Re: [8023-POEP] 802.3af Minimum Signature Voltage
Hi Joe,
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---
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>: --------------
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
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| "<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"
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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