Q and D values of capacitors

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Q and D values of capacitors [Copy link]

When working with RF, we pay special attention to the Q value when selecting inductors and capacitors. So what is the Q value? What does the 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.) or a resonant circuit to the energy lost per cycle. 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 Q factor 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 the ratio of the inductive reactance of the coil to the DC resistance of the coil when the coil operates under an AC voltage of a certain frequency. It can be expressed by the following formula:

Where: W——operating angular frequency L——coil inductance R——total loss resistance of coil

　　Depending on the application, the requirements for the quality factor Q are different. For the inductor 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, no requirement is required.

In fact, the improvement of 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 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 tens to one hundred, and the highest is only four or five hundred.

The Q value is equivalent to the inverse of the D value. The loss angle is the D value: Generally, electrolytic capacitors have a higher D value because of their larger internal resistance. Their 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 their material and is below 0.001-0.01. Ceramic capacitors are determined by their material and are below 0.025 for Hi-K type and S/C type. The specifications of T/C type are expressed in Q value and must be higher than 400-1000.