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PoE PD Modules – Disable PoE No Load
Protection in Dual Inputs Mode
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A. Scope
This document can be applied on Befact PoE /
PoE+ PD modules. Current models of Befact
PoE/PoE+ PD modules are TPD modules (TPD-12,
TPD-50, TPD-33), SPD modules (SPD-12,
SPD-50), THPD modules (THPD-12, THPD-50,
THPD-33).
B. How to Disable
PoE No Load Protection
PoE no load protection is a safety
requirement in IEEE802.3af/at standard. PSE
side detect a valid PD device will deliver
power to PD via LAN cable after PD signature
being finished. At mean time, PSE shall go
for a PD detection loop if PD is in a no
load condition. It means PD has no need to
sink power from PSE side. This mechanism is
performed by PSE side and PD can’t disable
this protection by itself.
Basically, there are three methods to
disable this protection in PoE application.
First one, control PSE device by software
configuration to disable PSE port if this
PSE device is software controlled PSE
device. Second way, use a little load
(normally is resistance load) to maintain
the “minimum load” to let PSE “think” that
PD needs power and PSE shall output power
stable and continuously. Third way, use two
double pole double throw (DPDT) relays to
“cut off” the inputs of PD device. At this
condition, PSE shall recognize “no” PD
device being presented in PoE link and this
PSE port shall turn off because this PoE
link is in open circuit condition. The
reference design is for the third method.
These two relays will be deactivate in no
external 12Vdc input condition and maintain
PoE link. These two relays shall be
activated when the external 12Vdc is
injected in DC jack. The circuits in PD
input shall be “cut off” from PoE link. And
PSE will stop delivering power and recognize
the PD is not presented. The PoE link shall
be recovered when the external 12Vdc is
removed from DC jack.
C.
Reference design :
PoE PD Modules – Disable PoE Power
In Dual Inputs Mode |
