The hottest Ti lithium ion battery charging protec

2022-08-02
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Ti's lithium-ion battery charging protection integrated circuit ucc3957

introduction: This paper mainly introduces the 3/4-cell lithium-ion battery charging circuit using ucc3975, the working principle of relevant protection circuits and the main characteristics of lithium-ion battery charging

key words: lithium ion battery, charging, protection

I. characteristics of lithium ion battery

compared with other batteries, lithium ion batteries mainly have the following advantages

1. the open circuit voltage marked by high voltage

is usually 3.6V, while the open circuit voltage of Ni MH and Ni Cd batteries is 1.2V

2. large capacity

high energy and high storage energy density are the core values of lithium batteries. In terms of the same output power, lithium ion batteries are not only half lighter than nickel metal hydride batteries, but also 20% smaller

3. discharge rate

the charging speed of lithium-ion battery is fast. It only takes 1~2 hours (H) to charge and reach the best state; At the same time, the leakage of lithium-ion batteries is very small. Even if they are placed at will for 1~2 weeks and then taken out for use, they can still give full play to power and work as usual; The self discharge rate of lithium-ion battery is low. 2. Charging characteristics of lithium-ion battery

during the charging process of lithium-ion battery, the battery voltage and charging current will change with the charging time, and the change law is shown in Figure 1

Figure 1 charging characteristic curve of lithium-ion battery

charging of lithium-ion battery requires controlling its charging voltage, limiting charging current and accurately detecting battery voltage. The charging characteristics of lithium-ion batteries are completely different from those of nickel cadmium and nickel hydrogen. Lithium ion battery can be charged at any point in its discharge cycle, and can effectively maintain its charge. Its holding time is more than twice as long as that of nickel hydrogen battery, and its weight is only 1/2 of that of nickel cadmium battery with the same capacity, and its specific mass density is 4 times that of nickel cadmium battery. When the lithium-ion battery starts charging, the voltage rises slowly and the charging current decreases gradually. When the battery voltage reaches about 4.2V, the battery voltage basically remains unchanged and the charging current continues to decline. The method to judge whether the lithium-ion battery charging is over is to detect its charging current. When its charging current drops to a certain value, the charging is over. For example, when the charging current of the lithium-ion battery drops to 40mA (the typical value is about 5% of the initial charging current), the charging can be ended, or the timer can be started when it is detected that the lithium-ion battery reaches 4.2V, and the charging can be ended after a certain time delay. At this time, the charging circuit shall have a battery voltage detection circuit with high accuracy to prevent overcharging of lithium-ion battery. It should be noted that:; Lithium ion batteries do not require trickle charging

III. main features of ucc3957

ucc3957 is a control integrated circuit for charger protection of 3/4 lithium ion battery pack using BiCMOS process. It charges the battery pack together with the external p-channel MOSFET transistor to realize two-stage overcurrent protection. If the first stage overcurrent threshold potential is reached, the protection circuit will discharge the external capacitor according to the protection time set by the user. If the first stage protection time is up, the battery overcharge and discharge fault has not been eliminated, and the external protection timing capacitor discharge MOSFET will be turned off at 17 times the first stage protection time to implement the second stage protection, This is useful for capacitive loads. The power consumption of ucc3957 in sleep mode is only 3.5 μ A. Typical operating current is 30 μ A. The DC working voltage range is 6.5 ∽ 20V. When the charging overcurrent protection is delayed, the mixed hydrogel flame retardant can also be dropped from a higher air room by adjusting the parameters of external components. The advantage of using external p-channel MOSFET transistor is that it can protect any battery from over discharge and overcharge, and protect the battery pack and ucc3957 integrated circuit itself

3.1 the working principle block diagram and pin function of ucc3957 are shown in Figure 2

Figure 2 the working principle block diagram of ucc3957

it can be seen from Figure 2 that the working state of ucc3957 can be selected by using the internal working state selector of ucc3957. When working in the continuous working state, each lithium-ion battery can be protected from overcharge and over discharge. The over-current controller is used to protect the battery pack from excessive discharge current and damage the battery pack

in order to cooperate with lithium batteries produced by different manufacturers, ucc3957 series integrated circuits have four different overvoltage protection threshold values as shown in Table 1

Table 1 overvoltage protection threshold value (V) of ucc3957-x

ucc 4.20

ucc 4.25

ucc 4.30

ucc 4.35

ucc3957 pin diagram is shown in Figure 3

Figure 3 pin diagram of ucc3957

ucc3957 each pin has the following functions:

pin 1vdd: this pin is the power supply input pin of ucc3957, the input voltage range is 6.5~20v, and is connected with the high potential terminal of the battery pack

pin 2clcnt: this pin is used to set ucc3957 to work in the charging state of three or four batteries

pin 3WU: this pin is used to wake up ucc3957 to enter the normal working state when ucc3957 is in the sleep working state. This pin should be connected to the drain of n-channel level transfer MOSFET transistor

pin 4an1: this pin is connected to the negative pole of the first battery and the positive pole of the second battery with the highest potential

pin 5an2: this pin is the pin connected to the negative pole of the second battery and the positive pole of the third battery with the highest potential

pin 6an3: this pin is the pin connected to the negative pole of the third battery with the lowest potential and the positive pole of the fourth battery. When there are only three batteries, it is connected to the low potential end of the battery pack and an4 pin

pin 7 and 11 pin an4: this pin is connected to the low potential end of the battery pack and the high potential end of the current detection resistance

