TPS23753A
SLVS933B –JULY 2009–REVISED JANUARY 2010
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The least popular technique is PoE priority. It is implemented by placing a diode between the PD supply voltage,
VDD, and the dc/dc controller bias voltage, VDD1. The diode prevents reverse biasing of the PoE input diode
bridges when option 2 adapter ORing is used. The PSE may then detect, classify, and provide power to the PD
while a live adapter is connected. As long as the PoE voltage is greater than the adapter voltage, the PSE will
power the load. The APD function is not used in this technique.
The IEEE standards require that the PI conductors be electrically isolated from ground and all other system
potentials not part of the PI interface. The adapter must meet a minimum 1500 Vac dielectric withstand test
between the output and all other connections for options 1 and 2. The adapter only needs this isolation for option
3 if it is not provided by the converter.
Adapter ORing diodes are shown for all the options to protect against a reverse-voltage adapter, a short on the
adapter input pins, and damage to a low-voltage adapter. ORing is sometimes accomplished with a MOSFET in
option 3.
Protection
A TVS across the rectified PoE voltage per Figure 1 must be used. An SMAJ58A, or a part with equal to or better
performance, is recommended for general indoor applications. If an adapter is connected from VDD1 to RTN, as
in ORing option 2 above, voltage transients caused by the input cable inductance ringing with the internal PD
capacitance can occur. Adequate capacitive filtering or a TVS must limit this voltage to be within the absolute
maximum ratings. Configurations that use DVDD as in Figure 25 may require additional protection against ESD
transients that would turn DVDD off and force all the voltage to appear across the internal hotswap MOSFET.
CVDD and DRTN per Figure 25 provide this additional protection.
CVDD
0.01mF
DVDD
DEN
CLS
VSS
Figure 25.
Outdoor applications require more extensive protection to lightning standards.
Frequency Dithering for Conducted Emissions Control
The international standard CISPR 22 (and adopted versions) is often used as a requirement for conducted
emissions. Ethernet cables are covered as a telecommunication port under section 5.2 for conducted emissions.
Meeting EMI requirements is often a challenge, with the lower limits of Class B being especially hard. Circuit
board layout, filtering, and snubbing various nodes in the power circuit are the first layer of control techniques. A
more detailed discussion of EMI control is presented in Practical Guidelines to Designing an EMI Compliant PoE
Powered Device With Isolated Flyback, TI literature number SLUA469. Additionally, IEEE 802.3at sections 33.3
and 33.4 have requirements for noise injected onto the Ethernet cable based on compatibility with data
transmission.
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