Anyone who has done development work has such experience. Test the switching power supply or the sound of leakage sound or high-voltage arcing that is similar to the product in the experiment. The sound is loud or small, or Sometimes it is not; its rhythm is deep or harsh, or it is impermanent.
The audio noise generally refers to the audio signal generated by the switching power supply itself during operation, and can be heard by the human ear at an audio frequency of 20-20 kHz. When the oscillation frequency of the electronic and magnetic components is within the hearing range of the human ear, an audible signal is generated. This phenomenon has been known in the early days of power conversion research. Transformers operating at 50 and 60 Hz power frequencies often produce annoying ac noise. If the load is modulated with an audio component, a switching power converter operating at a constant ultrasonic frequency will also produce audible noise.
At low power levels, the audio signal is usually independent of the converter. However, designers may wish to reduce the acoustic emissions of their circuits. In low-power AC-DC converters, the core sheets of a 50 or 60 Hz transformer are soldered together to reduce the AC noise to an acceptable level. Similar techniques are used for ferrite transformers in high frequency switching converters.
In the past, advanced audio engineering equipment was used to study the acoustic radiation of switching power supplies. This device can measure absolute sound pressure level and sound spectrum very accurately, but human perception of sound is very subjective. It's hard to say how much sound can be heard, and it's harder to determine how much sound in a particular application would be considered unbearable noise.
Acoustic radiation is similar to electromagnetic radiation, but there is no universal benchmark for measuring hearing tolerance. Therefore, the designer can deal with the problems related to audible noise according to the following guidelines, and reduce the sound radiation of the product.
Power supply audio noise generation and suppression method
One: audible noise generated by the transformer
In most flyback converter applications, the transformer is the primary source of audio noise. The noise generated by the first transformer prototype on the test board is often surprising. Using well-known appropriate structural techniques will essentially eliminate noise without adding extra cost. Pay attention to the repeatability of the finished product performance when assembling the prototype transformer.
There are some mechanisms that generate transformer noise, each of which produces a mechanical displacement that produces a sound. These mechanisms include:
Relative motion—The attraction between the two parts of the core moves it, pressing the medium that separates it.
Impact—If the surfaces of the two cores are in contact, they move in response to flux excitation, causing them to collide or scratch.
Bending—The cracks that exist only in the middle leg of the core of the EE or EI structure can cause the various parts of the core to follow the direction of attraction.
Magnetostrictive—the size of the core material varies with the flux density. The rate of change of normal power ferrite is less than 1 ppm.
Skeleton Movement—The displacement of the magnetic chip can be transmitted and amplified through the skeleton.
Coil Movement—The current in the coil creates the attractive and repulsive forces that move these wires.
The mobile sources work together to form a complex mechanical system that produces strong resonances at one or several frequencies within the human ear's hearing range. The commonly used structure of offline flyback converters below 10W generally produces resonances from 10 kHz to 20 kHz. When the fundamental frequency of the magnetic flux excitation or its harmonics passes through the mechanical resonance region, the movement emits a sound. The designer should change the load throughout to verify the audio noise, especially if dynamic loading is required.
The magnitude of the noise generated by these mechanisms is determined by the different locations in which they are located. Fortunately, designers can apply simple structural techniques to effectively attenuate the audible noise generated by various mechanisms.
The following is a brief explanation of common methods that can effectively attenuate the audible noise generated by various mechanisms.
Firstly, the transformer should be uniformly impregnated, so that the inherent gap between the coil and the coil, between the coil and the skeleton, between the skeleton and the magnetic core can be effectively filled, and the possibility of displacement of the movable member can be reduced, and the magnetic component and the line can be re-required if necessary. The plate contact surface is filled with white glue or sprayed with three anti-paint to further reduce the space of mechanical vibration and effectively reduce noise.
Try to reduce the peak magnetic flux density as far as possible, and fully consider the saturation magnetic flux density at high temperature, leaving enough margin to prevent the working curve from entering the nonlinear region, which can effectively reduce the audible noise of the transformer. Decreasing the density from 3000 Gauss to 2000 Gauss reduces the noise emitted by 5dB to 15dB.
The conditions allow the use of soft magnetic materials such as amorphous and ultrafine crystal alloys, their magnetic uniformity is much better than that of general ferrite, and the magnetostrictive effect tends to zero, so it is not sensitive to stress.
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