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About new battery charging solutions suitable for lithium-ion batteries

by:dcfpower     2021-03-24

With the increasing demand for high-efficiency, light-weight, fast-charging, safe, and cost-effective portable power supplies, the market has already developed nickel-metal hydride batteries NIMH, rechargeable alkaline lithium batteries and lithium polymer batteries (Li-poly). Have been developed. In order to maximize performance and ensure safety, advanced semiconductor devices of the same level that can charge and protect these batteries have also been developed on the market. Today we talk about the new battery technology, such as: Maxim Integrated, Linear Technology and Texas Instruments and other semiconductor suppliers' new charging solutions for lithium-ion batteries.

Battery technology

In the field of portable electronic products, some new rechargeable battery chemicals are emerging, which will compete with the most popular nickel-cadmium battery (NiCd) technology. Nickel-cadmium battery (NiCd) technology is still suitable for applications such as power tools that require short-term high current and low impedance characteristics.

However, designers of new portable applications such as smartphones, tablets, and digital cameras are looking for batteries with higher capacity and lower discharge rates than nickel-cadmium batteries (NiCd). In addition, these applications require fast-charging, light-weight batteries. Battery technologies that meet this requirement include nickel metal hydride batteries (NiMH), lithium ion batteries (li-ion) and lithium polymer batteries (Li-poly). NiMH batteries (NiMH) have high capacity and fast charging capabilities, but the disadvantage is that they have a relatively high discharge rate, which is twice that of nickel-cadmium batteries (NiCd) (Table 1).

Table 1: Main battery parameters of chemical composition type

Li-polymer exhibits higher capacity and higher capacity than nickel-cadmium batteries and nickel-hydrogen batteries A significantly lower discharge rate. In addition, according to the application note, lithium-ion batteries are lighter. As a result, it is estimated that the nickel-metal hydride battery (NiMH) capacity per unit mass of lithium-ion batteries is almost twice that.

However, there are several issues with lithium-ion batteries that need attention. Maxim pointed out that lithium-ion batteries are very sensitive to overcharging and undercharging. Excessive voltage will cause permanent damage to the battery, but repeated discharge due to severe under-voltage will cause capacity loss. Therefore, in order to protect the battery, the charging solution must limit the battery's current and voltage and the charge during discharge.

Therefore, lithium battery pack packs usually include circuits to protect any form of undervoltage and overvoltage, as well as fuses to prevent exposure to overcurrent. According to Maxim, these battery packs also include a switch to turn on the battery during high-pressure bleed.

In addition, lithium-ion batteries must use a combination of current and voltage sources for charging, which is different from nickel-cadmium batteries (NiCd) and nickel-hydrogen batteries (NiMH), which are nickel-cadmium batteries (NiCd) and nickel-hydrogen batteries. (NiMH) A current source is required for charging. In order to maximize charging without damage, most lithium-ion chargers maintain an error within 1% of the output voltage. Lower tolerances are generally not recommended because they are difficult to maintain and increase costs. Generally, the benefit of capacity is less than these efforts.

Single-cell Li-ion charger

The preferred method for charging mobile phones and other similar devices is to use a stand-alone device called a 'cradle charger' to hold the device or battery pack. According to Maxim, single-cell Li-ion or Li-polymer chargers are suitable for use with the cradle. Since the battery pack or charger unit is separate, the heat generated is not a problem because it is not integrated into the device. In this case, the linear regulator allows the voltage difference between the DC power supply and the battery to pass through the pass transistor in a linear range. Since the charger is used in a small space, ventilation is recommended to prevent overheating due to power consumption.

Maxim's linear single-cell lithium-ion charger uses MAX846A, which is designated as. The 0.5% reference accuracy can safely charge lithium-ion batteries that require high voltage accuracy. The voltage and current regulation loops that control low-cost external PNP transistors (or P-channel MOSFETs) work independently of each other. As shown in Figure 1, Fairchild Semiconductor's external power transistor FZT749 reduces the power supply voltage to the battery voltage and regulates most of the power consumption of the circuit. Therefore, the internal reference is more stable and provides a more stable battery voltage limit.

Figure 1: Linear single-cell lithium-ion charger MAX846A drives an external power transistor (Q1) to reduce the power supply voltage to the battery voltage.

In this circuit, R1 and R3 determine the output current. R1 detects the charging current, and R3 determines the level of current regulation. The output current of the ISET terminal is equal to 1/1000 of the voltage between CS + and CS-. The current regulator controls the ISET voltage at 2V. As a result, the current limit [2,000 /(R3 * R1)] is 1A. As mentioned in the Maxim application notes, the control loops for voltage and current limits have separate compensation points (CCV and CCI) to simplify the task of stabilizing these limits. The ISET and VSET terminals can be used to adjust the current and voltage limits.

Texas Instruments also provides linear chargers for space-constrained portable applications. TI's highly integrated bq24040 series are charger ICs for single-cell Li-ion and Li-polymer batteries. The charger can handle a high input voltage range and can be driven by a USB port or a low-cost AC adapter. According to TI, the bq2404x device charges the battery in three stages: management-constant current-constant voltage. During all charging stages, the internal control loop monitors the IC junction temperature and reduces the charging current when the internal temperature exceeds a certain level.

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