Topic: Power Isolation Transformers - Any Recommendations?

[H₂O]

I am think about building a DC-DC Converter with Isolated output.   Does anyone have some recommendations on some 4-6 coil transformers to use in this application?  I found a few but they are all surface mount - any other available packages?

http://www.cooperbussmann.com/3/CoiltronicsFinder.html

[H₂O]

Bump

[H₂O]

I am trying to ground isolate several devices after lowering the voltage with a prebuilt non-isolated DC-DC converter. 

I am running into some ground loop issues using a single power source with common ground.

The reason I am looking for 4-6 coils is provide power to two devices (possibly 3). 

I was thinking For 2 devices 4 should work with  2 coils for input in parallel and 1 coil for each device; for 3 devices 3 devices in parallel for input and 1 coil each for devices.

Thoughts?

[H₂O]

That's not true!

[H₂O]

The issue I was running too was when using a Linear Regulator attached to a single 15.5V battery and connected to two different 5V sources.  One of the 5V sources was always on regardless of the power switch setting when the signal cable (i.e. 1/8" stereo -> 2 1/8" monos) was connected between the two devices.


I am getting a sample of a DC-DC converter that has true power isolation - but this is an expensive part (about $75) and only has a single output - and was only able to get 1 for free.

I am looking into getting this companies other DC-DC converters that are not isolated but cost substantually less ($12-$20 a piece) and have higher efficiencies (up to 97%).  The thought was to take the cheaper converter make it's 5V out into 2 isolated 5V outs.

[Church-Audio]

Well, I don't know what to tell you there, but if you want isolated power, you'd need a transformer where there is AC.  Probably the isolated converter chops up incoming DC to AC (probably at a high frequency), sends it through a transformer, and then rectifies it.  You could feed the isolated converter from the non-isolated converter, but you might as well just feed one non-isolated converter and one isolated converter direct from the battery, as the efficiency will probably be higher.

I don't have enough information to guess if that will solve your problem or not.  I certain don't recommend using a linear regulator to drop 2/3 of your voltage, but if you feed two devices from the same linear regulator, you have a noise problem that you can isolate to the power supply?  If this is 50/60Hz hum, and you are running off of batteries, it's interference, not a ground loop, plain and simple.  The cause of that must be inadequate shielding and/or unbalanced connections, or just maybe really bad interference.

Anyway, do NOT connect one of those transformers you linked to any DC converter or battery, as the DC resistance of their windings is less than 0.1 ohm . . . that may do bad things to the converters, the battery, and/or the transformer.

Don't forget about the switching noise that you will get if your not careful with a dc to dc converter unless the switching freq is very high it could get passed into the audio band... I would love to see a dc transformer um but like you have said they do not exist.

[H₂O]

The switching frequency of the DC-DC converter that's coming in the mail is a min of 300Khz and max of 380Khz - so I think I should be covered - right?

[SparkE!]

The switching frequency of the DC-DC converter that's coming in the mail is a min of 300Khz and max of 380Khz - so I think I should be covered - right?

Only to the degree that the anti-aliasing filter ahead of your A/D conversion actually attenuates a 300 kHz signal.  You'd only get about 22 or 23 dB of attenuation at 300 kHz per pole used in the anti-aliasing filter if the cutoff frequency was 20 kHz.  That's one reason that switching supplies are so hard to use in audio work.  I'm not saying it can't be done, but you do have to be careful to reduce your ripple enough that your anti-aliasing filter does enough good to keep the ripple inaudible.  (Remember that higher frequencies can alias back into the audible portion of the spectrum if not sufficiently reduced in magnitude before the signal hits the A/D in your recorder.)

[H₂O]

Is there a good way to see this noise?  (i.e. spectrum anaylsis of a recording of silence on the recorder or an ocilloscope)

I haven't used an ocilloscope in almost 15 years when I was in school, and obviously I am very rusty on the EE portions of my degree.

Would the noise be consistent with use of converter or inconsistent throughout usage?


The 300-380Khz was the freq of the non-isolated DC-DC converter I was looking at.  The isolated one is running at a typical 400khz according to the spec sheet.

[SparkE!]

The easiest way to see the interference is to record silence coming from a device that is powered by the power supply and look at the output on a spectrum analyzer or do a spectrum analysis on the digital recording.  You'll see a narrow bump or a spike in the noise floor if there is a significant source of interference due to the use of a switching supply.

[H₂O]

So I am now looking at filtering, and based on the spec sheets provided by the manufacturers, they are only showing filtering on the input voltage and not the output.   Is this all you need or do you also need filtering on the outputs?   

On the Isolated Converter they do show 1nF CAPs (rated at 2kV) between the In's and Out's.   Would this provide some output filtering?

Should ripple be detectable on both ins/outs of either/both isolated/non-isolated DC-DC Converters?
 

[H₂O]

The suggested input filters are shown in the attached pics.

The isolated values are:
C1 = 3.3uF/50V
C2 = Not Required
Inductors = 325uH/3.3A
C3/4 = 1nF/2kV

Would a similar LC circuit be required on the output for both?

[H₂O]

Actually the inductor part for the isolated (as the converter manufacturer makes a dual coil part specifically for the converter) is pretty small, but the coils I found for the non-isolated converter (from a different manufacturer) is pretty big at about 6/10 on an inch in diameter.  since you need two you end up with a pretty big layout.

The non-isolated link is:
http://www.recom-international.com/pdf/Innoline-2008/R-78Bxx-1.5_L.pdf

The isolated is at:
http://www.recom-international.com/pdf/Powerline-DC-DC-2008/RP12_AW.pdf

The manufacturer's application guide where I got the input filter specs from is:
http://www.recom-international.com/pdf/RECOM-Application-Notes-2008.pdf