pin 8batlo: this pin is connected to the negative potential end of the battery pack and to the low potential end of the current detection resistance

pin 9chgen: this pin creates new materials and technologies that can meet the development of the plastic industry for the charging enable pin. This pin starts charging the battery pack at high level

pin 10cdly1: this pin is the delay time control pin for short-circuit protection. The value of the capacitor connected between this pin and an4 pin determines the time of overcurrent. When overcurrent occurs, it controls the off time of the discharge MOSFET transistor. The value of the capacitor also determines the time of hiccup overcurrent protection

pin 12chg: this pin is the pin connecting the external controllable n-channel MOSFET transistor, and the external n-channel MOSFET transistor can be used to drive the external p-channel MOSFET. If the voltage of any battery is higher than the overvoltage protection threshold potential, this pin is set to a low potential relative to the an4 pin; Only when the voltage of all charged single batteries is lower than the threshold potential, the pin is set to high potential

pin 13dchg: this pin is used to prevent battery over discharge. If the internal working state detector of ucc3957 determines that any battery is under voltage, pin dchg is set to high potential to turn off the external discharge p-channel MOSFET transistor. However, when the voltage of all batteries is higher than the minimum threshold potential, pin dchg is set to low potential

pin 14cdly2: a capacitor is connected between this pin and an4 pin to extend the protection setting time of the second stage overcurrent protection

pin 15avdd: this pin is connected with the power supply pin for the internal analog circuit power supply through 0.1 μ F capacitor is connected to an4 pin, and the normal working voltage is 7.3v

pin 16dvdd: this pin is connected with the power supply pin for the internal digital circuit power supply through 0.1 μ F capacitor is connected to an4 pin, and the normal working voltage is 7.3v

IV. working principle of ucc3957 and typical application circuit

4.1 working principle of ucc3957

ucc3957 can provide comprehensive protection against battery overcharge, over discharge, over-current charge, discharge and other faults for 3 or 4 lithium batteries. It samples the voltage of each battery in the battery pack and compares it with the internal precision reference voltage. When any battery is in overvoltage or undervoltage state, Ucc3957 will take appropriate measures to prevent the battery from further charging or discharging. Ucc3957 is externally connected with two independent p-channel MOSFET transistors, which respectively control the charging and discharging current of the battery

the following takes Figure 4 as an example to introduce the characteristics of its four lithium battery charging protection circuit using ucc3957

1. connection of battery pack

pay attention to its sequence when connecting the battery pack to ucc3957. The low potential end of the battery pack is connected to pin 7an4, the high potential end is connected to pin VDD, and the connection points of each two batteries are connected to pins 4an1, 5an2 and 6an3 in corresponding order

2. select the charging status of 3 or 4 batteries

when the battery pack is 3 batteries, pin 2clcnt should be connected to pin 16dvdd, and pin 6an3 and pin 7an4 should be connected together. When the battery pack is 4 batteries, pin 2clcnt should be grounded (i.e. connected to pin 7an4), and pin An3 should be connected to the positive pole of the lowest battery in the battery pack

3. undervoltage protection

when it is detected that any battery is in the over discharge state (lower than the undervoltage threshold potential), the state detector turns off two MOSFET transistors at the same time, making ucc3957 enter the sleep working mode. At this time, the power consumption of ucc3957 is only 3.5 μ A. Ucc3957 exits the sleep mode only when the voltage of pin 3WU rises to 1vdd

4. battery charging

when the charger is connected to the charging power supply, as long as the voltage of pin 9chgen is pulled to 16dvdd, the charging FET transistor VT1 is turned on and the battery pack is charged. However, if pin 9chgen is open or connected to pin 7an4, the charging FET transistor VT1 is turned off. During charging, if ucc3957 is in sleep mode, the discharge FET transistor vt2 is still off, Charging current flows through discharge Fe "The body diode of T transistor vt2 in plastic bags used by express industry; until the voltage of each battery is higher than the undervoltage threshold voltage, the discharge FET transistor vt2 is turned on. During sleep operation, the charging FET transistor VT1 is in a periodic on and off mode, and the on time is 7ms and the off time is 10ms.

5. protection of abnormal battery connection

ucc3957 has the protection function of abnormal battery connection in the charged battery box. For example If pins 4an1, 5an2 or 6an3 connected to the battery are abnormal or disconnected, ucc3957 can detect and prevent overcharge of the battery pack

6. overvoltage protection and intelligent discharge characteristics

if the charging voltage of a battery exceeds the normal overcharge threshold potential, the charging FET transistor VT1 is turned off to prevent the battery from overcharging. Turn off and hold until the battery voltage drops to the overcharge threshold potential. In most protection circuit designs, in the overvoltage protection band (between the normal value and the overcharge threshold value, or vice versa, between the overcharge threshold value and the normal value), the charging FET transistor VT1 is always in the fully off working state of protection. At this time, the discharge current must pass through the body diode of the charging FET transistor VT1, and the voltage drop of the diode is as high as 1V, thus generating great power consumption in the charging FET transistor VT1, Consume valuable battery power

ucc3957 has unique intelligent discharge characteristics, which can make the charging FET transistor VT1 turn on the discharge current (only for discharge) while still within the overvoltage return difference range. This greatly reduces the power consumption on the charging FET transistor VT1. This measure is completed by sampling the voltage drop across the current detection resistance rsenes. If the voltage drop exceeds 15MV (0.025 Ω, the current detection resistance corresponds to 0.6A

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