Understanding the Q and D values of capacitors

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Understanding the Q and D values of capacitors [Copy link]

When doing RF, we pay special attention to their Q value when choosing inductors and capacitors. So what is Q value? What does Q value mean and why is it important? Quality factor Q: A quality indicator that characterizes the ratio of energy stored in an energy storage device (such as an inductor, capacitor, etc.) and a resonant circuit to the energy lost per week. The larger the Q value of a component, the better the selectivity of the circuit or network composed of 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 it can be. The factor Q is an important parameter to indicate the quality of the coil. The size of the Q value indicates the size of the inductor coil loss. The larger the Q value, the smaller the coil loss; conversely, the greater the loss. The quality factor Q is defined as: when the coil works under an AC voltage of a certain frequency, the ratio of the inductive reactance presented by the coil to the DC resistance of the coil. It can be expressed by the following formula:

Where:W——operating angular frequencyL——coil inductanceR——coil total loss resistance The requirements for the quality factor Q are different depending on the application. For the inductor coil in the tuning circuit, the Q value is required to be higher, because the higher the Q value, the smaller the loss of the circuit and the higher the efficiency of the circuit; for the gooseneck coil, the Q value can be lower; and for the low-frequency or high-frequency choke coil, no requirement is made. In fact, the improvement of the Q value is often limited by some factors, such as the DC resistance of the wire, the dielectric loss of the coil frame, the loss caused by the core and the shielding, and the skin effect during high-frequency operation. Therefore, the Q value of the coil cannot be very high. Usually, the Q value is from tens to one hundred, and the highest is only four or five hundred. Q value is equivalent to the reciprocal of D value. Loss angle is D value: Generally, electrolytic capacitors have a higher D value because of their large internal resistance. Its specifications are determined by the capacitance value and are below 0.1-0.24. Plastic film capacitors have a lower D value, which is determined by its material and is below 0.001-0.01. Ceramic capacitors are determined by their material, and the Hi-K type and S/C type are below 0.025. The specifications of T/C type, expressed in Q value, must be higher than 400-1000.