Nickel Hydride No. 5 battery

　　Discussing lithium-ion battery charge protection integrated circuits

　　Compared with other batteries, Nickel Hydride No. 5 battery have the following main advantages:

　　The marked open circuit voltage is usually 3.6V, while the open circuit voltage of nickel metal hydride and nickel cadmium batteries is 1.2V.

　　It has high energy and high storage energy density. For the same output power, Nickel Hydride No. 5 battery are not only half lighter than nickel-metal hydride batteries, but also 20% smaller in volume.

　　Nickel Hydride No. 5 battery charge very quickly and only take 1 to 2 hours to charge and reach optimal condition. At the same time, Nickel Hydride No. 5 battery have very little leakage. Even if they are left for 1 to 2 weeks and then taken out for use, It can still exert power and work as usual; the self-discharge rate of Nickel Hydride No. 5 battery is as low as <8%/month, which is much lower than 30% of nickel-cadmium batteries and 40% of nickel-metal hydride batteries.

　　During the charging process of Nickel Hydride No. 5 battery, the battery voltage and charging current will change with the charging time.

　　Charging a lithium-ion battery requires controlling its charging voltage, limiting the charging current and accurately detecting the battery voltage. The charging characteristics of Nickel Hydride No. 5 battery are completely different from those of cadmium-nickel and nickel-metal hydride. Nickel Hydride No. 5 battery can be charged at any point during its discharge cycle and can maintain its charge very effectively. The retention time is more than twice as long as that of nickel-metal hydride batteries. It is lightweight and only weighs 1/2 of a cadmium-nickel battery of the same capacity. , its mass density is 4 times that of cadmium-nickel batteries. When the lithium-ion battery starts to charge, the voltage rises slowly and the charging current gradually decreases. When the battery voltage reaches about 4.2V, the battery voltage remains basically unchanged and the charging current continues to decrease. The way to determine whether the lithium-ion battery charging is completed is to detect it. charging current, and ends charging when its charging current drops to a certain value. For example, charging ends when the charging current of a lithium-ion battery drops to 40mA (typically about 5% of the initial charging current). You can also start a timer when it detects that the lithium-ion battery reaches 4.2V and end it after a certain delay. Charge. At this time, the charging circuit should have a battery voltage detection circuit with higher accuracy to prevent overcharging of the lithium-ion battery. It should be noted that Nickel Hydride No. 5 battery do not require trickle charging.

　　Lithium-ion battery charge protection integrated circuit UCC3957

　　UCC3957 is a 3/4-cell lithium-ion battery pack charger protection control integrated circuit using BiCMOS technology. It works with an external P-channel MOSFET transistor to charge the battery pack to achieve two-level overcurrent protection. If the first-level 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-level protection time is up, and the battery overcharge and discharge faults have not been eliminated. The external protection timing capacitor discharge MOSFET turns off at 17 times the first-level protection time to implement the second-level protection, which is very useful for capacitive loads. . The power consumption of UCC3957 in sleep mode is only 3.5mA, the typical operating current is 30mA, and the DC operating voltage range is 6.5~20V. The charging overcurrent protection delay time can be achieved by adjusting the parameters of external components. The advantage of using an external P-channel MOSFET transistor is that it protects any cell from overdischarge and overcharge, as well as protecting the battery pack and the UCC3957 integrated circuit itself.

　　In order to match the lithium batteries produced by different manufacturers, the UCC3957 series integrated circuits have 4 different overvoltage protection thresholds.

　　UCC3957 typical application circuit

　　UCC3957 can provide comprehensive protection functions for 3-cell or 4-cell lithium battery packs to prevent battery overcharge, over-discharge, over-current charge and discharge, etc. It samples the voltage of each cell in the battery pack and compares it with the internal precision reference The voltage is compared, and when any battery is in an overvoltage or undervoltage state, the UCC3957 will take appropriate measures to prevent the battery from further charging or discharging. UCC3957 has two independent P-channel MOSFET transistors externally connected to control the charging and discharging current of the battery respectively.

　　Typical protection circuit for 4-cell battery pack using UCC3957

　　Battery pack connection

　　Pay attention to the order when connecting the battery pack to the 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 every two batteries are connected to pins ④AN1, ⑤AN2, and ⑥AN3 in the corresponding order.

　　Choose 3 or 4 battery charging working status

　　When the battery pack is 3 cells, pin ②CLCNT should be connected to pin DVDD, and pin ⑥AN3 and pin ⑦AN4 should be connected together. When the battery pack is 4 cells, pin ②CLCNT should be connected to ground (that is, connected to pin Pin ⑦AN4), AN3 pin is connected to the positive terminal of the bottom battery of the battery pack.

　　Under voltage protection

　　When it is detected that any battery is in an over-discharge state (lower than the under-voltage threshold potential), the status detector turns off two MOSFETs at the same time, causing the UCC3957 to enter the sleep mode. At this time, the power consumption of the UCC3957 is only 3.5mA. Only when the voltage of pin ③WU rises to ①VDD, UCC3957 exits the sleep mode.

