Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom

StealthRT · Jan 3, 2020

Hey all I have been searching for a already-built solution that would output 3.3vdc, 5vdc and 12vdc @ 5a. I have found that here:

However the issue is that I will be feeding this 48vdc (POE) and it seems that the max this thing can take in is 40v.
Do you think it will be fine with 48v? Is there anything I could add to the board so that it can take up to 50v?

LM2596 datasheet: https://www.ti.com/document-viewer/LM2596/datasheet

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tronitech · Jan 3, 2020

The LM2596 rated maximum is 40VDC. Can't see much you can do about this?

You could perhaps drop the voltage from 48VDC to 40VDC using a number of appropriately rated diodes?

SeekerOfSecrets · Jan 5, 2020

Be aware that 48v 'POE' is only about 2Amps max, so you have to judiciously pick and choose which of your 4 outputs you 'really' want to powersomething on at that time.

Here's one idea: Feed your 48v DC source thru a 50-80v to 25-6v out step down 'buck' regulator. Put a nice fat 4000uf/160v cap with a 1 Meg, 5W resistor across the output to somewhat buffer and stabilize it, and then precisely adjust its DC output to 38vDC. Now attach/connect that CAP +/- across the input of your 40v to 4-way DC splitter gizmo thru a 4A diode to keep the far end from toasting your input side, to get those 4 DC voltages you need. Personally, I'd go to Goodwill, chat with the manager and get yourself an old 36v 8A HP Printer DC pack. It'll directly feed your 4-way and allow you to pull ~2a thru each rail (but not all at once)

StealthRT · Jan 5, 2020

SeekerOfSecrets said:
Be aware that 48v 'POE' is only about 2Amps max, so you have to judiciously pick and choose which of your 4 outputs you 'really' want to powersomething on at that time.

Here's one idea: Feed your 48v DC source thru a 50-80v to 25-6v out step down 'buck' regulator. Put a nice fat 4000uf/160v cap with a 1 Meg, 5W resistor across the output to somewhat buffer and stabilize it, and then precisely adjust its DC output to 38vDC. Now attach/connect that CAP +/- across the input of your 40v to 4-way DC splitter gizmo thru a 4A diode to keep the far end from toasting your input side, to get those 4 DC voltages you need. Personally, I'd go to Goodwill, chat with the manager and get yourself an old 36v 8A HP Printer DC pack. It'll directly feed your 4-way and allow you to pull ~2a thru each rail (but not all at once)

Doesn't the board produce the ~5a itself and not what the poe cable can carry? I would need to use 3 of the 4 outputs for what I'm doing.
-power 2nd gen echo dot (5v)
-power 100w mono audio amp module (between 12-24v)
-power an esp32 (3.3v)

rjenkinsgb · Jan 6, 2020

I think you may have to do it in two stages; a 12V output unit, 10A or more with a suitably high input voltage rating - 60V seems common - then run a separate 12V to 5V + 3.3V one from that.

picbits · Jan 8, 2020

Don't you have to negotiate with the 48V POE Switch first to ask it to provide the power otherwise it behaves like a normal switch ?

ronsimpson · Jan 8, 2020

Here is a high voltage version. HV 0.8a I know it is not what you want.

Another version: 60V 1A

Visitor · Jan 8, 2020

Silvertel has a variety of PoE power supply modules at a decent price. They handle the PoE negotiations without microcontroller help. I believe all of their products provide a single output voltage, so the best bet is to get a module that provides the highest voltage you need and use additional switching or linear regulators to get the other voltages you need.

Standard PoE (IEEE802.3af) can provide a maximum of 13 watts. PoE+ (IEEE802.3at) can supply 30 watts. IEEE802.3bt can supply 85 watts, but I'm not sure how common it is. Driving a 100 watt amp using PoE isn't possible.

kubeek · Jan 13, 2020

You need just a few components to create a circuit that can negotiate PoE, I am sure at least the AF variant. IIRC it was something like 15k resistance directly across the wires to signal the device and power requirement, then a mosfet switch with a zener in gate to turn on the load above 18V or so.

edit: see figure 5 **broken link removed**

Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom

StealthRT

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