RE: [802.3af] Late comment
All,
I agree with Yair and Dave on most points but oppose the rule out of item
(d) below. In multiport system, the ports common can very well be the
negative output (Vout-) with the switching N-Channel MOSFET on the postitive
rails. We don't have to use P-Channel MOSFET for this.
So, here is my suggested text to the standard:
1. The PSE port outputs shall be isolated from system ground,
frame
ground and data circuits.
2. In multi port system all ports common point can be either
the positive
output (+Vport) or the negative output (-Vport).
Thong Huynh
> -----Original Message-----
> From: Yair Darshan [SMTP:YairD@xxxxxxxxxxxxxx]
> Sent: Tuesday, January 14, 2003 12:22 AM
> To: 'Dave Dwelley'; Geoff Thompson
> Cc: stds-802-3-pwrviamdi@xxxxxxxx
> Subject: RE: [802.3af] Late comment
>
>
> All,
>
> I agree that if all ports are floating than you may have 2*Vport between
> any
> two ports however these ports would never touch each other under normal
> operating conditions and installation.
>
> In any case the preferred lead to be ground is the positive lead from
> practical point of view (easy to design when the + is the common for a
> single power supply) and from system point of view (positive lead to
> system
> earth).
>
> Currently in our spec we didn't defined if:
> a. we stay floating in both leads or
> b. ground the positive or
> c. use the positive lead as a common point without grounding it to the
> system ground/chassis.
> d. ground the negative or
> e. using mixture of possibilities.
>
> It is clear that option e is disaster to the standard.
>
> option d is bad from practical design aspects and cost issues. (P channel
> mosfets etc)
>
> Option c : the best option by having a positive common point which is
> floating too.
>
> Option b is proffered if we find good reasons why we should ground the
> port.
>
> Option a is very good due to max completely floating outputs.
>
>
> Summary:
>
> I suggest the following text to the standard:
>
> 1. The PSE port outputs shall be isolated from system ground, frame
> ground and data circuits.
> 2. In multi port system all ports common point shall be the positive
> output (+Vport).
>
> Yair.
>
>
>
> -----Original Message-----
> From: Dave Dwelley [mailto:ddwelley@xxxxxxxxxx]
> Sent: Tuesday, January 14, 2003 3:47 AM
> To: Geoff Thompson
> Cc: stds-802-3-pwrviamdi@xxxxxxxx
> Subject: Re: [802.3af] Late comment
>
>
>
> All -
>
> Geoff is right. There's nothing to stop someone from building a system
> with
> a +/-48V supply, and wiring half the ports to -48 and half the ports to
> +48. The fact that the supply is floating relative to earth ground doesn't
>
> matter - there are at least two terminals that are 96V apart, and
> something
> could short them together.
>
> Technically, because we're isolated, it's completely arbitrary which end
> we
> treat as "ground". Practically, there are arguments both ways:
>
> - most non-telecom engineers (read: networking engineers) are most
> comfortable with ground on the bottom, i.e., +48
> - most telecom engineers are used to -48
> - most engineers of both stripes like buying 100V NFets in preference to
> PFets, suggesting switching the negative rail is best.
>
> I'd vote we specify that the more positive rail (environment A only) is
> "common to all ports" (don't call it "ground"), and the more negative rail
>
> is switched. If we refer to that switched rail consistently as "isolated
> -48V", it would be even more clear.
>
> Now, what to call the rail that the logic on the isolated side runs from?
> -44.7V? :-)
>
> Dave
>
>
>