How to Reduce Core Losses in 800V EV Architectures: Nanocrystalline vs Ferrite Selection
Quick Selection Guide:
Selecting the right core for a DC fast charger isn't just about efficiency—it's about thermal management in tight spaces. In a recent Level 3 DCFC project (150kW), we found that switching from an N87 ferrite to a CC-Series Nanocrystalline core reduced total footprint by 35% while lowering core temperatures by 12°C under full load.
The #1 Challenge in EV Charger Magnetics Today: Size vs Heat
800V EV architectures demand higher switching frequencies (65-100kHz) to shrink transformer size, but this creates a thermal nightmare. We've seen too many 150kW+ chargers fail thermal testing because engineers ignored DC bias saturation or failed to account for acoustic noise from magnetostriction in high-power applications.
Nanocrystalline cores solve this by maintaining low losses even at 100kHz with 0.2T AC + 0.1T DC bias—something ferrite simply cannot do without massive derating.
Comparison of Nanocrystalline vs Ferrite for EV Chargers (20kHz-100kHz)
| Parameter | Nanocrystalline | N87 Ferrite | Impact |
|---|---|---|---|
| Saturation (Bs) | 1.2T | 0.39T | 3x higher → 35% smaller core |
| Permeability (μ) | 80,000 | 2,200 | Lower μ = better DC bias performance |
| Temp Stability (Tc) | 570°C | 215°C | Stable to 130°C vs 100°C limit |
| Cost per Watt | $0.12 | $0.08 | 50% premium justified by size reduction |
Common Engineering Mistakes in EV Charger Core Selection
These design pitfalls have cost our clients months of redesign time and failed thermal testing
❌ Ignoring DC Bias Saturation
Many engineers size cores using only AC flux density, forgetting that EV chargers have significant DC bias from current ripple. This causes 40-60% inductance drop in ferrite cores.
✅ Solution: Nanocrystalline maintains 90% inductance even with 0.1T DC bias
❌ Acoustic Noise in High-Power DCFC
Magnetostriction causes audible noise (>40dB) in 150kW+ chargers, especially with ferrite cores operating near saturation. This noise issue is closely related to EMI filter design challenges.
✅ Solution: Nanocrystalline has 10x lower magnetostriction coefficient
How to Calculate Core Requirements for 400V vs 800V Systems
Step-by-step math for real-world EV charger designs
Switching Frequency (20-100 kHz)
Raising frequency reduces required core size but increases core and switching losses. For many EV charger topologies, a practical operating window is 20-100 kHz, with 50-65 kHz often providing a strong balance between size and efficiency.
Lower end (20-40 kHz): Larger core, lower core loss, potentially higher copper loss. Mid range (50-65 kHz): Good compromise for most 20-150 kW designs. Upper end (65-100 kHz): Very compact transformers but tighter constraints on loss and thermal design.
Power Level (3.3-350 kW)
The charger's output power drives core cross-section, window area, and thermal strategy.
3.3-22 kW (Level 2 AC): Single nanocrystalline core is often sufficient, with moderate thermal complexity. 50-150 kW (DC fast charging): Larger cores or optimized toroidal/C-core designs are needed, with careful cooling. 150-350 kW (ultra-fast): Multiple parallel cores are typically used to distribute thermal load and ease manufacturing.
Isolation Voltage (3-10 kV)
Chargers must maintain safe galvanic isolation between grid and vehicle. Required isolation voltage influences insulation thickness, clearance, creepage, and how much of the core window can be allocated to copper.
Higher isolation demands reduce effective window utilization factor Ku. Winding arrangement and inter-winding insulation must be coordinated with the core geometry to meet standards.
Ambient Temperature (-40°C to +50°C)
Outdoor DC fast chargers typically see wide ambient swings. Since core loss and temperature rise are related, designs often derate flux density to provide margin.
