Capacitor placement for EMC
Summary¶
A decoupling capacitor absorbs voltage spikes before they propagate as noise. Its effectiveness is determined by how fast it can respond to a spike — which is limited by the inductance of every millimetre of wire between the capacitor and the noise source. Every millimetre of wire or trace adds approximately 1 nH of inductance. At a 50 A/µs current transient, each 1 nH adds 50 mV to the unclamped spike. Capacitors must be soldered directly to the noise source pads with no pigtail wire. A capacitor at the end of a 100 mm lead is nearly useless for fast transients. Two complementary capacitor stages at different positions address different frequency ranges: the 1000 µF electrolytic on the ESC pads for slow large spikes, and 100 µF MLCC ceramics on the FC logic supply for fast small spikes.
Concept¶
Why placement dominates performance¶
A capacitor is a charge reservoir. When a voltage spike occurs on the power bus, the capacitor should supply or absorb charge to hold the voltage steady. But before the capacitor can respond, the spike must travel from the noise source to the capacitor along the connecting wire. That wire is an inductor — it resists changes in current. The inductance of the wire limits how fast the capacitor can respond.
The voltage spike that appears at the unprotected node before the capacitor can respond:
V_spike = L × dI/dt
For a 50 A/µs motor deceleration transient through 1 nH of inductance:
V = 1×10⁻⁹ × 50×10⁶ = 50 mV per nH per A/µs
A 100 mm pigtail wire to the capacitor has approximately 100 nH of inductance. The unclamped spike has ~5 V of unclamped energy before the capacitor responds. This spike propagates into the ESC MOSFET drain-source junction. On 6S (25.2V full charge), this spike can bring the MOSFET's Vds close to or beyond the breakdown voltage, causing failure.
Solder the capacitor directly onto the ESC pads with lead lengths under 5 mm: 100 nH → under 5 nH. V_spike reduced by 20×.
Low-ESR requirement¶
Equivalent Series Resistance (ESR) is the parasitic resistance of the capacitor. A capacitor with high ESR has a voltage drop across it proportional to the current flowing through it — for a fast, high-current transient, a high-ESR capacitor has a proportionally high drop, reducing its effective clamping. At 100 kHz, a generic electrolytic capacitor may have ESR of 1–5 Ω. A low-ESR type (Panasonic FM or low-Z series) has ESR <0.1 Ω at 100 kHz — 10–50× better for fast transients.
For the 1000 µF electrolytic on the ESC pads, low-ESR is mandatory, not recommended. A generic capacitor will appear to work but will fail to clamp fast spikes that a low-ESR type would suppress.
Two-stage filtering¶
A single capacitor cannot cover the full frequency range of noise: - 1000 µF electrolytic (low-ESR) on ESC pads: handles large, moderate-speed spikes from motor deceleration (µs timescales). Its ESL (equivalent series inductance, typically 5–20 nH for a leaded electrolytic) limits effectiveness at very high frequencies. - 100 µF MLCC ceramic on FC 5V supply pads: handles fast, small spikes that propagate onto the logic supply (ns timescales). MLCC ceramics have very low ESL (<1 nH) and respond faster than any leaded component.
The two stages address different frequency bands and complement each other. The electrolytic does not make the ceramic redundant (too slow for ns-scale spikes); the ceramic does not make the electrolytic redundant (too small for µs-scale large current transients).
Reference¶
Capacitor specification¶
| Capacitor | Location | Value | Type | ESR requirement |
|---|---|---|---|---|
| ESC bulk decoupling | ESC VBAT/GND pads | 1000 µF 35V | Low-ESR electrolytic | <0.1 Ω at 100 kHz |
| Logic supply decoupling | FC 5V pads | 100 µF | MLCC ceramic | Inherently low |
Recommended: Panasonic FM series (1000 µF 35V, 8×11.5 mm), or equivalent low-Z type. Part number: EEU-FM1V102.
Wire inductance reference¶
| Wire/lead length | Inductance (approx) | V_spike at 50 A/µs |
|---|---|---|
| 0 mm (direct on pad) | ~1–2 nH | 50–100 mV |
| 10 mm | ~10 nH | 500 mV |
| 50 mm | ~50 nH | 2.5 V |
| 100 mm (typical pigtail) | ~100 nH | 5 V |
On 6S with 25.2V full charge and MOSFETs rated to 30V Vds, a 5V unclamped spike leaves only 0V margin — the MOSFET is at its limit.
Procedure¶
Installing the 1000 µF capacitor¶
- Verify the capacitor polarity (positive lead = longer lead on electrolytic).
- Trim leads to ~5 mm — the minimum needed to reach the ESC VBAT and GND pads.
- Position the capacitor so its body lies flat across the ESC PCB.
- Solder directly to the VBAT and GND pads. The joint between lead and pad should have no visible gap — the capacitor must be mechanically seated on the pad.
- Verify: no lead length between capacitor body and pad exceeds 5 mm from the solder joint.
- Secure the capacitor body with a drop of hot glue or a cable tie to prevent vibration fatigue on the solder joints over hundreds of flight hours.
Rationale¶
Why "no pigtail" is a hard rule¶
A pigtail — even a short 20 mm wire — adds enough inductance to allow a significant portion of the motor deceleration spike to appear on the MOSFET drain before the capacitor can respond. ESCs fail silently at first (reduced maximum safe current) and then catastrophically (MOSFET avalanche). The cost of an ESC failure is €30–50 and days of repair time. The cost of soldering the capacitor correctly the first time is zero.
Connections¶
requires: - emc-noise-sources - star-grounding related: - electronic-speed-controllers - power-rail-architecture leads_to: - power-signal-separation - ferrite-beads