High power PoE+ based applications have become very common. Especially in the domain of PoE lighting and PoE display applications. Synchronous rectifier for high power PoE+ applications helps reduce losses and cut on the heat generation in isolated PoE PD circuits.
You can use any ethernet controller like ESP32 or STM32 to go along with the PoE design covered here. The PoE class negotiation is all handled by the PoE PD controller and has nothing to do with the ethernet phy chip and MCU.
Main Source of Power Loss and Heat in PoE Designs
PoE standards have gotten more and more ambitious over the years and now with PoE+ and IEEE 802.3bt, you can send out 90 watts of power over ethernet cables. However, even with 802.3at systems that are relatively low in power output, there can be a ton of heat generated in the secondary side of isolated power supply designs.
For example, if we assume 24W of total power being consumed by a PoE powered system. If the system uses 3.3V to run its electronics, at 24W, the output current can reach 8 amps!
Suppose the output rectifier diode has a voltage drop of 500mV at 8 amperes, you are literally burning away 8×0.5 = 4 watts of power as heat in the rectifier diode!
The result – less than about 70% efficiency!
Using Synchronous Rectifiers in High Power PoE+ Designs
Synchronous rectifiers can be very useful in eliminating diode losses in the secondary side rectifier. The value of adding a synchronous rectifier is high when the output current is very high.
For PoE+ modules with output voltage of 12V or lower, adding a synchronous rectifier is the best way to make measurable impact on isolated PoE converter efficiency. These MOSFET switch based rectifiers often add a little to the end cost for the PoE device, but the cost addition is almost negligible if you are manufacturing more than 100 pcs of your design.
The figure below shows a basic PoE synchronous rectifier schematic.
The above circuit gets rid of almost all the heat dissipation that occurs in the secondary side rectifier of the flyback transformer.