Indeed, the TBS5930 can’t be powered only using USB. External +12V must be supplied.
I actually got the TBS5520SE before, but had to return it unfortunately.
While it had the advantage to use USB power as you mentioned I had issues with the blind scan on several satellites.
The box always got stuck on some (always the same) transponders.
In these cases, the box no longer responded and had to be disconnected from the computer.
My hope was that the TBS5930 works better as it is advertised as having a “built in hardware blind scan capability”. Strangely enough, the TBS blind scan tool (v3.0.2.2) doesn't work with the box at all. In other words, the functionality can’t be used currently with the manufacturer's tools.
What I also found interesting is that the box doesn’t seem to support USALS with the current driver (v1.0.0.2) despite the +12V/2A power supply.