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