Question E5D06:
What parasitic characteristic creates an inductor’s self-resonance?  

Choices:  
A: Skin effect  
B: Dielectric loss  
C: Coupling  
D: Inter-turn capacitance    

Correct Answer is  D: Inter-turn capacitance  

To understand an inductor’s self-resonance, remember that real inductors are not ideal—they have parasitic capacitance between turns of the winding. This “hidden” capacitance forms an LC resonator with the inductance.

Why D (Inter-turn capacitance) is correct:
- Adjacent turns act like capacitor plates, creating distributed or inter-turn capacitance.
- That capacitance in combination with the inductance produces a natural resonance (self-resonant frequency, SRF) where:
  $$X_L = X_C \quad \Rightarrow \quad \omega_0 L = \frac{1}{\omega_0 C} \quad \Rightarrow \quad f_0 = \frac{1}{2\pi\sqrt{LC_{\text{parasitic}}}}$$
- Below SRF, the inductor looks inductive; at SRF, impedance peaks; above SRF, it looks capacitive.

Why the other choices are incorrect:
- A: Skin effect — Increases AC resistance with frequency; it is a loss mechanism, not a reactive element that creates resonance.
- B: Dielectric loss — Another loss (dissipative) effect in insulation or core; it adds resistance, not resonance.
- C: Coupling — Mutual inductive coupling to nearby components can shift behavior but does not inherently create the inductor’s self-resonance; the core cause is the inductor’s own distributed capacitance.

Useful formulas and rules of thumb:
- Inductive reactance: $$X_L = 2\pi f L$$
- Capacitive reactance (from parasitic C): $$X_C = \frac{1}{2\pi f C}$$
- Self-resonant frequency: $$f_{\text{SRF}} = \frac{1}{2\pi\sqrt{L\,C_{\text{parasitic}}}}$$
- Rule of thumb: Choose inductors with SRF at least 5–10× higher than your operating frequency (for chokes/series inductors) to avoid unintended capacitive behavior.