Electronics Guide

Switch-Mode Power Supply Noise

Switch-mode power supplies generate differential-mode noise through the repetitive switching of transistors that chop DC or rectified AC into high-frequency pulses. The fundamental switching frequency, typically 50 kHz to several MHz, and its harmonics constitute the primary differential-mode noise spectrum. The magnitude of this noise depends on the switching frequency, duty cycle, and the impedance of the input source.

The input current waveform of a switch-mode power supply contains significant harmonic content even at low frequencies. For supplies with simple rectifier inputs, the current flows only during peaks of the AC waveform, creating harmonics at multiples of the line frequency. Power factor correction circuits reduce these low-frequency harmonics but may increase high-frequency differential-mode noise due to their additional switching activity.

Reverse recovery transients in rectifier diodes contribute to high-frequency differential-mode noise. When a diode switches from conducting to blocking, reverse recovery current flows briefly before the diode fully blocks. This current spike, combined with circuit inductance, produces voltage transients and high-frequency ringing that propagate back to the input.

Motor Drive Noise

Variable frequency drives and motor controllers generate substantial differential-mode noise through PWM switching of motor current. The switching frequencies, typically 2 kHz to 20 kHz for low-voltage drives, produce current harmonics that flow through the supply lines. Higher switching frequencies reduce motor torque ripple but increase EMI filtering requirements.

Motor starting transients and regenerative braking create low-frequency differential-mode disturbances on the supply. Large current variations during these events can cause voltage dips affecting other equipment on the same supply. Soft starters and controlled acceleration profiles reduce but do not eliminate these effects.

Digital Circuit Noise

High-speed digital circuits generate differential-mode noise on power supply connections through dynamic current consumption. Each logic transition draws current from the supply, producing noise at the clock frequency and its harmonics. The aggregate switching of thousands of gates in modern processors creates complex noise spectra that require careful power supply filtering.

The supply current waveform of digital circuits contains components at the clock frequency and subharmonics related to the switching activity of the circuit. DDR memory interfaces, with their synchronous data transfers, produce particularly strong noise components at the data rate and related frequencies.

Series Inductors

Series inductors provide increasing impedance with frequency, attenuating high-frequency differential-mode current while passing low-frequency power or signal current. The inductance value determines the filter cutoff frequency in combination with associated capacitance. Larger inductance provides greater attenuation but may affect voltage regulation under changing load conditions.

Core material and construction affect inductor performance. Powdered iron and ferrite cores provide high inductance in compact packages. The saturation current rating must exceed the maximum expected DC or peak AC current to maintain inductance under operating conditions. Some cores exhibit significant inductance variation with current level, requiring derating for applications with high peak-to-average current ratios.

Inductor parasitic capacitance creates a self-resonant frequency above which the component behaves capacitively rather than inductively. The self-resonant frequency should be well above the highest frequency requiring attenuation. Multiple smaller inductors in series may provide better high-frequency performance than a single larger inductor.

X Capacitors

X capacitors (also called line-to-line or across-the-line capacitors) shunt differential-mode noise while blocking the power frequency. Connected between line conductors, they provide a low-impedance path for high-frequency currents while presenting high impedance at the power frequency. The capacitor impedance decreases with frequency, providing increasing attenuation of higher-frequency noise.

Capacitor selection involves balancing several factors. Larger capacitance provides lower impedance and better high-frequency attenuation. However, capacitance must not be so large that it creates excessive reactive current at the power frequency. The voltage rating must exceed the peak line voltage plus any expected transients with appropriate margin.

Equivalent series resistance (ESR) and equivalent series inductance (ESL) limit capacitor performance at high frequencies. ESL creates a series resonance with the capacitance, above which the capacitor becomes inductive. ESR determines the minimum impedance at the resonant frequency. Film capacitors typically offer lower ESR and ESL than electrolytic capacitors, providing better high-frequency performance.

Safety Considerations for X Capacitors

X capacitors in AC mains applications require safety certification because they are connected directly across the line. Failure must not create a fire or shock hazard. Safety-certified X capacitors are designed to fail safely and maintain their ratings through specified stress tests.

