Turning off the discharge MOSFETs quickly is necessary, especially in the event of a short-circuit discharge (SCD) fault. When an SCD fault occurs with the MOSFET on, discharge current increases to a very large value and triggers the SCD protection to turn off the discharge MOSFET. The discharge current drops quickly, generating significant negative voltage on PACK+ because of the parasitic inductance of the short-circuit connectors and PCB traces. Negative voltage on PACK+ prevents current into the Zener D30 cathode and makes Q49 turn off, slowing down the discharge MOSFET turn off process. Diode D37 is necessary to limit the maximum negative voltage. Considering the discharge current can be huge and the D37 forward voltage can potentially reach the gate-source threshold voltage (Vgs_th) of the discharge MOSFET, Q48, R171, D34, R174, D36, and R173 are added to supply another path to discharge the input capacitance (Ciss) of the discharge MOSFET and speed up the turn off process. When Q48 is on because of negative voltage on PACK, Ciss of the discharge MOSFET discharges through Q48 and R171. To make sure the circuit works normally, Q48 has to be on before gate-source voltage of discharge MOSFET reaches the Vgs_th of the discharge MOSFET, considering the voltage divider of R174, R173, and D36. This condition is also necessary to make sure the discharge MOSFET is off for the reverse voltage tests on PACK side. D36 eliminates leakage current when PACK has normal high voltage and R173 limits current when the PACK side has a very high negative voltage, protecting D34 and Q48.

Figure 3-2 TIDA-010247 SCD Protection

Figure 3-3 Turning Off Discharge MOSFET With Q49 at SCD

Figure 3-4 Turning Off Discharge MOSFET With Q48 at SCD