LNK304 BUCK EMC FAILURE

Posted by: net-flyer on 03/27/2013

Hello,

the prototypes of a device we are ready to produce have failed the EMC testing, especially for conducted emissions.

The SMPS, which can be the only culprit, uses a LMK304 in buck configuration.

We have to use a simple buck configuration because the device is small and cheap. Also, we can't place a second rectifier diode on the Neutral wire because the circuit uses the Neutral as a voltage reference.

Input Voltage is 230V and out is 24V with 80-100mA current.

The EMC tests fail between 300KHz and 3MHz for conducted emissions and around 33MHz for radiated emissions (by a very small amount in this case).

We are wondering if we can solve the problem just placing a capacitor somewhere or if the PCB has to be re-drawn, which would be a problem.

Schematics of the SMPS section and diagrams of failed tests are all attached.

Thanks for any help... currently we are locked!!

Andrea / Italy

Files

B-O-M 3.3.0 SMPS.pdf (99.7 KB)

133043E-Ce01.pdf (88.17 KB)

133043E-Re.pdf (67.19 KB)

Comments

Would you like to test with uF4007 for D2? Hopefully this canimprove EMI.

Regards

Hi, thanks for your answer.

It turned out I had swapped by mistake the freewheeling diode and the feedbackk diode; that's why D2 on the schematics is a slow 1N4007.

Now I swapped back the diodes, using for the FW an SF18G which, although different from the one you suggested, is a fast diode with trr=35ns.

Unfortunately the emission level doesn't seem to get affected...

Would you like to try another stage inductor before C1? Actually common mode inductor can be used for this inductor, and your reference to netural should be ok.

regards

It was necessary to add a 68nF capacitor on the AC input. That lowered the conducted emissions diagram by almost 20dBuV !