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source needed for very low ripple isolated DC-DC converters

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graybeard:
I am working on a fast turn around project in which I need very low noise PCB mounted isolated 3.3V output DC to DC converters.  I need sub mV output ripple.  I can use multiple converters for lower noise.  Multiple converters with output of 200mA-1.1A or a single converter with an output of 650mA to 3.2A.   Based on DRE I choose an input voltage of 24V, but I am not married to that.  I only need 40V maximum of isolation.  Low output noise is my primary concern.

Linear regulators do not do a great job on noise in the 100 KHz to 100s of Mhz that comes out of the converters I have looked at.

I was hoping to buy something off-the-shelf since I have many design challenges and I do not want to spend time designing filters for the converters.  It seems I may have no choice but to devote time to filter design.

I have talked with the technical reps from several companies, but they just tell be I will need to design my own filters and refer me to their application notes.

Do you know of any companies that I should contact?

Thank you
Chris

fourtytwo42:
DC-DC modules always have limited internal filtering as that is sufficient for the bulk of applications, best get on and add the external filter components, I am sure the vendor will provide you with the required components to suit there module rather than you having to do the design. Don't forget input noise can be a problem with these devices too.

tszaboo:
Those modules are not going to be low noise. The small transformer is driven really hard, sharp edges, and often driven into saturation. What you do is: take one which gives you higher voltage than needed, then burn the voltage with an LDO or linear regulator. Depending on your noise level, you may need some common mode noise filtering inductors.

nctnico:

--- Quote from: graybeard on May 31, 2023, 09:35:34 pm ---I am working on a fast turn around project in which I need very low noise PCB mounted isolated 3.3V output DC to DC converters.  I need sub mV output ripple.  I can use multiple converters for lower noise.  Multiple converters with output of 200mA-1.1A or a single converter with an output of 650mA to 3.2A.   Based on DRE I choose an input voltage of 24V, but I am not married to that.  I only need 40V maximum of isolation.  Low output noise is my primary concern.

Linear regulators do not do a great job on noise in the 100 KHz to 100s of Mhz that comes out of the converters I have looked at.

I was hoping to buy something off-the-shelf since I have many design challenges and I do not want to spend time designing filters for the converters.  It seems I may have no choice but to devote time to filter design.

I have talked with the technical reps from several companies, but they just tell be I will need to design my own filters and refer me to their application notes.

--- End quote ---
What I normally use is a common mode filter at the outputs of DC-DC converter modules (preferably shielded models) with a large electrolytic (say 330 uf) at the DC-DC side and an MLCC at the side of the circuit. Don't forget a ceramic capacitor between in / out of the DC-DC converter to short the leakage current. For sensitive circuits, I use linear regulators with an RC filter at the input so only load / line regulation are an issue.

Peabody:
You might look at Dave's video #1116 on using a capacitance multiplier to reduce ripple.

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