Capacitors also whistle

In the project, a DCDC current source is used for the projected laser light source. Since it is a single DLP, the RGB three-color light needs to be modulated and output at 4k/60Hz. The way to achieve 4K depends on dithering, so the modulation frequency of the light source is 240Hz, and the final result is There was a loud buzzing sound on the power supply board. My unreliable experience told me that it must be the whistling sound of the inductor. After the subsequent adjustment, who knows that it will be impossible to fix it later.

mlcc-scream-2.jpg The above picture is the current of the G channel

Looking for manufacturers, Taobao, buying inductors of various crafts and models, and testing them back, it still doesn’t work. At this point, I began to wonder if the inductor is whistling, and which inductor is whistling. Occasionally, my colleague led the inductor out through the flying wire. When I listen to it, the inductor has no sound at all!

mlcc-scream-1.png

The picture above is the test diagram of the inductor flying out

Is the capacitor whistling? I have never heard of capacitor whistling before. As a result, I checked the Internet and found that it really exists. MLCC capacitors are also ceramic capacitors, and piezoelectric ceramic chip speakers are not ceramic technology. They are Not the same principle.

I have used the GSM module before. After the power is turned on and needs to be connected to the Internet, the power supply will whistle. It is said on the Internet: The PA power supply used by GSM, the power line is characterized by large power fluctuations and a typical fluctuation frequency of 217Hz. When it falls within the range of human hearing (20Hz~20Khz), when GSM is talking, listen to the capacitor on the power line with a special stethoscope, and it is easy to hear the “ZZZ” whistle. It seems that this situation is the same as what we encountered.

1. Now the switching frequency of the board-level DCDC is above 500k, and it is unlikely that the switching frequency will cause noise. Our application does not belong to this situation.

2. When the power supply is in light-load mode, the switch PWM is discontinuous and will work intermittently, outputting several pulses intermittently. The frequency of this intermittent pulse may also be heard by the human ear. Therefore, it is also necessary to optimize the working frequency of intermittent pulses during PFM operation from the perspective of power supply or load to avoid howling. The driving power supply of our light source will not work in the light-load mode, because everyone wants the output brightness of the projection to be bright enough, even if it is dimmed, it is not a light-load mode.

3. When the saturation current of the BUCK inductor is not selected properly, it may increase the output current, which will trigger the power supply to enter the overcurrent protection by mistake. The power supply switches repeatedly between the normal working mode and the overcurrent protection mode, which may also cause whistling. , the inductor selection must be appropriate. This problem is a design parameter problem, we also checked this situation, also replaced many types and parameters of inductors.

4. The layout plan mentioned on the Internet is not feasible. We finally chose the most thorough plan, instead of using MLCC capacitors.

Replace capacitors with electrolytic capacitors, or solid capacitors, polymer capacitors

mlcc-scream-3.jpg

Replace the inductor

The board whistling is gone!

This article is reprinted from: http://loonlog.com/2022/4/24/mlcc-scream/
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