Introduction
Capacitor howling is because the electric field force is generated after the capacitor is energized; the capacitor expands slightly under the action of the electric field force; if the ripple on the capacitor is too large, the change of the electric field force is relatively large; the changing electric field force is applied On the PCB board, vibration is caused; if the ripple frequency is within the range of human hearing, the sound from the capacitor is heard.
The source of the current noise turned out to be it! totally unexpected!
Earlier we talked about a lot of common knowledge about capacitors, and the links are at the end of this article.
First tell a story:
Friends who are familiar with 2G/GSM modules know that the GSM transmit power is very high. When a GSM mobile phone makes a call, you often hear a "squeaky" current sound .
A 2G mobile phone project that "Yi Smart" did a few years ago, encountered the sound of electric current. When making a phone call, the earpiece position creaks non-stop.
Engineers have done a lot of experiments, including copper foil shielding, audio line string magnetic beads, and capacitors. Various solutions have been tossed for several days without improvement.
Later, I don't know which buddy cramped, and took a circuit board to test, only the circuit board did not have a circuit board with peripheral devices, not even an earpiece speaker.
As a result, the circuit board even heard a current sound!
There are no earphones and speakers. Where does the sound of a single circuit board come from?
The engineer came up with a way to use a chopstick , one end in the ear and one end on the circuit board, and he just found the source of the sound: the ceramic capacitor on the main power supply!
↑Picture. Rusty old antique
Why do capacitors ring?
Capacitor makes a sound, also called "capacitor whistle"
Capacitor howling is because the electric field force is generated after the capacitor is energized ; the capacitor expands slightly under the action of the electric field force; if the ripple on the capacitor is too large, the change of the electric field force is relatively large; the changing electric field force is applied On the PCB board, vibration is caused; if the ripple frequency is within the range of human hearing, the sound from the capacitor is heard.
Why does the capacitor of a GSM mobile phone ring?
GSM transmits in pulses, 217 times per second, and the time for each transmission is only a few milliseconds, but the pulse current can reach more than 2A.
所以GSM手机的电源系统，在打电话的时候抖动的非常厉害，所以人耳就能听到电容发出的217Hz的声音了。
↑图.GSM发射时的电池纹波
其他电容发声的场景：
电容啸叫，一般出现在DC-DC电路、周期性变化的大电流线路的叠层陶瓷电容上。
例如LCD屏幕的背光驱动，一般采用2K-20KHz的PWM调光，会发出PWM频率的声音。
再有就是DC-DC进入睡眠模式时，一般会掉入轻载模式，此时DCDC处于断续导通状态，在此状态下电源噪声频率很可能调入声音频率范围，在选择DCDC芯片时需要关注整机睡眠功耗和DCDC是否支持强制导通模式，避免DCDC开关调入20HZ-20KHZ声音频率范围。
还有就是像GSM这样的瞬间大功率的电源系统。
如何解决？
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空间大的，用液态电解电容。体积大，膨胀小，介质软，几乎没声音。
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不差钱的，用钽电容或聚合物电容。没有陶瓷电容那么硬，所以声音也小。
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选择开关型的元器件，尽量避开20-20KHz的频率范围，就听不到电容的声音了。
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选择带支架的陶瓷电容，把电容和电路板隔开，避免声音传导和放大。
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实在不行，只有把陶瓷电容和听筒喇叭离远点了。
总结和提问：
电容啸叫有两种处理结果：啸叫完全消除和啸叫减弱。
除了变换开关频率、使其在20HZ-20KHZ范围以外，换电解电容或者钽电容，其它处理方式都只是减弱啸叫而非消除。不管采用哪一种做法，能达到结果就行：最终电容啸叫的噪声分贝达到设备的要求。