Taming Conducted EMC on a Budget
Buck Converter
A CISPR 25 Filter Case Study
Cheap LM2596-based buck
converter modules are everywhere in DIY 12V-to-5V conversions — camper van
builds, car electronics, small embedded projects. They're inexpensive, easy to
wire up, but generally not designed with automotive EMC compliance in mind.
This article walks through a hands-on CISPR 25 conducted-emissions
pre-compliance investigation into exactly how bad that problem is, and what it
actually takes to fix it.
The Problem: An Off-the-Shelf Converter That Was Never Designed for
Vehicles
The test subject is a common
low-cost buck converter built around the LM2596S switching regulator,
configured to step 12V vehicle power down to 5V for powering small electronics
— the kind of module widely used in camper van conversions and similar builds.
In this setup it was powered from a maintenance-free lead-acid battery, with
the output set to roughly 5V and loaded with a small LED floodlight drawing
about 400 mA — well under the module's rated 2A output.
While these converters work
fine electrically, they are not designed to meet automotive EMC requirements,
and using them in a vehicle can be non-compliant at best and capable of
disrupting other vehicle electronics at worst.
Two Kinds of Emissions: Conducted vs. Radiated
Electromagnetic interference
from a device generally falls into two categories:
●
Radiated emissions — interference radiated
outward via electric or magnetic fields.
●
Conducted emissions — interference that travels
directly along power or signal wiring.
This investigation focuses
entirely on conducted emissions, measured according to the CISPR 25 standard
used for automotive components.
Measurement Setup
Measuring conducted emissions
properly requires a LISN (Line Impedance Stabilization Network). A LISN
does two jobs: it presents a standardized, known impedance to the device under
test, and it lets interference signals be tapped off cleanly for analysis by a
spectrum analyzer or measurement receiver.
The test bench used a
symmetrical, largely standards-compliant configuration with two LISNs mounted
on a grounded copper plate, conductively bonded to the plate and fitted with
the required input capacitors. The available copper plate was somewhat small,
so the full minimum spacing and cable-routing requirements from the standard
couldn't be strictly met — a compromise judged acceptable given how large the
eventual limit violations turned out to be, and given that the goal was to
compare relative filter performance rather than produce a certified
compliance report. Because of this, the results throughout should be read as a
pre-compliance investigation rather than a certified CISPR 25 compliance test.
Instrumentation:
●
A Siglent SSA3021X Plus spectrum analyzer fitted with
the EMI option, providing the standardized 9 kHz resolution bandwidth and
quasi-peak detection needed for CISPR-style measurements.
●
Techbox EMC View software to automate the process —
configuring CISPR 25 limits, setting correct resolution bandwidths, and
compensating for frequency-dependent insertion losses of the LISNs and
measurement cables, rather than configuring all of this manually on the
analyzer. EMC View is free to use for measurements up to 10 MHz and works with
a range of common spectrum analyzers; since the LISNs used were also from
Techbox, their correction factors were already built into the software.
●
The conducted-emissions measurement range for this
setup spanned 150 kHz to 108 MHz.
On the resulting plots, the blue
line shows the CISPR 25 peak limit, while the red line shows the
applicable quasi-peak/average limit — the exact detector and limit depend on
the frequency band and CISPR 25 test category, and CISPR measurement systems
distinguish peak, quasi-peak, and CISPR-average detectors as separate things,
not interchangeable labels for the same reading. Both limits must be satisfied
where applicable — exceeding either one at any frequency point constitutes a
failure.
Baseline Result: A Massive Failure
Tested with no filtering, the
unmodified buck converter failed immediately and badly. Violations exceeded 30
dB above the limit at some frequencies — and because the display is
logarithmic, that number understates how severe it looks visually. To put it in
perspective: a 20 dB excess means the interference voltage is 10 times higher
than allowed; a 30 dB excess means it's roughly 32 times over the limit. In
total, the converter exceeded the allowed limit at 238 separate measurement
points across the swept range.
