IEEE802.3at Task Force
Technical and market considerations
regarding PoEp output voltage range
Vancouver BC, November 2005
Arkadiy Peker
Daniel Feldman
Yair Darshan
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Objectives
„ To discuss optimum voltage range for PoEp
„ To show advantages of having PoEp minimum voltage of 51V
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HP POE voltage range
„ Maximum voltage: 57V (safety consideration)
„ What is most optimum value for minimum voltage?
„ PoEp group objective; maximum power within practical limit (Power
Dissipation, Safety consideration)
„ Power dissipation is function of the current
„ Increasing minimum voltage reduces current level for the same load
power or
„ Increasing Power delivery to the load for a given current value.
„ Hence same infrastructure delivers more power to PDs for given
PSE PS size and cost
„ Bottom line: more applications may supported
– Larger POEp market
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PoEp minimum voltage
„ Lets assume current is 0.35A and calculate power delivered to the load by four
pairs
44V
51V
Power dissipation
on 4 pair
3.06W
3.06W
Maximum power
27.74W
32.64W
delivered to the PD
„ It means, that at 51V, for the same power dissipation on the cable, power
delivered to the load will be 4.9W more then at 44V.
„ The advantage of higher voltage is further increased for higher currents than
0.35A
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PoEp minimum voltage
„ Let us look on this issue under another angle assume minimum load need
to be delivered to PD by four pair is 40W
„ Power dissipation on a cables for 51V PSE will be 2.45W less then for 44V
„ Advantages of higher voltage is increase with increasing power level
44V
51V
Pd power
40W
40W
Current per pair
0.54A
7.3W
0.44A
Power dissipation
on a cables (4P)
4.85W
PSE power
47.52W
0.84
44.88W
0.89
System efficiency
for ~500W PSE PS
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PoEp minimum voltage
„ 51V required smaller PSE power supply then at 44V, for the same
load.
„ The technical advantage is clear for using higher voltage minimum
voltage level for POEp PSE.
„ Let consider different options in PoEp Power architectures :
- AC input and battery back up
- AC UPS input
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PoEp Power Architecture-AC Input
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PoEp Power Architecture-AC Input
„ One of the arguments for not using 51V power supply is that back up
battery operates up to ~ 43 - 44V.
„ However most batteries used in telecommunication have one of the
terminal grounded (mostly positive)
„ This would prevent direct use of this batteries in POE and would
require to have isolated converter between batteries and PSE
regardless of output voltage considerations
„ Therefore increasing output voltage to ~51V would not add any
system cost
„ But boost converter would be useful even in case of isolated
batteries (DC UPS), which really is not very popular power
architecture.
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HP POE Power Architecture-AC Input
„ Taken at account the voltage drop on a high current battery cable
and PSE voltage drop (Mosfet and CS resistor), it would require
Low Voltage Disconnect to shutdown battery at ~46V in order to
have 44V on the PSE output
„ It means battery life would not be fully utilize
„ Adding boost converter, which would keep output voltage of the
system constant, in our case 51V,would extend batteries
capabilities on ~15% in POE systems
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PoEp Power Architecture-AC UPS
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PoEp Power Architecture-AC UPS
„ Cost of power supply with 44V or 51V outputs is approximately the
same. But under the same conditions, 51V output power supply will
have higher efficiency then 44V PS
„ Estimated cost of additional boost circuitry for supporting the change
from 45V to 52V for 500W power supply is about 10% cost of power
supply.
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PoEp with Boost circuit
Lb
Db
R1
R2
Qb
Cb
PWM
AC
POE PS
Rs
Battery
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PoEp with Add Power
+
+
Add Power
AC or Battery
Circuit
-
Vout 51V
+
POE PS
V<51V
-
-
•For 48V 250W power supply, maximum power for add power converter
would be (52V-48V)*(500/48)=42W or about 9% of 500W PS cost
•However taken in account improved system efficiency, total cost of
ownership would remain about the same
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Conclusions
„ Increasing minimum PoEp voltage from 44V to 51V would
increase overall system efficiency
„ Costs of 51V PS and 48V PS are approximately the same and
from technical point of view would be beneficial to have minimum
51V in PoEp PSE
„ Regardless of output voltage consideration, adding boost circuit for
system with batteries back up, would increase battery useful life in
POE systems
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Part B
Market considerations regarding
802.3at output voltage range
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802.3af modular installed base considerations
„ 802.3af minimum PSE voltage: 44V
„ Modular switches
– 44V power supply is deployed with chassis
– Customer expectation: 802.3at-blades added to 802.3af-chassis
– 51V minimum at PSE Æ Boost converter MUST be placed on blade
„ More power dissipation at blade
„ More components added to crowded blade
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How to solve the conflict
„ The Conflict is:
– 51V is the Technically preferred solution
„ More power at low cost
„ Bigger addressable market
– 44V addresses the existing modular installed base
„ Impossible to replaced 44V power supplies on the field
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How to solve the conflict – PD side
„ Proposed Solution
„ Effects on PD specifications
– PD operating voltage range: No effect. Remains 37-57V.
– PD classification in 802.3at (including Class 4) should be
mandatory and not optional
– The enhanced classification table will be divided to two parts:
(See details in “An Extended Classification Proposal- Proposal #1, Vancouver BC, Nov. 2005)
„ Part a: power classes that can work with either 44V or 51V.
„ Part b: power classes that can work only with 51V minimum at the
PSE.
– This may be documented in the Informative section of the
standard.
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How to solve the conflict – PSE side
„ Effects on PSE specifications
– PoEp PSE minimum voltage is 51V.
– However PSE's that have a 44V minimum output and are 802.3at-
complaint in any other aspect, are anyway able to power 802.3at-
complaint PD's with classes of Part (a). (Hence all OK so far..)
– Supplies power only if capable of (Like we do today in
IEEE802.3af)
– 802.3af PSE classification including class 4 should be mandatory
(In order to PSE to know if to power or not)
„ To differentiate between 802.3af PD and PoEp PD.
– 802.3at PSE extended classification is optional.
„ Like classification is currently optional in 802.3af
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Summary
„ The conflict level is pretty much reduce by the previous
recommendations
„ Compliant 802.3at PSE is the one with 51V minimum
„ Physics allow to support most of the higher power classed even
with 44V ((enhanced classification part A group). which is built in
benefit however it doesn't concerns the other standard
requirements nor affect it.
„ It is perfectly OK that Switch/PSE is 802.3at compliant as long as
its Power Supply supports 51V.
„ If PSE PS Power Supply is 44V, then 802.3at PSE is not compliant
but working and covered most of high power classes
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Discussion
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References
„
An Extended Classification Proposal-Proposal #1. Yair Darshan, Vancouver BC, November 2005
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Annex A: Enhanced Classification table
„ Example for group A and group B
„ With Pmax=40W
Group B
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