Suggested Remedy for comment #183, 184
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IEEE802.3at Task ForceVport ad hocSuggested Remedy for comment #183, 184October 2007Yair DarshanMicrosemi CorporationSuggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page 1www.microsemi.comBackground for comments 183 and 184 Based on Vport ad-hoc meeting on October 11,2007 the group agree on the following baseline concept to address those comments:– (1) PD Model is constant power model Iport=Ppd/Vport– (2) Iport_avg and Iport_peak are function of Vport at PD input/PSE output– (3) Ipeak/Iavg = Ki_CLASS determined by the specifications (design constrains).– (4) Specification shall support legacy PD peak current numbers as presented in table 33-12 item 4. As a result of the above baseline, the following text corrections are proposed to meet (1) to (4) objectives.Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page 2www.microsemi.comEq-2 is just IEEE802.3 Draft D0.9 StatusInformative..It is better to allow higher Idc for Ki ⋅(Vpse−Ki ⋅Iport_avg⋅Rc)class class(Eq−2) Kp =class 1,2 and to use the same class(Vpse−Iport_avg⋅Rc)0.4/0.35 ratio for all classes(Eq−3) Iport_ peak = Ki ⋅Iport_avgclass2Vpse−(Vpse −4⋅Ppd _avg⋅Rc)(Eq−1) Iport_avg =2⋅RcShown in the specification or derived directly from itCalculated values based on PD constant power equationsSuggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page 3www.microsemi.comKey inputs Ki_class=Ipeak ...
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IEEE802.3at Task Force
Vport ad hoc Suggested Remedy for comment #183, 184
October 2007
Yair Darshan Microsemi Corporation
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page1 www.microsemi.com
Background for comments 183 and 184
Based on Vport adhoc meeting on October 11,2007 the group agree on the following baseline concept to address those comments: – (1) PD Model is constant power modelIport=Ppd/Vport – (2) Iport_avg and Iport_peak are function of Vport at PD input/PSE output – (3) Ipeak/Iavg = Ki_CLASSdetermined by the specifications(design constrains). – (4) Specification shall support legacy PD peak current numbers as presented in table 3312 item 4.
As a result of the above baseline, the following text corrections are proposed to meet (1) to (4) objectives.
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page2 www.microsemi.com
Eq2 is just IEEE802.3 Draft D0.9 StatusInformative.. It is better to allow higher Idc for Ki(Vpse Ki Iport_avg Rc) class class (Eq−2)Kp= class 1,2 and to use the same class pse−Iport_avg⋅Rc) 0.4/0.35 ratio for all classes
(Eq
3)
(Eq−1)
Iport_
Iport_avg=
Shown in the specification or derived directly from it Calculated values based on PD constant power equations
_
2 pse−4⋅Ppd_avg⋅Rc) 2⋅Rc
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page3 www.microsemi.com
Key inputs Ki_class=Ipeak/Iavg.Ki_class determines Ipeak, Ppeak. Kp_class is a function of Ki_class, Vpse and Iport_avg. It is not a constant actually it is a complex variable. – Kp transforms current to power in the PD constant power equations. Using only Ki as proposed in the Table below allows meeting all our objectives
Table TBD1
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Ki Kp
1.350 1.300 1.250 1.200 1.150 1.100 1.050 1.000
Ppd[Watts]
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page4
Suggested Text changes – Comment #183,184
Suggested Remedy principles:
For maintaining consistency with Table 3312 item 4 and 33.2.8.4: 1. Table 3312 item 4: Change symbol from “Iport” to “Iport_peak” 2. Replace the equation “Pport_max/Vport” with “Iport_peak as defined by Table 3312 item 4”. 2. Add the text “Iport_peak is approximated by Eq3” 3. Add equation Eq3, Eq1 and Ki_class constants from Table TBD1 to table 3310 in separate column. 4. Update accordingly 33.2.8.4 and 33.2.8.4a by removing “17.6W/Vport” and referring to Eq1 and Eq3. 5. Update 33.2.8.6 to reflect the above changes. 6.See attached word document for detailed remedy
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page5 www.microsemi.com
Suggested Remedy Text for comment #183, 184
See attached word document for full description of text changes.
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page6 www.microsemi.com
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Additional Information
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page7
Issues to be solved with the original suggested remedy
Ipeak used in Table 3312 item 4 contains margins for class 1 and 2 i.e. Ki>0.4/0.35.Hence the equation as proposed tobe used needs some modifications to present Ipeak as function of Ki_class. – The actual PD constant power model is not pure constant power model since efficiency is not 100%. This is the reason for the margin taken for Class 1,2 at Iport_eak per table 3312 item 4.
Ipeak numbers per table 3312 item 4 can not be changed due to the above rational and to keep legacy PDs compliant.
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page8 www.microsemi.com
1 Simplified PD constant Power Model Assuming efficiency of PD is100% Vpse−Vpd Ppd = Iport Rc VpdRc 2 0.5 Vpse+(Vpse−4⋅Ppd⋅Rc) Vpd= Vpse 2 Ppd=Constant Vpd I=P/Vpd 2 0.5 Vpse−Vpd Vpse−(Vpse−4⋅Ppd⋅Rc) Iport= = Rc2⋅Rc Iport_peak Ki= Iport_avg Iport_peak=Ki⋅Iport_avg Vpd_peak=(Vpse−Iport_peak⋅Rc) Vpd_avg=(Vpse−Iport_avg⋅Rc) Ppd_peak Vpd_peak⋅Iport_peak Kp= = = Ppd_avg Vpad_avg⋅Iport_avg 2 0.5 Vpse−(Vpse−4⋅Ppd_avg⋅Kp⋅Rc) class Iport_peak= ⋅(− ⋅_⋅)2 Ki Vpse Ki Iport avg Rc⋅Rc Kp= or (Vpse−Iport_avg⋅Rc) 2 0.5 Vpse−(Vpse−4⋅Ppd_avg⋅) Rc Iport_peak=Ki class 2⋅Rc
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Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page9
Actual PD constant power model Assuming efficiency of PD is avariable,eff: Pout Pout eff= = Ppd Pout+Ploss Pout⋅(1−eff) Ploss= eff Pout⋅(1−eff) Reff= 2 eff⋅Iport Vpse−Ipd⋅Rc−Ipd⋅Reff=V P = Ipd2 0.5 Vpse−(Vpse−4⋅P⋅Kp_class⋅(Rc+Reff)) V Ipeak= 22⋅(Rc+Reff) V−V⋅Vpse+P(Rc+Reff)=0 or 2 0.5 Vpse−(Vpse−4⋅P⋅(Rc+Reff)) 2 0.5 V=Vpse−(Vpse−4⋅P⋅(Rc+Reff)) Ipeak=Ki 2class 2⋅(Rc+Reff) Vpse−V Ipd= Rc+ReffFor each Ppd we need to find its Reff, P numbers on order to get Vpd and Ppd. Next we have to use Ki, Kp techniques as in the simplified model Vpd=V+Reff⋅Ipd
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Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page10
Efficiency v.s. Power
In conventional DC/DC converters efficiency is optimized near max. power. If output power is decreased, efficiency is decreased due to the growing effect of constant losses that are not function of the load power (housekeeping etc.) As a result, the simplified model is good enough at max. power range. At lower power range additional design margin is required. Hence Table 3312 item 4 is using higher peak current than Ki=0.4/0.35 for class 1 and 2.eff
Power
Suggested Remedy for comment #183, 184, Yair Darshan, October 2007 Page11 www.microsemi.com
