EMI with a switching regulator

kevin101

I do not have much knowledge about how magnetic fields affects a circuit, except that it is usually bad. With this in mind I have a 3 amp switcing regulator( LM2576T-ADJNS-ND) positioned in close proxinity to a propeller (1-2 Inches, DIP prop). The load would not be constant and would have have relatively large peaks. I have not tested my setup with a prop and now I am worried about the flux of the inductor scrambling the EEPROM signal or anything else. Would the inductor be a problem?

Much appreciated,
Kevin

Peter Jakacki

Never heard of a strong magnetic field scrambling an EEPROM, where did you get that idea? (come on, we are not the general-public). The EEPROM holds it's information as charges on floating gates which are not affected by magnetic fields. Anyway the field is very weak and is in fact a electromagnetic field. If there is going to be a problem then it would likely be with current spikes and noise if the pcb has not been laid out properly. Industrial designs use switching regulators all the time.

Also, when it comes to switching regulators the 2576 is a rather low-frequency device which requires larger inductors and capacitors. Surely there must be others available, even the LM2677 handles 5As and only needs a 22uh inductor and is more efficient (less heat). Transient response is also better with higher frequency devices.

*Peter*

kevin101

Um, the pc board is probably not been laid out properly because its on a perf board. I am mostly worried about the propeller itself. My programs already have problems and I want to avoid more.

Tubular

Kevin, tell us about the load you're switching. Is it audio? lamps? inductive relay coils? How much current when they're steady on?

Generally its just big inrush currents from inductive loads that might cause trouble. There are many simple things you can do with bypass capacitors and diodes to keep these in check. But for general low voltage circuits taking 1 or 2 amps and not switching inductive loads you can go as close to the prop as you like.

kevin101

I am running 100 RGB leds off the switcher. The way I have them configured, they could rapidly turn on and off very fast. For the scrambling the eeprom thing, I meant the signal going between the prop and eeprom.

stevenmess2004

I would just make sure that you have some capacitors between the supply leads nice and close to your propeller chip. This should solve any potential problems.

Joerg

If you use a closed inductor (not just a coil with a ferrite inside) i see not much problems caused by the magnetic flux.
The simple switchers do normally cause trouble if you make sure to have a minimal load current. If you tell me the values of the components used in the design i will calculate the minimum current. (it is not difficult, but since i have some experience it will be a fast task!)


Saluti Joerg

kevin101

I do have an input capacitor pretty close to the propeller already, and it is a closed inductor. The component values are...

LM2576T-ADJNS-ND
390 uf output capacitor
100 uf input capacitor
47 uH 10khz 3A inductor (digikey 257-1094-ND)
4A diode
r1=2.26K
r2=4.53k

hope this helps

Joerg

With your values and a guessed input voltage of 12V your minimum current is 0.5A! This seems quite high to me. I would suggest to use a higher inductance value.


The formula is: I(minimum) = (Uin -Uout)*Uout/2/Uin/f/L

The regulator will work with lower currents than Imin, but in the so called discontinued mode which causes a lot of ringing and therefore needs a better output filter.

Saluti Joerg

kevin101

The input would be 5V from an old computer power supply that I have laying around. This should decrease the minimun current. I think thats right. If theres still too much ringing, would I just have to add some more capacitors or a combination of inductors and capacitors?

Thanks for the help,
Kevin

Joerg

Hello

With 5V the minimum current will be around 250mA. For lower output noise you can add some more capacitors, since it is always a good idea to connect some in parallel to lower the inner resistance of the output filter (called ESR). If you add a inductor and some more capacitors you get the best results.
I think a 100uH inductor (or bigger) instead of the 47uH inductor would give a even better result.

Having looked up the datasheet i noted that for the chosen output of 3.7V the 5V input voltage is rather LOW, since for 3A current the saturation voltage of the chip is specified with 1.4V so in theory you need an input voltage of at least 5.1V!!

Saluti Joerg

kevin101

Hi,

I can definately add more capacitors to the output to reduce noise, but I can't change inductors, not in the budget. Also, the power supply that I have isn't very good... The 5V side measures 5.7V under load and the 12V side measures 10.4V under load. Both are still better than an actual 5V or 12V supply, as they are better voltages for the project.

Paul Baker

Hi Joerg,
I am in the process of designing the switching supplies for the next Parallax Oscilloscope. The formula you provide for minimum current, I assume that U is voltage potential (what we Yanks would label as V or E), is this correct? Is the formula applicable to all topologies or just a subset? I have been leaning towards using the TL497 because it can be used for all voltages needed (3 boost, 1 buck and 2 inverting) and is the cheapest when including the cost of passives. However the datasheet states it's a fixed on time/variable frequency setup and does not provide an oscillation frequency (obviously). How would I determine what the lowest current is for a given setup?

Also do you have a formula for efficiency? Datasheets typically give a number, but don't show how it was calculated. I found them on the net a few months back, but I haven't been able to seperate the wheat from the chaff in my searches when I tried to find them again.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 3/25/2008 1:06:07 AM GMT

Joerg

Dear Paul
You are right the U is a American V !!! (i am used to Swiss/German literature so please apologise!)
The formula is valid for step down converters and does not include any loses (for simplifying the formula). I have done a lot of calculations for switched mode power supplies and the drafts are just waiting for being put into one of my computers!!
The formula for minimum current for step up converters can be found in this document:

www.systech-gmbh.ch/pdfElektronik/usb-wandler1-11.pdf

The IC i use often is the LM3578 which allows also all the configurations, is low cost and available in different packages (SMD AND DIL). It is also an oldie like the TL497 (i have use it about 25 years ago in a EPROM programmer) but still on market.

If you mail me the schematics i do the formulas for you since the datasheets are mostly no big help and the normal literature is much to sophisticated (at least for daily use).

Let me know!

Saluti Joerg

to Kevin: if you use the 5V side of your PS it will be OK. I am always sceptical on power supplies dimensioning because i had to resolve too much problems caused be bad dimensioned power sources in my life (50 years - 1 day).

Paul Baker

Thanks Joerg, I'll probably take you up on your offer.·I'll give you schematics after I've drawn them out next week.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 3/26/2008 1:29:36 AM GMT