I recently had a question about the layout for an LM2678-ADJ switching regulator. Since, I have built the circuit and layout as follows:
So far, I have only had moderate success with the circuit. With an input voltage of 30V and up to 6A available, everything works well (at least, output voltage constant), up to a current draw of up to about 2.5A. Above that, the voltage drops dramatically and the diode starts getting hot(ter).
The following waveforms were taken at different current draws, with the rheostat set to produce 12V out: Yellow is measured across diode D1, and blue across capacitor C3 at the output on the right.
Current Draw: 0.1A
Current Draw: 2.4A
Current Draw: 2.5A
Current Draw: 3.0A
Aside from the slight ringing at 0.1A, it seems to normalize out to a square waveform by about 0.15A and everything looks well to about 2.4A.
However, at around 2.5A, where the voltage drop occurs, there seem to be "misses" in the waveform, and going further in 0.1A increments to 3.0A, the waveform only shows "misses".
I have read on other threads that the inductor needs to be large and have low resistance.
L1 is a torodial 47 µH with a DCR max = 13.5 mΩ and rated 10A.
D1 is an STPSC10H065D 10A Schottky Diode
I have tried a small ceramic across D1's terminals without success. Also, this thread says ringing should be checked, but there doesn't seem to be any in the above output (except under low/no load conditions).
So finally, my question is very basic: What is causing this, and how can it be fixed?
The datasheets for the components are here:
I have measured the input voltage, and am seeing some nasty feedback/ringing that I think is causing my issue: The blue is the input measured across Input and GND pins on the regulator (AC coupling), and yellow measured on Switch_Output pin (DC coupling)
I have tried:
Adding more input capacitance (to lower RMS)
Adding more output capacitance (to lower RMS)
Trying different Schottky diode
Different inductors as suggested and accepted as answer at first, one 3A 47uH for 255kHz switching regulators, and a 68uH stolen from a DC-DC boost converter. There was no measurable effect.
Resoldering on another prototype board with different layout in case ground plane or whatever on the board is causing this. Effect was about 0.3A more load before breakdown.