Author Topic: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom  (Read 437 times)

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Offline Stealthrt

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Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« on: January 03, 2020, 06:10:44 pm »
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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Across post:
https://forum.allaboutcircuits.com/threads/step-down-48v-input-poe-to-4-outputs-3-3v-5v-12v-custom.166040/
https://www.electro-tech-online.com/threads/step-down-48v-input-poe-to-4-outputs-3-3v-5v-12v-custom.158171/
 

Offline andy3055

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Re: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« Reply #1 on: January 03, 2020, 06:21:52 pm »
48 volts will be too high for those regulators. Also the caps are 50 volts max. Not sure if any of them are on the input side and if they are, there is not enough safety margin.
 

Offline Stealthrt

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Re: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« Reply #2 on: January 03, 2020, 06:43:38 pm »
48 volts will be too high for those regulators. Also the caps are 50 volts max. Not sure if any of them are on the input side and if they are, there is not enough safety margin.

Could I exchange the LM2596 with LM2576 (http://www.ti.com/lit/ds/symlink/lm2576.pdf)? That ladder seems to be tolerant up to 60v.
 

Offline Vovk_Z

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Re: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« Reply #3 on: January 03, 2020, 07:29:03 pm »
There is "HV" version of those dc-dc IC. It has 60V max.
 

Offline andy3055

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Re: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« Reply #4 on: January 03, 2020, 08:29:47 pm »
Yes, as Vovk_Z stated, you can do that. But I thought you wanted a ready made thing. If you plan to replace the regulators in what you buy, you need to consider the other stuff also like the caps and anything else there may be. Then the whole idea of "ready made" goes out the window!

In this case, you might as well design your own circuit based on the HV versions of the regulator as in the spec sheet you referenced. Why buy something that you need to modify? Not to mention you will be replacing the most expensive parts on that board. Does not make any sense.
 

Offline ttt

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Re: Step down 48v input POE to 4 outputs 3.3v/5v/12v/custom
« Reply #5 on: January 03, 2020, 08:55:42 pm »
1. Just to check: are you using real PoE (802.3xx) from a switch or just power injecting into Cat5/6 where you control the voltage? PoE voltages can go up to 57V. Ideally you should use 64V or better 100V rated parts for PoE applications.

2. How much power do you think you will use? You will need a proper PoE PD controller chip (802.3at), rectifiers and a transformer to reliably achieve anything above 25W or so over Cat5.
 


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