X1 capacitors are rated for applications with high voltage transients, typically in industrial environments. X2 capacitors are suitable for general applications with lower transient requirements. Selection between classes depends on the expected transient environment and the location within the equipment relative to surge protection.

LC Filter

The basic LC low-pass filter combines a series inductor with a shunt capacitor to provide second-order attenuation (40 dB per decade above the cutoff frequency). This simple topology effectively attenuates differential-mode noise above the cutoff frequency while passing lower frequencies with minimal loss.

The cutoff frequency is determined by the LC product: f = 1/(2 pi sqrt(LC)). Selecting L and C values involves trading off cutoff frequency, component size, and damping. Lower cutoff frequencies provide attenuation starting at lower frequencies but require larger components and may affect system response to load changes.

The quality factor (Q) of the LC filter determines the resonant peak at the cutoff frequency. High Q produces a sharp peak that can actually amplify noise near the resonant frequency. Adding damping through resistive elements or selecting components with appropriate ESR reduces the peak but also reduces the ultimate attenuation rate.

Pi Filter

The pi filter topology adds capacitors on both sides of the series inductor, providing a third-order response with steeper roll-off than a simple LC filter. The input capacitor presents low impedance to high-frequency source noise, while the output capacitor provides a low-impedance source for the load. This topology is particularly effective when source and load impedances are both relatively high.

Component values in a pi filter can be optimized for different source and load impedances. Equal input and output capacitors provide a symmetric response. Unequal capacitors can match different source and load impedances for optimal performance. The inductor value primarily determines the cutoff frequency.

T Filter

The T filter uses capacitance in the center with inductors on each side, providing a third-order response optimized for low source and load impedances. The series inductors increase the impedance seen by the noise source, while the central capacitor provides a shunt path to the return conductor.

T filters are effective in power applications where the source impedance (utility connection) and load impedance (power supply input) are both relatively low. The symmetric structure with equal inductors on each side provides balanced attenuation regardless of noise direction.

Multi-Stage Filters

Cascading multiple filter stages provides steeper attenuation than single-stage filters of practical component sizes. Each stage contributes its filter order to the overall response, achieving high attenuation without the extreme component values that would be required for a single-stage filter with equivalent performance.

Interaction between stages must be considered in multi-stage filter design. Inserting a resistor or lossy component between stages prevents resonance between stage resonant frequencies that could create attenuation nulls. Careful design ensures each stage contributes effectively to the overall attenuation.

Frequency Range Analysis

Identifying the frequency range of differential-mode noise guides filter specification. For switch-mode power supplies, this typically spans from the fundamental switching frequency through several hundred MHz. Conducted emission standards specify limits from 150 kHz to 30 MHz, while radiated emission concerns extend above 30 MHz.

The noise spectrum may contain distinct peaks at switching harmonics or broadband noise from switching transients. Filter attenuation should provide adequate margin across the entire noise spectrum to ensure compliance under varying operating conditions and production tolerances.

Impedance Matching

Filter performance depends on the source and load impedances, which may vary significantly with frequency. The line impedance stabilization network (LISN) used for conducted emission testing provides a defined impedance, but actual installation impedances may differ. Filter design should consider the range of expected impedances to ensure adequate performance across applications.

Mismatched impedances can cause reflections that reduce effective attenuation. In severe cases, impedance mismatches can create resonances that amplify noise at certain frequencies. Adding resistive damping reduces sensitivity to impedance variations at the cost of some power loss.

Current and Voltage Ratings

Inductor current ratings must accommodate both the continuous operating current and any peak or transient currents. Core saturation under peak current reduces inductance, potentially degrading filter performance exactly when it is most needed. Derating for temperature rise ensures reliable operation over the equipment operating temperature range.

Capacitor voltage ratings must exceed the maximum expected voltage including transients. For AC mains applications, the peak line voltage plus any surge or switching transients defines the minimum voltage rating. Temperature derating and aging effects should be considered for long-term reliability.