GaN and SiC devices push switching frequencies from 100 kHz into the 300 kHz–2 MHz range. That shrinks the passive components in theory — but only if the magnetic material and the winding are chosen for that frequency. Reusing a 100 kHz ferrite and a round-wire winding at 500 kHz produces a part that is smaller on paper and hotter in practice.
1. What changes at high frequency
| Effect | Consequence | Design response |
|---|---|---|
| Core loss rises with f^α (α ≈ 1.2–1.8) | Material loss density dominates | Low-loss ferrite grade, or powder core at lower flux |
| Skin depth falls | AC resistance rises sharply | Litz or foil; keep single layer where possible |
| Proximity effect between layers | AC/DC resistance ratio above 3 | Interleave, reduce layers, use planar |
| Flux swing falls with smaller core | Core loss per volume can stay flat | Verify loss at the actual B and f |
| Winding capacitance becomes visible | Resonance inside the switching band | Section the winding, control the layer order |
2. Material selection rules
Below 200 kHz, standard power ferrite is usually optimal. Between 200 kHz and 1 MHz, a low-loss high-frequency grade reduces loss density by 30–50% versus a general-purpose grade — worth the price premium because it can remove a core size. Above 1 MHz, ferrite loss becomes steep and the design often moves to a smaller flux swing with a lower-loss material, or to an air-core inductor where the inductance requirement is small.
Powder cores are attractive for high-frequency inductors with heavy DC bias because their soft saturation and stable permeability matter more than absolute loss, but their loss at 500 kHz is generally higher than a good ferrite at the same flux.
3. Winding at 500 kHz and above
Skin depth in copper at 500 kHz is about 93 µm, at 1 MHz about 66 µm. A 0.4 mm wire is already several skin depths thick, so most of the copper is not carrying current. Practical choices, in order of cost:
- Multiple parallel strands of fine wire, twisted, if the layer count stays at one or two.
- Litz wire with a strand diameter of 2–3 skin depths and a serve that fits the window.
- Foil for low-turn, high-current windings — foil has minimal proximity loss in a single layer.
- Planar windings on the PCB, which give excellent repeatability and low profile at the cost of copper thickness limits.
4. Parasitics become functional parameters
At 500 kHz a few picofarads of interwinding capacitance can create a resonance near the switching frequency, and 2 cm of lead length is a nanohenry-scale inductor. In a resonant topology this is not a parasitic to tolerate; it is part of the tank. Specify leakage inductance and capacitance explicitly, and measure them on the first sample.
5. Thermal density is the real limit
A 5 mm × 5 mm core running at 1 MHz may dissipate 1 W in a volume of 0.1 cm³. Surface area, not loss percentage, sets the temperature. High-frequency designs live or die on how well the heat leaves the component: planar windings with a thermal path through the PCB, potting to a heatsink, or forced air.
6. Practical design sequence
- Fix the frequency from the device and the EMI budget, not from the material catalogue.
- Calculate flux swing for the smallest core that fits the window at the required turns.
- Check core loss at that B and f, at the expected hot-spot temperature.
- Check AC copper loss with the chosen conductor; if the ratio exceeds 2, change the winding.
- Verify the resonant frequency of the winding is well above the switching band.
Shaanxi SiRui Technology supplies Magnetic Core series high-frequency ferrite grades with loss curves to 1 MHz, plus planar and Litz-wound assemblies for GaN and SiC converters.
Send your frequency and flux target to sales@sirui-tech.com.