Key magnetic cores, CMC chokes & power inductors for 3.6kW–22kW EV chargers
Introduction
The on-board charger (OBC) is one of the most power-dense converters in an electric vehicle. Converting AC grid power to high-voltage DC battery charging requires a carefully optimized magnetic component stack—including power factor correction (PFC) inductors, high-frequency transformers, common mode chokes (CMC), and output inductors.
As EV power levels rise from 3.6 kW to 11 kW and even 22 kW three-phase systems, magnetic material selection has become a critical differentiator for efficiency, size, and cost. In this guide, we break down the magnetic components inside a modern OBC and how engineers choose between ferrite, powder cores, and nanocrystalline materials.
Shaanxi SiRui Technology Co., Ltd. supplies magnetic cores and custom magnetic components for OBC applications worldwide. Read on to understand how each magnetic part contributes to overall charger performance.
1. Typical OBC Topology & Magnetic Component Overview
Most modern OBCs use a two-stage architecture:
Stage 1: PFC (Power Factor Correction)
- Boost-type or totem-pole PFC
- Converts AC input to ~400V–800V DC bus
- Key magnetic part: PFC Boost Inductor
Stage 2: Isolated DC/DC Converter
- LLC resonant converter (most common)
- DAB (Dual Active Bridge) for bidirectional V2G
- Key magnetic parts: High-Frequency Transformer, Output Inductor
EMI Filter Stage (Input & Output)
- Conducted EMI suppression per CISPR 25 / CISPR 11
- Key magnetic part: Common Mode Choke (CMC)
Magnetic Component Count in a Typical 11 kW OBC
| Position | Component | Typical Material |
|---|---|---|
| AC Input EMI | Common Mode Choke | Nanocrystalline |
| PFC Stage | Boost Inductor | Powder Core (Sendust / High Flux) |
| Isolation Stage | LLC Resonant Transformer | Ferrite (PC95) or Nanocrystalline |
| Output Filter | Output Inductor | Powder Core |
| DC Output EMI | Common Mode Choke | Nanocrystalline |
2. PFC Boost Inductor: Powder Core vs. Ferrite
The PFC boost inductor sees high DC bias current plus high-frequency ripple. This is where magnetic material selection matters most.
Why Powder Core Dominates in PFC Inductors
Distributed Air Gap Advantage:
- Powder cores (MPP, Sendust, High Flux) have a distributed air gap built into the material
- This eliminates the fringing flux losses seen in gapped ferrite cores
- DC bias characteristics are far superior: inductance roll-off is gradual and predictable
Typical Materials for OBC PFC:
- Sendust (Kool Mμ equivalent): Best cost-performance, moderate loss
- High Flux: Higher saturation flux density (15,000 G), for high-current designs
- MPP: Lowest loss, highest cost, for premium efficiency targets
Ferrite Alternative
Ferrite cores (EE, EC, PQ shapes) with a physical gap are still used in lower-power OBCs (3.6 kW). They offer:
- Lower core loss at high frequency
- Lower cost
- But suffer from: sharp inductance drop under DC bias, fringing flux losses near the gap, audible noise risk
Design Tip: For 11 kW OBCs switching at 50–100 kHz, powder core toroids are the standard choice. A typical PFC inductor uses 2–4 stacked toroidal powder cores.
3. LLC Resonant Transformer: Ferrite vs. Nanocrystalline
The LLC resonant transformer provides galvanic isolation and voltage step-down. Its design is tightly coupled to the resonant tank.
Ferrite Transformers (Most Common)
Core Materials:
- PC95 / PC96 grade Mn-Zn ferrite (TDK, Hitachi, or equivalent)
- Core shapes: PQ, ER, EE
- Switching frequency: 100–300 kHz
Advantages: Mature, low-cost supply chain; low core loss at high frequency; well-established winding techniques
Limitations: Saturation flux density only ~4,000–5,000 G; requires larger core cross-section for high power
Nanocrystalline Toroidal Transformers (Premium Design)
More advanced OBCs are switching to nanocrystalline toroidal transformers:
Advantages:
- Saturation flux density ~12,000 G (3x ferrite)
- Permeability up to 100,000+
- Core volume reduced by 40–60% vs. ferrite
- Lower no-load loss
Trade-offs: Higher material cost; winding on toroids requires specialized equipment
Trend: As OBC power density targets move above 4 kW/L, nanocrystalline transformers are gaining adoption in premium EV platforms.
