1、The principle of noise generation

Noise is generated by the vibration of objects. Let's take a speaker as an example to understand the principle of vibration.

A speaker does not directly convert electrical energy into sound energy. Instead, it uses a carrier current (voice coil or coil) to interact between magnets, making the voice coil vibrate and drive the diaphragm to vibrate: electrical energy - mechanical energy - sound energy.

A speaker: When the current direction at both ends of the coil changes, it directly interacts with the magnetic field of a permanent magnet, causing the voice coil to vibrate and drive the diaphragm to vibrate. Electrical energy - mechanical energy - sound energy.

A speaker produces sound by converting the mechanical energy generated when the current in the coil changes. Can an inductor be made into a speaker？

If a vibrating membrane is added to the inductor's coil and a small sound cavity is made, the inductor becomes a speaker. In fact, even without adding a vibrating membrane and sound cavity to the inductor coil, if a large enough driving current is applied to the inductor's terminals, it can also produce sound. However, the efficiency of converting mechanical energy to sound energy is very low, the sound is very small, and the volume is low, making it difficult to hear!

2、Will inductors also produce noise?

If you can hear a whistling (squeaking) sound, it's certain that there's a switching current of around 20Hz - 20kHz (the human ear's range) across the inductor. For example, in the case of inductor whistling in a DC - DC converter, due to excessive load current, there's a current - limiting protection circuit inside the DC converter. When the load exceeds the current capacity of the internal switching (MOS) of the IC, the current - limiting detection circuit will determine that the load current is too high. It will then immediately adjust the duty cycle of the internal switches in the DAC or stop the switching operation altogether. The switching will only resume normal operation after the load current is detected to be within the standard range. The time cycle from stopping the switch to restarting it is exactly in the frequency range of a few kHz, and it's this periodic switching frequency that generates the whistling noise.

The magnitude of the whistling noise is somewhat related to the quality of the inductor winding. Looser windings will generate louder whistling sounds.

3、Conditions for an inductor to emit sound

① A change in the magnitude of the current passing through the inductor → This causes a change in magnetic flux.  

② The presence of a conductor around the inductor, sufficient to induce Lenz's effect → The conductor senses the inductor's magnetic flux and generates a repulsive magnetic field → The aluminum housing/capacitor of a lamp provides such a condition. As we know, like poles of magnets repel each other while opposite poles attract. When an inductor/transformer operates, it generates a strong alternating magnetic field internally. The magnetic core and coils within this field are subjected to magnetic forces. If these forces cause periodic vibrations, friction, or material deformation, noise is produced. The vibration system formed by a high - frequency excitation source and a complex mechanical structure may generate audible noise.

4、Coil vibration causes inductor noise

If the gaps between turns of the inductor coil are large and the arrangement is not tight enough, and if the glue fails to fully penetrate and secure the coil gaps, it is prone to generating noise. The direction of alternating current continuously changes with frequency. As a result, mutual attraction and repulsion occur between coil turns. As the frequency increases, this attraction - repulsion evolves into vibration. When the vibration frequency falls between 20Hz and 20kHz (the audio range audible to human ears), noise is produced.  

Solutions:  

① Lenz's Law between the Coil and Magnetic Core → Strengthen the fixation of the coil to restrict its movement. Impregnate the coil or increase the wire diameter.  

② Lenz's Law between Magnetic Cores → Use glue to secure the cores and limit their movable space.

5、Magnetostriction (magnetic distortion) causes inductor noise

The magnetic core materials used in inductors are generally soft magnetic materials. The magnetic powder raw materials of magnetic materials exhibit a phenomenon of magnetic lattice distortion (magnetostriction), that is, when the magnetic powder inside the core is magnetized, the volume of the material will undergo slight changes. As the voltage increases and the frequency rises, this change becomes more intense, eventually developing into vibration. If there are gaps between the combined parts of the magnetic cores, resonance is likely to occur, generating noise.  

Solutions:  

① When assembling, minimize the gap between the bonding surfaces of the magnetic cores. The clamping force should be appropriately uniform to ensure close contact between the cores. In addition, the gap space at the air gap of the core's central column is most prone to resonance. The best approach is to completely fill it with glue.  

② Replace with a magnetic core material with high magnetic flux density and low magnetostriction: smaller distortion and vibration can effectively reduce noise.  

③ Replace with core materials made of finer magnetic powder. We can use iron powder with smaller particle sizes to reduce the gap distance between particles and increase the number of gaps. This action causes the vibration frequency generated by friction between magnetic walls to exceed the general hearing range of 20kHz.  

Note: When the vibration frequency exceeds 20kHz, it becomes inaudible to the human ear.

6、Noise Caused by Circuit Resonance  

Parasitic capacitance exists in the circuit. When the power supply frequency reaches or is very close to the natural LC frequency of the circuit, resonance occurs. If the resonance frequency happens to fall within the audio range, noise is generated.  

Solutions:  

① Adjust the output frequency of the power management IC to avoid the resonance frequency point.  

② Adjust the inductor value to avoid the resonance frequency point. (For example, taking the upper and lower limits of inductance values, which aims to change the resonance frequency).

7、Corona Effect Induced Noise

Partial discharge occurs due to poor insulation in materials, typically manifested as defects in the enameled wire insulation such as damage, scratches, or pinholes between turns. Under certain high - voltage conditions, this leads to electrical discharge into the surrounding environment, exciting resonance in adjacent cavities.

Solutions:

Coil Impregnation Treatment: Enhance the insulation performance of the coil through impregnation.

Replace with Higher - Quality Enameled Wire: Use enameled wire with better insulation properties.

8、Inductor Overload Operation

If the actual operating current is too large, reaching or exceeding 1/3 of the rated current, it may cause the inductor to emit noise.  

Solutions:  

① Reduce the effective magnetic permeability of the core and increase the number of coil turns.  

② Increase the effective cross - sectional area of the core's window.

9、Noise Caused by Uneven Grinding of Magnetic Core 

During the production process, the magnetic cores of high-current inductors generally require grinding for air gaps. If the air gap grinding is not smooth (especially the air gap of the central column), the direction of the nearby magnetic flux will be distorted, causing magnetic flux congestion, which is likely to lead to noise generation.  

Solution:  

Grind the air gap of the magnetic core smoothly.

10、Magnetic Core Material Damage

If the finished magnetic core is cracked or the central column is broken, when the magnetic powder inside the core is magnetized, noise is generated due to the magnetostriction phenomenon (magnetic distortion: explained previously).

Solutions:

① Select magnetic core materials with high strength for production.

② Use glue with low expansion coefficient and flexibility for filling.

11、PCB Trace Design and Nearby Magnetic Field Radiation

Unreasonable PCB trace design, such as traces forming a closed loop, can cause strong EMI radiation that interferes with inductors. Improper trace design may also lead to circuit resonance, both of which generate noise. Additionally, magnetic field radiation from nearby components can cause inductors to emit noise.  

Solutions:  

① Communicate with clients to adjust the circuit design.  

② Relocate the inductor to avoid interference and radiation sources.  

Conclusion: The above briefly analyzes common inductor noise issues. As we know, sound is produced by vibration—and inductor noise follows the same principle. To address such problems, we must identify the source of vibration and then adopt scientific and reasonable countermeasures.