bob said:
I've been wary of self-powered USB hubs because I didn't want the power to short-circuit,
go into my computer, and fry it. Am I being paranoid?
Is there a safeguard against this scenario of the hub accidentally cooking your computer?
A hub with its own wall adapter, can backfeed into the computer.
The Polyfuse on the motherboard, would limit the flow to <2 amps
or so.
On an Alcor hub schematic I've got (application note), they use
a small relay to isolate power. When the wall adapter is present,
the relay coil is energized, and upstream bus power is disconnected
(upstream VBus to hub path broken). When the wall adapter is
unplugged from the wall, the relay coil is no longer energized,
and VBus from the computer powers the hub. This has the advantage
of relatively low voltage drop in the power path (as it's using
switch contacts). If your hub had this, you'd hear a muted
"click" from inside the hub, as you plugged and unplugged
the attached wall wart.
(See PDF page 16 for the clever usage of a relay for backfeed protection...)
http://logout.sh/computers/projects/usbradio/AU9254 R2-020108.pdf
Other possibilities are the usage of a diode, but that might
cause too much DC droop. The USB bus has a voltage droop budget,
and I doubt they included room for a diode in the power path.
A diode would also change the supply impedance, so as the
USB devices changed current loading, the bus voltage would
end up with more ripple on it.
Another scheme, is to use a P-channel MOSFET. But that might
have the same problem. That might be used in a self-contained
electronics project (one with no downstream USB ports), but
if you put downstream USB ports on the box, then the USB bus
has to meet the minimum voltage spec.
Yet another solution, would be a boost-buck power converter,
providing enough voltage so a diode could be used. But that's
not likely to happen. Too expensive.
The relay idea is the most practical, but I don't have a clue
as to what percentage of hubs even consider this to be an
issue. I can find user reports on Newegg, about hubs
that do cause backfeed. It may prevent the computer from
shutting off properly (when using soft power control).
Paul
