Detailed application of power MOS transistor in lithium battery protection circuit
Due to the characteristics of lithium iron phosphate battery, its charge and discharge process needs to be protected in application to avoid overcharge, over discharge or overheating, so as to ensure the safe operation of the battery. Short circuit protection is an extremely bad working condition in the process of discharge. This paper will introduce the characteristics of power MOS transistor in this working state, and how to select the model of power MOS transistor and design an appropriate driving circuit.
The simplified model of the discharge circuit of the lithium iron phosphate battery protection board of the electric bicycle is shown in the figure. Q1 is a discharge tube, using n-channel enhanced MOS tube. In actual work, according to different applications, multiple power MOS tubes will be used in parallel to reduce on resistance and enhance heat dissipation performance. RS is the equivalent internal resistance of the battery and LP is the lead inductance of the battery.
During normal operation, the control signal controls the MOS tube to open, and the terminal P + and P - output voltage of the battery pack are used by the load. At this time, the power MOS transistor is always in the on state. The power loss is only the on loss and there is no switching loss. The total power loss of the power MOS transistor is not high and the temperature rise is small. Therefore, the power MOS transistor can work safely. However, when the load is short circuited, due to the small loop resistance and strong discharge capacity of the battery, the short-circuit current suddenly increases from tens of amps to hundreds of amps. In this case, the power MOS tube is easy to be damaged.
In the process of application, in order to prevent the misoperation of short-circuit protection circuit due to transient overload, the short-circuit protection circuit has a certain time delay. Moreover, due to the error of current detection resistance, the delay of current detection signal and system response, the short-circuit protection time is usually set at 200 according to different applications μ S to 1000 μ S. This requires that the power MOS transistor can work safely in this time under high short-circuit current, which also improves the design difficulty of the system.