I am making a PoE design with a silvertel AG5200.

This design has two inputs, PoE and a 'aux' power input. The AG5200 is a flyback and is isolated from the ethernet, so it has a isolated ground. When the PCB gets power from the AG5200 and I connect the 'aux' power source I get a ringing effect. This ringing effect kills my Ethernet switch IC. Because it bounces through the whole system.

I noticed I get the ringing of 3V above of the 12V when I am connecting a external ground. (I removed D1 and D2.) I think it is because the ground is isolated and "floats" and when I am connecting a external ground the AG5200 has to adjust his output because his reference is changing.

I added a resistor of 8ohm between the external ground and the ground of the PCB. The ringing effect is then smaller, but I need a better solution.

The AG5200 is a blackbox, so i have to do it externally.

What is the best approach to fix this problem?


simulate this circuit – Schematic created using CircuitLabenter image description here

  • \$\begingroup\$ So you are hot-plugging the aux supply and getting this waveform? Sounds like maybe your aux supply has poor phase margin/transient response. Before anything else I would put a TVS device on Vout to clamp the output voltage low enough so that it doesn't damage your switch IC. Might be tough to find one with tight enough tolerance, so maybe design a clamp circuit instead. Then do some load step testing on the aux supply to see if it's the cause of the problem. \$\endgroup\$
    – John D
    May 30, 2017 at 15:39
  • \$\begingroup\$ Going to order a TVS Diode, coming back on it. \$\endgroup\$
    – pmaal2
    May 31, 2017 at 8:56

1 Answer 1


The flyback module requires a minimum load of 200 mA. You need to ensure this load on its output or you will get audible and conducted noise.

If you can afford the power loss and heat generation, tacking a 50-60 ohm resistor on its output will solve your problem. If that isn't an acceptable solution, you will need to ensure that the flyback has priority when operating. You could do this by sensing its status outputs and only switching on the aux supply when the flyback isn't able to supply the required power.

  • \$\begingroup\$ +1. Maybe a 20-25mA dummy load would do too. On p.13 of DS is said 200mA for audible noise and 20mA absolute minimum. @pmaal2 what is the period from the ringing chain on the graphic to the next one? if it is more than 100-200us you could try a 25mA dummy load. \$\endgroup\$ May 30, 2017 at 19:58
  • \$\begingroup\$ When we are adding a load for 20mA we still getting the same ringing. There is no second ringing, it is only when i hot plug the aux. It also happens when I only connect the external ground on the aux. \$\endgroup\$
    – pmaal2
    May 31, 2017 at 8:47
  • \$\begingroup\$ I doubt 20 mA will do it - try 200 mA as I recommended. Also note that this load must be directly on the output of the flyback module - before any other components. \$\endgroup\$
    – Glenn W9IQ
    May 31, 2017 at 12:41
  • \$\begingroup\$ My schematic wasn't complete... i forgot to draw a diode in series with the + of the aux input. \$\endgroup\$
    – pmaal2
    Jun 1, 2017 at 7:48
  • \$\begingroup\$ You will still require a minimum 200 mA load on the output of the flyback whenever it is powered. \$\endgroup\$
    – Glenn W9IQ
    Jun 1, 2017 at 7:50

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