What is the difference between the different discharge current protections, OCD1, OCD2, OCD3 and SCD?
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OCD1, OCD2, OCD3 and SCD are discharge current protections with different delay and threshold ranges. OCD1, OCD 2 and SCD use comparators that monitors the differential voltage across the SRN – SRP pins and triggers an alert or fault when the voltage exceeds its programmable thresholds and delay. OCD3 uses the CC1 current measurement from the coulomb counter ADC to determine a fault condition. The threshold/delay ranges might be represented as follows, not to scale.
They all operate with the same principle, if the current crosses its corresponding voltage threshold the delay is evaluated, if the delay time is reached the action is taken, as described in Section 5.2.6 Short Circuit in Discharge and Section 5.2.8 Overcurrent in Discharge of the Technical Reference Manual.
If current builds, each delay is evaluated as the voltage threshold is crossed. An example of this is shown in the following figure.
If the SCD threshold is set too high or the cells can't produce enough current to reach the set threshold during the short circuit event, the part will time until the next delay is reached. In the figure below the short circuit current does not reach the SCD threshold and the OCD2 delay is reached at a later time, where DSG turns off. OCD1 would provide a similar operation with lower threshold ranges and longer delays. Risks should be evaluated by the system designer. Risks might include heating of the system or cells, or discharge of the cells beyond useful condition before reaching the delay time.