0
\$\begingroup\$

1I'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 is 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 ?

SMPS Datasheet

\$\endgroup\$
  • 1
    \$\begingroup\$ Why do you think the noise is from the SMPS? Have you put the capacitor across the SMPS GND pins as recommended for most modules? What type of SMPS is it you're using? \$\endgroup\$ – Puffafish Apr 8 at 13:11
  • 2
    \$\begingroup\$ What type of SMPS? Mains in and low voltage DC out? Add a link to a datasheet. It is possible that the SMPS has too much (capacitive) coupling to the mains which then disturbs the capacitive touch. Filtering will not fix that. A "better" SMPS or a 2nd isolated DCDC converter or a "better" touch screen controller might fix it. \$\endgroup\$ – Bimpelrekkie Apr 8 at 13:17
  • \$\begingroup\$ If SMPS body is not earthed, LCD touch signal affecting. I checked with battery supply, LCD function working fine with battery mode. That's why am doubting on SMPS. Unable to find capacitor recommendation for GND pins in datasheet or any design notes. Can you tell me between where we need to connect capacitor ? \$\endgroup\$ – Boopathi Apr 8 at 13:32
  • 1
    \$\begingroup\$ Per page 2 or 3 of the SPEC, there is 300uA of leakage current. We may interpret that as 3,333 ohms of leakage per volt, or about 390,000 ohm of impedance between primary and the output pins (VDD and GND/RTN pins). Use a DVM, set on AC, to measure the level of trash on your touch-screen RTN. This will be the potential between the screen and the fingers. \$\endgroup\$ – analogsystemsrf Apr 8 at 14:20
  • \$\begingroup\$ @Bimpelrekkie How about using LDO after the SMPS would PSRR eliminate such noise? Would isolated DC DC converter after an SMPS reduce 50Hz related noise? \$\endgroup\$ – user16307 Apr 19 at 18:37
0
\$\begingroup\$

Do you NEED to use a medical grade supply there?

You can get class II supplies that are not medical (low leakage) designs. The advantage is that they have a class Y cap across the primary/secondary isolation barrier to reduce high frequency common mode nose at the cost of higher line frequency (and harmonics) leakage.

Unless you need it, medical grade is NOT better here.

\$\endgroup\$
  • \$\begingroup\$ Yes, I want to use medical grate SMPS. i checked with common mode choke and Ferrite core(6 turns) at SMPS output side still the issue is reproducing. I connected Copper plate at SMPS earth point, but there is no improvement. \$\endgroup\$ – Boopathi Apr 10 at 13:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.