Noob question about dc-dc converters and common mode filters

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donzo93

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Joined
Feb 2, 2015
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17
Location
Italy
Hi everyone! I have a +24v 4a linear power supply and I want to add a module to provide phantom power to 6 preamplifiers. Every +48v feed must have a led that indicates the status. I think I could do it with this dc/dc converter: http://www.farnell.com/datasheets/1897342.pdf?_ga=2.22753787.1719564225.1530013216-424264997.1529698549 .
The specific model is JCD0624D24. I'm doubtful about two things: the model is a double out one (+-24v/+-125ma). Can I use the -24v rail as ground and obtain a +48v or it'll mess the converter? On the datasheets of models from other vendors the viability of this configuration is stated clearly, but there's nothing in this one. Provided that the configuration is feasible, this will change the output current from +-125 to +250ma, am I right? One last thing: I'm following this guide https://www.coilcraft.com/pdfs/doc191_CMFiltDesign.pdf to design a proper common mode filter for both the input and the output of the converter. Regarding second order filter at page 3 it says: "Identify the load resistance at the cutoff frequency: Assume RL= 50Ω". How I identify the load resistance at the cutoff frequency of the filter? Wouldn't different microphones have different impedances at those frequencies?

Thank you,
Marco
 
donzo93 said:
The specific model is JCD0624D24. I'm doubtful about two things: the model is a double out one (+-24v/+-125ma). Can I use the -24v rail as ground and obtain a +48v or it'll mess the converter?
These converters have fully floating outputs, so yes, you can use them as a 48V source.


Provided that the configuration is feasible, this will change the output current from +-125 to +250ma, am I right?
No; with the outputs in series, the limit is still 125mA, which is enough for 6 mics and 6 LED's running at 10mA..

One last thing: I'm following this guide https://www.coilcraft.com/pdfs/doc191_CMFiltDesign.pdf to design a proper common mode filter for both the input and the output of the converter. Regarding second order filter at page 3 it says: "Identify the load resistance at the cutoff frequency: Assume RL= 50Ω". How I identify the load resistance at the cutoff frequency of the filter? Wouldn't different microphones have different impedances at those frequencies?
It's not the mic that counts, it's the current draw. If each mic drew 10mA (maximum according to standards and each LED 10mA, the total load would be seen by the PSU as 120mA, which computes at about 400ohms. Since the number of active mics is variable, this figure varies from 400 (6 mics) to infinity (no mics).

BTW, I don't think you need a CM filter; you need a plain old LC low-pass filter.
You need to make sure that the characteristic frequency of the filter is much lower than the switching frequency, so the variations of the damping factor do not degrade the attenuation.
 
abbey road d enfer said:
You need to make sure that the characteristic frequency of the filter is much lower than the switching frequency, so the variations of the damping factor do not degrade the attenuation.

Ok, now I see, and it makes perfect sense. Thinking about it my question was kinda stupid. So basically I have to choose the L and C value of the filter that will guarantee that the loss at the switching frequency will be between a certain interval with all the possible loads applied to the PSU.

No; with the outputs in series, the limit is still 125mA, which is enough for 6 mics and 6 LED's running at 10mA..

This question was even more stupid than the one on the common mode filter...  :-\ :-\ :-\ I'm studying too much linear algebra and that is driving me nuts  :( :( :( :(

Thank you very much,
Marco
 
I don't think you should call your questions stupid. They are just questions to help you through your endeavor into electronics.

All the best,
 
donzo93 said:
This question was even more stupid than the one on the common mode filter... 
Thank you very much,
Marco

I don't get what was stupid about the CM filter question.
It all depends on the details - stray / parasitic reactances but some degree of CM filter never goes amiss with switching supplies ime.
 
Phantom doesn't draw that much current so you will have a big RC filter in which case there's probably no need for a choke. I only say "probably" because it might be desirable to keep high frequency noise from radiating out into nearby sensitive circuitry. But if you put the module in the back corner and maybe even also put the 48V RC filter right next to it (so that the noise isn't on the wire delivering 48V to the RC filter), it would almost certainly not be a problem.

Now for a general purpose supply that can make multiple voltages and supply tons of current, common mode choke + cap filters after quality ACDC SMPS in a separate enclosure is going to be a really solid and very quiet supply that can power multiple pieces of gear.

Although, someone here once said that the leakage inductance of transformers in good ACDC converters is so low that a regular choke would work fine too. It doesn't need to be common mode.
 
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