Providing high-performance battery chargers for portable products

    Portable products such as laptops have strict requirements on battery size, weight and battery life. The standard battery configuration is Li+ battery. Recently, lithium polymer batteries have also become one of its development directions. This type of battery can be made into various complex shapes as needed and can be widely used in mobile phones, handheld PCs and various small handheld products. Since the chemical characteristics of lithium polymer batteries and Li+ batteries are similar, their charging methods are almost the same. The main difference is only the termination of charging voltage and charging current.

    For Li+ batteries, the traditional charging method is to use constant current and constant voltage charging. The constant current charging method is used in the initial stage of battery charging, when the battery voltage is low. When the battery voltage gradually increases and reaches a predetermined limit level, the charger switches from the constant current mode to the constant voltage charging mode. The charging process is not terminated until the charging current decreases to close to zero to ensure that the battery is fully charged. During the constant voltage charging process, the charging current gradually decreases in an exponential function curve due to the internal resistance of the battery and the series parasitic resistance, similar to the charging process of a capacitor through a resistor. Because it decreases exponentially, a full charge process takes a long time to complete. Since the battery has voltage limit protection during charging, constant current charging does not require accurate charging current. Instead, it requires high accuracy for the limiting voltage, which is higher than 1%. This is because: the higher the battery voltage, the higher the storage voltage. The more energy there is, the higher the voltage will cause battery damage. Even if the battery is charged with a large current during constant current charging (more than 1C, C is the battery capacity, expressed in ampere-hours), there will be little improvement in shortening the battery charging time. This is because the battery is affected by the physical and chemical characteristics of the battery. Limitation, the constant voltage charging process takes up most of the battery charging process, while the constant current process only takes up a small part.

    Using a linear charger is one of the charging methods for Li+ batteries. As shown in Figure 1, the input power supply usually uses a DC power supply or an AC adapter, and IC1 is used to drive an external PNP tube to generate the voltage and current required for charging. The PIC microcontroller in the circuit controls the charging voltage and current through its different PWM outputs. Programmed to produce a variety of output voltages and currents, the circuit can charge batteries with a variety of different chemistries. In many systems, the charger can be programmed using the system's existing microcontroller PWM output. If the charger is designed specifically for a certain type of battery, the charging circuit can be further simplified by changing external resistors to control the charging voltage and current.

    Although the linear charger circuit is simple and small in size, its power consumption is large. Considering that the voltage range of a single Li+ battery is: 2.7V~4.2V, in order to ensure that the battery is charged at 4.2V, the input DC source voltage must be greater than 4.5V, so a cheap AC adapter with 10% margin is used , its output voltage range should be between 4.5V~5.5V. The maximum power consumption of the charger PNP tube occurs when the input voltage is the highest (5.5V) and the battery voltage is the lowest (2.7V). Considering the typical charging current: 1A (10%), the maximum power consumed by the PNP tube is: (5.5-2.7) , may cause the system to not work properly.

    When a built-in charging structure must be used, only a switch-mode charger can be used to solve the efficiency problem (see Figure 2). The IC in the figure uses two external power MOSFET tubes to chop the source voltage. The current and voltage required for battery charging are then processed by the filter circuit to obtain the current and voltage required for battery charging. These MOSFET tubes function like switches, either turned on or off. When turned on, the voltage difference generated by the charging current on the MOSFET is very small; When turned on, the MOSFET withstands a large voltage difference but the current is zero, so the power consumption consumed by the power device is very small: P=VxI, and is not greatly affected by changes in input voltage, charging voltage, and current. Using the circuit in Figure 2, you can High conversion efficiency (>90%) can be obtained within a wide range of input voltage, charging voltage, and charging current, but the price is that the circuit is more complex, larger in size, and higher in cost.

    Figure 3 is a new constant current pulse Battery charging method, which combines the advantages of DC and switching charging methods, uses an external current-limiting AC adapter to limit the charging current. The AC adapter is switched to the battery for constant current charging. When the battery voltage rises to the limiting voltage, the current The source is connected to the battery in a pulse mode, and its average current is the charging current required by the battery, and the battery voltage will not be overvoltage. The power consumption generated by this method is very low, because the working principle is similar to a switching power supply, either connected or disconnected , and the circuit is simple, like a linear charger, without the need for an output filter circuit. Although the external AC adapter may consume larger power (related to the way the external adapter works), it will not cause any damage to the battery if it does not exceed the maximum allowable safe temperature range. and bring any adverse effects to portable devices.

    The PMOS tube in Figure 3 realizes the switching of the current limiting source and the battery. Since the charger chip in the circuit uses a micro uMAX package, and the external PMOS tube uses an SOT-23 package, Therefore, the circuit is simpler and smaller than the circuit in Figure 2. In addition to MAX1679 and PMOS, only two capacitors and one resistor are needed to form a complete Li+ battery charger. (Adjustable resistor, LED, Schottky diode and thermal The sensitive resistors are all optional devices). The MAX1679 also has a certain battery protection function. If the over-discharged battery (less than 2.5V) is quickly charged, the battery life will be shortened. The MAX1679 detects the battery voltage before charging, and uses The internal 5mA current source precharges the battery, increases the battery voltage to 2.5V, and then starts the fast charging process. Another function of the MAX1679 is to detect the temperature through an externally connected thermistor. For Li+ batteries, if the temperature during charging Exceeding the 0~50(C range will affect battery life. Once the MAX1679 detects that the battery temperature exceeds the allowable range, it will terminate battery charging and will not start charging until the battery temperature returns to the normal range. In order to indicate the charging process, MAX1679 also includes a charging process monitoring output, which is used to drive an external LED to reflect the charging process.

    As portable products become smaller and smaller and circuits become more complex, stricter requirements are placed on battery chargers. Chargers are the key to ensuring maximum utilization of battery capacity. By shifting power consumption outside the portable device and away from the battery, the performance of the portable product can be improved and battery stress reduced. Although traditional linear and switching charging methods still have a certain market, current-limited pulse chargers reduce the size of portable products and improve their performance, so they have a wider range of applications.