I am seeing a problem with Micrel Ethernet PHY KSZ8721BLI.

Because of some mechanical constraints, the PHY is located near the edge of the main board and a small card edge connector. The board slides into a housing, and at the end of its travel, a smaller board mounted the back of the case and sporting a card edge connects to the card edge connector of the larger board mentioned above. This small board is home to the Ethernet RJ45 connector and magnetics, and no particularly long traces are present.

My problem: I am seeing failures of the TX+/- and RX+/- pins on the Ethernet PHY, all shorts. I have not observed a cable discharge event (CDE) when plugging in an Ethernet cable once the board is in place, and my cable is only 3 m. I've looked at the behavior of the connector during insertions and extractions of the larger board and haven't seen anything strange. Also, this is a design that I've built repeatedly and never had a problem with before.

The circuit is basically a Bob Smith connection with 0 Ω resistor instead of the typical 75 Ω resistor.

Assuming that this isn't just a bad lot of Ethernet PHY ICs, does anyone have any ideas of what might kill the PHY TX and RX pins?

  • \$\begingroup\$ We cannot see your shorts 👀but some dimensions or tolerances have shifted📐 and it is up to you to compare 🔍design with specs to find the fault 💰 \$\endgroup\$ Commented Jun 18, 2018 at 19:34

1 Answer 1


If your blowing out your ports and you have magnetics, then your doing something wrong. Make sure you have the proper magnetics and that they are terminated appropriately. Otherwise a static spike can get through the magnetics and blow out the transistors on your phy.

Ethernet cables can pick up a lot of static from triboelectric charging if moved across the floor, humans also come into contact with the cables so you need to have the magnetics be able to block a large differential voltage. The magnetics should also have a good isolation level like 4kV or 6kV (or more)

enter image description here Source: https://www.researchgate.net/publication/270883694_An_Ethernet_Cable_Discharge_Event_CDE_Test_and_Measurement_System/figures?lo=1

You can also put ESD protection diodes on the back end towards the phy.

enter image description here Source: https://incompliancemag.com/article/designing-ethernet-cable-ports-to-withstand-lightning-surges/


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