Basic POE design question

You are on the right track. The LM5072 (application #4) could work with some adjustments for output voltage (C16, R12) as could PMP4679 (http://www.ti.com/tool/pmp4679). You could also use something like that shown in (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slva440&fileType=pdf) using TPs23753 and adjusted for the higher output voltage.

Thanks for the reply.  A few comments:

The PMP4679 does indeed seem reasonably applicable.  However it doesn't specify the Vin range for the auxiliary supply input, I suspect that it won't support my 12-24Vdc adapter (since I'm assuming that will trigger the UVLO circuit).  I could simply diode-or the outputs and ignore the Aux Power portion of the reference design as it stands.  However, if I do that it's not clear that the grounds won't be a problem, since there are 3 separate distinct grounds it appears, and it looks like the output ground floats up to the input rail in order to regulate.   It's an unusual way to do this, at least for me (though it is confined to a reasonably small area of the board).

Looking at the LM5072 again I see that they are doing that there as well, perhaps it's something particular to POE circuits?

None of the datasheets or reference designs I've found are particularly detailed in component selection or the rational behind the design.  Also, as I noted WebBench doesn't appear to support any of the POE parts.   Is there an app-note or document that captures the design implementation in more detail?

Thanks for the input,

Dave.

Dave, while I do not have a readily available POE application type buck app note, there are quite a few other TI buck type switcher parts with the essential design steps such as the TPS54620 (see the design example starting on pg 21 http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=tps54620&fileType=pdf). For a wide input range (using a 12V adapter for example) you will need a controller whose duty cycle can achieve the required output voltage at minimum Vin (Vout = Vin_min x D). LM5072-80 and TPS23753 can achieve ~80% duty cycle.

Another option is a low cost TPS23753 flyback design (it is isolated, but you could tie secondary and primary grounds together if need be) and a design example can be found in (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slva305c&fileType=pdf). Regarding OR'ing adapters using TPS23753 you can refer also to (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slva306a&fileType=pdf) for many OR'ing type options.

We were doing an outdoor fixed wireless implementation powered over POE and I was wondering why I need to do an isolated design.  We used one of your chips in the past, the TPS23754PWP-1 on one of the more high end designs.  A new, smaller design has come up which is extremely price and space sensitive and I was wondering about getting away from the 4 transistors + transformer + opto + 2000V creepage.  Does this seem feasible from a regulatory and safety point of view?

There are no user touchable components or metal with the possible exception of the Ethernet shell.

I provide connection to earth ground through either the CAT5e cable shield or a ground lug with a grounding wire.

Please advise.

Thanks

Mark Devon

The best starting point is the IEEE POE standard for isolation guidelines/requirements (section 33.4.1 of  https://standards.ieee.org/about/get/802/802.3.html).

Generally speaking, the IEEE wants the ethernet network isolated form all other potentials, and also from high-surge environments.  The safety rules treat the ethernet like a telecom potential and have requirements for eliminating personal exposure to hazzardous telecom potentials including direct touch and metallic interfaces exposed to the telecom potentials.  Many safety-conscious customers use the isolation when in doubt to be "safe." 

There are no user touchable components or metal with the possible exception of the Ethernet shell. This sounds like a good candidate for a non-isolated type of product. In this case, the ethernet connector shell would be isolated from the POE power (using the traditional ethernet data line terminations to PD housing with the HV capacitor).

I provide connection to earth ground through either the CAT5e cable shield or a ground lug with a grounding wire. This sounds like a good approach but will depend on the installation, EMI/EMC/ESD/Surge requirements etc.

The IEEE standard covers operational requirements.  The safety rules usually come from EN/UL/IEC 60950-1.  Ethernet is usually treated like a TNV voltage.   Requirements are spread throughout the standard, with section 6 specific to telecom equipment.