Conducted Emissions in Switching Power Electronics

This article examines how differential-mode and common-mode noise currents are generated within switching power electronics and how they propagate back onto AC mains wiring as conducted emissions. It breaks down the rapid switching transitions that act as noise sources, details the distinct physical and parasitic pathways each noise mode takes to reach the input power lines, and explains how these unwanted high-frequency currents interact with system impedances to pollute the AC grid.

The Source of Conducted Emissions

Switching power electronics rely on the rapid turn-on and turn-off of semiconductor devices, such as MOSFETs and IGBTs, to achieve high efficiency. This high-frequency switching creates steep voltage transitions (\(dv/dt\)) and rapid current changes (\(di/dt\)). While essential for power conversion, these transitions generate a broad spectrum of high-frequency harmonic energy.

Conducted emissions generally occupy the frequency range from 150 kHz to 30 MHz. Instead of radiating through the air, these high-frequency currents travel along the physical conductors of the power supply, eventually finding their way out of the power converter and onto the AC mains wiring. Once on the mains, they can interfere with other equipment sharing the same power grid. These emissions are categorized into two distinct propagation modes: differential-mode (DM) and common-mode (CM).

Differential-Mode Noise Propagation

Differential-mode noise, also known as symmetrical or normal-mode noise, travels along the same path as the functional power supply current. It flows out to the power supply via the Line (Phase) conductor and returns via the Neutral conductor, meaning the noise currents on the two wires are equal in magnitude and opposite in phase (180 degrees out of phase).

Generation of Differential-Mode Currents

The primary driver of DM noise is the rapid change in current (\(di/dt\)) demanded by the power stage. When the main power switch turns on and off, it draws pulsing, non-sinusoidal current from the input bulk capacitor.

The Propagation Path onto the AC Mains

Ideally, the input bulk capacitor would supply 100% of this high-frequency pulsing current, isolating the AC mains from the switching action. However, real-world capacitors possess parasitic Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL).

  1. At high switching frequencies, the ESL causes the capacitor's impedance to rise, making it behave like an inductor rather than a capacitor.
  2. Because the input capacitor cannot instantaneously supply or absorb the high-frequency current spikes due to its internal impedance, a voltage ripple is generated across its terminals.
  3. This high-frequency voltage ripple forces a portion of the switching current to seek an alternative path of lower impedance.
  4. The current flows backward through the input bridge rectifier (whenever the rectifier diodes are forward-biased and conducting) directly onto the AC Line conductor.
  5. The current then travels through the impedance of the AC mains wiring and returns to the power supply via the AC Neutral conductor, completing the loop.

Differential-mode noise generally dominates the lower end of the conducted emissions spectrum, typically below 2 MHz.

Common-Mode Noise Propagation

Common-mode noise, or asymmetrical noise, is fundamentally different because it does not use the intended circuit paths. Instead, CM noise currents flow in the same direction (in phase) on both the Line and Neutral conductors simultaneously, and they return to the power supply via the safety Earth ground wire or an external reference ground plane.

Generation of Common-Mode Currents

The primary driver of CM noise is rapid voltage transitions (\(dv/dt\)). In a switching power supply, the drain of a primary-side MOSFET or the collector of an IGBT experiences extreme voltage swings—often hundreds of volts in a matter of nanoseconds. This node is known as a "switching node" or "hot spot."

The Propagation Path onto the AC Mains

CM noise relies entirely on parasitic (stray) capacitances to propagate. According to the capacitor current equation (\(I = C \cdot dv/dt\)), a rapid change in voltage across even a tiny, unintentional capacitance will generate a significant displacement current.

  1. Heat Sink Coupling: Power switches are often mounted to metal heat sinks for thermal management, and these heat sinks are frequently connected to the grounded metal chassis for safety. The thin insulating pad between the switch and the heat sink acts as a dielectric, creating a parasitic capacitance. The high \(dv/dt\) at the switch pushes high-frequency current through this capacitance directly into the chassis ground.
  2. Transformer Coupling: In isolated power supplies, high \(dv/dt\) on the primary winding couples across the inter-winding parasitic capacitance to the secondary winding. From the secondary side, this current couples to the chassis or Earth ground through mounting hardware, output cables, or deliberate grounding connections.
  3. Returning via the AC Mains: Current must always return to its source (the switching node) to complete the circuit. Once the displacement current enters the chassis or Earth ground wire, it travels out of the power supply onto the facility's AC ground wiring.
  4. Through the inherent impedance of the AC grid (or a Line Impedance Stabilization Network during compliance testing), this noise current couples back onto both the Line and Neutral wires simultaneously.
  5. The noise current travels back into the power supply along the Line and Neutral conductors, passes through the input rectifier, and returns to the switching node to complete the loop.

Because parasitic capacitances present a lower impedance as frequency increases, common-mode noise typically dominates the higher end of the conducted emissions spectrum, usually from 2 MHz up to 30 MHz.

Mode Conversion: Mixed-Mode Propagation

In practical applications, differential-mode and common-mode noise do not always remain strictly separated. A phenomenon known as "mode conversion" occurs when imbalances exist in the AC input circuitry.

If the impedance of the Line path does not perfectly match the impedance of the Neutral path—due to asymmetrical PCB trace routing, unequal component tolerances, or single-pole filtering—common-mode currents will experience different voltage drops on each line. This imbalance converts a portion of the common-mode noise into differential-mode noise (and vice versa), further complicating how emissions propagate onto the AC mains wiring.