Trying to build a 12V-to-5V DC-DC Buck converter

Sorry for posting in English because I'm poor at German...Hope someone can help me..I'm trying to build a 12V-to-5V DC-DC Buck converter using the LM2576 IC(http://www.kynix.com/Detail/861148/LM2576.html).

This is the circuit recommended in the datasheet:



I've built it (without the optional ouput ripple filter) in a perfboard.

Now I'm doing some tests in it, and it works well when I draw little current from the output. For example, with a 40 Ohm test load, y get an output of 5V and 0.125 Amps, as expected.

But as I test the converter with smaller resistors as loads (20 ohm, 10 ohm, 5 ohm, etc) the output voltage drops from 5V to 4V, 3V, etc., till eventually I get an output voltage of 0V and no current! And also, as the output voltage drops, I start noticing a high frequency sound coming from the converter (from the inductor, I believe).

I don't really have a clue about what might causing this problem. The datasheet of the LM2576 IC says the IC can put up with up to 3 Amps.

Could it be a inductor problem? I've read that the inductor might saturate with high current. But I think that in case of saturation, I would have a ripple problem, not a current problem, am I right?

So, what could be the problem? Thanks if someone can help me!

I don't want to discourage you but there a many posts on elecronics boards all over the internet. People which tried to just quickly build a DC/DC converter and in the end had to realize it's not that easy.
Such circuits involve high switching frequencies and with it trouble heads your way. At least if you do not have the proper knowledge in such designs and the proper measuring equipment (most important!).

Usually such circuits are often problematic and without the proper measuring equipment. To diagnose such a circuit you would need at least a LCR meter which can also measure ESR and an oscilloscope. It can work if you are lucky but often is does not as in your and many other cases.

What you are acustically experiencing when increasing the load on the circuit is most likely an instabillity. It starts to oscillate. Try to check what I have written below and you might succeed.


So what could we do to bring your circuit to real life?

*) Adhere to the design guidelines which are mentioned in the datasheet of the IC -->

- All PCB traces as short as possible
- Use of low ESR capacitors (e.g. Panasonic FC); but ESR not below 0,03 Ohm or you could get instability! Choose for 5V output voltage a capacitory with min. 10V DC. For instance Panasonic FM type would not be qualified as its ESR value is well below 0,03 Ohm.
- Use an appropriate inductor as mentioned in the datasheet. Did you do this? Not every inductor with the right value is also capable to handle high switching frequencies!

Finally there is also the slight possibillity you have damaged the IC during soldering. If you have a second one try to exchange it.

Best regards
using_e