Repair of DELL Latitude E5570 motherboard

[dzseki]

At work we wanted to program an FPGA board with an Altera USB Blaster and using a DELL Latitude E5570 laptop.
The FPGA board is rather complex and it seems something seriously went wrong with it. Upon powering up, the FPGA board seemengly took out the USB Blaster, in turn the USB Blaster took out the laptop as it went dark immediately -go figure!
The laptop is not switching on anymore, more precisely if we push the power button the power on LED comes on for 20 seconds or so, but nothing else happens.

I have disassembled the laptop and found out that with two out of the three USB connectors have the data+ and data- legs (of USB 2.0) shorted to ground (through ~2 Ohm).
On the data lines there was a surge protector diode, I hoped it was that easy, removed to no avail. The short remained.
I have traced these lines to a QFN16 device marked „sZZHE”, again I hoped after removing this the short would go away, but again no.

So I am curious how USB is done on modern(ish) computers. This one has an I5-6300U CPU which contanis pretty much everything in one BGA package soldered on the motherboard.

[SilverSolder]

I know this is too late...  but there is such a thing as a USB isolator!

[dzseki]

Yes, we realized this too -just a bit too late eh...
Anyway I was able to find a DELL schematic diagram that was also a Latitude series, but a few years older. Anyway it was very similar to what I saw. The USB data lines are protected with ESD surge arrestor diodes and with a common mode coil, the mystic IC was probably a multiplexer that is used to decide how much current should the USB allow for the device. Other than that the data lines went in to the chipset on the old schematic, in our case there is not even a chipset IC anymore, everything is integrated into the CPU, the CPU on the other hand is soldered on the motherboard, so that was about it...