Next-gen PoE solution targets high-power applications

Power over Ethernet (PoE), defined by the IEEE 802.3at specification, is a method to safely deliver application data and power over a single CAT-5 Ethernet cable. It has been increasing in popularity due to its flexibility for installing equipment almost anywhere, without the constraint of ac-power proximity or requiring installation by an electrician.

The original IEEE 802.3af PoE specification limited the power delivered to the powered device (PD) to just 13 W, which in turn limited the scope of applications to devices such as IP phones and basic security cameras. In 2009, the IEEE 802.3at specification increased this available power to 25.5 W. However, this was still insufficient to satisfy the growing number of power-hungry PoE applications, such as picocells, wireless access points, LED signage and heated pan-tilt-zoom (PTZ) outdoor cameras. In 2011 Linear Technology released a new proprietary standard, LTPoE++, which extends the PoE and PoE+ specifications to 90 W of delivered power, while maintaining 100% interoperability with the IEEE PoE standards. Four different power levels are available (38.7-, 52.7-, 70-, 90-W), allowing the power supply to be sized according to the application's requirements.

LTPoE++ PSEs employ a clever PSE isolation architecture to minimize component count and enable the use of less expensive external components. Comprehensive cable discharge protection and 80-V absolute max pins ensure high reliability in the field. Use of external FETs allows thermal performance to be matched to the application requirements, improves the efficiency of the system, and increases long term reliability. The LTPoE++ architecture requires only one power sourcing equipment (PSE) and PD controller to deliver up to 90 W over a four-pair 100-m CAT-5e cable.
System isolation requirements
Implementation of PoE requires careful architecture and component selection to minimize system cost, while maximizing performance and reliability. A successful design must adhere to IEEE isolation requirements, protect the Hot Swap FET during short-circuit and overcurrent events, and otherwise comply with the IEEE specification. The PoE specification clearly lays out isolation requirements, guaranteeing ground loops are broken, maintaining Ethernet data integrity and minimizing noise in the PD application circuit.
Traditional PSE isolation architectures isolate the digital interface and power at the host-to-PSE controller interface. Digital isolation elements such as opto-couplers are inherently expensive and unreliable. ICs capable of performing the isolation function are cost-prohibitive or do not support fast I2C transfer rates. In addition, isolated dc/dc converters needed to power the PSE logic increase board space and system cost.

 Isolation made easy
The 12-port (LTC4270/LTC4271) and 8-port PSE (LTC4290/LTC4271) chipsets take a different approach to PSE isolation by moving all digital functions to the host side of the isolation barrier (see Fig. 1 ). This significantly reduces the cost and complexity of required components. There is no longer the need for a separate, isolated dc/dc power supply; the LTC4271 digital controller can use the host’s logic supply. The LTC4271 controls the LTC4290 or LTC4270 using a transformer-isolated communication scheme. An inexpensive and ubiquitous Ethernet transformer pair replaces six opto-couplers. I² C communication including port management, reset and fast port shutdown are encoded in a protocol designed to minimize radiated energy and provide 1,500 V of isolation.

Fig. 1: The LTC4290/LTC4271 chipset achieves isolation without any opto-isolators and eliminates the need for a dedicated isolated dc/dc converter.

 Robust cable discharge protection
It is important to consider the robustness of your PoE design, especially when dealing with high cable counts, high voltages, high currents or high temperatures. Linear Technology’s circuit protection scheme is scalable to match IEC61000 cable discharge voltage requirements. Only a single TVS is required to protect the high voltage analog supply while a pair of cheap clamping diodes is used on each output port (see Fig. 2 ).

The diodes at the ports steer harmful surges into the supply rails, where they are absorbed by the surge suppressor and the VEE bypass capacitance. The surge suppressor has the additional benefit of protecting the PSE controller from transients on the VEE supply. Linear’s PSE controllers also have an 80-V absolute max rating on all analog pins to provide native protection against transients.

Fig. 2: Robust cable discharge protection.
 

Reduced power dissipation
Fourth-generation PSE and PD controllers from Linear support IEEE 802.3at compliant operation in addition to LTPoE++ power levels of up to 90 W , while minimizing heat dissipation through the use of low R_DS (_on) external MOSFETs and 0.25-Ω sense resistors. This is important in high-power systems where thermal design and power loss can be extremely costly, as well as in power-limited applications where the application needs to maximize the delivered power to operate within the application’s power budget. PSE and PD controllers with integrated MOSFETs have higher R_DS(on) characteristics, making thermal design more difficult as the heat is dissipated inside the device. Damage to a single port can bring down the whole chip.
The LT4275 (see Fig. 3 ) is currently the only PD controller on the market that controls an external MOSFET to reduce overall PD heat dissipation and maximize power efficiency, which is especially important at higher power levels. This novel approach allows users to size the MOSFET to meet the application’s exact heating and efficiency requirements, enabling the use of low R_DS(on) MOSFETs on the order of 30 mΩ. The LT4275 can support any power level up to 90 W.
A single TVS and 100-V absolute max port pin provides more than enough protection against cable discharge events. The LT4275 operates from –40° to 125°C temperature range and is equipped with overtemperature protection that protects the device during momentary overload conditions. With this much protection, it’s easy to see how rugged applications can benefit.

Fig. 3: LTPoE++ PD controller uses external MOSFET for increased power efficiency

LTPoE++ plug-n-play solution
LTPoE++ provides a safe and robust plug-n-play solution that dramatically reduces engineering complexity in power sourcing equipment (PSEs) and powered devices (PDs). The benefit of LTPoE++ over other power-extending topologies is that only a single PSE and PD is required to deliver up to 90 W over a single CAT-5e cable, resulting in significant space, cost and development time advantages.

The LTPoE++ solution significantly lowers the cost of ownership not only by reducing the bill of materials and associated component cost, but also by providing the most power efficient end-to-end solution available today, maximizing the power delivered to the application while minimizing heat dissipation and costly heat sink design.

LTPoE++ uses a three-event classification scheme to provide mutual identification handshaking between the PSE and PD while maintaining backward compatibility with the IEEE 802.3at standard. The LTPoE++ PSE determines if a PD is a Type 1 (PoE), Type 2 (PoE+), or LTPoE++ device by the PD response to the three-event classification scheme. One important distinction for LTPoE++ is that it does not require the use of the Link Layer Discovery Protocol (LLDP) that was mandated in the IEEE PoE+ specification for software-level power negotiation.

LLDP requires extensions to standard Ethernet stacks and can represent a significant software development effort. LTPoE++ PSEs and PDs autonomously negotiate power level requirements and capabilities at the hardware level while remaining fully compatible with LLDP-based solutions. This gives LTPoE++ system designers the choice of whether or not to implement LLDP. Proprietary end-to-end systems may choose to forgo LLDP support. This creates time-to-market advantages while further reducing BOM costs, board size and complexity.

Advanced features
Linear’s Power over Ethernet PSE controller family incorporates a wealth of PoE experience and expertise backed by well over 200 million shipped ports. New fourth-generation features include field-upgradable firmware for future-proofing designs. Also new is optional 1-s current averaging, which simplifies host power management. Advanced power management includes prioritized fast shutdown, 12-bit per-port voltage and current read back, 8-bit programmable current limits and 7-bit programmable overload current thresholds. A 1MHz I2C interface allows a host controller to digitally configure the IC or query port readings. “C” libraries are available to reduce engineering costs and improve time to market.