IEEE802.3at Task Force
Vport ad hoc
Constant Power Model = Worst Case
Jan 2008
Yair Darshan
Microsemi Corporation
www.microsemi.com
Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 1
Objectives of this presentation
Showing that using the PD as a constant power model covers all cases of PD
power supply implementations
– Linear regulators
– Switching Power Supplies
– Any other combination
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Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 2
List of facts
1 of 2
– Linear Regulator
– Vout and Pout determine Pout=constant. Iout = Iin = constant
– Pin=Vout*Iin+(Vin-Vout)*Iin = Vin*Constant. – Pin_max=min {Vin*Constant , 12.95W} Pin_max = 12.95W
– Efficiency=Pout/Pin=Vout*Iin/Vin*Iin= Vout/Vin !!
– Switching Regulator
– Vout and Pout determine Pout=constant. Pin=Pout/Efficiency =~ constant (at >80% efficiency)
– Pin=Pout/Efficiency =~ constant.
– Pin_max = 12.95W
In all cases:
– Pin_max=12.95W. Pport_max.
– Iport_max=0.35A. Icable_max.
– Iport=Pport/Vport must be maintained to meet the 12.95W and 0.35A limitations
The differences between the models are: In switching regulator the output power is much
higher then in linear regulator due to better efficiency!
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Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 3
Constant Power Model allows max power utilization
Constant Power Model
0.35
0.3
Maximu
m
0.25
Constan
t
0.2
0.15
Power M
odel op
e
rating ra
802.3af Example
Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 4
55
53
51
49
47
45
43
41
0
PD Input Voltage [V]
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nge
Maximum Linear Regulator operating
range (Constant Current Model)
0.1
0.05
37
Iin_Linear_Reg [A]
0.4
39
Iin_SMPS [A]
PD maximum Input Current
Constant Current Model
List of facts
2 of 2
802.3af uses PD constant power model assumption in various locations
– Table 33-12 item 5: Input Current DC or RMS.
Iport=0.35A for Vport=37V 12.95W/37V=0.35A, Pport=constant, Vport=Variable
Iport=0.23A for Vport=57V 12.95W/57V=0.23A , Pport=constant, Vport=Variable
Constant power model.
– 33.3.5.4: Iport_max=Pport_max/Vport
– PSE section: similar representations as in PD section
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Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 5
Summary and conclusions
802.3af is our baseline
802.3af is using constant power model load for
average/rms power/current.
Constant power model load allows optimal utilization of
PD power supply.
802.3at is should continue using the same concept.
Constant Power Model Definition:
Iport=Pport/Vport=Pclass_max/Vport
Pclass may be received from Physical Layer or Data
Link Layer classification.
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Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 6
Discussion
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Constant Power Model = Worst Case, Yair Darshan, Jan 2008
Page 7