Re: [8023-POEP] 802.3af Minimum Signature Voltage
Joe,
The
key is to use a depletion-mode N-chan MOSFET. These
switches are fully turned on at Vgs=0, so Rsig is seen at the
RJ45 even when the PD is completely unpowered. After the PD is
fully powered, you turn off the FET (removing Rsig from the circuit) by making
Vgs=5V or so.
Chip
makers don't seem to be having trouble with this so far. I don't think the
spec needs to be changed.
Steve
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 room 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>"
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03/16/2005 10:35 AM
Please respond to "IEEE 802.3 Power over Ethernet
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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 802.3af was placed so low, at 2.8V. With
the input diode bridge drop 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