Lithium-ion battery protection circuit design

Circuit Overview
Lithium-ion ibm thinkpad g40 laptop battery protection circuit including over-charge protection, over current / short circuit protection and over-discharge protection, the circuit is to ensure that such over-charging and discharging state security, and to prevent property deterioration. It mainly consists of an integrated protection circuit IC, chip resistors, chip capacitors, FET (MOSFET), and some have thermistor (NTC), identify the resistance (ID), fuse (FUSE) and other accessories. The circuit shown in Figure 1. Lithium-ion thinkpad t61 7663 battery protection circuit, the integrated protection circuit IC is used to detect the current protection circuit voltage, current, time and other parameters in order to control the FET switch state; field-effect transistor (MOSFET) is to protect the IC control circuit according to whether have to be open or closed; chip resistors for current limiting; SMD capacitor acts as a filter, adjust the delay time; thermistor to detect the ambient temperature inside the cell block; fuse to prevent the current flowing through the battery is too large, cut off the current Loop.
Over-charging protection
When the charger on the battery over-charging, the dell 6y270 battery temperature rises due to increased internal pressure caused by the need to terminate the current state of charge. At this point, the integrated circuit IC test is required to protect the battery voltage reaches 4.25V, when (assuming the dell vostro 1700 battery charging voltage threshold is 4.25 V) that is activated over-charge protection, will be to cut off the power MOS from the open, then closing charge. In addition, to prevent excessive noise generated by the miscarriage of justice charge for the overcharge protection, so the need to set the delay time and delay time can not be shorter than the duration of the noise in order to avoid false positives. Overcharge protection delay time tvdet1 calculated as: t vdet1 = {C3 × (Vdd - 0. 7)} / (0. 48 × 10 - 6) (1) where: Vdd for the protection of the overcharge detection voltage N1 value. Simple calculation of delay time: t = C3 / 0. 01 × 77 (ms) (2) Should the C3 content is 0.22 F, then the delay value is: 0. 22 / 0. 01 × 77 = 1694 (ms)
Over-discharge protection
In the case of over-discharge, the electrolyte decomposition due to deterioration caused by cell characteristics, and resulting lower number of charge cycles. Over-discharge protection IC principle: In order to prevent excessive dell inspiron 700m battery discharge status, assuming lithium battery connected to the load when battery voltage is lower than the over-discharge voltage detection point (assumed to be 2.3 V) will activate the over-discharge protection, the power MOS FET from the open end into the cut and discharge, in order to avoid excessive battery discharge phenomenon, and will keep the battery toshiba pa3331u-1brs low quiescent current standby mode, when the current is only 0.1μA. When the lithium battery connected to the charger and the battery voltage is higher than at this time when the over-discharge voltage, over-discharge protection be lifted. In addition, taking into account the case of pulse discharge, over-discharge detection circuit has a delay time in order to avoid malfunction.
Over current and short circuit protection
For unknown reasons (positive and negative discharge or metal objects being inadvertently) caused by over-current or short circuit, to ensure safety, you must immediately cease its discharge. Principles of over current protection IC, when the discharge current is too large or short-circuit condition occurs, protect the IC will be activated off (short circuit) current protection, over current detection time is the power MOSFET's Rds (on) as a sensor for monitoring the impedance The voltage drop in the case, if the ratio set by the over-current detection voltage is higher then stop the discharge, computing formula is: V_ = I × Rds (on) × 2 (V_ for the over-current detection voltage, I is discharge current) (3 ) Suppose V_ = 0. 2V, Rds (on) = 25 mΩ, the protection of the current size of I = 4 A. Similarly, over-current detection delay time must also be equipped to prevent current flow when unexpected malfunction. Usually over-current is generated, if removal of over-current factors (such as immediately and load out) will resume its normal state, you can charge and discharge then normal action.
Conclusion
During the protection circuit design to charge the sony pcg-nv100 battery to full state of the user's concern, taking into account the security issue, so need to allow the voltage cut-off state of charge. To simultaneously satisfy these two conditions, there must be a high precision detector, current detector precision of 25 mV. Also must take into account the integrated protection circuit IC power consumption, resistance to high voltage problem. In addition to the power MOSFET's Rds (on) the charge current and discharge current in the effective use of time is required so that the impedance values ​​as low as possible, now that the impedance is approximately 20 ~ 30 mΩ, so over-current detection voltage can be lower.

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