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Thread: Don't blow your Teensy 4.1 Ethernet PHY when using passive PoE

  1. #1
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    Don't blow your Teensy 4.1 Ethernet PHY when using passive PoE

    Hot-plugging a passive PoE connection will blow your teensy PHY. Even though the PHY is DC-isolated from the jack, the unbalanced condition that occurs while hot plugging a powered cable will cause a transient spike of the order of your PoE voltage on your PHY. If you actually make this mistake, only the PHY is damaged and you can fix your teensy by replacing it (PN:dp83825irmqr). Robin very generously sent me some spares they had around and I was able to bring my dead teensies back to life. Thanks Robin!!!

    The solution to this is easy: TSV diodes on the ethernet data lines on the PHY side + small series resistors. Implementing this on the next teensy revision or the ethernet adapter board or on your custom PCB will protect your PHY from hotplugging passive PoE.

    This application note explains all this in detail. http://ww1.microchip.com/downloads/e.../00002157B.pdf

  2. #2
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    Thanks for finding this, very interesting. I use a Netgear GS305P POE that I'm always connecting and disconnecting test equipment, rPi and computers to, as if it was a regular ethernet connection. While the simplistic visual aids in the instructions show plugging in the power last no specific mention is made of the necessity. So far no damage to my stuff, but I will now take precautions, thanks.

    Looking at schematics of the Raspberry Pi 3+ and 4 that were designed for POE, I see no TSV. In fact looking at 4 devices so far I see no protection, unless it is built into the magJack or hidden somewhere.

  3. #3
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    You probably wont have this problem with active PoE because the power isn't enabled until negotiation, so hotplug events wont happen the same way they would with passive PoE.

  4. #4
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    @mamdos,
    Thanks, wondered what passive meant.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    @mamdos - Can you tell me which hardware you're using for passive PoE? If I wanted to experiment with various diodes and resistors, which gear should I buy to give me the PHY-destroying scenario you had?

  6. #6
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    Hey, just saw you responded. I came back to report that the adding a parallel TSV diode (P/N: UCLAMP3301D.TCT with no resistor) as described in the article didn't work for me. I blew another PHY after hotplugging with a load. I'm going to hook thinks up to a scope to get more measurements.

    My setup: I've designed a custom baseboard for the Teensy 4.1. I'm using the ETH1–460 PoE magnetics, and FDMQ8205A for the rectifying bridge (because "PoE" is what happens when you fail at making a standard ). I've attached the schematic below. This schematic is for the "fixed" board and it contains series resistors in addition to the TSVs. For my tests, I've only been patching the first revision of the board by soldering just the TSV on the teensy ethernet header.
    For the PoE injectors, I'm using these https://www.tycononline.com/GigE-4Pa...45V_p_361.html and a 54V power supply. The load is mostly capacitive. I'll post results once I figure out a solution.

    Click image for larger version. 

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    Click image for larger version. 

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  7. #7
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    Some updates. I took the teensy out and probed the data lines on one of the pairs. I used a 6" and a 100ft CAT 5 to see the effect of input LC and I tested with and without the UCLAMP diodes. Here are the results. As you can see the TSV diodes make a huge difference, but not enough to protect the teensy. Without the diodes, the transient spikes at the PHY are in the order of 70V!!! the diodes help bring those down to 9V. Some series resistance should hopefully provide some protection. Thats up next.Click image for larger version. 

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