High PoE made easy

Posted on Feb 5, 2014

The IEEE standards for so-called `high power-over-Ethernet` (HPOE) have yet to be resolved, but most designers expect a 53 VDC source, usable current from each line of about 750 mA, and a total cable resistance of 12. 5 ohms. If you design for what will probably be the worst-case scenario, expect 46 VDC (48 V nominal) at 720 mA and 12. 5 ohms. At 72

High PoE made easy
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0 mA, the cable would produce a drop of 9 volts, leaving 37 VDC to work with. In this case, about 26. 6 watts would be available at the end of the cable. The typical power stage running from this input would yield a bit over 20 watts. Unfortunately, that`s not nearly enough power for some applications. One solution is to use multiple Ethernet lines, but that topology raises the problem of power sharing. Here`s how to deal with that issue. Conventional HPOE interfaces consist of a polarity-protecting bridge rectifier and a hot-swap section with a power-over-Ethernet interface, followed by an isolated converter with regulated outputs. Preferably, these outputs won`t be load-dependent and will have good transient response. Typical designs use isolated feedback to produce one voltage, typically 5 VDC, which is then converted to various other voltages for the given application. In some cases multiple outputs are attempted from one feedback loop, but the regulation is very dependent on load. In either case, the losses in the bridge rectifiers and converters leave you with rather poor efficiency. Also, isolated feedback tends to provide rather poor transient response. The point of HPOE, on the other hand, is to get as much usable power from as few Ethernet lines as possible without sacrificing performance. That`s possible in the example below, which addresses both the HPOE interfaces and the power converters to to provide an...

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