48V bus topics

["Walker, Ed" <ed_walker@xxxxxx>]

Title: RE: power delivery question from Liaison report

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For phone infrastructure they use a -48V bus, supported primarily from an AC input converted to a -48V output that is isolated from the AC line. The secondary means of supporting the -48V bus is from a battery bank. The 2 sources are diode or-ed together.  In actuality the system sometimes runs off the batteries all the time and the AC source just keeps the batteries charged. Typical -48V bus design criteria call for the voltage range to be from 36 to 72V. Converters operating off this bus are designed to handle the input swing.

ALL STANDARD OFF THE SHELF PRODUCT NOW.

 

There are lots of folks that make modules that operate from a 48V (-48V) to produce an isolated output voltage (12v, 5v, 3.3v, etc). The main difference with the 802.3 requirement is going from -48V to a 55V output.  This is easily done with todays power technology but not commonly available, YET. Waiting on a standard I guess.

 

Regulation tolerance can easily be controlled to within +-5% at the source.  55 x 1.05 = 57.75, below the 60V SELV limit.

What the load will see depends solely on its current draw and the resistance of the power path. The PowerDsine team did an excellent job of describing the path resistive drop effects.

 

So what you will have in this scenario is a Power supply (module):

Vin=-36 to -72V

Vout=55V +-5% isolated

Iout=350mA current limited per port.

 

For multiple port powering there are folks who isolate each port and some who group several ports together. If each port needs isolation then to maintain the 5% regulation some sort of regulator module will be needed for each port.  If each port does not need isolation, say groups of 5 to 10, then the power supply design get easier, all you have to do is limit the current to each (probably using the circuit that actually applies power to the line after discovery).

 

Ed Walker
Technical Staff

Analog Field Applications 
Texas Instruments Incorporated
7001 Weston Parkway #100
Cary NC 27513

INTERNET: ed_walker@xxxxxx
WEB SITE: http://www.ti.com
Office = 919-677-7061
Fax = 919-677-7030

 

-----Original Message-----
From: Rick Brooks [mailto:ribrooks@xxxxxxxxxxxxxxxxxx]
Sent: Wednesday, July 19, 2000 10:06 AM
To: Nick Tullius
Cc: 'stds-802-3-pwrviamdi@xxxxxxxx'
Subject: RE: power delivery question from Liaison report

Just to be clear, we are almost certainly not putting a battery voltage directly on the RJ-45.
At the very least each ethernet UTP port to frame ground needs 2250 VDC of isolation, this is an existing spec.
When the working group talks about 48 VDC, we are talking about what the PSE (power sourcing equipment)
is putting out on the RJ-45, and it will need to be at least somewhat regulated.
We are now discussing what the voltage range needs to be.

If someone wants to regulate a 48V battery voltage to obtain the PSE output voltage, then fine.
If someone wants to regulate a 120V/240V, or 208, 3 phase line voltage to obtain the PSE output voltage, then that's also fine.

The working group will not be involved in that decision; that it outside the scope of IEEE802.3af.
IEEE802.3af will only spec the output V, I  of the PSE, not the input power.

Also, the Liaison report was talking about restrictions beyond SELV.
Some (many?) of us would desire the absolute maximum voltage to be above 48 VDC, like maybe 52 to 56 VDC,
otherwise we will have to limit the available power.

- Rick

-----Original Message-----
From:   Tullius, Nick [CRK:0M90-I:EXCH]
Sent:   Wednesday, July 19, 2000 6:25 AM
To:     Brooks, Rick [SC5:321:EXCH]
Cc:     stds-802-3-pwrviamdi@xxxxxxxx
Subject:        RE: power delivery question from Liaison report

Team,

just a reminder that 48 Vdc is a NOMINAL voltage (number of battery cells x 2) and has little to do with the limits of the operating range. The actual float voltage of the battery is 52.08 V (2.17 V/cell x 24 cells) for flooded batteries, and typically 54.00 V (2.27 V/cell x 24 cells). This is obviously the highest voltage ever appearing at the power-to-telecom equipment interface.

These voltages are all within the maximum SELV value of 60 Vdc (see IEC 60950).

For an example of the parameters needed to define a generic 48 V bus, see ANSI T1.315 Voltage Levels for DC-Powered Equipment Used in the Telecommunications Environment.

Best of luck,

Nick Tullius
Astec Advanced Power Systems
Tel 613 763-2359
Fax 613 763-7155
ntullius@xxxxxxxxxxxxxxxxxx

-----Original Message-----
From:   Brooks, Rick [SC5:321:EXCH]
Sent:   Tuesday, July 18, 2000 7:00 PM
To:     stds-802-3-pwrviamdi@xxxxxxxx
Subject:        power delivery question from Liaison report

I was reading in the Draft Liaison report from ISO/IEC JTC 1/SC 25/WG 3 to IEEE802.3 on power feeding that was
handed out at the July Plenary.

IEEE802.3af had question 4: Info on parameter limits (voltage, current, power, source impedance, ...) for world wide standards.

i.e. what are the restriction beyond SELV.

The response back was 48 VDC max, 175 ma max per pin.




My question is:
Is the 48VDC output from the port really 48VDC max as the response to the question indicates?





If so, my thoughts are the following:

We would have to spec our power output at the PSE as 48 VDC + 0%, - 8%, or something like that,
so that it never exceeds 48 VDC continuously.

This will further limit the available load power;
it would be less than the load power that was discussed at the last meeting namely 14.6 watts.

So, in that case the PD must be designed to draw at most 350 ma, as we discussed.
And the power delivered at 100 meter cable would then be:
Pwr = [44.2 - (12.5 x 0.35)] 0.35 =  13.9 Watts. (where 44.2 VDC is the lowest output voltage to still be in spec)
For long cable lengths, the current per pin will be balanced, and we don't exceed the 175 ma per pin.

For short cable lengths, we probably need an additional power spec, so that neither RJ-45 pin exceeds 175 ma.
Say that due to connector imbalance, one pin is 175 ma, and the other is 20% below that, or 140 ma, which is a total of 315 ma.

Then the power for a short cable would be (at least) 13.9 watts (44.2 * 0.315).

This would say that the PD device should be designed not to draw more than 350 ma,
and at the same time not to draw more than 13.9 watts.
That way we never exceed 48 VDC nor 175 ma per pin on a continuous basis.

This puts the burden on the PD end to meet these current and power requirements.
The PSE end would have a max voltage of 48 VDC, but it's current limit would be set slightly higher than 350 ma
by some appropriate margin.

If, on the other hand, we put the burden at the PSE end, then the available power goes down even more, but that may be OK also.

comments?