Attempt 1: Decoupling Capacitors
One of the most common fixes
suggested by viewers of an earlier short video on this topic was to simply add
decoupling capacitors to the supply line. Four capacitors — 100 µF, 10 µF, 100
nF, and 10 nF — were soldered in parallel on a small perfboard and inserted
between the LISNs and the converter.
Result: Some
improvement, but nowhere near enough to pass. This was expected: shunt
capacitors alone provide limited attenuation when there's little series
impedance between the noise source and the supply — which is the case here,
with both the battery and the converter's input presenting low impedance.
What's actually needed is a series element, such as an inductor or
ferrite component, to create the impedance discontinuity that isolates the
noisy converter from the source. Capacitors alone can't provide that.
Attempt 2: A PI Filter
The next step up was a PI
filter in the supply line, made from a 47 µH inductor and two 150 nF capacitors
(mounted on the underside of the board). The video cites a resulting cutoff
frequency around 120 kHz for this filter; for reference, the ideal
single-L/single-C corner frequency for these values (fc = 1/(2π√LC)) works out
closer to 60 kHz, so the 120 kHz figure likely reflects the actual PI topology
or an equivalent-capacitance calculation rather than a simple single-pole
estimate. This filter replaced the decoupling-capacitor board in the same test
position between the LISNs and the converter.
Result: Noticeably
better attenuation at lower frequencies compared to the plain capacitors, but
still short of full compliance. Notably, a persistent noise bump just above 10
MHz remained essentially unchanged. To check whether near-field coupling across
that region was responsible, an additional measurement was taken with a
grounded conductive shield placed between the filter and the converter. The
shield produced essentially no change, suggesting the peak was not primarily
caused by direct near-field coupling across that particular region — though
this doesn't rule out every possible near-field mechanism, just that one.
Understanding Why: Differential Mode vs. Common Mode Noise
To get past this plateau, it's
necessary to distinguish between two propagation modes on the supply lines:
●
Differential mode (DM) noise — interference
current flows in opposite directions on the positive and negative lines.
●
Common mode (CM) noise — interference current
flows in the same direction on both lines and returns through parasitic
capacitances, chassis/ground structures, shielding, or other unintended paths —
in this case, capacitive coupling into the ground plane. (Radiated emission is
more a consequence of this coupling than the return path itself.)
Each mode requires a different
filtering strategy, and a filter tuned for one may do little for the other.
Because the test setup already
used two LISNs, it was straightforward to adapt the measurement to isolate each
mode separately: a TBLM1 LISN mate (also from Techbox) was connected to
the two LISNs, with either the differential-mode or common-mode output routed
to the spectrum analyzer while the unused output was terminated in 50 ohms.
Result: A clear pattern
emerged — differential-mode noise decreases with increasing frequency, while
common-mode noise increases with frequency. This is consistent with the
fact that the coupling and radiation mechanisms behind common-mode noise become
more effective at higher frequencies. The conclusion: effectively suppressing
common-mode noise needs substantially more deliberate filter design than
differential-mode noise does.
Attempt 3: A Proper CM/DM Filter Topology
To address common-mode noise
directly, the investigation moved to components from Würth Elektronik's
"Design Your EMC Filter" kit — described as a practical toolkit of
proven filter topologies plus the components to build and test them.
The key component here is the common-mode
choke: two coils wound symmetrically on a shared magnetic core. For
common-mode signals (identical, in-phase on both lines), the choke presents
high impedance and blocks the noise. For differential-mode currents, the
magnetic flux from the two windings largely cancels, so the choke presents
relatively low impedance to that current — though not zero, as the next point
explains.
"Sample circuit 1"
from the kit is a classic filter structure intended to suppress both CM and DM
noise simultaneously: Y-capacitors provide common-mode noise a direct
path to ground, working alongside the common-mode choke's high impedance; from
the differential-mode perspective, the same circuit behaves as a simple LC
filter. In practice, common-mode chokes always have some leakage inductance
(non-ideal behavior for differential-mode signals) — normally undesirable, but
here it's actually a small bonus, since it adds some differential-mode
suppression too. Because the test setup, unlike a mains-powered appliance, has
no protective earth, the Y-capacitors were instead tied to the negative supply
return conductor rather than to a separate chassis or earth reference — a
distinction that turns out to matter a great deal, as the next section shows.
