Touch screen power supply problem

Question

I am using Raspberry PI 4 Model B (4GB) with an official capacitive 7" display. I am powering the device using an external AC-DC (220VAC - 12V) power supply. I converter 12V to 5V using a buck converter to power the Raspberry Pi and LCD.

I have two power supply MeanWell GS60A12-P1 (End of life product) and GST60A12-P1J (new product). Everything is fine using the end of life power supply GS60A12-P1, but using the new power supply the touch screen is almost unresponsive. If I touch the backplate of the LCD or any power pin, the screen works. If I ground the backplate of the LCD, everything works (but grounding is impossible using this AC-DC adapter). Both power supplies have almost the same specification.

I am guessing, that there is a problem with common-mode noise. I tried to place a common mode choke on the 12V power line and common mode filter - Did not help.

I tried to disconnect the earth connection for the AC plug. Helped a little bit, but the screen still was unusable.

What could cause this problem?

P.S. I tried to measure the noise of both power supplies, both look similar, except GST60A12-P1J have 890Hz (100mV) noise beside 480kHz switching noise (50mV). I don't know from where the 890Hz noise is coming.

Answer 1

It's most likely an EMI problem, as capacitive touch screen are very sensitive to it.

Make sure you have connected the ground wire to the power supply (VAC; N; GND). If it doesn't help, try to add a common mode choke and a ferrite at the 12V cables.