the thermal design. Thermal design is the input voltage, charging current, and thermal resistance between the pass transistor and the ambient cooling air. The worst-case scenario occurs when the device transitions from the trickle charge phase to the constant-current charge phase, where the pass transistor must dissipate maximum thermal energy, and a trade-off must be made between charge current, system size, cost, and thermal requirements.

　　For example, the application requires a 5V±5% input power supply to charge a 1000mAh single-cell lithium-ion battery at a constant current of 0.5C or 1C. Figure 3 shows how Microchip's MCp73843 can be used to form a low-cost, stand-alone solution that requires only a minimal number of external components to implement the required charging algorithm. MCp73843 perfectly combines high-precision constant current charging, constant voltage regulation and automatic charge termination functions.

　　To further reduce the size, cost and complexity of linear solutions, many external components can be integrated into the charge management controller. Advanced packaging can provide higher integration, of course, but also sacrifice some flexibility. Such packaging requires advanced production equipment and in many cases avoids rework. Charge current detection, pass transistors, and reverse discharge protection are typically integrated. In addition, this type of charge management controller will also implement certain thermal regulation functions. The thermal regulation function can limit the charging current according to the device die temperature, thereby optimizing the charging cycle time while ensuring the reliability of the device. The thermal regulation function greatly reduces the workload of heat dissipation design.

　　The fully integrated linear solution based on Microchip MCp73861 is shown in Figure 4. MCp73861 contains all the functions of MCp73843, plus current sensing, pass transistor, reverse discharge protection and battery temperature monitoring.

　　Charging cycle waveform

　　The entire charging cycle using MCp73843 at 1C and 0.5C constant current charging rates is shown in Figure 5. When charging at a 0.5C rate instead of a 1C rate, the charge ends about an hour later. MCp73843 will reduce the charge termination current in proportion to the charging current during fast charging. The result is a 36% increase in charging time and a 2% increase in battery capacity while also reducing power loss. The charge termination current dropped from 0.07C to 0.035C, causing the final battery capacity to increase from ~98% to ~100%. System designers must make tradeoffs between charging time, power consumption and available battery capacity.

　　Switched charging solutions

　　Applications with wide input voltage fluctuations or large differences between input and output voltages often use switching charging solutions. In such applications, switching solutions have the advantage of increased efficiency, but the disadvantages are system complexity, relatively large size and high cost. For example, in an application, a car adapter needs to be used to charge a 2200mAh single-cell lithium-ion battery with a constant current of 0.5C or 1C. Due to heat dissipation and other issues, it is extremely difficult to use a linear solution. Of course, a linear solution that supports thermal regulation can also be used. , but the extension of the charging cycle caused by reducing the charging current is unacceptable.

　　The first step in designing a successful switch-mode charging solution is to choose a design architecture: buck, boost, buck-boost, flyback, single-ended primary inductor (SEpIC) or other formats. Based on input and output requirements and experience, the choices for the application can be quickly narrowed down to two architectures: buck or SEpIC. The advantage of a buck converter is that it requires only one inductor, while the disadvantage is that it requires additional diodes for reverse discharge protection, high-side gate drive and current sensing, and pulsed input current (which can cause EMI). The advantages of the SEpIC topology are low-side gate drive and current sensing, continuous input current, and DC isolation between input and output. Its main disadvantage is the need for two inductors and an energy transfer capacitor.

　　MCp1630 is a high-speed pulse width modulator (pWM) that can be used with a microcontroller. With the microcontroller, MCp1630 can control the duty cycle of the power supply system and provide output voltage or current stabilization function. The pIC16F684 microcontroller can be used to regulate output voltage or current, as well as adjust switching frequency and maximum duty cycle. The MCp1630 generates duty cycle and provides fast overcurrent protection based on different external inputs. External signals include input oscillator, reference voltage, feedback voltage, and current sense. The output signal is a square wave pulse. The power supply structure used by the charger is SEpIC. Microcontrollers provide great design flexibility. In addition, the microcontroller can also communicate with the battery monitor (Microchip's pS700) in the battery pack, thereby greatly shortening the charging cycle time.

　　Charging cycle waveform

　　The entire charging cycle using a switched charging solution is shown in Figure 6. By using a battery monitor in the charging system, the charging cycle can be greatly shortened. Using a battery monitor eliminates the need to detect the voltage at both ends of the battery pack protection circuit and the contact resistance of the charging current.

　　in conclusion

　　Properly implementing battery charging in today's portable products requires careful design considerations. This article discusses linear and switching charging solutions for CR1220 battery. It discusses the guiding principles and design considerations that virtually all battery charging system designs need to consider.

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