Planar Transformer Design for High-Frequency EV Chargers

What Makes a Transformer "Planar"?

A planar transformer replaces traditional wound bobbins with flat copper conductors (PCB traces or flat copper foil).

PCB Integrated: Windings are traces on a multi-layer PCB.

Copper Foil: Flat foil conductors wound in planar configuration.

Hybrid: PCB primary + heavy foil secondary (optimal for high-current).

Advantages Over Wound Magnetics

Low Profile: Reduces height from 60-80mm to 10-18mm.

Consistent Leakage Inductance: Controlled to ±3–5% in production, vs ±20–30% for wound.

Thermal Performance: Flat geometry allows excellent coupling to heatsinks (3–8°C/W).

EMI: Interleaved structure contains flux better, reducing interference.

Winding Design and Interleaving

Interleaving is the most critical design decision. It determines AC winding loss.

Non-interleaved: Rac/Rdc factor of 3–8 at 150 kHz.

Fully Interleaved: Rac/Rdc factor of 1.1–1.4 at 150 kHz.

Design Rules Summary

Use planar-specific cores (EQ, EL, ER).

Always interleave windings to minimize proximity effects.

Specify heavy copper (2-3 oz for primary, foil for high-current secondary).

Use 3C95 or N87 ferrite for 100-300 kHz.

Validate parameters with an impedance analyzer on the first prototype.