PLANET WGSD-10020 - Manual (Page 528)

User’s Manual of WGSD-10020 Series
528
Classification
Once a PD is detected, the PSE may optionally perform classification, to determine the maximal power a PD is to consume. The
PSE induces 15.5-20.5 VDC, limited to 100 mA, for a period of 10 to 75 ms responded by a certain current consumption by the
PD, indicating its power class.
The PD is assigned to one of 5 classes: 0 (default class) indicates that full 15.4 watts should be provided, 1-3 indicate various
required power levels and 4 is reserved for future use. PDs that do not support classification are assigned to class 0. Special
care must be employed in the definition of class thresholds, as classification may be affected by cable losses.
Classifying a PD according to its power consumption may assist a PoE system in optimizing its power distribution. Such a
system typically suffers from lack of power resources, so that efficient power management based on classification results may
reduce total system costs.
Start-up
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 high frequencies) would introduce noise on the data 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 provision
startup.
Operation
During normal operation, the PSE provides 44-57 VDC, able to support a minimum of 15.4 watts power.
Power Overloads
The IEEE 802.3af standard defines handling of overload conditions. In the event of an overload (a PD drawing a higher power
level than the allowed 12.95 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 this period to protect the cabling infrastructure.
Immediate voltage drop is avoided to prevent shutdown due to random fluctuations.
Power Disconnection Scenarios
The IEEE 802.3af standard requires that devices powered over Ethernet be disconnected safely (i.e. power needs be shut down
within a short period of time following disconnection of a PD from an active port).
When 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 non-PoE
notebook computer. What’s sure to follow is not a pretty picture.
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.