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RE: PSE vs. PD power dissipation again

Rick, I agree with you.

This is part of important data that we need in order to help us deciding
which option is best.
Our info from marketing says that 5-12watts is the big market, what others


> -----Original Message-----
> From:	Rick Brooks [SMTP:ribrooks@xxxxxxxxxxxxxxxxxx]
> Sent:	ו, מרץ 23, 2001 5:31 PM
> To:	Dave Dwelley; stds-802-3-pwrviamdi@xxxxxxxx
> Subject:	RE: PSE vs. PD power dissipation again
> Dave, 
> that is a concise summary alright. 
> One more thing that we should try to consider is the overall market. 
> We should try to give the best tradeoffs to the center of mass of that
> market. 
> Let's try to make the center of mass better. 
> Otherwise, it is easy for each of us to try to make the spec favor our
> part of the market (PSE or PD). 
> Since I would do PSE's, say, I would favor lower PSE cost and higher port
> density, so option 1. 
> In a way, I hate to ask for marketing input here, but that might be the
> best way to help 
> to choose the best of the 2 options. 
> For example: 
> If we all think that the largest part of the market will be rather large
> PD's (maybe 8 to 12 watts) 
> that need a big input capacitors, then Option 2 might be the best. 
> If we all think that the PD designs will be all over the map in terms of
> power 
> and required DC/DC capacitance, then option 1 might be the best. 
> I'm talking here about numbers of units shipped, not just the fact that
> there will be 1W PD's and 12W PD's. 
> We should lower overall (relative) cost of the system: 
> # of PSE ports shipped 
> cost per PSE port 
> # of PD ports shipped 
> cost per PD port 
> thoughts? 
> - Rick 
> -----Original Message----- 
> From:   Dave Dwelley [SMTP:ddwelley@xxxxxxxxxx] 
> Sent:   Thursday, March 22, 2001 5:46 PM 
> To:     stds-802-3-pwrviamdi@xxxxxxxx 
> Subject:        RE: PSE vs. PD power dissipation again 
> At the risk of repeating some of this discussion, let me summarize the 
> PSE-PD dissipation issue as I see it. If I've made a mistake in any of the
> following points, please correct me! 
> We seem to be split into two camps: 
> Inrush limit by PD: 
> - No dissipation in PSE, which means we can integrate multiple switches 
> - Requires inrush circuit in PD = more $$ in PD (amount of $ subject to
> debate) 
> - Puts power dissipation in PD FET always = bigger PD FET 
> - Requires rapid overcurrent disconnect in PSE 
> - A PSE with this design cannot power up a PD with no inrush limit 
> Inrush limit by PSE: 
> - Requires big FETs in the PSE to survive 500mA/100ms wire short 
> - Can power any PD - with or without inrush protection 
> - Dissipation can be in PSE, PD, or shared 
> - Must allow extended over-current faults before turn-off - adds to PSE 
> dissipation 
> - Can power big PD cap faster (500mA vs 350) if the PSE is sized to 
> dissipate the additional power 
> We need to endorse only one of these two, since they have mutually 
> exclusive features. 
> Option 1 really only has one compelling feature, which is low watts in the
> PSE. We can integrate multiple option 1s in one chip. Multiple option 2s 
> can't be integrated without some accommodation - sequential turn on, 
> dynamically controlled current limit - something. There are secondary 
> benefits to option 1 - it won't power up non-inrush-controlled PDs, which 
> almost gets us the "second check" that Roger has been asking for, and it 
> won't put a heavy load on a power-managed PSE for long durations during a 
> wire short. 
> Option 2 has some nice features, most notably the ability to power up 
> nearly any PD. It can also ride out a brief short on the wire without 
> disconnecting the PD. A minor downside is that the PSE power supply must 
> absorb a fair-sized overload if a PD classified as a low power device
> (with 
> power allocated thusly) suffers a wire short. If we chose option 2, we 
> encompass a wider range of PD designs, including some very low cost 
> options. But it limits the ability to integrate multiple channels down the
> road. 
> As an IC designer, I naturally favor option 1 - I'd like to sell PSE chips
> with many integrated channels. As an engineer, I'm willing to weigh the 
> pros and cons of each (including ones I haven't thought of yet) and vote 
> for the best solution. Let's continue to air out the pros and cons until 
> Don's vote - coming soon, right, Don? 
> Dave