Coupled inductors are the secret weapon of high-current multiphase voltage regulators (VRMs) used in CPUs, GPUs, FPGAs and AI accelerator power delivery. By magnetically coupling the phases, designers achieve faster transient response, smaller overall size and lower output ripple compared with using separate inductors per phase.
This guide explains how coupled inductors work, the role of coupling coefficient (k), core material selection, and design tradeoffs for multiphase buck converters.
Shaanxi SiRui Technology Co., Ltd. manufactures custom coupled inductors and integrated magnetic modules using ferrite, powder core and nanocrystalline materials. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. What is a Coupled Inductor?
A conventional multiphase buck converter uses N independent inductors, one per phase. A coupled inductor replaces them with windings on a shared magnetic core, so the phases interact magnetically.
- Faster transient response: energy transfers between phases immediately
- Lower output ripple: ripple currents partially cancel
- Smaller size: shared core vs. N discrete inductors
- Better thermal distribution: heat spread across the core
2. Coupling Coefficient (k)
The coupling coefficient k (0 to 1) determines how strongly phases interact. k = 0 means no coupling (same as separate inductors). k = 1 means perfect coupling. Typical range for multiphase VRMs is k = 0.5–0.9.
Higher k = faster transient response, but also more cross-regulation between phases.
3. Coupled vs. Uncoupled Comparison
| Parameter | Uncoupled | Coupled |
|---|---|---|
| Transient response | Slow (per-phase) | Fast (energy shared) |
| Output ripple | High (adds linearly) | Lower (cancellation) |
| Size | N separate inductors | Shared core, smaller |
| Cross-regulation | None | Present |
| Design complexity | Simple | Requires careful geometry |
4. Core Materials
Mn-Zn ferrite is best for high-density VRM applications at 300 kHz–1 MHz. Powder cores (Sendust, Kool Mμ, High Flux) offer stable inductance under DC bias and are preferred for 20 A+ per phase. Nanocrystalline is the premium choice for AI accelerator VRMs where size is critical.
5. Design Steps
- Determine phase count (2–16 typical)
- Choose coupling coefficient (0.5–0.7 general, 0.8–0.9 fast transient)
- Calculate leakage inductance (sets per-phase ripple)
- Calculate mutual inductance (sets transient speed)
- Select core geometry (EE, UI, toroidal with multiple slots)
- Verify DC bias and thermal performance
6. Typical Applications
| Application | Phases | Current/Phase | Frequency |
|---|---|---|---|
| CPU VRM | 4–8 | 20–40 A | 300–800 kHz |
| GPU VRM | 6–12 | 30–60 A | 300–1 MHz |
| AI accelerator | 8–16 | 40–80 A | 500 kHz–1 MHz |
| Server POL | 2–4 | 10–20 A | 500 kHz–2 MHz |
7. Sirui-Tech Capabilities
- Custom coupled inductors: 2- to 8-phase, up to 100 A per phase
- Core materials: Mn-Zn ferrite, Sendust, Kool Mμ, High Flux, nanocrystalline
- Integrated magnetic modules: fully potted with mounting hardware
- Design support: coupling coefficient optimization and core selection
Conclusion
Coupled inductors are essential for modern high-current multiphase power delivery. The coupling coefficient k is the key design parameter. For AI and GPU power designs, coupled inductors are no longer optional.
Need a custom coupled inductor? Contact Sirui-Tech at sales@sirui-tech.com with your phase count, current per phase and switching frequency.