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I'm reviewing the schematics for an evaluation board, which uses an LC filter on the DC input as shown here:

enter image description here

PWR_IN is defined for the evaluation board as being 7-27V. The issue I'm facing, is where the evaluation board is defined for current draw less than 7A, my device is designed for up to 15A draw (external 15A fuse on the PWR_IN positive lead). Ferrite beads aren't typically available above 10/10.5A rating, so I need an alternative means for providing a similar level of EMI filtering.

Can I put 2 10A ferrite beads in parallel on each of the positive and negative traces, or would a dedicated automotive EMI IC (such as the BNX02301L) be the best option?

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    \$\begingroup\$ Where did this terrible circuit come from? Context would help. \$\endgroup\$ – Techydude Sep 15 '18 at 6:45
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    \$\begingroup\$ It's from the Toradex Ixora schematic. \$\endgroup\$ – jars121 Sep 15 '18 at 6:49
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This is not a job for ferrite beads. What you're looking for here is a common-mode choke.

This random google result is what I have in mind: enter image description here

I'd relocate C3 (in your/Toradex' circuit) to be adjacent to the CM-choke.

If this is a automotive 12V-battery application, then that SMAJ-series TVS might not be up to the job of dealing with cranking, jump-starting, & load-dump events, so perhaps look into re-spec’ing that.

I'm guessing D2 is supposed to be reverse-voltage protection? But it's only rated for 3A, so guess who might pop first: D2, or the 7A fuse? lol. Either re-spec it or get rid of it.

C4, C5, C6 are 'yeah, whatever' - depends on what's being driven. You really need to do a proper impedance analysis of what kind of noise you're trying to protect against, what your regulator(s) rejection-ratio is, and what, if anything, in the powered circuit actually cares for this level of clean power.

Just replace those silly ferrite beads with a proper wire-wound common-mode choke, whose continuous DC current exceeds your expected load (15A). Yes, CM-chokes at 15A will be MUCH bigger than a pair of pissy little pointless ferrite beads.

I'd be wary about adding LF2 (from the circuit I've pasted in) without further careful consideration of what's downstream (load). Many switch-mode regulators/PMICs won't like significant amounts of L on their input like that, unless there's plenty more C on the load-side of LF2.

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  • \$\begingroup\$ Common mode choke was going to be my very next thought, you beat me to it! Everything else you've posted makes sense. The downstream PMIC is a TPS51120, which, as you stated, would probably do better without LF2. As for D1, D2, etc., they have all been rated appropriately for my particular circuit. Thanks for your valued input! \$\endgroup\$ – jars121 Sep 15 '18 at 7:43
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    \$\begingroup\$ While the alternator voltage may not go much higher than 14V, you will regularly see transients of 60V+ on a standard automobile 12V bus due to load dumps (like turning on and off the headlights). Pick a higher voltage (50V+) diode / find a dedicated automotive protection diode that has done the work of being ISO 16750 compliant for you. You can't drop the voltage rating as suggested, unfortunately. \$\endgroup\$ – Krunal Desai Sep 15 '18 at 8:58
  • \$\begingroup\$ I've currently specified the TPSMC30CA as D1 (in the original schematic), which doesn't carry the ISO 16750 certification; would this still be a suitable option @KrunalDesai? \$\endgroup\$ – jars121 Sep 15 '18 at 11:07
  • \$\begingroup\$ actually @jars121 yes, Kaunal’s quite right. automotive: cranking, jump-starting, & load-dump events, all put way more than nominal recharge voltage on the line, and for relatively long periods that could pop a SMAJ-series TVS. \$\endgroup\$ – Techydude Sep 15 '18 at 16:32
  • \$\begingroup\$ Thanks @Techydude. I've opted for TPSMC20CA for D1 (Vc = 27.7V, which is just below my PMICs Vin(max) of 30VDC. Would the Eaton FPT705-200-R coupled inductor be a good choice? It complies with the current requirements, but it's inductance seems really low (200nH). The layout diagram in the datasheet doesn't make any sense either... \$\endgroup\$ – jars121 Sep 16 '18 at 0:10

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