　　Charging batteries

　　When the charger is connected to the charging power supply, as long as the voltage of pin ⑨CHGEN is pulled to (16) DVDD, charging FETVT1 is turned on and the battery pack is charged. But if pin ⑨CHGEN is open or connected to pin ⑦AN4, the charging FETVT1 is turned off.

　　During charging, if the UCC3957 is in the sleep mode, the discharge FETVT2 is still turned off, and the charging current flows through the body diode of the discharge FETVT2; until the voltage of each battery is higher than the undervoltage threshold voltage, the discharge FETVT2 is turned on. During sleep operation, the charging FET transistor VT1 is in a periodic turn-on and turn-off mode, with a turn-on time of 7ms and a turn-off time of 10ms.

　　Protection against abnormal battery connection

　　UCC3957 has a protection function in case of abnormal battery connection in the charged battery box. If the pins ④AN1, ⑤AN2, or ⑥AN3 connected to the battery are not connected properly or disconnected, UCC3957 can detect and prevent overcharging of the battery pack.

　　Overvoltage protection and intelligent discharge characteristics

　　If the charging voltage of a certain battery exceeds the normal overcharge threshold potential, the charging FETVT1 is turned off to prevent the battery from overcharging. Shutdown remains until the battery voltage drops to the overcharge threshold level. In most protection circuit designs, in this overvoltage protection band (between the normal value and the overcharge threshold, or conversely, between the overcharge threshold and the normal value), the charging FETVT1 is always in complete shutdown of the protection In the working state, the discharge current must pass through the body diode of the charging FETVT1. The voltage drop of the diode is as high as 1V, which causes great power consumption in the charging FETVT1 and consumes precious battery power.

　　UCC3957 has unique intelligent discharge characteristics, which can make charging FETVT1 conductive to the discharge current (only for discharge) while still being within the over-voltage hysteresis range. This greatly reduces the power consumption on charging FETVT1. This measure is accomplished by sampling the voltage drop flowing through the current sensing resistor RSENES. If this voltage drop exceeds 15mV (0.025Ω current sensing resistor corresponds to a discharge current of 0.6A), the charging FETVT1 is turned on again. In this example, if the body diode voltage of a 20mW FET drops to 1V, corresponding to a 1A load, the power consumption of VT1 will be reduced from 1W to 0.02W.

　　Over current protection

　　UCC3957 uses a secondary overcurrent protection mode to protect the battery pack from overcharge current and battery pack short circuit. When the voltage drop on the current detection resistor RSENSE (connected between pin AN4 and pin BATLO) exceeds a certain threshold potential, The over-current protection enters the hiccup protection working mode. In this operating mode, the discharge FETVT2 is turned off and on periodically until the fault is eliminated. Once the fault is eliminated, the UCC3957 automatically resumes normal operation.

　　In order to adapt to large capacitive loads, UCC3957 has two over-current threshold voltages, and different delay times can be set corresponding to each threshold voltage. This secondary overcurrent protection provides fast response to short circuits while allowing the battery pack to withstand certain surge currents. This can prevent unnecessary overcurrent protection actions caused by large filter capacitance.

　　The first-level overcurrent protection threshold potential is 150mV, corresponding to a 0.025Ω current detection resistor, and the overcurrent threshold is 6A. If the peak discharge current lasts longer than the time set by this value (set by the capacitor connected between CDLY1 and ground), UCC3957 enters the hiccup protection operating mode. The duty cycle of the hiccup protection working mode is about 6%, that is, the off time is about 17 times the on time.

　　The second-level overcurrent threshold potential is 375mV, corresponding to a 0.025Ω current detection resistor, and the overcurrent threshold is 15A. If the peak discharge current exceeds the time set by this potential value (set by the capacitor connected between CCDLY2 and ground), UCC3957 enters the hiccup protection operating mode, and the duty cycle is generally less than 1%. The off time tOFF is still determined by the capacitor connected between CCDLY1 and ground. This technology greatly reduces the power consumption on FETVT2 during short circuit, thereby reducing the requirements for FETVT2.

　　When CDLY1=0.022mF, the on-time tON corresponding to the first-stage overcurrent (when the current is greater than 6A and less than 15A) is approximately 10ms, the off-time tOFF is approximately 160ms, and the duty cycle is 5.9%; when the current If CDLY2 is not used when exceeding 15A, the duty cycle of the second-level overcurrent protection is 0.1%; if CDLY2 is 22pf, the corresponding conduction time is 800ms and the duty cycle is 0.5%.

　　VR1 and R2 are used to protect the charging FET transistor VT1 when the charger's open circuit charging voltage is too high. In this application circuit, the discharge FET transistor VT2 is turned off during a short circuit. Due to the distributed inductance of the battery pack output, a negative sudden change in voltage will occur; this negative sudden change will exceed the withstand voltage value of the discharge FET transistor VT2, which results in A negative mutation also damages UCC3597. VD1 in Figure 2 clamps this negative mutation to protect discharge FETVT2, and C5 should be placed directly on the top and bottom of the battery pack.

　　Because during discharge overcurrent protection, the negative voltage overcharge generated by discharging FETVT2 is related to the size and the rise and fall time of the on and off driving pulses of discharging FETVT2. Therefore, R3, C5, and R4 are used to control the size in Figure 2.

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