Lithium battery protection circuit works
Circuit with overcharge protection, over discharge protection, over-current protection and short circuit protection, its working principle is as follows:
1 Normal state
In the normal state of the circuit in the N1's "CO" and "DO" feet are high-voltage output, the two MOSFET are in the conduction state, the battery can be free to charge and discharge, MOSFET on-resistance is small, usually less than 30 milliohms, so its on-resistance of the circuit performance is very small. In this state, the current consumption of the protection circuit is μA, usually less than 7μA.
2 Overcharge protection
Lithium-ion battery as a rechargeable battery, the required charging method for the constant / constant voltage, the initial charge, the constant current charging, with the charging process, the voltage will rise to 4.2V (according to the cathode material, Battery requirements constant voltage value of 4.1V), to constant voltage charging, until the current getting smaller and smaller. When the battery is charged, if the charger circuit out of control, the battery voltage will continue to exceed 4.2V constant current charge, then the battery voltage will continue to rise when the battery voltage is charged to more than 4.3V, the battery chemistry Side effects will increase, will lead to battery damage or security problems.
In the battery with protection circuit, when the control IC detects the battery voltage reaches 4.28V (the value determined by the control IC, different IC have different values), the "CO" pin will be converted from high voltage to zero voltage, So that V2 turns from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the battery, play a role in over-charging protection. At this point because the V2 comes with the presence of the body diode VD2, the battery can discharge the external load through the diode.
In the control IC detects the battery voltage exceeds 4.28V to send off V2 signal between, there is a delay time, the length of the delay time determined by the C3, usually set to about 1 second, to avoid interference caused by errors judgment.
3 over-discharge protection
Battery discharge in the process of external load, the voltage will gradually decrease with the discharge process, when the battery voltage dropped to 2.5V, its capacity has been fully shine, then let the battery if the load continues to discharge, will cause the battery Permanent damage.
During the battery discharge, when the control IC detects that the battery voltage is lower than 2.3V (this value is determined by the control IC, the different IC has different value), the "DO" pin will be changed from high voltage to zero voltage, V1 From turn-off to turn off, thus cutting off the discharge circuit, so that the battery can no longer discharge the load, played a role in over-discharge protection. At this point because of the presence of body diode V1 VD1, the charger can charge the battery through the diode.
As in the over-discharge protection state can not lower the battery voltage, so the protection circuit requires a very small current consumption, this time will enter the low-power control IC, the entire protection circuit will be less than 0.1μA power consumption.
The control IC detects the battery voltage is lower than 2.3V to send off between the V1 signal, there is also a delay time, the delay time determined by the length of C3, usually set to 100 milliseconds or so, to avoid interference caused by errors judgment.
4 over-current protection
Due to the chemical nature of lithium batteries, the battery manufacturer provides the discharge current can not exceed the maximum 2C (C = battery capacity / hour), when the battery exceeds 2C current discharge, will lead to permanent damage to the battery or security problems.
When the discharge current flows through two MOSFETs connected in series, the voltage across the two MOSFETs will generate a voltage at the two ends of the MOSFET due to the MOSFET's on-resistance. The voltage value U = I * RDS * 2, RDS is a single MOSFET on-resistance, the control IC on the "V-" feet of the voltage value of the test, if the load for some reason cause an exception, so that the loop current increases, when the loop current to U> 0.1V (the value from Control IC decision, different IC have different values), the "DO" feet will be high voltage to zero voltage, so that V1 from on to turn off, thus cutting off the discharge circuit, so that the current loop is zero, Play over-current protection.
There is also a delay time between the occurrence of the overcurrent detected by the control IC and the generation of the turn-off V1 signal. The length of the delay time is determined by C3, which is usually about 13 milliseconds to avoid false judgments due to interference.
In the control process, we can see that the size of the over-current detection value depends not only on the control value of the control IC, also depends on the MOSFET on-resistance, when the MOSFET on-resistance is greater, the same control IC, the over-current protection The smaller the value.
5 short circuit protection
When the battery is discharging the load, if the loop current is large enough to make U> 0.9V (this value is decided by the control IC, different IC has different value), the control IC will judge the load short-circuit, the "DO" Rapidly from high voltage to zero voltage, so that V1 from turn-off to turn off the discharge circuit, play a short-circuit protection. Short-circuit protection delay time is very short, usually less than 7 microseconds. Its working principle and over-current protection is similar, but different judgments, protection delay time is not the same.
A detailed description of the above-mentioned single-cell lithium-ion battery protection circuit works, multi-series lithium-ion battery protection principle is similar, not repeat them here.
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