The magnetic core is the single component that determines whether a high-frequency transformer can hit its efficiency target, survive thermal limits, and fit the enclosure. Choose the wrong material and you either leave 2–3 points of efficiency on the table or watch core loss push the temperature past 105°C. Choose the right core for the topology and frequency, and the transformer shrinks, cools down, and runs quieter.
This guide walks through the four core families used in high-frequency transformers—ferrite, nanocrystalline, amorphous and powder cores—where each wins, how to estimate core loss, and how to match the geometry to the converter.
Shaanxi SiRui Technology Co., Ltd. manufactures all four core families and custom magnetic components for EV charging, industrial power, renewable energy and telecom. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. Why the Magnetic Core Defines High-Frequency Transformer Performance
At line frequency (50/60 Hz), transformers are dominated by winding resistance and copper loss. Once you push switching frequency above 20 kHz, the balance flips:
- Core loss scales with frequency—roughly with f^(1.3–2) depending on material.
- Flux density swings shrink—you can run far fewer millitesles per cycle than at 50 Hz.
- Winding loss moves to proximity and skin effect, not DC resistance.
The core is no longer a passive path for flux. It is an active loss source that sets the thermal budget, and its permeability and saturation flux density set the inductance and energy-handling capability. A well-designed high-frequency transformer is therefore a core-material decision first and a winding-layout decision second.
2. Core Materials for High-Frequency Transformers
2.1 Ferrite Cores (Mn-Zn and Ni-Zn)
Ferrite is the workhorse of high-frequency power conversion. It is a ceramic material—manganese-zinc (Mn-Zn) for frequencies up to ~500 kHz, and nickel-zinc (Ni-Zn) for 500 kHz and above.
- Permeability: 1,500–15,000 (Mn-Zn); 100–1,000 (Ni-Zn)
- Saturation flux density: ~0.35–0.5 T at 25°C, dropping sharply above 100°C
- Core loss: the lowest per unit volume in the 20 kHz–1 MHz range
- Cost: low to moderate, widely available
- Best for: SMPS flyback, forward, half-bridge, LLC resonant converters up to ~10 kW
The weakness of ferrite is its low Bsat and its temperature sensitivity—a Mn-Zn core can lose 30% of its flux capacity between 25°C and 120°C. Thermal design has to derate accordingly.
2.2 Nanocrystalline Cores
Nanocrystalline material is a thin ribbon of iron with nanoscale grains (10–20 nm), annealed into a toroid or cut core. It sits between ferrite and amorphous in performance.
- Permeability: 10,000–100,000
- Saturation flux density: ~1.2 T
- Core loss: very low at 20–100 kHz, higher than ferrite above 200 kHz
- Cost: high (premium over ferrite)
- Best for: high-current common-mode chokes, PFC inductors, EMI filters, and high-efficiency transformers in EV on-board chargers and solar inverters
Nanocrystalline is the preferred material when size and efficiency both matter—its high Bsat lets you run a smaller core at higher flux, and its high permeability makes it excellent for CM filtering.
2.3 Amorphous Cores
Amorphous iron ribbon is glass-like metal with no crystal structure. It has higher Bsat than nanocrystalline but higher loss.
- Permeability: 800–1,500
- Saturation flux density: ~1.5–1.6 T
- Core loss: moderate to high, typically used below 20 kHz
- Cost: moderate
- Best for: line-frequency transformers, high-current inductors, wound-core distribution transformers
Above ~20 kHz, amorphous loss climbs quickly, so it is rarely the first choice for modern high-frequency converters.
2.4 Powder Cores (Sendust, High Flux, MPP, Iron Powder)
Powder cores are iron-based particles pressed with a distributed air gap. They are mostly used as inductors rather than transformers, but they appear in coupled inductors and boost inductors.
- Permeability: 14–200
- Saturation flux density: 1.0 T (Sendust) to 1.5 T (High Flux)
- Core loss: higher than ferrite at high frequency, but soft saturation and excellent DC bias
- Best for: buck/boost inductors, PFC chokes, output filters—not typically transformer cores
3. Core Loss in Practice: The Steinmetz Equation
Core loss is predicted with the generalized Steinmetz equation:
Pv = k · f^α · B^β
where Pv is loss per unit volume (mW/cm³), f is frequency, B is peak flux density, and k, α, β are material constants published by the core vendor.
Practical takeaways:
- Halving B cuts loss by roughly a factor of 4–8 (β ≈ 2–3). Running lower flux density is the fastest way to cool a transformer.
