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Adom Fields

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by adom

Adom Fields: where the current and the heat go on your board.

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adom-fields / skills / adom-fields-emc / SKILL.md
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John Lauer 0.10.1: the skill's topics as installable sub-skills d982a7e

name: adom-fields-emc description: >- Adom Fields EMC, compliance and ESD modules (--with emc, --with esd): hot-loop and cable common-mode radiated emissions, conducted emissions through a LISN, FCC/CISPR limit sets, a compliance prediction with uncertainty bands, the cheapest fixes by dB, a bench plan and bench calibration, connector ESD/TVS checks. Use when asked about EMI, EMC, FCC or CISPR risk, "will it pass the chamber", "radiated emissions", "conducted emissions", "common mode cable", "pre-compliance", "ESD protection", "TVS".

Parent skill: adom-fields

Adom Fields: EMC, compliance and ESD (--with emc, --with esd)

Turn on emc when the board has a switching converter, clocks or cables and the product will face FCC/CISPR testing; esd when a connector faces the outside world (name those connectors in spec external).

The EMC screen (0.7.5)

ui view --quantity emc on a board with a modelled switching converter (see the energy cycle): three checks drawn on the board.

  • The hot loop: input capacitor → the switch's input pin → its ground pin → back to the capacitor, for every input capacitor; the smallest (fastest-edged) capacitor's loop is the red patch, pulsing with the switching, drawn over the parts; a green dashed outline is the smallest loop the switch allows (a capacitor straight across its pins). Areas in mm², in the board's plane between the pads.
  • The reference plane: copper of the loaded and switching nets on the top layer, green where ground copper lies on the next layer, red where it does not (the return current has to detour); the share and mm² per net in the panel.
  • Radiated emissions: the hot loop as a small loop antenna (Ott: E = 1.316·10⁻¹⁴ f² A I / r), its current's harmonics from the trapezoid (duty from the cycle, edge time from the spec's switching.<net>.riseNs, 8 ns by default), 30 MHz to 1 GHz at 3 m against the FCC Part 15 class B limit, with the worst margin and what the smallest loop would gain. A screen, not a chamber: differential mode only; cables (common mode), which often dominate, are not modelled.

Compliance prediction and bench calibration (0.9.5)

fields.json review.compliance puts every emission source the review models into one table against the chosen limit set (emcLimit):

  • The sources: the switching hot loop (loop), each connector's cable (cable:J2), an oscillator's copper (lo:<net>), a receiver's oscillator leaking out of its antenna port (lo-leak:<net>, FCC 15.111), and the conducted emissions (conducted).
  • What each row carries: level, limit, margin, an uncertainty band from the model's own error, and the basis:
    • loop ±6 dB;
    • cable 0 to +20 dB, because the common-mode model is an upper bound;
    • conducted ±4 dB;
    • antenna port ±3 dB.
  • Verdicts per test (radiated, conducted, antenna): likely fail (even the band's best case is over), at risk (the band crosses the limit), likely pass, thin margin (under 6 dB), likely pass.
  • fixes, each with the dB it buys per source:
    • slower switch edge (computed from the edge's envelope at each source's frequency);
    • spread spectrum (from the dither width over the receiver bandwidth: 9 kHz conducted, 120 kHz radiated);
    • a smaller hot loop (from the area ratio);
    • per cable: a clip-on ferrite (about 200 Ω), a board common-mode choke (about 1 kΩ), and moving the connector's ground to the converter's input-capacitor ground;
    • an input filter (sized to 6 dB);
    • for receivers: an RF amplifier's reverse isolation, or weaker LO coupling;
    • a shield can.
  • toSixDb: the cheapest set of those fixes that gets every source to 6 dB of margin, found by searching every combination (free and cable fixes cost less than layout and parts), with the margins after it.
  • benchPlan: how to measure each prediction, cheapest first:
    • conducted: LISN and analyzer, 150 kHz to 30 MHz;
    • each cable: a current clamp, with the predicted current and the current that puts the cable on the limit;
    • the hot loop: an H-field probe, which ranks sources and shows a fix working but gives no dBµV/m;
    • the antenna port: an analyzer.
    • Each step says what to expect and gives the measurements record to fill in.
  • Bench calibration: spec measurements: [{source, fMHz, value, unit}], with unit dBuV (LISN), dBuA (clamp), dBuVm (chamber or pre-scan at 3 m) or dBm (antenna port).
    • The source moves onto the reading, keeping the model's spectral shape and taking the bench's level. Its band narrows to ±2 dB at the measured frequency (±3 dB elsewhere).
    • A cable reading also moves the cables not measured by the same offset, ±4 dB.
    • calibration says what was used, by how much, and why a reading was not used.
  • analyze prints a Hint: Compliance line: each verdict with its worst source and band, and the cheapest set.

On the TPS54202 buck (12 V to 5 V, 1 A, 1 m cables, FCC B):

  • Radiated is at risk: J2's cable is −7.1 to +12.9 dB, while the loop itself is 55 dB under.
  • Conducted is a likely fail: −33.5 dB with no input filter.
  • The cheapest fix set: an input filter, spread spectrum and a 16 ns edge, which gets every source to 6 dB or more.
  • With a clamp reading of 9 dBµA on J2 (9 dB under the bound), radiated becomes "likely pass, thin margin", and the set drops to the input filter plus spread spectrum.

Also in this area

  • Conducted emissions (0.9.2, review.conductedEmissions, conducted-emissions).
    • The converter's trapezoid input current split between its input capacitors (at bias) and any input filter into a LISN (50 Ω ∥ 50 µH), against CISPR 32 / FCC 15.107 class B (or A with emcLimit *-a), 150 kHz to 30 MHz.
    • The fix sizes the input-filter inductor for the attenuation needed.
    • Differential mode only.
  • Emission limits (0.9.0): spec emcLimit picks the set the issues use: fcc-b (default), fcc-a, cispr32-b, cispr32-a, or a custom name. Spec emcLimits adds lines (CISPR 25, MIL-STD-461 RE102, a customer's) as {name, distanceM, points MHz, dBµV/m}. Every emission estimate also reports its margin against every set.
  • Cable emissions (0.9.0, review.cableEmissions, cable-emissions): common mode, the usual chamber failure.
    • The switching loop's harmonic current across the ground copper's inductance between the loop and each connector's ground pin (μ0·h × the solved squares) drives that connector's cable (spec cables {J1: metres}, 1 m) as a monopole.
    • It is an upper bound with no ferrite.