SMPS electrical noise issue

[BoopathiS]

Hi,
I'm using class 2 medical graded SMPS in our device. So earth connection not recommended. When connecting SMPS to Load(contains Capacitive Touch LCD),
Touch function getting affected by SMPS noise. When connecting SMPS earth point to Protective Earth, LCD touch function working properly.

I don't want to use earth connection because of class 2 constraints. I want to bypass that noise. I have added few decoupling ceramic capacitor at touch input supply to avoid high frequency noise to touch input. Still am facing issue.

Can anyone give me some idea to overcome above issue ?

 

[FvM]

I don't believe that power supply noise is the issue. It's rather common mode noise between earth ground and the local circuit ground. The disturbing voltage is sensed at the touch controller input. Y capacitors might help, also good common mode chokes. Or varying the touch controller carrier frequency.

 

[Easy peasy]

Yup large common mode ferrite toroid with say 8 turns of + & - insulated (pvc) wire for the output leads - your only hope really ...

 

[BoopathiS]

Thanks to FvM and Easy Peasy for giving valuable suggestion.

I checked with common mode choke and Ferrite core with 8 turn at SMPS output terminal. Issue reproducing.

I connected Copper plate( 3mm thickness and 1*1 inch dimension) to SMPS earth terminal, but no improvement.

I want to try with Y capacitor and LCD carrier frequency.

 

[Easy peasy]

more turns needed on CM choke ...

 

[BoopathiS]

more turns needed on CM choke ...

Thanks, Now I checked with 45 turns CM choke.Issue is repeating. Then added two 45 turns CM choke assembly in series with SMPS DC output, Not seeing much improvement.

 

[c_mitra]

When connecting SMPS earth point to Protective Earth, LCD touch function working properly...

Yes, I would suggest a Y capacitor first and next touch controller.

 

[Easy peasy]

Y caps to gnd at your problem board is the next step ...