This article was originally found on the TDK website and is reproduced here for technical reference.
Overview
When designing electronic products, especially large systems such as servers, switches, and communication equipment, audible "whine" can originate from passive components such as capacitors and inductors. This noise affects the acoustic environment and may also interfere with nearby high-speed signals. This article focuses on causes of audible whine from power inductors used in power circuits such as DC-DC converters and outlines practical mitigation measures.
Audible Whine from Power Inductors
Intermittent operation, frequency modulation modes, and load changes can produce vibrations in the audible range.
Sound is an elastic wave propagating in air, and humans typically hear frequencies roughly between 20 Hz and 20 kHz. In a DC-DC converter, when alternating current or pulsed currents within the audible range flow through a power inductor, the inductor body can vibrate. This phenomenon is often referred to as coil noise or whine (Figure 1).
As device functionality increases, DC-DC converter power inductors have become potential noise sources. DC-DC converters switch power devices on and off, producing pulsed currents. By varying the ON-time (pulse width), the converter regulates output voltage. This pulse-width modulation approach, PWM, is widely used.
Although switching frequencies in many DC-DC converters are high (hundreds of kHz to several MHz) and thus above human hearing, audible whine can still occur. Common causes include intermittent operation for power saving, switching between PWM and pulse-frequency modulation (PFM) modes, and other variable-frequency operation. Figure 2 shows the basic principles of PWM and PFM operation.
PWM Dimming and Intermittent Converter Operation
Mobile device backlight auto-dimming often uses intermittent DC-DC converter operation to save battery. PWM dimming controls LED on and off times to adjust brightness with minimal color shift. Typical PWM dimming frequency around 200 Hz causes the converter to operate intermittently. While the eye rarely perceives backlight flicker at this frequency, the intermittent current is within the audible range. When such intermittent currents flow through an on-board power inductor, the inductor body can vibrate, producing audible whine.
Note on duty cycle: In DC-DC converters, the duty cycle is the ratio of ON time to the total switching period. For LED PWM dimming, duty cycle equals on time divided by (on time + off time) and represents brightness.
Variable-Frequency Modes (PFM) and Whine
PWM-mode DC-DC converters can be efficient under normal loads, often achieving approximately 80% to 90% efficiency. Under light load or standby conditions, however, efficiency can drop because switching losses scale with frequency. To improve light-load efficiency, some converters switch from PWM to PFM. In PFM, with a fixed ON time, the switching frequency is reduced by extending the OFF time as the load decreases. This reduces switching losses and improves efficiency at light load. However, the resulting switching frequency can fall into the audible range (around 20 Hz to 20 kHz), where the power inductor may produce whine.
Load-Induced Whine
Various power-saving strategies in mobile devices can create periodic current variations that fall into the audible range. For example, CPUs often implement power states that periodically change current draw. If such current variation frequencies fall within the audible band, a power inductor may produce audible whine.
Role of the Power Inductor in a DC-DC Converter
An inductor allows DC current to pass while opposing changes in current through its inductance, temporarily storing energy in the magnetic field and releasing it back to the circuit. Power inductors, also called power chokes, are key components in switching power supplies, working with capacitors to smooth high-frequency pulses from the switching element.
Because power circuits carry large currents, wound inductors are common. High-permeability magnetic materials such as ferrite or soft magnetic metals are used as cores to achieve high inductance with fewer turns, enabling compact designs. Figure 3 shows the basic non-isolated buck converter circuit that uses a power inductor.
Mechanisms of Inductor Vibration and Noise Amplification
When currents in the audible range flow through a power inductor, the inductor body can vibrate and generate whine. The main vibration mechanisms and noise-amplifying factors include the following.
Vibration Mechanisms
- Magnetostriction of the magnetic core. Magnetization causes minute shape changes in the core material, producing vibration when an alternating magnetic field is present.
- Magnetic attraction between magnetized core parts. In shielded or closed magnetic structures, gaps between core parts can allow relative motion driven by magnetic forces when magnetized.
- Leakage flux acting on windings. In unshielded open magnetic circuits, leakage flux exerts forces on conductors carrying current, causing winding vibration according to Fleming's left-hand rule.
