Sendust and High Flux are the two most popular powder cores for DC/DC power inductors. Here is how they differ in composition, DC bias, core loss and cost — and how to pick between them.
Introduction
When your buck converter needs a powder core, the choice usually comes down to two materials: Sendust (Kool Mμ) and High Flux. Both are distributed-gap powder cores with soft saturation and excellent DC bias handling — but they are engineered for different operating points. Sendust is the cost-effective generalist. High Flux is the high-current specialist with the best DC bias of any common powder core.
Shaanxi SiRui Technology Co., Ltd. supplies both Sendust and High Flux cores for power inductors worldwide. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. What Is a Sendust Core?
Sendust (also sold as Kool Mμ) is an alloy of iron, silicon and aluminum — about 85% Fe, 9% Si and 6% Al. Its particles are coated and pressed into a distributed-gap core. Sendust is the most popular general-purpose powder core because it balances cost, loss and DC bias for most applications.
- Permeability range: 26 – 125 (typical)
- Saturation flux density: ~1.0 T
- Core loss: moderate — higher than ferrite and MPP, lower than iron powder
- Cost: low to moderate, very competitive for high-volume production
- No audible noise: unlike gapped ferrite, Sendust does not magnetostrict or buzz at typical ripple frequencies
2. What Is a High Flux Core?
High Flux is a nickel-iron alloy (about 50% Ni, 50% Fe). The high nickel content gives it the highest saturation flux density (~1.5 T) and the best DC bias stability of the common powder cores.
- Permeability range: 14 – 160 (typical)
- Saturation flux density: ~1.5 T
- Core loss: higher than Sendust at the same flux swing — nickel adds loss
- Cost: significantly higher than Sendust (nickel is expensive)
- Best DC bias: inductance holds up best under high DC current of any powder core
3. Sendust vs High Flux: Key Differences
| Property | Sendust | High Flux |
|---|---|---|
| Composition | Fe-Si-Al (~85/9/6) | Ni-Fe (50/50) |
| Permeability | 26 – 125 | 14 – 160 |
| Saturation flux density | ~1.0 T | ~1.5 T |
| DC bias stability | Good | Excellent — the best of common powder cores |
| Core loss | Low to moderate | Moderate to high (nickel) |
| Temperature stability | Very stable | Very stable |
| Relative cost | Low to moderate | High (nickel content) |
| Typical applications | Buck/POL inductors, PFC, output chokes, general filtering | High-current buck/POL, VRM, server & telecom power, inrush-heavy loads |
4. DC Bias: The Real Difference
DC bias is what separates these two materials in practice. When DC current flows through an inductor, it biases the core toward saturation. The question is how much inductance you keep at the worst-case current:
- Sendust: retains roughly 90% of its initial inductance at 50% of rated bias current, and about 50–60% near saturation. Fine for typical 10–30 A designs.
- High Flux: retains a flatter inductance curve under the same bias because of its higher Bsat. For the same size, a High Flux core can carry roughly 30–40% more DC current before reaching the same inductance roll-off.
That is why High Flux is the default for server VRM phases, 48 V POL converters and other high-current rails — the extra material cost pays for itself in a smaller core and lower winding resistance.
5. When to Choose Sendust
- General-purpose buck/POL up to ~30 A: Sendust covers most designs with the lowest cost.
- Cost-driven high-volume products: the Fe-Si-Al alloy is much cheaper than nickel-based High Flux.
- PFC boost and output chokes: moderate bias, moderate loss, proven reliability.
- Low audible noise requirements: both materials are quiet, but Sendust’s lower magnetostriction is a known advantage.
6. When to Choose High Flux
- High DC bias (30 A+ per phase): VRM, server and telecom power rails where inductance must hold at full load.
- Current spikes and inrush: the higher Bsat gives extra headroom before saturation.
- Size-constrained high-current designs: the flatter bias curve lets you use a smaller core for the same worst-case inductance.
- Negative-current / transient-heavy loads: processors, ASICs and GPUs demand stable inductance across fast load steps.
7. Common Design Mistakes
- Choosing High Flux for a low-current design and paying 2–3× more for capability you never use.
- Choosing Sendust for a 40 A+ phase and watching the inductance collapse at full load.
- Comparing only catalog permeability — under DC bias the “same permeability” Sendust and High Flux diverge sharply.
- Ignoring core loss: at high ripple current, Sendust’s lower loss can beat High Flux even in a high-bias design — always check both loss and bias.
- Not checking the bias curve at your actual worst-case current and temperature.
8. Powder Cores from SiRui-Tech
At Shaanxi SiRui Technology Co., Ltd., we supply the full powder core family with DC-bias-annotated data so you can select with confidence:
- Sendust cores (26–125 μ) for general-purpose power inductors and PFC
- High Flux cores (14–160 μ) for high-current POL, VRM and server power
- MPP cores for the lowest powder-core loss
- Iron powder cores for cost-driven line-frequency designs
Start with our Magnetic Core Selection Guide, see how powder cores compare with ferrite in our ferrite vs powder core guide, and understand the loss trade-offs with our core loss calculation guide. For buck converter design steps, see our Buck Converter Inductor Design Guide.
Web: www.sirui-tech.com | Email: sales@sirui-tech.com
FAQ
Q1. Is High Flux better than Sendust?
Not universally. High Flux wins on DC bias and saturation flux; Sendust wins on cost and, at moderate bias, on core loss. Choose by your worst-case DC current and budget.
Q2. Can I replace Sendust with High Flux directly?
Only if you re-check the design. The same part number in High Flux has different permeability, loss and size characteristics — always verify the bias curve and thermal performance.
Q3. Why is High Flux more expensive?
The 50% nickel content is a costly raw material. Nickel also raises core loss, so High Flux is only worth the price when DC bias is the limiting factor.
Q4. What permeability should I pick?
Start with 60 μ (Sendust) or 60–125 μ (High Flux) for typical buck converters. Lower permeability holds inductance better under bias but needs more turns; higher permeability gives more inductance per turn but saturates sooner.
Conclusion
Sendust and High Flux are complementary tools, not competitors. Use Sendust where cost matters and bias is moderate. Use High Flux where the current is high and inductance must hold. Checking the DC bias curve at your worst-case operating point — not the catalog permeability — is what separates a working design from a field failure.