Kinetic Technology AN162

Description

Most PoE PD applications use an isolated converter topology, and the available controllers are designed for this configuration. However, they may also be used for non-isolated topologies. The 802.3af/at/bt PoE standards specify input to output isolation, but many low-cost applications comply using other techniques. If the PD system ground is constrained to a single point (the source Ethernet cable), and there are no other connectors and no access to non-isolated conductors, a non-isolated power supply complies with the standard.There are two configurations possible for the non-isolated Buck. The Buck topology may be configured with Vin(-) as the common input to output connection, or it may be configured with Vin(+) as the common input to output connection. The former is referred to as a low-side Buck, and the latter a high-side Buck; in reference to the location of the common rail. Each has its advantages and disadvantages. Both have the PD/DC-DC controller IC referenced to Vin(-), so either the FET driver output and current sense signal must be level shifted to or from the floating gate-source, or the feedback signal must be level-shifted to IC ground reference Vin(-). For simplification and clarity, VIN is defined as Vin(+) – Vin(-) and VOUT is defined as Vout(+) – Vout(-).A non-isolated topology reduces the cost and complexity of the power conversion function in a PD, so its appeal is obvious. However, there are some disadvantages to the Buck topology regardless of configuration which will be discussed later in this application note.

Features

• Compliant to IEEE® Std. 802.3af-2003
• Supports Type 1 operation for IEEE® 802.3at2009 and IEEE® 802.3bt-2018
• Integrated DC-DC Controller with robust 150V, 0.7Ω typ. supporting both isolated and nonisolated applications
• Low RDS_ON Hot-Swap FET, 0.49Ω typ.
• Adjustable switching frequency from 100kHz to 800kHz
• Programmable DC-DC Current Limit
• Integrated Surge Protection for 15kV/8kV System level ESD Compliance
• Exceptional EMI performance -Integrated Short-Circuit Protection
• Over temperature protection
• RoHS & Pb Free 4x4 mm, 16 lead TQFN Package
• Industrial temperature range (-40°C to +85°C)

Typical applications

• security and web cameras, Voice over IP (VoIP) phones, Wireless LAN access points, Fiber-to-the-home (FTTH) terminals

More information

• Datasheet