thru hole vs smd inductors in high current low-pass filter

[sifu0832fjcm2w3fd]

Hello everyone,

I'd like to pick your expertise on inductors in the output low-pass filter of a DC-DC regulator scenario.

Is there any difference / advantage in using thru hole vs smd inductors? I DO produce high quality square wave that's being fed to LC but after the inductor I experience some instability. The circuit does not get hot, nor the interference is produced by the load. I'm set to operate around 500-600kHz and I draw up to 3.5A off my design. That makes inductor's current ~3.5A rms and ~4A peak. My calculations point to minimum ~6uH.

Back of an envelope: 5uH + 50uF -> ~10kHz

Now. At a first glance the answer is simple - as long as the parameters of the part are the same - there is no difference, right? Egh... but real life is some apart from theory. That's why I'm asking for a quick look in your history of experience. Is "add 30% above what you calculate (inductors have 10-30% tolerance) and you should be fine" the end of it?

Before anyone asks - yes, I did build the PCB circuit and already testing the measurements in it's target environment. And yes, I am using low ESR cap.

At this moment I'm more working on improving the design then making it work.

Thanks for your input.
Patrick.

[tggzzz]

By "instability" I presume you mean noise or oscillation. What frequency is any oscillation? What repetition interval is it?

Consider multiple capacitors in parallel, smaller capacitor -> better RF response.

Consider ferrites to increase high-frequency losses.

Consider common-mode inductors.

Ensure inductors aren't saturating - try a physically larger inductor.

Ensure grounding (or lack of it) isn't causing a measurement problem.

Lower ESR -> reduced ripple amplitude but increased ripple duration, since less energy is dissipated in the resistor.

[f5r5e5d]

Ensure inductors aren't saturating - try a physically larger inductor.

rather - read the data sheet for saturation performance - there's so much variation in materials and construction that "size" isn't your best guide - only applies to the same technology/material/construction all held constant

and read the datasheets in detail since the manu do "game" the specs - one may give "operating current" at 20% L reduction from saturation another 50%

[tggzzz]

Ensure inductors aren't saturating - try a physically larger inductor.

rather - read the data sheet for saturation performance - there's so much variation in materials and construction that "size" isn't your best guide - only applies to the same technology/material/construction all held constant

Agreed, but size is a good starting point for someone that is inexperienced in "interpreting" manufacturer's data sheets (or do I mean interpolating missing information?)

and read the datasheets in detail since the manu do "game" the specs - one may give "operating current" at 20% L reduction from saturation another 50%

Just so. See my sig for other examples

[TMM]

As above, the core of the smd inductor is probably saturating. If you need to keep the design low profile try multiple smd inductors of the same core size but lower inductance each in series. Watch out for excessive ESR/DCR though.

[sifu0832fjcm2w3fd]

Thanks everyone for your responses. Much appreciated and some great advice.

Patrick