AN2487 datasheet(6/32 Pages) STMICROELECTRONICS

6 / 32 page

Powered device demonstration board

AN2487

6/32

Powered device demonstration board

2.1

Design choices

The following points are considered as requirements for powered device supplies:

Input

Three inputs are provided; one for each of the two PoE powering schemes, and one

additional input for lab test and demo purposes.

Output

There are two outputs which cover most applications.

Low voltage DC, tightly regulated and relatively ripple-free, for logic circuitry.

Relatively high voltage DC for relays, actuators, motors, lighting, etc. High precision is

typically not required, nor is low ripple.

Selected voltages and currents:

●

3.3 V at 0.65 A, ripple less than 50 mV P-P

●

12 V at 0.65 A, ripple in the range of 250 mV P-P

The 12 V output is cross-regulated from the 3.3 V output. It will maintain ±10 to ±15%

regulation.

1500 V isolation of the output is included because it is usually required and is easy to

eliminate for additional small cost savings. Where appropriate, the demo board uses 2 mm

clearance between the Ethernet input and circuitry that may need to be grounded locally.

Power over Ethernet interface

The ST STHS4257 power over Ethernet interface controller was chosen for the signature,

classification, source connect, and converter start functions. This chip combines these

functions in a small SO-8 package.

DC-DC Converter

The converter accepts, at a low cost, a wide range of input voltage and load currents.

Topology, operating mode, and control scheme

The converter uses the flyback topology. It is the simplest, lowest cost solution at the 10 W

power level. It also supports multiple cross-regulated outputs at minimum possible cost.

Transition mode is used to maximize transformer utilization and minimize electro-magnetic

interference (EMI). The converter starts each transformer charge cycle (transistor on)

immediately after the output diodes shut off. There are no stability problems over 50% duty

because there is no energy left in the transformer at the end of each cycle.

The converter uses peak current-mode control, in which the error between output and

setpoint controls the peak current in the transformer primary.

AN2487 Datasheet(PDF) 6 Page - STMicroelectronics