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AN1192.3
Intersil PoE Evaluation Board
Introduction
With advancements being made at a furious pace in the field
of networking, applications have started focusing on
achieving the most out of the existing infrastructure. Power
over Ethernet (PoE) is one such classic example that
integrates power and data lines. PoE consists of two power
entities: Power Sourcing Equipment (PSE) and Powered
Devices (PDs), for use with the physical layers. These utilize
the same LAN cabling (CAT-5 cables), that are used for
transmission of data, over spare twisted pair cables. Typical
applications include IP phones, WLAN access points,
security cameras and RFID tag readers, to name a few.
Scope
This document focuses on Intersil’s solution for the Powered
Device portion of the circuit, which complies with IEEE
802.3af, the standard for PoE. An inexpensive approach with
discrete circuitry has been adopted instead of an integrated
solution. With PoE soon turning into a commodity market,
low cost will be key, as compared to utilization of
sophisticated integrated solutions. The following solution has
been optimized for performance per the IEEE 802.3af
standard, with low cost and optimal performance as the
prime objectives. The evaluation board is currently available.
Powered Device (PD)
The PD needs to be able to operate with a maximum
average input power of 12.95W, and should be operational
between 36V and 57V DC. Class 0 (default) was selected for
this design, to provide the end user with maximum flexibility
with their output power requirements. Discrete classification
circuit may be added, as required, at a very low additional
cost. Other specifications selected are in line with the
requirements as specified in Table 33-12 of IEEE802.3af,
and are listed below:
Specifications Chosen for the PD Design
Operating Input Voltage range: 36V-57V DC
Max. Input Operating Current: 350mA
Max. Input Inrush Current: 400mA
Output Voltage: 3.3V
Output Current: 3.35A
Ripple: 50mV
P-P
Typical Efficiency: 87%
Isolation: 1500V DC
Selection of the Power Topology
While selecting the topology for this application, the emphasis
was placed on two key features: optimized performance and
low cost. A simple low-power design requirement, combined
with low cost target dictated the use of a flyback converter for
this application. Given that the input power available is limited
by the PSE, high efficiency becomes an essential
requirement. The converter is being operated in the
continuous current mode to achieve this goal.
IC Selection
Intersil’s superior industry-standard ISL684x family of PWM
controllers would best serve the needs of this design. Some
key features of this family of parts include:
• Tight internal voltage reference of 1% over temperature.
• 40ns peak current sensing.
• 1A MOSFET driver
ISL6844 was selected for its large UVLO hysteresis, UVLO
start threshold, and the fact that the converter has been
designed for a maximum operational duty cycle of 50%, thus
protecting the IC by limiting the duty cycle in case of extreme
fault conditions.
Transformer Design
The design of the flyback transformer is an iterative process,
and involves the following steps:
• Calculate the energy storage needed.
• Select core geometry and material.
• Select the maximum desired flux density of operation.
• Select core size.
• Calculate the maximum flux density.
• Calculate the turns ratio.
• Select the wire gauge, number of strands, winding order
and insulation requirements.
• Estimate the losses.
• Verify the design.
While the details of the design are not discussed in this
document, as a summary, it was decided to proceed with an
EP-13 core, in order to achieve a balance of good
performance and low cost. The evaluation board provides
the end users with the option of using two other low-profile
cores for their transformer design, and can be tailored to
meet the customers’ specific requirements.
Application Note March 12, 2007
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774
| Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005-2007. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
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