PoE

How should a PoE flyback transformer look on the oscilloscope

Measure at the pin not connected to the 48VDC line i.e. the pin of the transformer connected to the MOSFET / integrated flyback regulator IC (If unsure, try it out).

Depending on how exactly the IC works and the load (and the circuitry), these waveforms look quite differently. So the following pictures are mostly for basic reference.

Posted by Uli Köhler in Electronics, EMI, PoE

Is your PD device 802.3bt PoE++ Type-3 or Type-4?

Note: Do NOT confuse Type-3 or Type-4 with PoE class 3 and class 4!

If your PD uses a maximum of 51W (class 1-6 – up to 60W at the PSE), your device is Type 3. If your device uses 62W or 71.3W (class 7 or 8 – 75W or 90W at the PSE), your device is Type 4.

In case your device uses more than 71.3W (or 90W at the PSE), your use-case is not covered by IEEE 802.3bt aka PoE++.

Note that “uses” does not refer to the actual power draw of your device but whatever class you present during the PoE classification process. Typically, this is configured using a resistor, depending on your PoE PD controller.

For reference, see this Ethernet Alliance Whitepaper

Posted by Uli Köhler in Electronics, Networking, PoE

Does 802.3bt PoE++ work with 2-pair power or does it require 4-pair power?

802.3bt always requires four pairs for power >= 40W (class 5+) and does not support powering over two pairs. Powering 2-pair devices using 802.3bt will only when powering devices with up to 30W (i.e. in “802.3at backwards-compatible” mode).

For reference see this whitepaper by the Ethernet Alliance.

Posted by Uli Köhler in Electronics, Networking, PoE

What does “PSE” mean in PoE / Power over Ethernet context?

PD means Power sourcing equipment. This refers to a device which injects power in a PoE port – in other words, it provides Power over Ethernet as opposed to a PD (powered device) that consumes power injected by a Power sourcing equipment device.

Also see: What does “PD” mean in PoE / Power over Ethernet context?

Posted by Uli Köhler in Electronics, Networking, PoE

What does “PD” mean in PoE / Power over Ethernet context?

PD means Powered device. This refers to a device which is powered by a PoE port – in other words, it consumes Power over Ethernet as opposed to a PSE (power sourcing equipment) that injects power that can be consumed by powered devices.

Also see: What does “PSE” mean in PoE / Power over Ethernet context?

Posted by Uli Köhler in Electronics, Networking, PoE