This filter was inserted into
the converter's supply line and measured.
Result: Disappointing.
There was a modest reduction in differential-mode noise, but the common-mode
interference — the thing this filter was specifically meant to address —
remained almost unchanged.
Diagnosing the Failure, and Scaling Up
The first instinct was to scale
up the design rather than change its topology: much larger 1000 µF X-capacitors
were added along with a bigger common-mode choke, on the reasoning that bigger
impedance jumps should help more.
Result: Noise at lower
frequencies dropped significantly this time — but the stubborn peak around 10
MHz still refused to move.
The eventual explanation traced
back to how the Y-capacitors were grounded. A common-mode capacitor only
helps if it provides a low-impedance high-frequency return path without
simultaneously providing a path that bypasses the common-mode choke. The
reference itself can vary with the system architecture, but in this setup the
critical issue was that tying the Y-capacitors to the same supply return the
choke was meant to isolate created a path that bypassed the choke rather than
routing common-mode current usefully — a genuinely common real-world design
mistake, included here deliberately as a lesson.
To investigate further, a metal
housing was placed over the buck converter and bonded to the Y-capacitors.
Result: Common-mode
noise was suppressed far more effectively. However, leaving the Y-capacitors
completely disconnected produced a nearly identical measurement result — which
strongly suggests it was the enclosure/shielding that changed the common-mode
current path, not the Y-capacitors themselves that were responsible for the
improvement. This particular shielding approach wasn't pursued further here,
partly because radiated emissions and shielding are planned as a separate
topic, and partly because the result above shows this specific fix isn't fully
understood or robust yet.
The Working Solution: A Multi-Stage Filter
The best result up to this
point had come from the combination of two X-capacitors and one common-mode
choke. Building on that, a second common-mode choke stage was added to create a
multi-stage filter, while skipping an additional X-capacitor and relying
instead on the capacitance already present at the converter's input.
Result: Success — the
converter measured below the selected CISPR 25 limits across the measured
range, apart from a few narrowband peaks in the FM broadcast band. A close-up
look at those FM-band peaks pointed to local FM radio station signals coupling
into the unshielded test setup rather than something generated by the converter
itself. A DUT-off ambient scan should be considered part of the diagnostic
process whenever unexplained narrowband peaks like this remain — comparing
DUT-on and DUT-off scans is the standard way to confirm ambient signals aren't
being mistaken for (or masking) device emissions.
It's worth restating the caveat
from earlier: this demonstrates that the modified converter can meet the
selected CISPR 25 limit levels under this particular pre-compliance setup — it
does not, on its own, establish formal CISPR 25 compliance.
Key Takeaways
1.
A common, low-cost buck converter can be wildly out
of spec. Limit violations exceeding 30 dB (roughly 32× over the limit) at
over 200 measurement points show this isn't a marginal issue — it's a
fundamental design gap for automotive use.
2.
Differential-mode and common-mode noise are
different problems. Filters aimed at one may do almost nothing for the
other; measuring them separately (via a LISN mate) reveals which one actually
dominates and where.
3.
Simple decoupling capacitors rarely solve conducted
EMC issues when source and load impedances are already low — you need a
genuine impedance discontinuity, not more capacitance.
4.
Common-mode chokes are the right tool for
common-mode noise, but topology and grounding matter enormously. A textbook
filter circuit can fail completely if a Y-capacitor's return path ends up
bypassing the common-mode choke instead of routing noise usefully to a proper
reference — a subtle but very common mistake.
5.
Bigger components help, but only once the topology
is correct. Scaling up capacitor and choke values gave real gains, but only
after the underlying grounding error was identified and understood.
6.
Multi-stage filtering (cascaded common-mode chokes)
can close the final gap once single-stage designs plateau.
7.
Not every remaining peak is the device's fault.
Ambient signals (like local FM broadcasts) can couple into an unshielded test
setup and should be identified before being blamed on the device under test.