Hello i am using a ACT4088US-T (datasheet here) Buck converter and i made it to run in constant current mode 500[mA] through a Diode (D5): Everything works fine. It draws 500mA no matter the load i attach to it and the Voltage at FB is 0.81V all the time.


simulate this circuit – Schematic created using CircuitLab

But i get noise of about 280[mV] p-p (Peak to peak) max before the diode (Point A at my schematic). If i remove C2 and C3, the noise increases (thats why i have them there). 250mVNoise

What are other ways to reduce the noise At point A?

Reading the datasheet of ACT4088, i think i can decrease the noise by using a larger Inductor (L1). Maybe using a larger C1 would help? Would bootstraping the output with a diode (Page 8 of datasheet, D2 diode) would help?

(Just for the record i tried adding a large 3300uF Cap after the diode, it didnt help much)

Probe is x10 Oscilloscope settings are:


V/div Offset: NORMAL

Probe attenuation: x1

Thank you.

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    \$\begingroup\$ Make sure you are properly scoping the voltage - anything other than a spring probe is going to give false results. \$\endgroup\$ – Andy aka Jan 20 at 14:08
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    \$\begingroup\$ You can prove this by seeing what the noise is when you touch the probe tip onto where the alligator clip is when connected to the circuit. \$\endgroup\$ – Andy aka Jan 20 at 14:14
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    \$\begingroup\$ Do not use the ground clip of the probe because the cable can pick a lot of noise. This is the correct way of probing anywhere in the SMPS. \$\endgroup\$ – Rohat Kılıç Jan 20 at 14:22
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    \$\begingroup\$ No matter how "good" your buck converter design is, there will always be some ripple due to the switching. Indeed a larger inductor value decreases the current peaks and can help. Also the filtering capacitors (C2, C3) need to be low ESR type. If you still need less noise / ripple after doing everything "correctly", consider filtering. I'm thinking of a large value inductor in series with D5. \$\endgroup\$ – Bimpelrekkie Jan 20 at 14:40
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    \$\begingroup\$ If changing the grounding of the probe doesn't appear to help, consider measuring differentially using two probes. Probe 1: anode of D5, Probe 2: kathode of D5. Connect the grounds of both probes to the ground of your circuit. Then on the oscilloscope subtract the two signals. \$\endgroup\$ – Bimpelrekkie Jan 20 at 14:44

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