4. Common Mode Chokes (CMC): Nanocrystalline is King
Common mode chokes suppress high-frequency conducted noise on both AC and DC lines. For automotive OBCs, nanocrystalline CMCs have become the standard.
Why Nanocrystalline for CMC?
Extremely High Permeability:
- Initial permeability: 50,000–100,000+
- Provides much higher common mode impedance than ferrite at the same size
High Frequency Performance:
- Effective from ~10 kHz to 30 MHz
- Covers the entire CISPR 25 conducted noise band
Compact Size:
- A nanocrystalline CMC can be 1/3 the size of a ferrite equivalent
- Critical for space-constrained OBC designs
CMC Design Considerations
- Rated current: Must handle continuous phase current (e.g., 16A, 32A)
- Leakage inductance: Used as differential mode inductance—don’t over-optimize
- Winding arrangement: Balanced winding for good CM/DM impedance ratio
- Shielding: Some designs require copper tape shielding to reduce inter-winding capacitance
5. Output Inductor: The DC/DC Filter
After the LLC resonant stage, the output inductor filters the rectified high-frequency ripple to smooth DC battery voltage.
Design Requirements
- Handles full battery voltage (400V–800V)
- Carries high DC current (up to 20A–40A)
- Small inductance (typically 10–100 μH)
- High saturation current rating
Material Selection
Powder Cores Again:
- Sendust or high-flux toroidal cores
- Distributed gap handles DC bias well
- Lower core loss than gapped ferrite at the switching frequency
SMD Inductors (Lower Power):
- For 3.6 kW OBCs, shielded SMD power inductors are sometimes used
- Easier automated assembly
- But limited current handling vs. through-hole toroids
6. Custom Magnetic Components from SiRui-Tech
At Shaanxi SiRui Technology Co., Ltd., we work directly with OBC manufacturers to optimize magnetic component designs.
Our OBC-Relevant Product Lines
| Product Line | OBC Application | Customization Options |
|---|---|---|
| Nanocrystalline Cores | CMC chokes, resonant transformers | Toroidal, cut cores, various dimensions |
| Powder Cores (Sendust, High Flux, MPP) | PFC inductors, output inductors | Toroidal cores with various permeability |
| Ferrite Cores | LLC transformers, auxiliary inductors | PQ, ER, EE shapes, Ni-Zn / Mn-Zn grades |
| Common Mode Chokes | Input/output EMI filtering | Custom winding, rated current, impedance profile |
| Power Inductors | PFC boost, output filter | Through-hole SMD, toroidal |
Engineering Support
- Material selection: Match your frequency, power level, and efficiency target
- Prototype samples: Fast turnaround for design validation
- Custom tooling: For volume production programs
- Cost optimization: Balance performance and BOM cost
7. Design Checklist for OBC Magnetic Components
Before freezing your OBC magnetic design, verify:
- PFC inductor DC bias curve at peak load + transient
- Transformer saturation margin at maximum current and temperature
- CMC impedance curve covers the full noise band
- Output inductor saturation current above worst-case peak
- Temperature rise for all magnetic components at worst-case thermal
- Audible noise — powder cores are quieter than gapped ferrite
- Datasheets available with loss curves at your switching frequency
Conclusion
Magnetic components typically account for 20–30% of an OBC’s total cost and a similar share of its volume. Optimizing magnetic core selection—especially between powder cores, ferrite, and nanocrystalline materials—directly impacts:
- Efficiency: Lower core loss → fewer cooling requirements
- Power density: Better DC bias handling → smaller inductors
- EMI performance: High-permeability CMCs → fewer filter stages
- Cost: Balanced material selection → competitive BOM
Whether you’re designing a 3.6 kW entry-level OBC or a 22 kW premium three-phase unit, Shaanxi SiRui Technology Co., Ltd. has the magnetic core expertise to support your program.
Contact us today to discuss your OBC magnetic component requirements and request engineering samples.