Noise Amplification Mechanisms
- Contact with nearby components. Small inductor vibrations can be amplified if the inductor touches other components or enclosures.
- Leakage flux acting on nearby magnetic parts such as shields or enclosures, causing them to vibrate.
- Resonance with the natural vibration modes of the assembly including the PCB. If an inductor's vibration coincides with an assembly resonance, the noise can be greatly amplified.
Figure 4 summarizes the vibration and amplification mechanisms that can lead to audible whine.
Magnetostriction and Domain Behavior
Magnetic materials consist of small regions called magnetic domains where atomic magnetic moments align. In an initially demagnetized state, domain orientations cancel out. Under an external magnetic field, domains realign and domain walls move, causing subtle atomic-level displacements that result in macroscopic magnetostriction. Although dimensional changes are extremely small (on the order of 10^-4 to 10^-6 of the original size), repeated alternating magnetostrictive strain can produce audible vibration when a coil is wound on the core. On a PCB, if the inductor's vibration frequency matches a PCB resonance, the vibration may be amplified and perceived as whine.
Attraction Between Core Parts
In fully shielded power inductors with a closed magnetic path, there can still be gaps between core components such as a bobbin and shield. When these parts are magnetized by AC, magnetic forces can attract the parts toward each other. If the resulting movement occurs at audible frequencies, the gap area can emit noise. Manufacturing adhesives used to seal gaps are typically not rigid enough to fully prevent motion, so this vibration can persist.
Leakage Flux and Winding Vibration
Unshielded inductors have open magnetic circuits, so leakage flux interacts directly with windings. Currents in the windings experience forces from the flux, causing the windings to vibrate and produce audible noise (Figure 7).
Assembly Resonance
Small core magnetostriction alone is usually insufficient to cause noticeable whine. However, when the inductor assembly is mounted on a PCB, the combined structure has multiple natural vibration modes within the audible band. Finite element method analysis can identify PCB vibration modes and show how inductor placement and board fixation affect resonant frequencies. In a model with the inductor at the board center and two long edges fixed, lower-order modes can be strongly influenced by the mounted inductor. A mode that shows significant Z-direction displacement can shift to much lower frequency when the inductor is attached to the PCB, increasing susceptibility to audible whine.
Mitigation Strategies for Power Inductor Whine
The following summarizes practical countermeasures for audible whine from power inductors in DC-DC converters.
Key 1: Avoid Currents in the Audible Range
Preventing current components within the audible band is the most fundamental approach. However, intermittent operation or variable-frequency modes intended for power saving may produce audible-frequency currents. When such modes cannot be avoided, apply the additional silencing strategies below.
Key 2: Avoid Placing Magnetic Materials Near the Inductor
Keep magnetic shields, shields, or other magnetic parts away from the inductor to prevent leakage flux from exciting nearby magnetic structures. If proximity is unavoidable, choose an inductor with low leakage flux (shielded/closed magnetic path) and pay attention to placement orientation.
Key 3: Detune Natural Frequencies
Shifting assembly resonant frequencies or increasing them can reduce audible whine. Changing inductor shape, type, layout, or how the PCB is fixed can alter the combined assembly natural frequencies. Whine is more common with larger inductors (for example, above 7 mm). Using smaller inductors (under approximately 5 mm) raises the natural frequencies and can reduce audible noise.
Key 4: Use Metal-Integrated Molded Inductors
In shielded inductors, gaps between core parts can produce attraction-induced vibration. In unshielded types, leakage flux can cause winding vibration. One effective solution is a metal-integrated molded inductor, formed by embedding a wound coil in a soft magnetic metal powder and molding it into a single part. Because there are no gaps between core parts and the coil is fixed integrally to the magnetic material, core attraction and leakage-flux-induced winding vibration are suppressed. Selecting magnetic materials with low magnetostriction further reduces vibration. Replacing an unshielded or conventional shielded inductor with a metal-integrated molded type can reduce audible whine. These molded types also typically exhibit very low leakage flux, making them suitable for placement near signal lines.
Inductors using ferrite cores remain widely used; they offer a larger range of inductance values and are common in many devices due to maturity in mass production.
