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Take you to know the Q and D values of the capacitor
2019-05-23
 
When doing RF, choose the inductor and capacitor to pay special attention to their Q value, then what is the Q value? What does the Q value mean and why is it important?

Quality factor Q: A quality indicator that characterizes an energy storage device (such as an inductor, capacitor, etc.), the energy stored in the resonant circuit, and the energy lost per week. The larger the Q value of a component, the better the selectivity of the circuit or network formed by the component.

Or Q = reactive power / active power, or the ratio of characteristic impedance to loop resistance.

The higher the Q value, the smaller the loss and the higher the efficiency;

The higher the Q value, the higher the frequency stability of the resonator and, therefore, the more accurate. The factor Q is an important parameter representing the quality of the coil. The size of the Q value indicates the loss of the inductor coil. The larger the Q value, the smaller the loss of the coil; on the contrary, the greater the loss.

The quality factor Q is defined as the ratio of the inductive reactance exhibited by the coil to the DC resistance of the coil when the coil is operated at an AC voltage of a certain frequency. It can be expressed as follows:

公式

式中:W——工作角频率 L——线圈电感量 R——线圈总耗损电阻

  根据使用场合的不同,对品质因数Q的要求也不同。对调谐回路中的电感线圈,Q值要求较高,因为Q值越高,回路的损耗就越小,回路的效率就越高;对鹅合线圈来说,Q值可以低一些;而对于低频或高频扼流圈,则可以不做要求。

实际上,Q值的提高往往受到一些因素的限制,如导线的直流电阻、线圈骨架的介质损耗、铁心和屏蔽引起的损耗以及高频工作时的集肤效应等。因此,线圈的Q值不可能做得很高,通常Q值为几十至一百,最高也只有四五百。

Q值相当于D值的倒数。损失角即D值: 一般电解电容器因为内阻较大故D值较高, 其规格视电容值高低决定, 为0.1-0.24以下. 塑料薄膜电容器则D值较低, 视其材质决定为0.001-0.01以下. 陶瓷电容器视其材质决定, Hi-K type 及S/C type为0.025以下. T/C type其规格以Q值表示需高于400-1000.

 
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