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Original question:

UC2843: Emission Issue

UC2843: UC2843: Emission Issue

Nikhil F
Prodigy 110 points
Part Number: UC2843

Tool/software:

Hello,

We are facing issue for radiated emission test with below section of UC2843 IC.

Observed around 78dBuV emission at 55MHz-110MHz frequency. 

Below are the details:

Input supply: 24V, Output: Sec1- 24V/110mA, Sec2- 24V/70mA.

Analysis: We observed that there is ringing at gate pulse Q2. 

We have done some changes marked in below image. 

1. Added capacitor 0.01uF between Mosfet Q2 gate and ground as shown below.

After changes we observed there is reduction in emission by approx. 12dBuV.

Can you please confirm above solution is correct? 

Is any other way to reduce emission?

Thanks,

Nikhil

  • 0
    TI__Guru 64225 points

    Hi,

    Usually increase R8 instead of adding this capacitor. But it is ok to add the capacitor. Increase R8 is with flexible since you can add an anti-parallel diode to make rise and fall time different.

  • 0 in reply to Hong Huang
    Prodigy 110 points

    Hello Hong,

    Considering the 210 kHz operating frequency of the switching converter, we changed R8 to 22 ohms and tuned the capacitor value in parallel with R212 to 22 nF. We observed good emission results below 40 dBuV, as required.

    Please refer to the attached DSO waveform, which shows the MOSFET Q2 gate pulse (red color), the MOSFET Q2 drain pulse (blue color), and the transformer secondary pulse (yellow color).

    We have a query regarding the gate pulse of MOSFET Q2; it is not a perfect square wave due to the connected RC at the gate, as mentioned above.

    Is the shape of the gate pulse acceptable?

    We have verified that voltage of gate pulse is 8.2V which is higher than MOSFET gate threshold voltage (3.8V).

    Can we proceed with the above RC combination, or do you have any suggestions for improvement?

    Thanks,

    Nikhil

  • 0 in reply to Nikhil F
    TI__Guru 64225 points

    Hi,

    What are your oscilloscope channels bandwidth? It looks the bandwidth setup not good.

    Basically, you need to measure the efficiency and the temperature on your board. If these are ok, then the design will be ok.