# Choke Fundamentals
# Inductor/Choke Fundamentals
Inductors, commonly called "chokes" in VIC terminology, are the workhorses of the resonant circuit. They store energy in their magnetic field and, together with capacitors, determine the resonant frequency and voltage magnification capability of the VIC.
## What is an Inductor?
An inductor is a passive electrical component that stores energy in a magnetic field when current flows through it. The fundamental properties are:
#### Inductance (L):
Measured in Henries (H), inductance quantifies the magnetic flux linkage per unit current:
L = NΦ/I = N²μA/l
Where:
- N = number of turns
- Φ = magnetic flux
- I = current
- μ = permeability of core material
- A = cross-sectional area of core
- l = magnetic path length
## Key Inductor Parameters
| Parameter | Symbol | Units | Importance |
|---|
| Inductance | L | Henry (H) | Determines resonant frequency with C |
| DC Resistance | DCR, Rdc | Ohms (Ω) | Limits Q factor and causes losses |
| Self-Resonant Frequency | SRF | Hz | Must be > operating frequency |
| Quality Factor | Q | Dimensionless | Ratio of reactance to resistance |
| Saturation Current | Isat | Amps (A) | Max current before inductance drops |
## Inductor Construction
A practical inductor consists of:
1. **Wire:** Conductor wound into coils (turns)
2. **Core:** Material inside the coil (air, ferrite, iron, etc.)
3. **Form:** Structure that holds the winding
### Types of Cores
| Core Type | Permeability | Frequency Range | VIC Application |
|---|
| Air core | 1 (reference) | Any (no losses) | High-Q, low inductance |
| Iron powder | 10-100 | Up to ~10 MHz | Good for VIC frequencies |
| Ferrite | 100-10000 | 10 kHz - 100 MHz | Most common for VIC |
| Laminated iron | 1000-10000 | 50/60 Hz to ~10 kHz | Lower VIC frequencies |
## Inductance Formulas
#### Single-Layer Solenoid (air core):
L = (N²μ₀A)/l = (N²r²)/(9r + 10l) µH
Where r and l are in inches (Wheeler's formula)
#### With Magnetic Core:
L = AL × N² (nH)
Where AL is the inductance factor of the core (nH/turn²)
#### Toroidal Core:
L = (μ₀μrN²A) / (2πrmean)
## DC Resistance (DCR)
The DC resistance is determined by the wire properties:
Rdc = ρ × lwire / Awire
Where:
- ρ = resistivity of wire material (Ω·m)
- lwire = total wire length ≈ N × π × dcoil
- Awire = wire cross-sectional area
## Q Factor of Inductors
#### Inductor Q Factor:
Q = ωL/R = 2πfL/Rtotal
#### Rtotal includes:
- DC resistance of wire
- Skin effect losses (increases with frequency)
- Proximity effect losses
- Core losses (hysteresis + eddy currents)
## Self-Resonant Frequency (SRF)
Every inductor has parasitic capacitance between turns and layers:
SRF = 1 / (2π√(LCparasitic))
#### Design Rule:
SRF should be at least 10× the operating frequency.
At frequencies above SRF, the inductor acts like a capacitor!
## VIC Choke Design Goals
1. **Target inductance:** Sets resonant frequency with capacitor
2. **Low DCR:** Maximizes Q factor
3. **High SRF:** Ensures proper operation at intended frequency
4. **Adequate current rating:** Won't saturate or overheat
5. **Appropriate core:** Low losses at operating frequency
**Key Tradeoff:** More turns = more inductance, but also more wire = more DCR. The design challenge is achieving the target inductance with minimum resistance, which means selecting appropriate wire gauge, core material, and winding technique.
*Next: Core Materials & Properties →*