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My PSU design ripple and noise with picture measurements

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VEGETA:
Hello

I designed a PSU as a replacement PSU for Dreamcast using TPS62913 which promises total final noise and ripple of 1 mV p-p but I got around 80-100mV p-p despite following the datasheet and design layout recommendation. Other third-party PSU delivered near identical results despite having inferior parts and basic design. I added LC filters and ferrites plus bulk filtering caps 1000uF with 22uF ceramics for the input 12v rail but still I got this result.

Input 12v is supposed to come from cheap 12v PSUs, mine has 250mV p-p ripple and noise as shown in below pictures...

so:

1- does this mean that even good buck regulator design just cannot eliminate input ripple\noise? I mean it will get reduced but still will pass through?

2- what solution to achieve such low figure of 1mV or near it?

3- is adding huge inductor Pi filter will solve it? I only had 4.7uH for 12v filtering plus 1000UF elec cap and many ceramics.


pictures are labelled so you can recognize it.

for your kind help.

regards,

ezalys:
we’d need to see your schematic and layout. Also what’s the signal on the scope look like if you do the exact same measurement but with the power supply turned off?

ezalys:
additionally, I think it’s bad engineering to just throw LC filters at the thing. I’d like to know what the cutoffs are of the filters, the parasitics of them, the layout, all these things will contribute. It’s also true that no regulator will eliminate all noise, they just suppress it with a particular amount of suppression per frequency. LC filters, bulk caps, ferrites, and regulators themselves are tools in your toolbox to be applied deliberately to reduce noise.

floobydust:
Who promised 1mV p-p ripple out of a buck converter? Oh TI did TPS6291x 3-V to 17-V, 2-A/3-A Low Noise and Low Ripple Buck Converter with Integrated Ferrite Bead Filter Compensation using 8A ferrite bead BLE18PS080SN1.
I think your expectations are high and a Dreamcast doesn't require such low ripple.

Your PCB layout, scope grounding, choice of capacitors, ferrite bead- are all critical. You've got a 50mV noise floor so you cannot measure below that anyhow.

VEGETA:

--- Quote from: ezalys on August 22, 2021, 07:08:39 pm ---additionally, I think it’s bad engineering to just throw LC filters at the thing. I’d like to know what the cutoffs are of the filters, the parasitics of them, the layout, all these things will contribute. It’s also true that no regulator will eliminate all noise, they just suppress it with a particular amount of suppression per frequency. LC filters, bulk caps, ferrites, and regulators themselves are tools in your toolbox to be applied deliberately to reduce noise.

--- End quote ---

I didn't do calculations for LC filters but rather kept it straightforward and simple. I knew 12v will be very noisy, so I put them. I will put schematic once I return from work, even though it is not open source and should be commercial product... but it is ok.


--- Quote ---Who promised 1mV p-p ripple out of a buck converter? Oh TI did TPS6291x 3-V to 17-V, 2-A/3-A Low Noise and Low Ripple Buck Converter with Integrated Ferrite Bead Filter Compensation using 8A ferrite bead BLE18PS080SN1.
--- End quote ---

Yes, this is the IC. I have used different passives (from LCSC) than those of TI, but I think the ferrite is the same. why the results differ so much?


--- Quote ---I think your expectations are high and a Dreamcast doesn't require such low ripple.

--- End quote ---

I kinda went with their data and demonstration, or else what extra benefit to get from this IC over a regular one?

Dreamcast is an old console which uses analog video signals which are sensitive to noise. analog video is 1v only, so 100mV ripple and noise will have an effect. Generally, less noise and ripple the better. Making a cheap PSU which has low noise\ripple is good too.



--- Quote ---Your PCB layout, scope grounding, choice of capacitors, ferrite bead- are all critical. You've got a 50mV noise floor so you cannot measure below that anyhow.

--- End quote ---

Scope is not mine, it is in a fablab which is far away from me. Not accessible easily due to far distance and so on. It is gw instek gds-1152a-u 150 MHz 2 channels. I used normal probe with putting the probe ground to psu ground and then get the signal. I wonder why I got such a huge noise floor.

If I got 60-90mV noise for it, does that mean its real noise\ripple is just 10-40 mV p-p since the extra 50mV is just noise floor?


what solutions do you suggest to achieve this low values or near them?

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