Core Materials
Core Materials & Properties
The core material of an inductor dramatically affects its performance. Choosing the right core is essential for achieving the desired inductance, Q factor, and frequency response in VIC applications.
Why Use a Core?
A magnetic core increases inductance by providing a low-reluctance path for magnetic flux:
L = μ₀μᵣN²A/l
The relative permeability (μᵣ) of the core multiplies the inductance compared to an air core.
Core Material Comparison
| Material | μᵣ (typical) | Frequency Range | Saturation | Cost |
|---|---|---|---|---|
| Air | 1 | Any | N/A | Free |
| Iron Powder | 10-100 | 1 kHz - 100 MHz | High (0.5-1.5T) | Low |
| Ferrite (MnZn) | 1000-10000 | 1 kHz - 1 MHz | Low (0.3-0.5T) | Medium |
| Ferrite (NiZn) | 50-1500 | 100 kHz - 500 MHz | Low (0.3-0.4T) | Medium |
| Laminated Silicon Steel | 2000-6000 | 50 Hz - 10 kHz | High (1.5-2.0T) | Low |
| Amorphous Metal | 10000-100000 | 50 Hz - 100 kHz | High (1.5T) | High |
| Nanocrystalline | 15000-100000 | 1 kHz - 1 MHz | High (1.2T) | High |
Core Losses
All magnetic cores dissipate energy through two mechanisms:
1. Hysteresis Loss
Energy lost each time the core is magnetized and demagnetized.
Ph ∝ f × Bmaxn (n ≈ 1.6-2.5)
Proportional to frequency and flux density.
2. Eddy Current Loss
Circulating currents induced in the core material.
Pe ∝ f² × Bmax²
Proportional to frequency squared - dominates at high frequencies.
Steinmetz Equation
Pcore = k × fα × Bβ × Volume
Where k, α, β are material-specific constants from datasheets.
Ferrite Materials for VIC
Ferrites are the most common choice for VIC frequencies (1-50 kHz):
| Material | μᵢ | Optimal Frequency | Application |
|---|---|---|---|
| 3C90 (TDK) | 2300 | 25-200 kHz | Power transformers |
| N87 (EPCOS) | 2200 | 25-500 kHz | General purpose |
| N97 (EPCOS) | 2300 | 25-150 kHz | Low loss |
| 3F3 (Ferroxcube) | 2000 | 100-500 kHz | Higher frequency |
| 77 Material (Fair-Rite) | 2000 | Up to 1 MHz | EMI/RFI suppression |
Iron Powder Cores
Micrometals and Amidon iron powder cores are popular for their:
- High saturation flux density
- Gradual saturation (soft saturation)
- Good temperature stability
- Self-gapping (distributed gap)
Common Iron Powder Mixes
| Mix | μ | Color | Frequency Range |
|---|---|---|---|
| Mix 26 | 75 | Yellow/White | DC - 1 MHz |
| Mix 52 | 75 | Green/Blue | DC - 3 MHz |
| Mix 2 | 10 | Red/Clear | 1 - 30 MHz |
| Mix 6 | 8 | Yellow | 10 - 50 MHz |
Core Shapes
Toroidal
Doughnut shape with closed magnetic path. Excellent flux containment, low EMI. Harder to wind but very efficient.
E-Core / EI-Core
E-shaped halves that mate together. Easy to wind on bobbin. Can add air gap easily.
Pot Core
Cylindrical with center post. Shields winding from external fields. Good for sensitive applications.
Rod Core
Simple cylindrical rod. Open magnetic path, lower inductance per turn but no saturation issues.
Core Saturation
When the magnetic flux density exceeds the saturation limit:
- Permeability drops dramatically
- Inductance decreases
- Current increases rapidly
- Core heating increases
Avoiding Saturation:
Bpeak = (L × Ipeak) / (N × Ae) < Bsat
Always check that peak flux density stays below saturation limit of your core material.
Recommendations for VIC
| Frequency Range | Recommended Core | Notes |
|---|---|---|
| 1-10 kHz | N97/3C90 ferrite or iron powder | Low loss at these frequencies |
| 10-50 kHz | N87/3F3 ferrite | Good balance of μ and loss |
| 50-200 kHz | 3F3/3F4 ferrite or Mix 26 powder | Lower permeability, lower loss |
| >200 kHz | NiZn ferrite or Mix 2 powder | Designed for high frequency |
VIC Matrix Calculator: The Choke Design module includes a core database with AL values and frequency recommendations. Select your core and it will calculate the required turns for your target inductance.
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