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Answers to quiz questions

Figure 1 depicts capacitance between the windings of a common mode choke (the subject of question 1.) By definition, common mode signals are identical at both terminals; they both ride up and down together (remember, we're talking about common mode signals only.) The capacitance - all the little capacitors - see no change in voltage across them. There is no charging/discharging of the capacitance between windings and it has no affect on the common mode performance of the choke. Resonant frequency and Q do not depend on the capacitance between windings as far as common mode signals are concerned.

Figure 2 depicts capacitance to ground (the subject of question 2.) The capacitance forms a low pass LC filter in combination with the inductance of the choke. This is generally what the choke is used for anyway; for common mode rejection caps to ground are typically placed after the choke and sometimes before. There are a couple of interesting points:

- The capacitance is 'seen' by both common mode and differential mode signals, however the inductance of the choke is much greater for common mode signals, so the corner frequency of the LC filter is completely different for common mode and differential mode signals; very much lower for common mode.

- To first order, the capacitance to ground does not influence the resonant frequency or Q of the choke.

Figure 3 depicts capacitance between individual turns in each winding as well as between layers in the winding. This is the capacitance that is responsible for self-resonance; it is effectively in parallel with the inductance and forms a parallel resonant filter. At frequencies above resonance the filter appears a capacitive not inductive and the impedance falls with increasing frequency. That is in contrast to the rising impedance of an ideal choke.

Since this capacitance is the one that is responsible for self-resonance, it is also the one that most influences the Q, MUCH more than those discussed above.

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In answer to c-J's quiz question, loosely wound turns are used to minimize the capacitance between turns as described above. Whether or not there are just a few turns depends on the intended use. At very high frequencies the capacitance between turns needs to be very low and a few turns will yield enough inductance. At lower frequencies more - and consequently tighter - turns might be used.

-- Dave


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