Autopsy of an exploded USB power supply. (With skidmark)

Thanks to Gerard for sending in this pre-detonated multi-port USB power supply.
Given the isolation between the mains voltage primary windings and the low voltage secondary windings, perhaps it’s a good thing that it exploded.

It’s a shame, because the transformer is the only bit that really lets it down. The circuitry looks like a textbook switchmode based around a dedicated chip.
The reason for the failure is not clear, but a short has occurred that has made a track on the output of the rectifier blow like a fuse and cause a more serious short by virtue of the metal laden plasma.
The glass fuse has blown in half s they are not really suited to breaking high current faults.

I’d guess it was chip failure that initiated the explosive event.

The mystery chip is just an LM339 quad comparator for controlling the channel activity LEDs.