Analysis of the working principle of NiMH No.7 batteries circuit

　　NiMH No.7 batteries circuit working principle The circuit has overcharge protection, over-discharge protection, over-current protection and short-circuit protection functions. Its working principle is analyzed as follows:

　　1. Normal state: In the normal state, the CO and DO pins of N1 in the circuit both output high voltage, and both MOSFETs are in the on state. The NiMH No.7 batteries can charge and discharge freely. Since the on-resistance of the MOSFET is very small, usually less than 30 milliohms, so its on-resistance has little impact on the performance of the circuit. 7|The current consumption of the protection circuit in this state is (mu)A level, which is usually less than 7 (mu)A.

　　2. The charging method required for overcharge protection lithium-ion batteries is constant current/constant voltage. In the early stage of charging, constant current charging is used. As the charging process progresses, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require constant current (voltage value is 4.1V), switch to constant voltage charging until the current becomes smaller and smaller. During the charging process of the NiMH No.7 batteries, if the charger circuit loses control, the NiMH No.7 batteries voltage will continue to be charged with constant current after it exceeds 4.2V. At this time, the NiMH No.7 batteries voltage will continue to rise. When the NiMH No.7 batteries voltage is charged to exceed 4.3V, the NiMH No.7 batteries chemistry will Side reactions will be aggravated, causing NiMH No.7 batteries damage or safety issues.

　　In a NiMH No.7 batteries with a protection circuit, when the control IC detects that the NiMH No.7 batteries voltage reaches 4.28V (this value is determined by the control IC, different ICs have different values), its CO pin will change from high voltage to zero voltage, causing V2 It switches from on to off, thus cutting off the charging circuit and making the charger no longer able to charge the NiMH No.7 batteries, thus serving as overcharge protection. At this time, due to the existence of the body diode VD2 of V2, the NiMH No.7 batteries can discharge the external load through this diode. There is still a delay time between when the control IC detects that the NiMH No.7 batteries voltage exceeds 4.28V and sends the shutdown V2 signal. The length of this delay time is determined by C3 and is usually set to about 1 second to avoid errors caused by interference. judge.

　　3. Over-discharge protection When the NiMH No.7 batteries discharges to the external load, its voltage will gradually decrease with the discharge process. When the NiMH No.7 batteries voltage drops to 2.5V, its capacity has been completely discharged. At this time, if the NiMH No.7 batteries continues to discharge the load , will cause permanent damage to the NiMH No.7 batteries. During the NiMH No.7 batteries discharge process, when the control IC detects that the NiMH No.7 batteries voltage is lower than 2.3V (this value is determined by the control IC, different ICs have different values), its DO pin will change from high voltage to zero voltage, causing V1 to change from conductor to Turning on to off, thus cutting off the discharge circuit, making the NiMH No.7 batteries no longer able to discharge the load, thus playing the role of over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the NiMH No.7 batteries through this diode.

　　Since the NiMH No.7 batteries voltage cannot be reduced in the over-discharge protection state, the current consumption of the protection circuit is required to be extremely small. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1 (mu) A. There is also a delay time between the control IC detecting that the NiMH No.7 batteries voltage is lower than 2.3V and sending the shutdown V1 signal. The length of this delay time is determined by C3 and is usually set to about 100 milliseconds to avoid errors caused by interference. judge.

　　4. Over-current protection Due to the chemical characteristics of lithium-ion batteries, the NiMH No.7 batteries manufacturer stipulates that the maximum discharge current cannot exceed 2C (C=NiMH No.7 batteries capacity/hour). When the NiMH No.7 batteries is discharged with a current exceeding 2C, it will cause permanent damage to the NiMH No.7 batteries. or security issues arise. During the normal discharge process of the NiMH No.7 batteries to the load, when the discharge current passes through two MOSFETs connected in series, a voltage will be generated at both ends due to the on-resistance of the MOSFETs. The voltage value U=I*RDS*2, RDS is a single MOSFET on-resistance, the V- pin on the control IC detects the voltage value. If the load is abnormal for some reason, the loop current increases. When the loop current is large enough to make U(gt)0.1V (this value is determined by When the control IC determines (different ICs have different values), its DO pin will change from high voltage to zero voltage, causing V1 to turn from on to off, thus cutting off the discharge loop and making the current in the loop zero. Over current protection function.

　　There is also a delay time between the control IC detecting the occurrence of overcurrent and sending the shutdown V1 signal. The length of this delay time is determined by C3, usually about 13 milliseconds, to avoid misjudgment caused by interference. From the above control process, it can be seen that the overcurrent detection value not only depends on the control value of the control IC, but also depends on the on-resistance of the MOSFET. When the on-resistance of the MOSFET is larger, for the same control IC, the over-current protection The smaller the value.

　　5. When the short-circuit protection NiMH No.7 batteries discharges the load, if the loop current is large enough to make U(gt)0.9V (this value is determined by the control IC, different ICs have different values), the control IC will determine that the load is short-circuited. Its DO pin will quickly change from high voltage to zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge circuit and playing a role in short-circuit protection. The delay time of short circuit protection is very short, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, but the judgment method is different and the protection delay time is also different.

　　The above describes the working principle of the single-cell lithium-ion NiMH No.7 batteries protection circuit in detail. The protection principle of multi-cell series lithium-ion batteries is similar and will not be repeated here. The control IC used in the above circuit is the R5421 series of Ricoh Corporation of Japan. In actual NiMH No.7 batteries protection circuits, there are many other types of control ICs, such as Japan's Seiko S-8241 series, Japan's MITSUMI's MM3061 series, Taiwan's Fujing's FS312 and FS313 series, Taiwan's Analog Technology's AAT8632 series, etc. The working principles are similar, but there are differences in specific parameters. In order to save peripheral circuits, some control ICs have filter capacitors and delay capacitors built into the chip, and their peripheral circuits can be very few, such as the S-8241 series of Japan's Seiko. In addition to the control IC, there is another important component in the circuit, which is the MOSFET. It plays the role of a switch in the circuit. Since it is directly connected in series between the NiMH No.7 batteries and the external load, its on-resistance has a significant impact on the performance of the NiMH No.7 batteries. Influence, when the selected MOSFET is better, its on-resistance is very small, the internal resistance of the NiMH No.7 batteries pack is small, the load capacity is strong, and it consumes less power during discharge.

　　With the development of technology, the size of portable devices is getting smaller and smaller. With this trend, the requirements for the size of lithium-ion NiMH No.7 batteries protection circuits are getting smaller and smaller. In the past two years, control ICs and MOSFETs have appeared. Products integrated into a protection IC, such as DIALOG's DA7112 series, and some manufacturers even package the entire protection circuit into a small-sized IC, such as MITSUMI's products.

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