This note should be in the installation instructions of power supply firmware:
After a successful power supply firmware download, the managed system turns off for a duration ranging from three to twenty minutes to continue updating the firmware. It is imperative not to interrupt/restart manually during this time. Update time can vary but should be approximately 10 minutes. Margin should be added to assure the Dell Update Package (DUP) is not interrupted (some DUPs may take up to 20 minutes). Once the firmware update is complete, the managed system reboots automatically.
If you interrupt the update process then the firmware may be corrupted. You can attempt to update the firmware again. If the firmware will not update with both PSUs installed then you can try with just the PSU installed that needs the update. If the system will not power on or allow the update with both or just the one PSU installed then you will likely need to replace the PSU.
Long restarts are normal during BIOS and firmware updates. You should avoid interrupting the update process. I would wait at least an hour for updates to complete before resorting to pulling the power or any other forceful shutdown/restart.
Dell EMC, Enterprise Engineer
Get support on Twitter @DellCaresPRO