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I am using a step-down-converter to get a variable supply output in the range of 2-4 V from a 5 V input voltage from an adapter.

The problem I am facing is that I get a 200 mV ripple which looks like a 200 mV peak-to-peak sine wave with a 10 μs period. The voltage is 3 V (all this from a DSO).

How to smooth this ripple? Which filter is best to reduces this ripple? With a simple 470 μF capacitor in parallel the ripple peak reduces to half, but the results are not satisfactory because my application need no or minimum ripple.

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    \$\begingroup\$ There is not enough info. And no specs. Which step down converter you have and how does it work? Any schematics? What you can't do is remove the ripple completely, as it needs an infinite capacitance to do that. You need specs how low ripple is acceptable. The ripple can be calculated from switching frequency, capacitance, current, and acceptable ripple voltage. Some post filtering may be required. If the input 5V comes also from switch mode supply, it may pass through the second step down converter. Would a linear regulator work? \$\endgroup\$
    – Justme
    Mar 4, 2023 at 10:27
  • \$\begingroup\$ The input to the DC-DC converter is from a "mains" adapter - any chance to adapt directly to load? How much ripple is there without the load connected, with and without the additional 470 μF capacitor? What about the ripple at the output of the adapter/input of the DC-DC converter? \$\endgroup\$
    – greybeard
    Mar 4, 2023 at 10:29
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    \$\begingroup\$ There will always be ripple and noise. You need to make your application a tad more robust. \$\endgroup\$
    – Andy aka
    Mar 4, 2023 at 10:43
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    \$\begingroup\$ How did you measure 200 mV? Show your setup. \$\endgroup\$
    – winny
    Mar 4, 2023 at 11:57
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    \$\begingroup\$ Halving a 200 mV ripple was not satisfactory, but reducing to about 100 mV is? \$\endgroup\$
    – greybeard
    Mar 4, 2023 at 12:25

1 Answer 1

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There are two approaches to filtering DC:

  • use components capable to store energy.
    There will be a capacitor at the output of your DC-DC converter; you already got an additional one -
    filtering will improve with a series component between them.
    Resistors are low price, inductors more effective,
    both need a time constant with the next/output capacitor no smaller that the time between ripple peaks. To be effective, the energy stored in each component needs to be bigger than what's necessary to "fill the ripple valley". With parallel components like a filtering capacitor, PCB layout may get significant influence.
  • get a voltage "without ripple", use the "ripple ridden source" to power an amplifier to output something closely related to the first voltage.
    If the "reference voltage" is constant, this is a low drop-out /LDO voltage regulator. Keep an eye on power supply rejection ratio.
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