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'[EE] Low Cost, Low Power POE Device?'
2012\04\25@233509 by Harold Hallikainen

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I've got a product with a PIC32 in it that I'm currently driving with a
250mA 5V USB style power supply. I'm using a simple linear regulator to
drop it to 3.3V for the digital, while keeping the 5V for the analog.

Customers don't want to run two wires (Ethernet and USB Power). So,
there's always Power Over Ethernet. But, it looks complicated and
expensive, especially for the 2 watts or so we need (a lot more than we're
currently spending for the USB power supply, cable, connectors, and linear
regulator).

But, it seems like this should be a common problem with a low cost modular
solution. Linear and TI and others are making switching DC to DC converter
modules, some of which are pretty inexpensive. It SEEMS like there should
be such a module for POE. It'd have the two diode bridges, load switch, DC
to DC converter, and a DC output. But, I can't find anything like that
(well, I did find one for $35). Most of the POE stuff is emphasizing high
power (and cost).

Does anyone know of a low cost POE PD module?

THANKS!

Harold



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2012\04\26@052240 by Richard Prosser

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On 26 April 2012 15:34, Harold Hallikainen <spam_OUTharoldTakeThisOuTspamhallikainen.org> wrote:
{Quote hidden}

Harold

IIRC RJ45 connected ethernet - at least for PICs - use transformer
coupling at both ends and typically use only 2 of the 4 available
pairs.
So it should be possible to arrange a power feed by either using the
"spare" pairs in the RJ45 connection, or to arrange centre tapped
transformers to feed the power & return down the 2 used pairs. Just so
long as the voltage and power levels are low. And whtch out for high
surge currents when connecting and capacitors charge up.

Can't much more simple than that - or am I missing something?

R

2012\04\26@071114 by Mark Hanchey

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On 4/25/2012 11:34 PM, Harold Hallikainen wrote:
> But, it seems like this should be a common problem with a low cost modular
> solution. Linear and TI and others are making switching DC to DC converter
> modules, some of which are pretty inexpensive. It SEEMS like there should
> be such a module for POE. It'd have the two diode bridges, load switch, DC
> to DC converter, and a DC output. But, I can't find anything like that
> (well, I did find one for $35). Most of the POE stuff is emphasizing high
> power (and cost).

I think the problem you are having is looking for a pre-built module when most devices that will need POE have it implemented in the main board . The cost to build your own POE converter is really low, can easily do it for under $10.
Put two bridges, a MAX5940 and a MAX5014 , a few resistors and you are done.\

Mark

2012\04\26@072026 by Mark Hanchey

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On 4/26/2012 7:11 AM, Mark Hanchey wrote:
> On 4/25/2012 11:34 PM, Harold Hallikainen wrote:
>> But, it seems like this should be a common problem with a low cost modular
>> solution. Linear and TI and others are making switching DC to DC converter
>> modules, some of which are pretty inexpensive. It SEEMS like there should
>> be such a module for POE. It'd have the two diode bridges, load switch, DC
>> to DC converter, and a DC output. But, I can't find anything like that
>> (well, I did find one for $35). Most of the POE stuff is emphasizing high
>> power (and cost).
> I think the problem you are having is looking for a pre-built module
> when most devices that will need POE have it implemented in the main
> board . The cost to build your own POE converter is really low, can
> easily do it for under $10.
> Put two bridges, a MAX5940 and a MAX5014 , a few resistors and you are
> done.\
>
> MarK

I did a quick search and these modules sell for under $8.
http://www.semiconductorstore.com/cart/pc/viewPrd.asp?idproduct=13982

Mark

2012\04\26@091408 by Info2004
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If you have control of both ends, and are not too concerned with interoperability and isolation, then just use the spare pairs to provide to power.

....Andy
On 26/04/2012 04:34, Harold Hallikainen wrote:
{Quote hidden}

>

2012\04\26@134037 by Harold Hallikainen

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{Quote hidden}

Thanks for the ideas! I see the MAX5014 is not recommended for new
designs. Looking at the suggested replacement, MAX17500, it requires quite
a few external parts. It SEEMS that they'd be able to use an internal FET.
And it seems that they'd be able to do voltage regulation by watching the
peak primary voltage (not as tight regulation, but reasonable). That'd
save the opto. I look at some boost converter chips (I'm often using ones
from LT) that need very few external components. I look at a flyback
converter as being very similar to a boost. If you take an "output" of a
boost converter as across the inductor (instead of inductor to ground),
then you'd have the equivalent of a flyback with a 1:1 transformer.
Anyway... I guess I could use any flyback converter following the MAX5490,
which, by itself, does not look too bad.

The module you point out looks great EXCEPT the output is not isolated, as
required for POE.

So, I really appreciate the quick response! I'll consider these ideas and
keep looking for a POE module.

Thanks!

Harold


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2012\04\26@210539 by William \Chops\ Westfield

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On Apr 26, 2012, at 10:40 AM, Harold Hallikainen wrote:

> The module you point out looks great EXCEPT the output is not isolated

Sivertel makes isolated converters as well.  Somewhat more expensive, but still in the "about $10" range.  They're 6W; I'd really like to see a smaller, cheaper, 2W unit as well.

OTOH, I think the switch-side costs of PoE make $10 seem pretty cheap…

BillW

2012\04\26@232836 by Harold Hallikainen

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>
> On Apr 26, 2012, at 10:40 AM, Harold Hallikainen wrote:
>
>> The module you point out looks great EXCEPT the output is not isolated
>
> Sivertel makes isolated converters as well.  Somewhat more expensive, but
> still in the "about $10" range.  They're 6W; I'd really like to see a
> smaller, cheaper, 2W unit as well.
>
> OTOH, I think the switch-side costs of PoE make $10 seem pretty cheap…
>
> BillW
>
>

Thanks! I now see them at http://www.silvertel.com/poe_products.htm .
Looks like a possibility.

Harold



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