DC to DC buck converter inductor question. Square wave on output.

[Chris Wilson]

I have built a DC to DC buck converter on a breadboard, so none optimized lead lengths. I used the circuit attached. The chart, also attached, for deducing inductor values for 50V in and 24V out do not correlates though. I used what i have on the breadboard, a 680uH and a 1000uH. The 680uH gives the waveform superimposed on the output as in the scope photo also attached. I am using a x10 probe and the scope is not compensating for that, so multiply the levels by 10 please. Using the 1000uH I see a diminishing ringing sine wave also superimposed. How critical is inductor value, and what do people recommend I try? The 680uH is wound on a ferrite rod core, and I could remove some turns. It's rated at 1.5 amps DC. The 1000uH is wound on a toroidal core. I am seeing the square wave with a 50 Ohm load resistor across the output so about 0.5 Amps being drawn. That is about the maximum i expect the circuit that needs 24V to draw. I am not using the optional ripple filter. Thanks.

[rs20]

So you have 20 volts peak-to-peak ripple? That's not really ripple anymore, that's just a raw switching waveform. Are you sure you're probing the correct part of the circuit? (Ground clip and probe leads either side of the 2000uF capacitor, after the inductor?)

[tautech]

Often IC manufacturers have recommended inductors (brands/models) for various reasons including frequency and current ratings and you can "roll your own" inductors but saturation and efficiency also need be considered and optimised in a number of ways.
Current probes and shunts can be handy to check the relationship between on times, frequency and saturation.
Furthermore efficiency checks need current measurements (power analysis).

If you're fortunate enough to have something that works well enough and componentry is not heating you might just go with it. 

[Chris Wilson]

I am an idiot, I had the probe clipped on the diode leg, the breadboard has 2 other circuits I wanted to keep upon it and it's all very tight. Scrub the above, the output looks fine given lead lengths and board capacities and stuff, sorry!!!!

[damn_dirty_ape]

If you're doing this to learn, try to take time to understand the two different operating modes of almost all dc-dc converters.  You can be in either continuous or discontinuous conduction modes.  If you increase the resistance high enough you should hit discontinuous, which behaves a bit differently on the scope.

https://en.wikipedia.org/wiki/Buck_converter#Discontinuous_mode