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DC-DC converter in mixed signal application

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ZeroResistance:

--- Quote from: bson on June 18, 2019, 01:04:59 am ---What kind of converter and what currents?


--- End quote ---
Good question! :)
I need a converter primarily for isolation about 4200Vac rms / 6KV dc to comply with standards.

radiolistener:
use very high frequency dc-dc, for example 1-2 MHz, it will increase gap between noise spurs and will minimize risk of influence.

Yansi:
Also good to note, never ever use switchers with those stupid hysteretic controllers (34063, LT1074 atc... being the fine specimens of this). You won't get the in band noise away ever!

Modern MHz range converters can be made very silent with a tight good layout and a bit of postfiltering (LC, ferrite bead).

ZeroResistance:

--- Quote from: floobydust on June 17, 2019, 09:06:06 pm ---Minimizing Switching Regulator Residue in Linear Regulator Outputs - Banishing Those Accursed Spikes AN101F Jim Williams

Very good read on the topic.
Yes there are LDOs with high PSRR at 100kHz or higher. Since that changes with load and temperature, I play it safe and add a small inductor instead.

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Great link! Thanks for sharing. And inductor would work well to reduce the 100Khz switch ripple however what about the high frequency > 10Mhz noise spikes that appear along with the ripple at the output of the DC-DC converter would you need a ferrite bead for that?

floobydust:
For DC-DC output differential-mode noise going into the LDO, something like a 10uH inductor works, up to 10's MHz which is enough. I find ferrite beads alone don't attenuate enough in the low MHz.

Monitoring mains voltages using an optoisolator and DC-DC for power does have drama.

Most important and not known is the DC-DC converter's common-mode noise ends up getting out on the mains wiring that you are doing the voltage monitoring of. This is a big trap, also with the ADUM isolators too. Even Analog Devices misses the boat AN-0971 thinking the PCB layout is the fix  :palm:

The isolation voltage rating of the DC-DC is usually a 1 minute hi-pot test value, NOT an operating value. The insulation's working value, for years of use needs to be considered. It helps to have decent UL/CSA approvals and the proper overvoltage category i.e Cat. III for industrial products. Don't forget a fuse if the isolation (opto or DC-DC) fails, as raw mains will burn up a board badly. Even a 1A fuse is needed when you stare at the circuit for a long time.

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