Ingenious system installers developed a variety of ways to send power through an Ethernet cable along with the digital Ethernet signal to power various remotely-connected Ethernet devices even before Power-over-Ethernet (PoE) became a standard.
Fast forward to today where legacy PoE systems use an Ethernet Powered Device (PD). Since there were no standards when these legacy systems were developed, designers tended to use various polarities of the power supply voltage. And to complicate things further, crossover cables could have been employed so it is difficult to know what appears on which lines. So, it becomes necessary for an Ethernet PD to accommodate multiple types of connections and supply polarities.
There is a simplified way to sort this all out. Bourns has developed an easy-to-follow application note that outlines how to use AC rectifiers for this application. In it, you will learn about the two most common Ethernet system installation solutions used in legacy systems. Central to an effective solution for Ethernet PDs, there are two important questions to consider about using bridge rectifiers:
- How do their data sheet parameters relate to operation in a DC system such as PoE?
- How should the proper rectifier diodes be chosen for PoE operation?
Getting to the core of the Ethernet PD requirements, the app note covers how AC rectification works. It points out the fact that receiving power from PoE is very different than receiving power from AC as there is no sinewave. And, there is no AC to DC rectification. Once the PoE connection has been negotiated, the supply voltage is constant.
Tips on how to select a bridge diode are provided where the rule of thumb is to choose one rated at two times the load current. Examples of bridge diodes for the maximum PoE current are also shown.
Bridge diodes in PoE PD designs allow for compatibility with legacy Ethernet installations. Designers should remember that bridge diodes are characterized for AC rectification applications, not PoE. To this point, a helpful description of diode requirements for AC rectification and PoE is given.
A summary of the power and current requirements for the various PoE device types and classes is provided allowing a starting point for choosing appropriate bridge diodes. If you are designing PoE powered devices, it is worthwhile to read this useful app note.