- Doubling frequency roughly doubles to triples loss (α ≈ 1.3–1.6). Pushing from 50 kHz to 200 kHz is not free.
- Always use the vendor’s curves at your actual temperature. Ferrite loss spikes above 100°C on some grades.
A common design error is picking a core from a 25°C loss curve and then wondering why the transformer runs 20°C hotter than calculated.
4. Core Geometry: EE, ETD, Toroid and Planar
Material is half the decision; geometry is the other half.
| Geometry | Winding window | Benefits | Best for |
|---|---|---|---|
| EE / EI | Large, accessible | Low cost, easy winding, modular | Flyback, forward, general SMPS |
| ETD / EC | Rounded window | Better winding uniformity, lower proximity loss | Half-bridge, LLC |
| Toroid | Closed, low EMI | Zero air-gap leakage, best shielding | CM chokes, current transformers |
| Planar | Flat PCB windings | Very low profile, excellent thermal | High-frequency POL, telecom, EV |
| Cut C-core | Large window | High power, easy assembly | >10 kW industrial transformers |
For high-frequency transformers above 100 kHz, planar cores and ETD shapes consistently outperform simple EE because the winding window minimizes proximity-effect copper loss.
5. Matching the Core to Your Converter
Flyback / Forward (20–200 kHz)
Mn-Zn ferrite EE or ETD. Keep B below 0.2 T to control loss. Cost is the primary driver.
LLC Resonant Converter (50–500 kHz)
Low-loss Mn-Zn ferrite (e.g., PC95, 3C95 equivalent). LLC runs near-resonant with near-square-wave current, so low core loss at high B is essential. ETD or PQ shapes.
EV On-Board Charger (OBC, 50–200 kHz, >3 kW)
Nanocrystalline toroid for PFC and resonant inductors; Mn-Zn ferrite for the high-frequency transformer. Efficiency target >96% makes premium material worth the cost.
Solar Inverter (20–50 kHz, >5 kW)
Ferrite or nanocrystalline. Toroidal nanocrystalline for EMI filtering, ferrite EE for the isolation transformer.
Telecom / POL (200 kHz–1 MHz)
Ni-Zn ferrite or planar ferrite. Low profile dominates; efficiency >95% required.
6. Quick Selection Table
| Application | Frequency | Recommended Core | Why |
|---|---|---|---|
| Flyback SMPS | 20–100 kHz | Mn-Zn ferrite EE | Low cost, adequate loss |
| LLC resonant | 50–500 kHz | Low-loss Mn-Zn ferrite (PC95 class) | Minimal switching loss |
| EV OBC PFC / LLC | 50–200 kHz | Nanocrystalline toroid + ferrite | High efficiency, compact |
| Solar inverter | 20–50 kHz | Ferrite EE / nanocrystalline | Cost vs. efficiency balance |
| Telecom POL | 200 kHz–1 MHz | Ni-Zn / planar ferrite | High frequency, low profile |
| CM EMI filter | 10 kHz–1 MHz | Nanocrystalline toroid | High permeability, low leakage |
7. Sirui-Tech Magnetic Core Capabilities
Shaanxi SiRui Technology Co., Ltd. manufactures and supplies:
- Ferrite cores: Mn-Zn and Ni-Zn in EE, ETD, PQ, toroid and custom geometries
- Nanocrystalline cores: toroids, cut cores and common-mode choke cores from 10 mm to 120 mm
- Powder cores: Sendust (Kool Mμ class), High Flux, MPP and iron powder toroids
- Finished magnetic components: transformers, inductors and current transformers wound to customer specification
All cores are RoHS and REACH compliant, tested by SGS and CTI. Custom OEM and ODM programs are available for new energy, industrial and telecom customers.
Conclusion
Magnetic core selection for high-frequency transformers is a trade-off between core loss, saturation flux density, temperature stability and cost. Ferrite dominates the 20 kHz–1 MHz range for pure transformer applications. Nanocrystalline wins when size, efficiency and EMI filtering are top priorities. Powder cores remain the best choice for inductors with heavy DC bias.
Match the material to your switching frequency, keep peak flux density below the vendor’s recommended limit, and verify core loss at your actual operating temperature—not at 25°C.
Need help selecting the right core for your design? Contact Sirui-Tech at sales@sirui-tech.com with your topology, frequency, power level and target efficiency—our engineering team will recommend the optimal core material and geometry.