I'm trying to build a 12V-to-5V DC-DC Buck converter using the LM2576 IC.

This is the circuit recommended in the datasheet:

enter image description here

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?

  • \$\begingroup\$ With 20ohm i would expect current of 250mA, not 125mA \$\endgroup\$ – Gregory Kornblum Mar 8 '18 at 2:48
  • \$\begingroup\$ Sorry, I meant 40 ohm load. I'll edit it. \$\endgroup\$ – user246185 Mar 8 '18 at 2:49
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    \$\begingroup\$ Provide a photo of the perfboard, and highlite the various wiring inductances at pin 1, pin 2, and pin3. Also estimate the inductance (at 1nH per millimeter) in the GND path between bottom of Cin and bottom of Cout. and bottom of the diode. \$\endgroup\$ – analogsystemsrf Mar 8 '18 at 3:18
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    \$\begingroup\$ Perfboards are not usually conducive to good performance from switching power supplies. The layout is critical and parasitic elements can cause erratic behavior. \$\endgroup\$ – John D Mar 8 '18 at 4:03
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    \$\begingroup\$ What was the DCR of L1? < 50 Ohm ?? ESR of Cout? \$\endgroup\$ – Sunnyskyguy EE75 Mar 8 '18 at 4:06

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