Design Guides
Planar Transformers for EV On-Board Chargers (OBC)
Where the planar format fits in an EV on-board charger, why LLC and CLLC resonant topologies pair naturally with planar magnetics, the honest inter-winding-capacitance trade-off, and what to specify when bringing a custom OBC planar transformer to febetek — an ISO 9001 magnetics maker building PCB / lead-frame windings under a UL-recognized insulation system.
An EV on-board charger (OBC) converts AC from the grid into the DC that charges a high-voltage traction battery, and it has to do that inside a tight thermal and volumetric budget on the vehicle. The isolation transformer in that power train is where size, height, and high-frequency loss all collide — which is exactly why planar transformers have become a common choice for the isolated DC-DC stage of modern OBCs. This guide explains where the planar format fits in an OBC, why the LLC and CLLC resonant topologies pair so naturally with it, and what to specify when you bring a custom planar transformer for an on-board charger to febetek.
Where the Transformer Sits in an OBC
A typical OBC has two stages: a front-end AC-DC stage with power-factor correction (PFC), and an isolated DC-DC stage that provides galvanic isolation between the grid side and the battery and steps the voltage to the battery's range. The isolation transformer lives in that second stage. Because automotive packaging pushes hard on height and footprint, the transformer is often the component that decides whether a target power density is achievable at all.
This is general OBC architecture, not a febetek-specific design — but it is the reason the magnetic component is worth engineering carefully rather than picking from a catalog.
Why LLC and CLLC Resonant Topologies Favor Planar
The isolated DC-DC stage of high-density OBCs is frequently built as an LLC or CLLC resonant converter. These topologies are popular because they allow soft switching (zero-voltage switching on the primary, near-zero-current switching on the secondary) across a wide operating range, which keeps switching loss low as frequency rises — and the CLLC variant is bidirectional, supporting vehicle-to-grid (V2G) operation. The published research literature on EV OBCs documents this LLC/CLLC dominance in detail (see, for example, MDPI Energies, "Design of Planar Transformers for LLC Converters in High Power Density On-Board Chargers").
Resonant operation rewards a transformer with well-controlled, repeatable parasitics, because the leakage inductance and the inter-winding capacitance are part of the resonant tank — they shape the converter's behavior rather than just being parasitic nuisances. That is where the planar format earns its place:
- Repeatable leakage inductance. Because the windings are defined by PCB artwork or lead-frame tooling rather than hand-winding, the leakage inductance spread from unit to unit is tight — valuable when leakage is a designed resonant element (industry-general property of planar magnetics).
- High operating frequency. OBC resonant stages commonly run in the ~100 kHz to several-hundred-kHz band, where the thin, flat copper of a planar winding stays within the skin depth and interleaving cuts proximity-effect loss (industry-general).
- Low profile and thermal headroom. The flat ferrite set has a high surface-area-to-volume ratio, and the PCB copper spreads heat — both useful when the transformer must fit a shallow automotive enclosure and reject heat continuously (industry-general).
The one honest trade-off the literature also flags: planar windings have higher inter-winding capacitance than wire-wound, which matters in CLLC designs and is something the research above analyzes explicitly. A good planar OBC design manages that capacitance through winding arrangement rather than ignoring it.
What febetek Builds — and What You Define
febetek designs and manufactures custom planar transformers in Taiwan, with PCB and lead-frame windings, under a UL-recognized transformer insulation system (UL E533808 — scoped to the transformer insulation system). The company is ISO 9001 certified at the company level and has been building precision magnetics since 2016.
For an OBC planar transformer, the parameters below are the ones to lock down before an RFQ. febetek quotes the concrete numeric values against your actual design rather than publishing generic limits.
| Parameter | febetek capability / status |
|---|---|
| Winding construction | PCB windings and lead-frame windings — both available |
| Insulation system | Built under a UL-recognized transformer insulation system (UL E533808) — product-level scope, transformer insulation system |
| Topology | Defined per your converter (e.g. LLC, CLLC, phase-shifted full-bridge) |
| Turns ratio | Custom — defined to your battery and bus voltages |
| Isolation / creepage / clearance | Custom — specified to your working voltage and the relevant safety standard (e.g. per IEC 60664 creepage tables) |
| Target leakage inductance | Custom — specified when leakage is a designed resonant element |
| Power range | Defined per project — TBD against your spec |
| Switching frequency range | Defined per project — TBD against your spec |
| Profile height | Low-profile custom — TBD against your enclosure |
| Core material / shape | Planar E/I, ER, PQ and similar; ferrite grade selected for your frequency and loss target |
A note on power numbers: OBC designs in the public literature span roughly single-kilowatt to tens of kilowatts (commonly cited classes include 3.3 kW, 6.6 kW, and 11–22 kW), but those figures belong to the cited research and other manufacturers — not to any specific febetek part. febetek states only what it can build to your spec.
How to Engage febetek for a Custom OBC Planar Transformer
Custom planar work runs through our RFQ flow rather than a fixed catalog page. To get a useful first response, include:
- Electrical spec — battery voltage range, bus voltage, power, switching frequency, and topology (LLC / CLLC / PSFB).
- Resonant requirements — target leakage inductance and any magnetizing-inductance constraint, if leakage is part of your tank.
- Mechanical envelope — maximum height and footprint, mounting and termination.
- Isolation / safety — working voltage, isolation class, creepage/clearance or the standard you must meet (and whether the design is automotive-qualified at the system level on your side).
- Target volume — prototype quantity and expected annual production.
Request a Quote — Magnetics · Transformers →
Related Reading
- What Is a Planar Transformer? — the pillar definition and construction guide.
- Planar vs Conventional (Wire-Wound) Transformer — when each format wins.
- Planar Transformers — series page
Frequently Asked Questions
- OBCs push hard on height, footprint, and continuous heat rejection inside a vehicle. Planar transformers help because their flat PCB or lead-frame windings give a low profile, a high surface-area-to-volume ratio for cooling, and repeatable parasitics — and the LLC/CLLC resonant topologies common in OBCs run in the ~100 kHz to several-hundred-kHz band where planar construction performs well. These are general properties of the planar format, not measurements of a specific part.
- Yes. In LLC and CLLC resonant converters the leakage inductance (and, for CLLC, the inter-winding behavior) is part of the resonant tank, so it must be designed and held to a tight tolerance rather than treated as a nuisance. Planar windings help because PCB/lead-frame geometry gives repeatable leakage from unit to unit. febetek designs the turns ratio and target leakage to your converter.
- febetek builds custom planar transformers to your spec; the achievable power, frequency, and height are quoted against your actual design rather than published as generic limits. Power figures like 3.3 kW, 6.6 kW, or 11–22 kW that appear in OBC literature belong to that research and to other manufacturers, not to a specific febetek part. Send your electrical and mechanical requirements via RFQ and febetek will respond with what it can build.
- It is a real trade-off — planar windings have higher inter-winding capacitance than wire-wound, which matters in CLLC converters. The published research analyzes this explicitly. A good planar OBC design manages the capacitance through winding arrangement; febetek designs the winding stack with this in mind rather than ignoring it.
- febetek holds ISO 9001 (company-level quality management) and a UL-recognized transformer insulation system (UL E533808), whose scope is the transformer insulation system at the product level — not a company-wide or whole-product safety mark. System-level automotive qualification is defined on your side. febetek does not claim certifications it does not hold.