HV110
Power-over-Ethernet (PoE) Standards
IEEE802.3af standard, DTE Power via MDI, deals with the subsequently. The reason for all of this is that high voltages
specification of the interface that can supply/draw power us- (-48V) connected to many legacy devices can cause equip-
ing the same generic cabling as that used for data trans- ment damage. For this reason discovery takes place at volt-
mission. It allows both power and data to flow through the ages compatible with existing legacy equipment and high
Media Dependent Interface (MDI) (like 10Base-T, 100Base voltage DC is only applied once discovery is satisfied. The
TX or 1000BaseT) to the Data Terminal Equipment (DTE) IEEE802.3af discovery is based upon the sensing of a char-
safely and effectively. It defines the functional and electri- acteristic impedance. This impedance is defined nominally
cal characteristics of two optional power (non-data) entities as 25k (23.5k to 26.25k) with no more than 0.1µF of ca-
– the Powered Device (PD) and the Power Sourcing Equip- pacitance in parallel with the impedance, in a voltage range
ment (PSE) that makes this single interface possible. The from 2.8V to 10V. The presence of diode rectification at the
mechanical and electrical interface between PSE and PD PD end forces a slope impedance method, requiring at least
and the transmission line is achieved through the Power In- two operating point measurements, to eliminate the effect of
terface (PI) Devices (usually the LAN cables).
diode level shift.
PSE is defined as a device that provides a single portion
of the link (10BASE-T, 100BASE_TX or 1000BASE_T) with
both the data it requires and the power to process this data.
PSEs may be placed with the DTE/Repeater/Mid-Span. A
PSE that is located along with the DTE/Repeater is called
Endpoint PSE, while a PSE that is located within the link,
between the MDIs is called a Mid-Span PSE. All the specifi-
cations for the PSE sitting in the End Point (e.g. the router)
may not apply for the Mid span PSE.
Classification (optional)
As per the IEEE802.3af standards, the PSE has to deliver a
minimum of 15.4W to a PD connected to it while limited by
the 350mA maximum operating current. Not all PD devices,
however, require this much power to operate. For example
an IP Phone with a monochrome screen will require far less
power than an IP Phone with color display. By identifying
the power drawn through each port, PSE can assist in the
System Power Management protocol to determine the total
number of PDs it can support, depending on the output ca-
pacity of the system power supply.
Even though the HV110 is a PD device, it is closely associ-
ated with the operation of PSE, in fact it is dependent on the
PSE for its normal operation. The HV110, however, unlike
many other PD controllers, provides redundant PSE protec-
tions and timings for maximum protection while ensuring
compliance. Hence certain basic functionalities of the PSE
are included in this data sheet for better understanding some
of the features and operation of PDs.
To achieve this type of power management an optional step
was added to the IEEE802.3af standard called ‘Classifi-
cation’. Classification allows a device to communicate the
maximum power it will ever demand to the PSE so that the
Power Management Protocol can allocate the unused power
to other ports, enabling the full utilization of the installed ca-
pacity. Table 1 identifies the different Classifications included
in the IEEE802.3af standard.
PSE Power Standards
PSE powers a single link. It searches the link for a PD and
supplies power to the link only after a PD Signature is de-
tected. The PSE will reject any links with an invalid PD Sig-
nature. When the PD is removed, the PSE will also remove
the power from the link.
Table 1. PD Power Classification
Class
Usage
Default
PD Power (W)
0.44 – 12.95
0.44 – 12.95
3.84 – 6.49
0
1
2
3
Optional
Optional
Optional
PSE may be able to do an optional classification of the PD,
to detect the maximum power drawn by the PD, to do some
high level Power Management. PSE is limited to a continu-
ous maximum output of 15.4W.
6.49 – 12.95
In order to identify the class of the PD connected, the PSE
sends a second voltage signal of 15 to 20V, slightly higher
than signature detection voltage and measures the current.
Depending on the magnitude of the current drawn, the PSE
will classify the load to one of the four Classes as shown in
Table 2, and will assume that the load will not draw any ad-
ditional power than shown for the given Class.
Discovery
Key to all Power-over-Ethernet methods is discovery. Dis-
covery is the method used to determine if a device at the
end of the cable is capable of receiving high voltage DC,
before applying high voltage. Discovery also is used for de-
termining when a PD device is disconnected or removed
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SUPERTEX [ Supertex, Inc ]
