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How to filter DC-DC converter noise?


Kchula-Rrit

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A couple of months ago I adapted my main TI to run on batteries.  I used a 21V battery pack that feeds a couple of DC-DC converters.  One of them puts out +12V, and the other puts out +5V, along with -5V.  It seemed to work great for a bit, but the F18A video output started to change colors, so I immediately turned the power off.  Haven't started it since.

 

Before connecting the TI, I checked the converter outputs.  The 5V seemed a little low, with about 150mV ripple on it.  The converter data sheet suggested 100 microfarads would filter the output.  Being a firm believer in over-building, I put 200 microfarads on the +5V line, but still have the ripple.

 

My experience with these things is rather limited, so I'll have to ask the more-experienced (or at least less "rusty") than me how to flatten out this ripple.

 

K-R.

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I does depend on the DC-DC converters you're using I suppose; after my TI power board decided to quit I adapted it to use those cheap Ebay LM2596 modules (the ones you can adjust with a 10 turn pot) for +12V and +5V and some other type (have to check) for -5V. All had quite a bit of ripple to it (>200mV from memory) so I added an inductor and a low ESR elco to each output. Now the ripple is within 50mV for all of them and the TI seems to be happy (don't have a F18A though).

 

The +5V gets to ~40C with a small custom heatsink, +12V and -5V stay cool; better still the TI stays quite a bit cooler too!

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I was thinking of using an inductor, but figured it might ring or oscillate.  I'm a major-league worry-wart.  Looks like I need to get some inductors and do some experiments.  Do you remember, or have a rough idea, of the inductance you used?  Also, I assume that "elco" is an electrolytic capacitor.

 

The converters I'm using are from CUT, ordered from Digi-Key.

 

K-R.

 

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1 hour ago, GDMike said:

The PEB transformer allows 110-240 vac in and makes sure 110 vac is out to the pcb power supply, so can't I just connect 110vac line in and bypass the transformer?

 

No! Not unless you want a big bang, puffs of smoke, and possibly an extended visit to your favourite deity. The PEB transformer output to the power supply PCB is at most around 20 Vac. You can use different connections on the transformer for 110 or 240 Vac input, but the output voltage is far lower.

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34 minutes ago, Stuart said:

No! Not unless you want a big bang, puffs of smoke, and possibly an extended visit to your favourite deity. The PEB transformer output to the power supply PCB is at most around 20 Vac. You can use different connections on the transformer for 110 or 240 Vac input, but the output voltage is far lower.

I'm needing a general purpose power supply for this. To run a geneve computer + floppy controller + gotek + half height floppy drive.

Because the original transformer went out and I dug for a fuse and never found one,so I tossed out the transformer.

I suppose I'll need a fan too, usually a PS comes with a fan.

 

 

 

 

 

IMG_20210202_142811674.jpg

Edited by GDMike
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On 2/1/2021 at 9:22 PM, Kchula-Rrit said:

Thanks for the picture.  Those blue things with the stripes are inductors?  I have some of those in my parts box; thought they were resistors.  Boy, I have been out of touch...

 

K-R.

And those switches, the power on / off, I've noticed, and I haven't heard a lot about them, but on all my computers those switches wear and wear easily. On one computer the switch is so flimsy that it slides to off on its own. I'll have to look for a better type of switch soon.

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  • 2 weeks later...

An update...

 

I bought some 4 Amp, 2.2mH inductors and 470uF capacitors and made up a pi-filter (upper circuit in the picture below) on a breadboard.  That reduced the noise on the +/- 5V lines coming out of the converter from about 400mV peak-to-peak, to about 50mV p-p.  The noise did not appreciably increase when I removed the output-side capacitors to make the circuit in the bottom of the picture.  Since the computer has filter/bypass caps on the boards, I'm thinking the output capacitors may be redundant anyway.

 

Another thing I did was to accidentally let the batteries discharge to the point where they would not run the converters.  The data sheet says they need a minimum of 9V to run.  The power supply I used was 19V at 2.5A, and the charger was sucking all the current it could supply.  It got pretty warm to the touch!  More disturbing was the amount of power-supply noise that got passed through the charger to show up in the converter output.  The noise dropped off quite a bit after the batteries were topped-up.  I may need to put a filter on the converter input, or maybe the charger input.

 

Filters.thumb.jpg.e2b559ccf1847cf13f35de10121de1dd.jpg

 

The proto-boards I ordered to mount the parts should arrive in the next week, so I'll find out if it really works.

 

K-R.

 

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  • 1 month later...

It took a while, but I finally solved the problem! (I think).  The above schematic is not quite correct, as the DC converter data sheets advised a max of 100uF on the converter inputs to get them to start.

Supply noise is down to 100mV no-load.

While I had the supply torn apart, I took the opportunity to beef-up the wiring from 22 gauge to 18, and added a 100uF cap on the input to the chargers to dampen out possible noise from the power brick.

 

K-R.

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