LTPoE++ EXTENDS PoE 90W with reliable and easy-to

LTPoE++ EXTENDS PoE 90W with reliable and easy-to
SPECIAL REPORT : POWERING COMMUNICATIONS
LTPoE++ EXTENDS PoE
90W with reliable and easy-to-use standard
By Heath Stewart
Power over Ethernet, or PoE, is an increasingly popular way to
deliver both power and data over existing Ethernet cable, thus
freeing applications from the constraint of AC-power proximity.
A
s the number PoE
solutions has
grown, so has the
applications’ appetite
for power. A new proprietary
standard, LTPoE++™, satisfies
this hunger by extending the
PoE and PoE+ specifications
to 90W of PD delivered power.
LTPoE++ also dramatically
reduces engineering complexity
in power sourcing equipment
(PSEs) and powered devices
(PDs) when compared to other
power-expansion solutions.
Plug-and-play simplicity and
safe, robust power delivery
are hallmarks of LTPoE++. The
capabilities of this standard
expand the field of Ethernetpowered applications by several
orders of magnitude, enabling
entirely new classes of PDs, such
as power-hungry picocells, base
stations or heaters for pan-tiltzoom cameras.
The IEEE standard defines
PoE terminology, as shown in
Figure 1. A device that provides
Figure 1: Typical PoE system
power to the network is known
as a PSE, or power sourcing
equipment, while a device that
draws power from the network
is known as a PD, or powered
device. PSEs come in two types:
endpoints (typically network
switches or routers), which
send both data and power, and
midspans, which inject power
but pass data through. Midspans
are typically used to add PoE
capability to existing non-PoE
networks. Typical PD applications
are IP phones, wireless access
points, security cameras, cellular
femtocells, picocells and base
stations.
The IEEE PoE+ specification
specifies backward compatibility
with 802.3af PSEs and PDs. The
PoE+ specification defines Type
1 PSEs and PDs to include PSEs
and PDs delivering up 13W. Type
2 PSEs and PDs deliver up to
25.5W.
LTPoE++ Evolution
The IEEE PoE+ 25.5W
specification had not yet been
finalized when it became clear
that there was a significant
and increasing need for more
than 25.5W of delivered power.
In response to this need, the
LTPoE++ specification reliably
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SPECIAL REPORT : POWERING COMMUNICATIONS
allocates up to 90W of delivered
power to an LTPoE++ PD.
The LTPoE++ specification
provides reliable detection and
classification extensions to
existing IEEE PoE protocols.
LTPoE++ is backward compatible
and interoperable with existing
Type 1 and Type 2 PDs. Unlike
other proprietary powerextending solutions, Linear’s
LTPoE++ provides mutual
identification between the PSE
and PD. LTPoE++ PSEs can
differentiate between an LTPoE++
PD and all other types of IEEE
compliant PDs, allowing LTPoE++
PSEs to remain compliant and
interoperable with existing
equipment.
LTPoE++ PSEs and PDs
seamlessly interoperate with
IEEE 802.3at Type 1 and Type 2
devices. Type 1 PSEs generally
encompass 802.3af functionality
at and below 13W. Type 2 PSEs
extend traditional PoE to 25.5W.
•
•
•
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36
Type 1 PSEs will power all
Type 1, Type 2 and LTPoE++
PDs with up to 13W.
Type 2 PSEs will power Type
1 PDs with up to 13W and
provide 25.5W to Type 2 and
LTPoE++ PDs.
LTPoE++ PDs can power up
with limited functionality
even when attached to
traditional Type 1 and 2 PSEs.
LTPoE++ PSEs interoperate
with Type 1 and Type 2 PDs.
LTPoE++ PDs are powered
to the designed limit of the
Figure 2: IEEE 802.3at signature resistance ranges
LTPoE++ PSE. When an
LTPoE++ PD is identified,
the PD will be powered up if
the PSE power rating meets
or exceeds the requested PD
power. For example, a 45W
LTPoE++ PSE can power both
35W and 45W PDs.
IEEE-Compliant PD Detection
LTPoE++ physical detection
and classification is a simple,
backward-compatible extension
of existing schemes. Other power
extension protocols violate the
IEEE specification, as shown
in Figure 2, and risk powering
up known noncompliant NICs.
Any high power allocation
scheme violating the IEEEmandated detection resistance
specifications risks damaging
and destroying non-PoE Ethernet
devices.
The following rules define any
detection methodology for the
highest levels of safety and
interoperability.
Priority 1: Don’t turn on things
you shouldn’t turn on.
Priority 2: Do turn on things you
should.
Linear Technology PSEs provide
extremely robust detection
schemes utilizing four-point
detection. False positive
detections are minimized by
checking for signature resistance
with both forced-current and
forced voltage measurements.
LTPoE++ Advantages
Figure 3: The expensive way to extend PoE+ power. Dual Type 2 PD provides
more power than standard PoE+ PD, but it also doubles the cost and
component count.
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POWER SYSTEMS DESIGN NOVEMBER
2011
DC converter, resulting in
significant board space, cost and
development time advantages.
