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[on:]

I am trying to build a 5v 3amp switching regulator and things keep going awry. My latest failure involves an inductor that is obviously scorched (for the second time now). The voltage now oscillates between 5 and 6 volts. Before I go pop in another inductor, I was wondering if someone could clue me in to some of the pitfalls of this circuitry. What might be causing the inductor to burn up? Also, D1 is a pretty hefty diode which looks suitable for a three amp circuit. To contrary, the 100uH inductor (which I am assuming handles the load output) has some fairly thin leads- seems suspect to me. I am slightly ashamed to say but for a circuit that costs about $0.75, I have destroyed more than $2 in parts trying to get it right.  Any suggestions ? (besides using a 7805)
   

[Reply #1 on:]

You need to know the rated value of the inductor, that will tell you how much current you can take. yes it takes all the current. It needs to be rated so your maximum current is not more than 70% of the rating.

Switch mode power supplies are very sensitive to circuit layout, without a good PCB it is almost impossible to get a reproducible circuit. Even with a good PCB layout I have seen many professional engineers take two or three goes at getting it stable under all current draws.

That is why you are better off buying a ready built module for this sort of thing.

[Reply #2 on:]

Are you following this?

C. Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in Figure 3. Part numbers
are listed for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2576 switching frequency (52
kHz) and for a current rating of 1.15 × ILOAD. For additional inductor
information, see the inductor section in the Application Hints section
of this data sheet.

If you are smoking the inductors, I would hazard to guess you have not selected a hefty enough rated part.

[Reply #3 on:]

Wow, I didn't even take a look at the rest of that data sheet. It makes a little more sense now.

[Reply #4 on:]

This link will help you determine the relavant values.
http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equations.phtml

For Buck Converters, the Inductor must be able to handle the peak switching current without saturating.
The Peak Switching current is higher than the load current.

[Reply #5 on:]

You'll get better efficiency if you leave more of a margin - partly due to lower copper losses, partly due to smaller hysteresis loop.

[Reply #6 on:]

I am incredibly lazy, I just find one that works as I like then "Copy It"... Works every time. Even my first very first bread boarded switchers all worked. There is a "Suggested" layout from almost every Mfr that is a great guide... just don't deviate too far.

Bob

[Reply #7 on:]

I have built a couple of boards with the LM2576. I used a 3A Schottky diode and an inductor rated for 3A. The output capacitor also needs to be of the low ESR type. I'm using some expensive Panasonic capacitors. Even with a 2A load the voltage hardly fluctuates, I measured a peak to peak voltage of 240mV with my oscilloscope. That's within spec.

[Reply #8 on:]

Any suggestions for where to find said inductor? Shipping from Mouser is like 3-4 times the cost of the part itself. I think Digikey is even more. I found some inductors on ebay but they skimp on specs. Docedison, I am about 30 minutes from Anahiem; do you have local place you order from?

[Reply #9 on:]

Any suggestions for where to find said inductor? Shipping from Mouser is like 3-4 times the cost of the part itself. I think Digikey is even more. I found some inductors on ebay but they skimp on specs. Docedison, I am about 30 minutes from Anahiem; do you have local place you order from?

Well for $1.53 and free shipping you could buy one of the below and pull the inductor from it for your regulator. Or would you find that much too ironic? 

http://www.ebay.com/itm/251066005460?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Lefty

[Reply #10 on:]

I bought some through the hole 3A 100µH inductors on Ebay.

[Reply #11 on:]

Any suggestions for where to find said inductor? Shipping from Mouser is like 3-4 times the cost of the part itself. I think Digikey is even more. I found some inductors on ebay but they skimp on specs. Docedison, I am about 30 minutes far from Anahiem; do you have local place you order from?

I wonder if anyone sells a good inductor kit (like you can find for resistors and capacitors).  If I was going to be working heavily with inductors I would be thinking about getting a wide-range value kit for experimentation purposes just like the resistor and cap kits that I already have - three big ones each for electrolyic/ceramic/polyester(my newest) and 1 watt, 1/4 watt, and 1/2 watt resistors (Joe Knows) and a couple of bulk packs too.