SMPS chip with undocumented K factor

[Simon]

I have found this cheapish SWPS controller with integroted switch: https://www.mouser.co.uk/ProductDetail/Monolithic-Power-Systems-MPS/MP9486AGN-Z?qs=sGAEpiMZZMtitjHzVIkrqZzDzZChF5ZshnSD7svEkTwgJMOjvIusFQ==

On Page 13 bottom left the inductor formula has a "K" which can be 0.15 - 0.85, that's quite some range. I do as it happen need 5V and it will only be up to 150mA but the reference design is for 1A so I can probably make some adjustments.

The application notes talked about in the datasheet don't seem to exist.

[Simon]

So K is the ripple current factor with respect to the output current?

I am looking to run 12-28V input and while this is not intended for military I do want decent EMC and would have to pass automotive.

[Simon]

ZCS?

No I am not worried about the discotinuos mode. I once made a SMPS with discrete componets that was in dicontinuos mode and as far as I know it passed EMC.

Why is a bigger inductor bad? If i use a shielded inductor it will have low immisions anyway.

[T3sl4co1l]

I did a TPS40210* SEPIC, in DCM, and it passed almost every automotive EMC test.  Layout and filtering is far more critical than operating mode.

The one test that didn't pass, was an extremely sensitive electric near-field test, which picked up a few microvolts from the switch node.  A little metal over top is all that was needed.  We ended up with a proper RF shield can for production, though.

*Do not recommend. The maximum setpoint is above the current fault threshold. Retarded design. Workaround: remove soft-start cap so that, when it faults during startup, it still delivers enough average current to get going (a few 100 mA).  Have a thread on TI E2E that's characteristically unsatisfactory, documenting this behavior.

Tim

[T3sl4co1l]

Why is a bigger inductor bad? If i use a shielded inductor it will have low immisions anyway.

Expanding on my post further -- a magnetically shielded inductor may have lower stray magnetic field, but they never make them with the core grounded (even the ones with a mounting tab -- it's just floating in plasic), so the electric field from the core is basically just the switch node.

In my case, the transistor and SEPIC coupling cap and diode were all the active switch node, but the inductor is as big as all of them combined, so contributes significantly to the near field.

Tim

[Simon]

I'm thinking of just using a DC/DC: https://www.mouser.co.uk/ProductDetail/XP-Power/SRH05S05?qs=sGAEpiMZZMsc0tfZmXiUnQ%252BwKZhbvwnuh2Xt7v90y77VgETNir%2F5Hg%3D%3D
has everthing in it and has passet tests.

[Simon]

although it is 3x the cost of the other controller and still needs external filtering. Like you say I can just put a can over it all.

[Simon]

presumably i want my inductors resonant frequency to be above my switching frequency or it won't me much of an inductor anymore?

[T3sl4co1l]

Yes.

You'll need a stupendously large inductor to run into problems with SRF at most switching frequencies I think.

Tim

[Simon]

Yes, I worried myself breifly as i calculated that for a variety of input voltages and loads 1mH was about right at 100KHz, I then forgot the 100KHz and saw that it has a self resonant frequency of "just" 1.8MHz. Then I calculated an inductor for 1MHz and it was 330µH, I found an inductor and it had a resonant frequency of 6MHz, so yea there should be no problem.

[T3sl4co1l]

Can go lower, no problem. It's not like you have to worry about heat rise when the total output is under a watt.

Tim