Fairchild GreenBridge ™ Solution to Replace Conventional Diode Bridge in Power Over Ethernet Applications Summary

application requires a bridge circuit to regulate the polarity of the input power when the Power Source Equipment (PSE) supplying power to the PD is equipped with an Uninterruptible Power Supply (UPS). The simple diode bridge design has been used without major concerns at low cost. However, as PD gets hungrier for power, the conduction loss of the diode bridge caused by the forward-voltage drop becomes one of the main issues to be solved efficiently. Fairchild’s GreenBridge ™ solution reduces power losses in the bridge circuit and leads to an efficient and cost-effective PoE system. In this application note, the GreenBridge drive circuit is examined and test results with a typical diode bridge are presented. Power Over Ethernet Power over Ethernet (PoE) introduces a new facet to Ethernet networking, delivering DC power through a CAT5 network cable. Figure 1 shows a typical PoE system. It is convenient to install PDs; such as IP telephones, wireless LAN access points, and security cameras; without any wall power lines because the CAT5 cable delivers data as well as power required by those PDs. In addition, the PoE system is energy efficient because the PSE communicates to the PD first and provides regulated power PD for demands. The power management block of PD must be designed carefully to operate efficiently at limited input power from the PSE