Magnetics for EV On-Board Chargers: PFC Inductor, Isolation Transformer and Filter

The Magnetics Are the Bottleneck in On-Board Chargers

An 11 kW on-board charger in a modern EV typically contains four magnetic components that dominate volume: the bridgeless PFC boost inductor, the DC-link to battery isolation transformer in the DC-DC stage, an output filter inductor, and a common mode choke in the input EMI filter. Their combined volume frequently exceeds that of the power semiconductors, and their loss sets the thermal design of the whole enclosure.

Designing these four parts for an OBC means satisfying three constraints simultaneously: high switching frequency to shrink size, high ambient because the charger sits in an engine-adjacent or under-floor enclosure, and automotive-grade qualification with 15-year lifetime expectations.

Component-by-Component Design Notes

Typical OBC magnetics and the dominant design constraint for each
Component	Typical rating	Dominant constraint	Practical choice
PFC boost inductor	3–11 kW, 60–200 kHz	Ripple current and DC bias saturation	Powder core (sendust or iron powder) or gapped ferrite; distributed gap controls fringing
Isolation transformer (LLC / CLLC / DAB)	3–22 kW, 100–500 kHz	Loss at high flux, creepage for reinforced insulation	Ferrite with Litz wire, or nanocrystalline below 50 kHz; split bobbin for creepage
Output filter inductor	Output current 10–60 A	DC bias and ripple at battery voltage	Powder or gapped ferrite, high saturation alloy
Input CMC	AC line current 16–32 A	Impedance from 150 kHz to 30 MHz, leakage inductance	Nanocrystalline toroid with split winding

Thermal Reality

OBC enclosures are usually sealed to IP67 and cooled by a coolant plate, so the magnetic component’s heat path is conduction to the housing, not convection. That changes design choices: potted and thermally bonded assemblies with a defined thermal resistance outperform open-frame parts even when their copper loss is slightly higher. A useful planning number is a hotspot limit of 125 °C for Class H insulation with margin against a 105 °C coolant-plate temperature in derated conditions.

Loss budget in a typical 11 kW design: PFC inductor 60–120 W, isolation transformer 80–200 W, output inductor 20–60 W, CMC negligible. A charger targeting 96 % efficiency cannot afford a transformer at the top of that range without a larger core.

Automotive Qualification Points Buyers Overlook

Life at temperature. AEC-Q200 style qualification includes high-temperature operating life and temperature cycling; insulation systems must be qualified for the hotspot, not the average.
Vibration and potting. Sealed modules see road vibration; a loosely potted core can abrade enamel and fail hipot after cycling. Potting compound choice affects both thermal resistance and mechanical integrity.
Partial discharge. At 800 V battery architectures, the isolation transformer sees high working voltage; PD inception above the working voltage becomes a real requirement, not a formality.
Traceability. Lot-level traceability on core, wire, and potting material with retention for the vehicle program duration.
PPAP. Level 3 PPAP with control plans and MSA on the hipot and inductance test stations.

Wide-Bandgap Changes the Balance

With SiC in the PFC stage and GaN in the DC-DC stage, switching frequencies move from 60–100 kHz to 200–500 kHz. Core loss grows faster than copper loss falls, so the optimum shifts to smaller flux swing, thinner laminations, and Litz or foil windings. Above roughly 300 kHz, ferrite grades with low loss at high frequency and stranded wire become mandatory, while nanocrystalline falls out of contention for the transformer.

The practical consequence for buyers: an OBC magnetics specification written for silicon is not simply re-quoted for SiC. The core material, winding construction, and thermal path all change, and the supplier must be able to show loss curves at the new frequency, not extrapolations.