Power Electronics

PoE+ Attracting Compliant ICs

The IEEE recently approved IEEE802.3at ratification to the current IEEE 802.3™ standard for Power over Ethernet (PoE) systems. This version, also known as PoE+, promises improved power-management features and increases the amount of power available

Commonly known as PoE+, the recently ratified IEEE 802.3at standard supports up to 30 W per port, enabling PoE to be deployed across more power-hungry applications. In addition, there is also a growing need to make these applications greener through increased efficiency. Network administrators increasingly demand per-port power measurement and dynamic power allocation to maximize the overall power efficiency of their networks and minimize power-supply costs.

A host of IC manufacturers have followed these developments with devices designed to support the new PoE+ standards. This article will discuss several of these new products, as well as their support for PoE+.

REAL-TIME POWER MONITORING WITH SILICON LABORATORIES SI345X

The Si345x quad PoE power-sourcing equipment (PSE) controllers from Silicon Laboratories (Fig. 1) support four independent PoE (IEEE 802.3af) and PoE+ (IEEE 802.3at) PSE ports, as shown in Fig. 2. The Si345x controllers offer real-time power measurement capabilities, energy-efficient powered-device (PD) detection and disconnect algorithms, and low on-resistance power MOSFETs and sense resistors, as well as a transient voltage surge suppressor (TVSS) on each port. These features enable smaller, highly energy efficient, and lower-cost PoE/PoE+ Ethernet switches and midspans that can power multi-radio wireless access points, industrial automation, and building security and surveillance systems.

The Si3452x controllers incorporate high-precision current- and voltage-monitoring capabilities to measure the power consumed by each port in real time, enabling systems to maintain tighter control of power allocation. This approach enables smaller power supplies and lower supply overhead, and provides system administrators with visibility into how much power is allocated to and consumed by each port in their network.

The Si3452 and its Power Manager Software Development Kit fully supports IEEE's link layer discovery protocol (LLDP) power management to enable dynamic power allocation. With LLDP, powered devices can make requests to increase or decrease their consumption, and the PSE responds with an acknowledgement or denial based on system power budgets and constraints.

dV/dT DISCONNECT

The Si345x controllers leverage a patent-pending approach to provide a reliable alternative to traditional ac or dc disconnect methods, further improving system power efficiency. Using this innovative method, called dV/dt disconnect, an Si345x controller can detect the absence of a connected PD through proprietary measurement techniques to avoiding the use of dc disconnect, or power-hungry series diodes and extra components associated with ac disconnect.

The dV/dt disconnect feature helps eliminate expensive thermal-management components such as heat sinks and fans, making the Si345x family a truly green PoE solution. For example, compared to a PoE+ PSE system implemented with an ac disconnect-based PSE controller, a solution using the Si345x's dV/dt disconnect algorithm, integrated low-on-resistance FETs, and current-sense resistors saves nearly 500 mW per port in power consumption, or approximately 24 W in a 48-port system.

The integration of the Si3452 controller decreases the per-port solution cost and shrinks the layout area of the printed circuit board (PCB). Each Si3452 supports four PoE or PoE+ PSE ports, with proprietary support for power levels up to 40 W per port. Integrated on each port are a current-sense resistor, a proprietary TVSS, and a robust power MOSFET with a 0.3-Ω RDS(ON), simplifying the bill of materials to a single capacitor that reduces the per port costs by up to $0.20 and saves up to 55% in PCB space.

The Si345x family is supported by a complete set of development tools, including the feature-rich Power Manager Software Development Kit and an eight-port midspan evaluation system (Si3452MS8-KIT), shown in Fig. 3. It enables rapid integration with software protocol stacks offered by leading Ethernet switch chipset suppliers. Three complete reference designs are also available from Silicon Labs to support PSE daughter-card and motherboard implementations. The complete PSE subsystem solution includes Silicon Labs' Si840x family of bidirectional I 2C isolators, which isolate the Si345x's I2C bus to the host system controller.

A PSE CONTROLLER FOR POE+

Linear Technology's LTC4274 is a single-channel PoE controller for PSE that must support IEEE 802.3at (25.5 W), or proprietary higher power levels. Fig. 4 shows a typical LTC4274 circuit.

Providing up to 100 W over 4-pair Ethernet cabling, the LTC4274 is fully compliant with the new IEEE 802.3at PoE+ standard and backward-compatible with the prior IEEE 802.3af PoE standard. To help conserve power, the LTC4274 delivers low heat dissipation by using low RDS(ON) MOSFETs and 0.25-Ω current-sense resistors, eliminating the need for expensive heat sinks and providing a more robust PSE solution.

