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LM5023: Noise/Ripple Reduction in the output voltage

Mehdi
Prodigy 160 points
Part Number: LM5023
Other Parts Discussed in Thread: LM317

Hello,

I am using an LM5023-2EVM and with changing the transformer and some other modifications, the output voltage is 12V. The regulated 12V is then connected to LM317HVT/NOPB regulator to obtain +8V. 

The circuit works fine in a wide range of input voltages. Could you please advise how I can reduce the noise and ripple in the output voltage?

Please note that I have tried a few methods such as using copper foil shield in the transformer between primary and secondary windings, using an LC filter in the output, snubbers across the rectifier diodes. But still I have not been able to reduce the noise.  

 

+12V waveform (200mV/div)

+8V waveform (200mV/div)

 

Thank you.

  • +1
    TI__Expert 7900 points

    Hello Mehdi,

    How are you making the measurement?  The 3-4 inch ground lead on the 'scope probe picks up high frequency noise and gives a false reading.

    The best way to measure noise is directly across an output capacitor using the "tip-and-barrel" method shown below.  Note the small spring that has replaced the 3-4 inch ground wire to the 'scope probe.

    Also, make sure you use ceramic capacitors on the output of the LM317 that are located very close to the device.

    Regards,

    Eric

  • 0 in reply to Eric Reicher
    Prodigy 160 points

    Thanks Eric. That resolved my issue.