Ring wave at SMPS power line

Hi,

sometimes I´ve seen at SMPS a wave form on the power line instead of a straight clear line with the scope. I know probably it has to do with a bad filtering of signal at secondary but sometimes it´s very difficult to remove it even making filter better. Is it like a feedback from inductor of the transformer?

Type of signal:

https://fabmodules.com/wp-content/uploads/2013/04/squarewaveringing.jpg

It is usually caused by the ringing of the tank formed by the stray capacitance and inductance in the circuit. It can be minimized by short connections for all leads carrying the switching currents, including the switching transistor and flyback diode, and a good ground plane. A RC snubber circuit of the proper values can also be use to minimize the ringing.
It can also be from not connecting the scope probe ground directly to the ground near the switching elements.

Understood, thank you very much for the information!!!