The LTC4274 suits a wide variety of PSE applications, including single-port switches, cameras, and electronic message boards. The device's low power dissipation simplifies thermal design compared to using PSEs that integrate more fragile MOSFETs with higher on-resistance.

IEEE 802.3at PDs can request up to 25.5 W of power, responding to two-event classification from the LTC4274 and confirming that the PD is indeed a high-power device. PD discovery is accomplished using a proprietary dual-mode, four-point detection mechanism that ensures the best immunity from false detection.

Advanced power management features include: selectable high-speed 9.5-bit (1-kHz) or high-resolution, 14.5-bit current and voltage readback; a 7-bit DAC for overcurrent threshold; a unique nonlinear 8-bit programmable current limit; and versatile quick shutdown. PD discovery uses a proprietary dual-mode four-point detection mechanism ensuring the best immunity from false PD detection.

Midspan PSEs are supported with two-event classification and a 2 s back-off timer. Legacy high-capacitance device detection is available.

With an I2C serial interface operable up to 1 MHz, the LTC4274 comes in commercial and industrial temperature grades, and in an RoHS-compliant QFN-38 package.

PD CONTROLLER POWERS UP TO 30-W REMOTE DEVICES

Previously, several manufacturers referred their ICs to the draft version of the IEEE802.3at standard. Many manufacturers are now updating their datasheets to reflect the approved IEEE802.3at standard.

For example, Texas Instruments is now updating the datasheet for its TPS23754/6 PD controller. A combined PoE PD interface and current-mode dc-dc controller, the TPS23754/6 is optimized specifically for isolated converters. Its PoE interface supports the IEEE 802.3at standard. Fig. 5 shows the evaluation module for a TPS23754 system.

The TPS23754/6 supports a number of input-voltage ORing options including highest voltage, external adapter preference, and PoE preference. These features allow the designer to determine which power source will carry the load under all conditions. Its PoE interface features the new extended hardware classification necessary for compatibility with high-power midspan PSE per IEEE802.3at. The detection signature pin can also be used to force power from the PoE source off. Classification can be programmed to any of the defined types with a single resistor.

Two complementary gate drivers — with programmable dead time — are included in the dc-dc controller. This simplifies optimized gate drive for highly-efficient flyback topologies. The second gate driver may be disabled if it also features internal soft-start, bootstrap startup source, current-mode compensation, or 78% maximum duty cycle.

A programmable and synchronizable oscillator allows design optimization for efficiency and supply designs. Accurate programmable blanking with a default period simplifies the usual current-sense filter design trade-offs.

Three different versions are available. The TPS23754 has a 15-V converter startup whereas the TPS23756 has a 9-V converter startup. The TPS23754-1 replaces the PPD pin with a no-connect for increased pin spacing.

ABOUT PoE SYSTEMS

THE IEEE 802.3at standard for PoE allows data-terminal equipment (DTE) to receive power over the same cabling used for data in an Ethernet network (See the figure). The standard specifies the protocol for delivery of a nominal 48 Vdc over unshielded twisted-pair cables (such as CAT-5). This eliminates the need for a local power source for the remote device. 802.3at presents the requirements for providing and receiving power over existing Ethernet cabling. It involves power sourcing equipment (PSE) that provides the power on the cable for the powered device (PD).

Typical PD controllers provide an isolated, low-voltage power source to the remote device. These controllers incorporate all the necessary interface requirements to comply with IEEE802.3at, including PD detection and PD classification current signatures required for the PSE.

IEEE802.3at modified powering requirements of the original IEEE802.3af standard by allowing higher voltage and current. This requires compatible ICs and also OEM power supplies that meet the “at” spec. Higher power requirements mean power supplies will produce more heat, so cooling them properly will become important. Higher currents may produce more EMI, so PCB layout may become a greater design consideration. In addition, the voltage drop and operating temperature of network cables will increase because of higher currents, so CAT5 cable with four twisted pairs will be required to replace the existing CAT3 cable with two twisted pairs. Generally, a device compliant to the “af” standard is referred to as a type 1 device, whereas “at” devices are called type 2 devices.

The IEEE 802.3at standard for PoE allows data terminal equipment (DTE) to receive power over the same cabling used for data in an Ethernet network. Unshielded twisted-pair cables supply the power, eliminating the need for a local power source at the remote device.

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