Planar magnetics have moved from a niche telecom component to the default choice for high-frequency, high-power-density converters in EV charging, server power and industrial supplies. The reason is simple: flat PCB windings and low-profile ferrite cores cut height by 50–70% compared to wire-wound toroids, while improving thermal performance and repeatability.
But planar transformers and inductors are not just smaller versions of conventional magnetics. The winding is etched copper, not round wire. The core geometry is different. Proximity loss behaves differently. Thermal paths go through the PCB, not the bobbin. This guide covers what engineers need to know before starting a planar magnetic design.
Shaanxi SiRui Technology Co., Ltd. supplies planar ferrite cores (EE, ELD, EFD, PQ and custom shapes) and can deliver fully wound planar magnetics with PCB embedded windings. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. What Are Planar Magnetics?
Planar magnetics use flat, PCB-embedded copper traces as windings instead of round magnet wire. The core is a low-profile ferrite shape—often EE, ELD, EFD or PQ—that sits on top of the PCB or is embedded into a cutout in the board.
- Winding height: copper traces are 0.5–2 oz thick per layer, stacked across 2–6 PCB layers
- Core profile: typically 8–20 mm total height vs. 25–40 mm for through-hole toroids
- Thermal path: heat flows into the PCB copper and through the board, not into a plastic bobbin
- Manufacturing: windings are etched, not hand-wound—excellent repeatability for high-volume production
2. When to Choose Planar vs. Conventional
| Factor | Planar | Conventional Wire-Wound |
|---|---|---|
| Height | <15 mm critical | 25–40 mm acceptable |
| Frequency | >100 kHz | <100 kHz preferred |
| Power level | 50 W – 5 kW | Any |
| Volume production | >10k units/year | Low to medium volume |
| Thermal coupling to PCB | Excellent | Poor |
| Cost at low volume | High (PCB tooling) | Low |
Planar wins when height, repeatability and thermal density matter. Conventional wound components still win for low-frequency, high-voltage, low-volume or custom-turn-count applications.
3. Core Materials for Planar Magnetics
Nearly all planar cores are ferrite. At the frequencies planar magnetics operate (100 kHz – 1 MHz), ferrite is the only material with acceptable core loss.
Mn-Zn Ferrite (200–500 kHz)
The standard choice for LLC resonant, half-bridge and forward converters. Low-loss grades (PC95, 3C95) are preferred. Bsat ~0.45 T.
Ni-Zn Ferrite (>500 kHz)
For very high frequency POL converters and VRM. Higher resistivity than Mn-Zn, lower loss at 1 MHz and above, but lower permeability and lower Bsat.
4. Winding Layout: The Critical Design Decision
4.1 Interleaving
The single most important technique. Split the primary into two layers (P-S-P) to reduce proximity loss. Without interleaving, proximity loss in a planar transformer can be 3–5 times the DC loss.
4.2 Copper Thickness
Skin depth at 200 kHz is about 0.15 mm in copper. Use 1 oz or 2 oz copper per layer—thicker copper wastes skin depth and adds cost.
4.3 Leakage Inductance
Planar transformers naturally have higher leakage than tight wound toroids. For LLC resonant converters, leakage is often used as the resonant inductor—this is a feature, not a bug.
5. Thermal Design
Planar magnetics transfer heat into the PCB through the copper pads. This is their biggest thermal advantage.
- Design 2 oz or 3 oz copper planes under and around the core
- Add thermal vias from the core contact area into inner ground planes
- Typical thermal resistance: 5–15 °C/W for a 1 kW planar transformer vs. 20–30 °C/W for a wound equivalent
6. Common Topologies
LLC Resonant Converter (200–500 kHz, 1–10 kW)
The classic planar application. EE or ELD ferrite core, interleaved winding, leakage inductance designed into the transformer. Common in server PSUs and EV OBCs.
POL / VRM (500 kHz–1 MHz, 50–500 W)
Ni-Zn planar ferrite, very low profile (<8 mm), multi-layer interleaved design. High current, low voltage output.
7. Sirui-Tech Planar Magnetic Capabilities
- Planar ferrite cores: EE, ELD, EFD, PQ and custom geometries in Mn-Zn and Ni-Zn
- Fully assembled planar magnetics: PCB-embedded windings, tested and qualified
- Custom core geometries: tooling for customer-specific shapes
- Application support: from concept to production for LLC, POL and flyback converters
All cores are RoHS and REACH compliant, with SGS and CTI verification.
Conclusion
Planar magnetics are the right choice when height, thermal density and production repeatability matter. The core material is ferrite (Mn-Zn below 500 kHz, Ni-Zn above). The winding layout—especially interleaving—is what makes or breaks the design.
Need a planar magnetic design for your converter? Contact Sirui-Tech at sales@sirui-tech.com with your topology, switching frequency, power level and height limit.