I have designed a POE device where I used the MAX17690 flyback converter to implement an isolated PSU for my device. I used the reference design MAXREFDES1105 (5V/2A design) from Maxim to size the circuit. I used same nominal value components in my design. Previously the operation was tested on MAX17690 evalboard.

First Issue: I think the circuit is too hot for normal operation. At secondary side the board temperature is about 50-60°C with ~400mA load. At primary side it is 45-50°C. On evalboard, with same load, at secondary side it is like 40-45°C, primary side it is ~30-40°C.

For some resistors I used 5% resistors instead of 1%. Can it cause the issue?

I am not using the PSU on the rated current, can that be the problem?

Second issue: The POE interface controller dies after the first successful operation when I remove the Ethernet cable. So for the first plug-in, it works nice, after the second try the Network Switch does not recognize the device and POE supply is not starting. It works only if I replace the controller.

I am using a QFN package of LTC4257IDD-1#PBF. If I solder the LTC4257CS8#PBF, SOIC8 package, with air wires, it works well after multiple connection/disconnection of ethernet cable. With QFN package with the same wiring it does not..

Am I missing any protection circuit, or capacitor in the POE interface circuit?

Or am I just messed up the Layout?

POE isolated PSU Layout

  • \$\begingroup\$ Any reason you connected C318 to pins 9 and 10 of the primary instead of 7 and 6? The way it is placed now makes it amplify the switching noise on the output instead of the typical purpose of this capacitor, decreasing it. \$\endgroup\$
    – Ferrybig
    Commented Oct 1, 2021 at 13:26
  • \$\begingroup\$ Currently that capacitor is not assembled and yeah its wiring is messed up. The capacitor shall be wired between the two isolated GND. Does this capacitor have a key role in operation? \$\endgroup\$
    – D_Dog
    Commented Oct 1, 2021 at 13:35

1 Answer 1


The solution for First issue is found!

The reference design and MAX17690EVKIT evalkit has an unnecessary load circuit, D303 + R323, on the secondary side. And the heat source is that circuit. Resoldering the unnecessary circuit solves the heat issue.

Here are some pics before and after the modification: Before modification After removal of redundant circuit

However the primary side still seems hot. I will try to hunt it down. On the evalboard + breadboard POE + 400mA load, all sides are "cold":

enter image description here

The second issue is still not solved. If someone has idea, he will get accepted answer.


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