Core losses increase with temperature (often approximated with a positive temperature coefficient). A common guideline is to reduce maximum design flux density by about 10% for outdoor installations to keep temperatures under control.
Decision Matrix: When to Choose Nanocrystalline vs Ferrite
Based on 50+ EV charger designs we've optimized in the last 2 years
Level 2 AC Charging (3.3-22 kW)
Advantages:
DC Fast Charging (50-150 kW)
Advantages:
Ultra-Fast Charging (150-350 kW)
Advantages:
Design Walkthrough: 50kW DC Fast Charger Transformer
Real calculations from a production 50kW charger using 800V battery architecture
Core area calculation
For our 50kW, 65kHz LLC resonant converter with 800V output:
Why these specific values:
- • Bmax = 0.15T: Conservative for LLC with 0.05T DC bias
- • η = 0.98: Achievable with nanocrystalline at this power level
- • Ku = 0.35: Accounts for 5kV isolation requirement
Core loss estimation
Actual core loss calculation for CC-65 at operating conditions:
Thermal validation results:
- • Core hotspot: 92°C with 200 CFM forced air
- • Efficiency impact: 0.12% loss (59W / 50kW)
- • Ferrite equivalent would be 89W loss at same conditions
Temperature Performance: -40°C to 120°C Reliability Data
Field data from 500+ deployed EV chargers using nanocrystalline cores
Performance Curves: Nanocrystalline vs Ferrite
Inductance vs Temperature
- • -40°C: 102% of room temp value
- • +25°C: 100% (baseline)
- • +85°C: 98% of room temp value
- • +120°C: 95% of room temp value
Core Loss vs Temperature
- • -40°C: 85% of room temp loss
- • +25°C: 100% (baseline)
- • +85°C: 115% of room temp loss
- • +120°C: 130% of room temp loss
Thermal management
- • Keep core temperature below about 100°C for optimal performance and lifetime
- • Apply forced-air cooling for designs above roughly 50 kW, and consider liquid cooling at higher power densities
- • Use split cores or multiple cores in parallel on very high-power stages to improve heat spreading
- • Validate designs with hotspot temperature measurements or detailed thermal models
EMI considerations
- • Prefer toroidal cores where possible to minimize stray fields and leakage flux
- • Combine appropriate shielding, grounding, and layout practices to meet EMI limits
- • Match material permeability to the operating frequency and desired impedance characteristics
- • Optimize winding techniques (e.g., interleaving, foil windings) to reduce leakage inductance and radiated noise
Safety and standards
- • Design to meet applicable EV charging standards such as IEC 61851 and related documents
- • Ensure galvanic isolation with dielectric withstand levels typically greater than 3 kV, depending on system class
- • Consider fault current withstand and mechanical robustness of the transformer
- • Integrate overcurrent and overtemperature protection in the overall system design
Cost optimization
- • Balance core size against copper usage and cooling system complexity
- • Use standardized core geometries where possible to reduce tooling and procurement costs
- • Evaluate trade-offs between slightly larger cores and simpler cooling or reduced switching losses
- • Focus on overall charger cost per kW, not just the core cost in isolation
The Verdict: Final Recommendation by Charger Power Level
3.3-22kW (Level 2)
Use nanocrystalline if space is critical. Ferrite acceptable for cost-sensitive designs.
50-150kW (DC Fast)
Nanocrystalline strongly recommended. Thermal and size benefits justify cost premium.
150-350kW (Ultra-Fast)
Nanocrystalline mandatory. No viable ferrite solution at these power densities.
Based on thermal testing of 50+ EV charger designs, nanocrystalline cores consistently outperform ferrite in applications above 25kW. In our experience at CenturaCores, while the data sheet says Ferrite is stable to 215°C, we never recommend it for 800V systems above 50kW because the thermal runaway risk is simply too high for long-term reliability in outdoor stations.
Download EV Charger Core Selection Checklist
Step-by-step checklist with calculations for 400V and 800V EV charger designs.
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