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7.3 Start-up
h frequencies) would introduce noise on the data
uring normal operation, the PSE provides 44-57 VDC, able to support a minimum of 15.4 watts power.
The IEEE 802.3af standard defines handling of overload conditions. In the event of an dra
p el than the allo Watts), or an outright short circuit caused by a failure in cabling or in the PD, the PSE
must shut down power within 50 to 75 milliseconds, while limiting current drain during t otec
in Immediat om fl .
7 con on Scenarios
T 02.3af stand net be disconnect ely (i.e. power needs be shut
d ction of a PD from an active port)
hen a PD is disconnected, there is a danger that it will be replaced by a non-PoE-ready device while power is still on.
Imagine disconnecting a powered IP phone utilizing 48 VDC, then inadvertently plugging the powered Ethernet cable into a
r. What’s sure to follow is not a pretty picture.
ent of current. Naturally, a disconnected PD stops consuming current, which
an be inspected by the PSE. The PSE must therefore disconnect power within 300 to 400 ms from the current flow stop.
s important to prevent shutdown due to random fluctuations.
nals
port (disconnected PD).
d
Once line detection and optional classification stages are completed, the PSE must switch from low voltage to its full
voltage capacity (44-57 Volts) over a minimal amount of time (above 15 microseconds).
A gradual startup is required, as a sudden rise in voltage (reaching hig
lines.
Once provision of power is initiated, it is common for inrush current to be experienced at the PSE port, due to the PD’s input
capacitance. A PD must be designed to cease inrush current consumption (of over 350 mA) within 50 ms of power provi-
sion startup.
7.4 Operation
D
Power Overloads
overload (a PD wing a higher
ower lev wed 12.95
his period to pr t the cabling
frastructure. e voltage drop is avoided to prevent shutdown due to rand uctuations
.5 Power Dis necti
he IEEE 8 ard requires that devices powered over Ether ed saf
own within a short period of time following disconne .
W
non-PoE notebook compute
The standard defines two means of disconnection, DC Disconnect and AC Disconnect, both of which provide the same
functionality - the PSE shutdowns power to a disconnected port within 300 to 400ms. The upper boundary is a physical
human limit for disconnecting one PD and reconnecting another.
DC Disconnect
DC Disconnect detection involves measurem
c
The lower time boundary i
AC Disconnect
This method is based on the fact that when a valid PD is connected to a port, the AC impedance measured on its termi
is significantly lower than in the case of an open
AC Disconnect detection involves the induction of low AC signal in addition to the 48 VDC operating voltage. The returne
AC signal amplitude is monitored by the PSE at the port terminals. During normal operation, the PD's relatively low im-
pedance lowers the returned AC signal while a sudden disconnection of this PD will cause a surge to the full AC signal level
and will indicate PD disconnection.