Jump to content
Kchula-Rrit

How to filter DC-DC converter noise?

Recommended Posts

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.

Share this post


Link to post
Share on other sites

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!

  • Like 1

Share this post


Link to post
Share on other sites

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.

 

Share this post


Link to post
Share on other sites

See attached pic; the 2.2uH in parallel must mean I ran out of 1uH 🙂 Values are based on testing with various inductors, a capacitor decade box and a scope. Low ESR turned out to be essential.

20210202_100123.jpg

  • Like 1

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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?

IMG_20210202_075831028.jpg

Share this post


Link to post
Share on other sites
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.

  • Thanks 1

Share this post


Link to post
Share on other sites

Sorry, I don't have the old transformer anymore, but I was just told that there was another winding that puts out 18vac as well ...I forgot that..oookayy.

Thank you

 

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites
21 hours ago, jbourke said:

They are green but yes they look like 1W resistors 😊

Great!  Now I have to get an inductance meter and, probably, some more toys!  ;-)

 

K-R.

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

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.

 

  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...