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I'm planning a design where we'll use PoE to power a data acquisition board (essentially, an ADC + microcontroller + ethernet PHY and interface).

I look at DC/DC isolated converters to get the 5V or 3.3V from the, say 48V from the PoE wires, and I get a headache! I look at two reference designs (from TI and from Analog Devices / LT), and they have a components count of about 50 parts !!!! The Art of Electronics (3rd Edition), on Figure 9.83 show a "real world" example of a switching converter (in that case, it is AC/DC, but same principle), with approx. 40 parts, and they say that that is a "relatively uncomplicated" circuit .....

On the other hand, I see single-package DC/DC converters such as the CUI PDQ10-Q48-S_X_ — 1-inch by 1-inch by 10mm height, 10W, input DC between 18V and 75V. I don't see why something like this would not work. Any comments?

As a secondary question: I guess I will at least want to put the recommended EMI input filter (C-L-C, as per datasheet suggested circuit), and some TVS (say, a 60V or 70V Zener TVS?) just after the output of the rectifier bridge from the ethernet isolation transformers? Any other aspects I need to pay attention to?

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  • \$\begingroup\$ What’s the issue? SMD feeder line limitation? Very small run? \$\endgroup\$
    – winny
    Nov 18 '19 at 22:35
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The Art of Electronics example is for line voltage so it has some extra filtering, and regulated, not constant.

PoE is not just "here is 48V" too, you deal with long unshielded lines (ethernet is up to 100m) so you have to take care of some transient protection too.

There is also a matter of negotiating required power mode if you need to do bit more than standard ~15W

On the other hand, I see single-package DC/DC converters such as the CUI PDQ10-Q48-S_X_ — 1-inch by 1-inch by 10mm height, 10W, input DC between 18V and 75V. I don't see why something like this would not work. Any comments?

Those tiny converters have quite a few parts:

enter image description here

So not like they are significantly smaller on part count.

Biggest thing to watch aside from input filtering you've mentioned is that they can be quite noisy in high frequency region and your typical LDO is not fast enough to filter it

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  • \$\begingroup\$ ‒ "Those tiny converters have quite a few parts". I meant that from the perspective of my design, this is just one part. It didn't remotely occur to me that I could fit designs such as the two PoE reference designs (approx. 50 parts) within a 1-inch by 1-inch area.... Looking at the images you posted, I wonder. Then again, I may still have an advantage with these single-packaged units? This CUI model advertises to have 5-wall EM shield. That sounds interesting. Again, I wonder whether I have any concrete advantages with a discrete design vs. these types of packaged designs? \$\endgroup\$
    – Cal-linux
    Nov 19 '19 at 20:15
  • \$\begingroup\$ From what I've noticed it is pretty much tradeoff between, just buying ready-made box vs putting the effort to design basically same thing and feature size. You might use 2 sided board with bigger parts for ease of manufacture/manual build but have some high density multilayer board to save a bit on size. \$\endgroup\$
    – XANi
    Nov 22 '19 at 1:49
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I use Silvertel Ag5100 units to convert PoE to 12V (and after that 12V to 3.3V converters without galvanic separation). No headache.

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