LM5156: LM5156/55 output ringing with reflection.

Hi Faheem,

Thanks for reaching out. Your phase margin seems to be good in theory, but it is always necessary to tune the elements of the compensation loop when the PCB design is ready, in order to compensate uncertainties that arise from component placement and PCB routing.

Furthermore, the resolution of the schematic is not that great, so I can't really see confirm it, but it looks a little as though the secondary winding is connected in the wrong direction.
As you can see in the picture below, the secondary windings should be connected in the following way.

Thank you and best regards,
Bryan

LM5155EVM-FLY User's Guide.pdf

Hi,

Thanks for your reply. I guess the polarity is not a problem as you can cross check with this reference design. 

You can cross check the dot convention with the attached evl board. Please guide me on how to tune the tune the elements of the compensation loop.

Thanks.

Hi Faheem,

Thanks for the better image. We'll get back to you with some feedback.

Best regards,
Bryan

Hi Bryan,

Now its looking good at the output. But there is another problem. If the load is less than 8W its tend to do that. With load more than 8W. Its fine.

Hi,

Over more investigation its not ringing. Its saw tooth wave over the output giving voltage fluctuation from 5v to 15v. After removing the output Caps its working but now it has noise and noise increases as power drawn increase.

Is it something with soft start?

Its more ripple voltage.

Hi Faheem, 

Thanks for the continued inputs. Please have some more patience.

Best regards,
Bryan

Hi Faheem, 

It indeed seems as though the transformer and diode are connected in the right way.
Removing all of the output capacitors won't be a good idea.

Could you please summarize your latest findings and possible include an image of the schematic that is better readable?
The separate comments don't really make sense to me, so it would be great if you could give me a more elaborate explanation of the current situation.

Thank you and best regards,
Bryan 

Hi Bryan,

The output becomes OK as long as there is a load of at least 8W. As the load is less than 8W it behaves like capacitor charging discharging. By decreasing the output capacitance from 1mF to 500 uF the ripple is less for less power (e-g. It works fine after 5W with less output capacitance).

Hi Faheem,

So are you happy with the situation as it is now, since it sounds like a stable system, or would you like to decrease the noise?

Best regards,
Bryan