Linkcom Manufacturing Co., Ltd

In the laboratory late at night, the power supply engineer is staring intently at the waveform display on the test bench. Suddenly, a sharp, disturbing humming sound broke the silence, as if coming from deep within a transformer. This is not an uncommon phenomenon, but it is always a headache. This abnormal sound may hide a design flaw or even indicate a potential crisis in the performance of the entire system. In the PoE (Power over Ethernet) power supply system, the abnormal noise of magnetic components not only affects the product quality, but may also become a stumbling block to the user experience. This article will take you to an in-depth discussion of the causes, diagnostic methods and effective solutions for abnormal noise in magnetic components.
Causes of abnormal sound
The abnormal sound problem of PoE power supply may come from many factors. The following are some common reasons:
1. Electromagnetic Interference (EMI)
  The high-frequency signal between the PoE power supply device and the load may generate    electromagnetic radiation and affect surrounding components.
There are sources of high-frequency interference (such as Wi-Fi routers or industrial machinery) near the device.

2. Transformer or magnetic component saturation
In PoE equipment, poor transformer and power inductor design or improper core material selection may cause abnormal sound.
Core saturation or unstable flux changes.
3. High-frequency noise of switching power supply
PoE power supply uses DC-DC converter or switching power supply, these components may produce abnormal sound due to high-frequency pulses.
Significant fluctuations in output voltage or current may result in increased power supply noise.

4. Poor PCB layout and grounding design
Poor layout of the power supply line may cause common-mode noise or parasitic oscillation, causing abnormal sound in the entire power supply system.
A floating ground may cause noise accumulation.

5. Load characteristics
When some devices are powered by PoE, they may generate noise due to transient current changes.
Loads using high power devices (such as motors or LED drivers) may affect stability.

Although magnetic components such as transformers or inductors are often the source of audible and audible noise in PoE power systems, accurate analysis and testing are critical in resolving audible and audible noise issues caused by magnetic components. The following are some commonly used technical methods, focusing on the source, characteristics and possible solutions of abnormal sounds

1. Acoustic testing
- Use a highly sensitive directional microphone or acoustic pickup to monitor for unwanted sounds, especially in a laboratory environment. The microphone should be placed 10-20 cm away from the magnetic element to ensure accurate sound pickup.
- Perform spectral analysis on the acquired sound waves to identify the main frequency ranges. The noise is usually concentrated in the range of 500Hz to 5kHz.
2. Vibration test
- Use an accelerometer to measure the physical vibration of a magnetic component and determine the magnitude and direction of the vibration.
- Use spectrum analysis to understand the correlation between vibration and sound, whether it is caused by current or design defects.
- It is recommended to combine the data acquisition system to monitor the vibration of components under different power loads in real time.

3. Current waveform analysis
- Use an oscilloscope to capture the waveform of the power supply current and check whether there is high-frequency ripple or current distortion, which may be one of the root causes of abnormal noise.
- It is recommended to use it in conjunction with a current analyzer to record the ripple amplitude and its changes under different operating conditions.

4. Thermal imaging test
- Use a thermal imager to detect the surface temperature distribution of magnetic components. Abnormal noise is sometimes associated with overheating of the component, such as material expansion or structural instability.
- When the component is operating under high load, record the temperature changes and analyze whether there are local hot spots.

Through the above methods, if the magnetic component is determined to be the source of the abnormal sound, the following are several main causes of the abnormal sound. R&D personnel can understand the cause and take more effective improvement measures:

1. Magnetostrictive effect
Magnetostrictive materials undergo tiny deformations under the action of a magnetic field, causing periodic vibrations that produce sound.
2. Structural resonance
Certain structural parts of magnetic components (such as coils or iron cores) will resonate at specific frequencies, amplifying the intensity of abnormal sounds.

3. Current ripple and waveform distortion
Non-ideal current or waveform distortion may also stimulate physical vibrations in components.

In view of the above reasons, the abnormal noise problem can be solved by adjusting the resonance frequency, reducing vibration, absorbing/blocking sound waves and other different countermeasures. There are too many variations to list. At the end of this article, a recent case is briefly shared, hoping to help readers understand the practical approach.

To troubleshoot:
1. Initial testing: acoustic monitoring
Engineers used highly sensitive microphones to approach key components in the module (MOSFET, capacitors, transformers) one by one, and simultaneously adjusted the module's load conditions to simulate operating conditions.
- Result: The noise is most obvious near the transformer, and the audio frequency range is concentrated in 1.5kHz - 3kHz, indicating that this area may be the source of the noise.

2. In-depth analysis: waveform and component behavior detection
The engineer then performed waveform analysis on the relevant components to confirm the possible cause of the abnormal sound:
- MOSFET switching waveform: Check if the driving frequency has spikes or instability, especially if high-frequency ripple is triggered under heavy load conditions
- Filtering effect of capacitors: Observe whether there is excessive voltage ripple at the output, which may cause physical vibration of other components.
- Transformer operating waveform: Check the current waveforms on the primary and secondary sides to confirm whether there is magnetic saturation or waveform distortion, which may cause structural vibration.

- Results: The waveform data shows that the switching ripple of the MOSFET causes the transformer to resonate at a specific frequency, and the capacitor fails to filter it sufficiently, further exacerbating the effect of the ripple.

3. Final diagnosis and improvement measures
Based on acoustic and waveform analysis, the root cause of the problem was determined to be as follows:
- MOSFET switching frequency: too close to the transformer's resonant frequency, resulting in amplified abnormal sound.
- Transformer structure design: The winding is not fixed firmly enough and the resistance to vibration is insufficient.
- Insufficient capacitor filtering: Failure to effectively suppress high-frequency ripples, causing the ripples to further affect other components.

Solution:
- Adjust the PWM drive frequency of the MOSFET to avoid matching the natural resonance frequency band of the transformer.
- Strengthen the structural design of the transformer and perform glue dispensing on the winding part to reduce the possibility of coil vibration.
- Replace capacitors with higher performance low ESR (equivalent series resistance) products to improve filtering effect.

Through the above case sharing, we hope that readers can further understand the process of dealing with abnormal sound problems. In the current Flyback/Forward architecture, although abnormal noise is common in PoE power supply systems, it can be solved through scientific analysis and reasonable design. As technology continues to advance, future magnetic components will be quieter and more efficient, bringing more possibilities to the application of PoE systems.