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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 720 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 additional efficiency boost of several percent and excellent transient response. Figure 1 shows one of the two HPOE interfaces in a 47-watt dual Ethernet pair design. Two n-channel and two p-channel MOSFETs form each bridge rectifier. Each is biased on by a 150k resistor from the opposite polarity input line. The gates are protected by low-current zener diodes (test current is 50 microamps).
Figure 1 (Click on Image to Enlarge)
Only the two MOSFETs with the correct polarity will be turned on. The drain-source diodes in the MOSFETs act as the bridge rectifier until the circuit is able to charge the MOSFET gates through the 150k resistors. The integrated HPOE interface lends some simplicity to the circuit, providing all necessary interface and hot-swap functions.
Figure 2 shows one of the two DC/DC converters. The active-clamp, forward converter, using the LM5025 controller, delivers very high efficiency and does away with the need for isolated feedback. The LM5025 generates a ramp across capacitor C4, which in turn controls the duty factor. The duty factor becomes inversely proportional to input voltage and produces a nearly constant output voltage. Fortunately, capacitors with 1 percent accuracy cost only a few pennies these days, and as applied in this circuit provide excellent regulation without the need for feedback.
Figure 2 (Click on Image to Enlarge)
No components are needed for current sensing and thus any losses associated with them are eliminated. The basic current limiting circuitry in the hot-swap section ahead of the forward converter and the current-limiting in the post regulators at the output simplify the design while providing adequate protection. The current required by the LM5025 controller is only about 10 mA, so a large value of inductance is necessary to prevent peak charging since the rectifiers are not synchronous. But because the current is very low, the inductor is physically very small. Its DC resistance is about 32 ohms. A linear regulator could be powered from the high input voltage to provide this function, but the power loss would be significant while the cost would be about the same.
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