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Mike
Michael McCormack
Texas
Instruments, Power Interface Products
50 Phillippe Cote St.
Manchester, NH 03101 USA
Voice: +1 603 222 8686
Fax: +1 603 222 580
Email: mike_mccormack@xxxxxx
Web Site: http://www.ti.com
From:
owner-stds-802-3-poep@xxxxxxxx [mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of <Gerard> <Bachand>
Sent: Wednesday, March 16, 2005
3:22 PM
To:
STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 802.3af
Minimum Signature Voltage
Good point, if an "Enhancement-mode" MOSFET is used to
disconnect the signature resistor, when power is applied.
However, If a "Depletion-mode" MOSFET is used to switch out
the signature resistor, there will be no problem, because there will be
enough voltage to turn off the MOSFET, once the PD is powered.
Note: A "Depletion-mode" MOSFET is normally saturated with 0V
on the gate.
Once there are several volts (reverse bias) on the gate, the MOSFET
will be in its "open" state; like a normally-closed relay.
Supertex, as well as others make some real nice Depletion-mode MOSFETS.
Also Clare Theta J, as well as others make optocoupled solid state
relays.
One such unit is a six pin mini dip normally closed (form
"B" contact"), I believe its part number is LCB110,
the normally open version is LCA110.
The least expensive part would probably be the depletion MOSFET,
although both should work.
Also, there may be some depletion mode JFETs that may also be
suitable.
I'n not sure of its voltage rating, but the J110 comes to
mind. I'm not sure but It may have a 60V rating.
On Semiconductor also has some surface mount depletion mode JFETS.
Just some food for thought---
--
Gerard E. Bachand
Power Systems Engineer
7 Atwood Terrace
Cherry Valley, MA 01611
508-756-7738
bachand@xxxxxxx
-------------- Original
message from Joseph DeNicholas <Joseph.DeNicholas@xxxxxxx>:
--------------
Hi Jerry -
Thanks
for your reply, I'm sorry I wasn't more clear about which MOSFET I was
referring to. Many PD ICs are putting a MOSFET in series with the 25k
signature resistor to "shut it down" after signature mode is complete
because it wastes power at 48V. This MOSFET must be turned on for the PSE
to be able to "see" the 25k resistor. The MOSFET must also be
capable of standing off high voltage as it will see the full supply voltage. So,
the PD is responsible for turning this device on at a low enough voltage such
that the PSE sees the 25k resistor. But, with 2.8V at the connector and a
lot of drop caused by the input diode bridges, it doesn't leave a lot of
voltage head r! oom to turn the MOSFET on. Even just another 0.5V would
be extremely helpful. 2.8V doesn't seem to be an arbitrary number, seems
as if that was selected for a reason and I was wondering what drove that
voltage spec.
thanks,
Joe
|
|
"<Gerard>
<Bachand>"
Sent
by: owner-stds-802-3-poep@xxxxxxxx
03/16/2005 10:35 AM
Please
respond to "IEEE 802.3 Power over Ethernet
plus"
|
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
cc:
Subject: Re: [8023-POEP]
802.3af Minimum Signature Voltage
|
Joe,
The signature voltage is only used to validate the signature, not
to directly turn on the series MOSFET.
In other words, the signature voltage is used by the signature
validation circuit, at a low level.
Once the signature is validated, and after classification (if
used)! , the MOSFET is turned on.
Remember, there's always full supply voltage available within the
PSE circuitry to turn on the MOSFET.
Regards
Jerry
--
Gerard E. Bachand
Power Systems Engineer
7 Atwood Terrace
Cherry Valley, MA 01611
508-756-7738
bachand@xxxxxxx
-------------- Original message from Joseph DeNicholas
<Joseph.DeNicholas@xxxxxxx>: --------------
Greetings -
I was wondering why the minimum signature voltage in 8! 02.3af was placed so
low, at 2.8V. With the input diode bridge d rop of 1.5V or even 1.6V at
cold, this doesn't leave a lot of head room (1.2V) for the PD ICs to turn
a series high voltage MOSFET device on and get it into low RDS(ON). This
MOSFET disables the 25k signature resistor at high voltage to conserve power. I
noticed the majority of PSE ICs are supplying higher than that, around 3.5V or
more, but I was wondering why the spec is so low when there doesn't seem to be
a need for such low voltage operation when all we are trying to do is measure a
resistor.
Has any! one proposed changing this in PoEplus?
Thanks,
Joe