Why Placing a Ground Plane Under a Common-Mode Choke Often Makes EMI Worse

This is one of the most common layout mistakes I see during EMC reviews.

The schematic is correct.The common-mode choke is correctly placed at the interface.

But the PCB layout shows a solid ground plane directly underneath the choke.

Imagine a GND plane below the chocke.

The intention is usually good: keep the ground plane continuous.Unfortunately, at high frequency, this decision often defeats the common-mode choke entirely.

The Hidden Assumption: Inductors Are Not Ideal

A common-mode choke is selected to present high impedance to common-mode currents, especially at high frequency.This assumes the choke behaves as an inductor over the frequency range of interest.

In reality, inductors are not ideal components.

Every inductor includes:

• Winding-to-winding capacitance

• Winding-to-core capacitance

• Winding-to-PCB capacitance

As frequency increases, these parasitic capacitances become dominant. At a certain point — the self-resonant frequency — the inductive impedance stops increasing.

Above this frequency, the inductor no longer behaves inductively.

It behaves capacitively.

This behaviour is shown clearly in impedance-versus-frequency plots, where impedance peaks and then falls as frequency increases.

RED expert from Wurth is my favourite tools! we can see from theabove the effect of the parassitic winding capacitor.

What the Ground Plane Really Does

When a solid ground plane is placed under a common-mode choke, it creates an additional parasitic capacitance.

The choke winding and the ground plane form a parallel-plate capacitor.Even a small copper area can easily add tens of picofarads of capacitance.

This parasitic capacitance:

• Appears in parallel with the choke

• Lowers the self-resonant frequency

• Creates a high-frequency bypass path

At high frequency, common-mode current now has an alternative route:

• Through the parasitic capacitance

• Into the ground plane

• Back to the source

The current no longer needs to flow through the choke.

Why This Breaks the Common-Mode Filter

At low frequency, the choke still behaves as expected. At high frequency — where EMC problems usually occur — the situation changes.

Because:

• The choke impedance collapses above self-resonance

• The parasitic capacitive path has low impedance

The common-mode choke becomes progressively ineffective exactly where it should be most useful.

This is why many designs:

• Look correct on paper

• Pass low-frequency conducted tests

• Fail radiated emissions above a few MHz

At high frequency, current follows lowest impedance, not schematic intent.

This Is Why the Schematic Does Not Reveal the Problem

The schematic shows:

• A common-mode choke

• A ground reference

What it does not show:

• Parasitic capacitance to the ground plane

• Self-resonant behaviour of the inductor

• High-frequency displacement currents

As a result, this issue is:

• Invisible in the schematic

• Easy to miss during layout

• Very obvious in the EMC chamber

Practical Layout Guidance

A simple rule that works well in practice:

Do not place solid reference planes directly under common-mode chokes.

Instead:

• Clear copper underneath the choke

• Or strictly control the copper area and distance

• Especially on the layer directly below the component

This forces common-mode current to flow through the choke instead of bypassing it capacitively.

Final Thought

Common-mode chokes fail in real products not because they are wrongly chosen, but because their parasitic behaviour is unintentionally amplified by the PCB layout.

Understanding how inductors behave beyond their ideal model is essential for effective EMC design.