Figure 4: Less expensive, but flawed, alternative for extending PoE+ power.
This scheme is similar to the dual Type 2 set-up shown in Figure 3, but a diode
ORed power sharing architecture reduces some of the cost by eliminating one
DC/DC converter in the PD. However, due to intrinsic reductions in surge
protection tolerance, these solutions rarely meet PD design goals.
Standard PoE PSEs use two of
the four available Ethernet cable
pairs for power. Some powerextending topologies use two
PSEs and two PDs over one cable
to deliver 2 × 25.5W. This “dual
Type 2” topology is shown in
Figure 3. The main problem with
this strategy is that it doubles the
number of components, thus
doubling PSE and PD costs.
Additionally, robust design
considerations require two DC/
DC converters at the PD, one for
each component PD, where each
DC/DC converter is a relatively
complex flyback or forward
isolated supply.
One of the DC/DC converters
in a dual Type 2 set-up can be
eliminated by ORing the PD’s
output power as shown in Figure
4. This approach still requires
two PSEs and two PDs, with
the associated cost and space
disadvantages. The voltage drop
incurred by the power ORing
LLDP Interoperability and
Options
During selection and architecture
of a PoE system, many PD
designers are surprised to
discover the hidden costs of Link
Layer Discovery Protocol (LLDP)
implementations. LLDP is the
IEEE-mandated PD software-level
power negotiation. LLDP requires
extensions to standard Ethernet
stacks and can represent a
Figure 5: The LTPoE++ architecture is the only PoE power-extending solution
that provides 90W at the PD while keeping complexity and costs in check.
diodes might be considered a fair
trade-off for the savings gained by
using a single DC/DC converter.
In most cases diode ORed power
sharing architectures remain
attractive until surge protection
testing begins. Due to intrinsic
reductions in surge protection
tolerance, these solutions rarely
meet PD design goals.
In contrast, LTPoE++ solutions,
as shown in Figure 5, require
only a single PSE, PD and DC/
significant software development
effort. Unfortunately the opensource community effort to
provide LLDP support is still in
its infancy.
While Type 2 PSEs may optionally
implement LLDP, fully IEEEcompliant Type 2 PDs must
implement both physical
classification and LLDP power
negotiation capabilities. First,
this places the burden of LLDP
software development on all Type
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SPECIAL REPORT : POWERING COMMUNICATIONS
2 PDs. In addition, designs are
complicated by the dual power
requirements inferred by the
LLDP requirement. Specifically,
the PD-side processor must
be fully functional at the 13W
power level and then have the
ability to negotiate, via LLDP,
for the delivery of additional
power. Clearly this requirement
can increase development and
system costs and complexity.
LTPoE++ offers LLDP
implementation options.
LTPoE++ PSEs and PDs
autonomously negotiate
power level requirements and
capabilities at the hardware level
while remaining fully compatible
with LLDP-based solutions.
In short, LTPoE++ gives
system designers the choice to
implement or not implement
LLDP. Proprietary end-to-end
systems may choose to forgo
LLDP support. This creates timeto-market advantages while
further reducing BOM costs,
board size and complexity.
Power Claims Demystified
PoE power paths can be divided
into three main components:
the power produced by the PSE,
the power delivered to the PD
and the power delivered to the
application. Claims of PSE and
PD power delivery capabilities
must be carefully examined
before useful comparisons can be
made. One vendor may describe
the power as delivered by the
PSE, another the power delivered
to the PD, while the PD designer
38
typically cares about power
consumed by the application.
Although the PSE power metric
is the least useful of the three,
it is the one most often cited in
marketing materials. PSE power
is generally defined as the power
delivered at the PSE end of the
Ethernet cable. Power capability
is sometimes further distorted
when vendors specify power
at the maximum rated voltage,
which is rarely achieved.
PD power or “delivered power”
is the power delivered to the PD
end of the Ethernet cable, prior
to the diode bridge. Quoted PD
power is a more useful metric
than PSE power, since it must
account for significant losses
over 100 meters of CAT-5e
cable. PD power claims make
no assumptions about the
application’s DC/DC converter
and diode bridge efficiencies,
which are unknown to PSE and
PD silicon vendors.
A PD designer is
most interested in the
power delivered to
the application when
all system effects are
considered, including
the resistance of the
Ethernet magnetics,
diode bridge voltage
drops and DC/DC
converter efficiency.
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This metric, although
the most telling, is
the most difficult to
accurately specify.
PSE Availability
Linear Technology is committed
to LTPoE++ technology and
provides an entire family of PSE
and PD solutions. A full family
of PSEs, spanning 1- to 12-port
solutions is now available.
Conclusion
LTPoE++ offers a robust, end-toend high power PoE solution with
up-front cost savings. Combined
with Linear Technology’s
excellent application support,
proven delivery record and
reputation for reliability, LTPoE++
is the most comprehensive high
power solution on the market.
LTPoE++ systems simplify
power delivery and allow system
designers to concentrate their
design efforts on their high value
applications.
Author: Heath Stewart
Senior Design Engineer
Linear Technology Corporation
www.linear.com
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