A planar high frequency transformer uses PCB copper or flat conductors to create repeatable, low-profile windings. The approach can reduce assembly variation and improve integration, but copper thickness, layer sequence, vias, AC resistance, winding capacitance, leakage inductance, insulation, and thermal spreading must be designed as one system.
Start with current density and available copper
PCB copper thickness, trace width, number of parallel layers, and allowable temperature rise determine DC resistance. At high frequency, skin and proximity effects redistribute current. Simply adding parallel copper layers may not reduce loss as expected if the magnetic field drives unequal current sharing.
Field-aware layer arrangement is especially important near gaps and winding transitions. Finite-element analysis can help, but measured winding resistance and temperature on representative hardware remain valuable.
Vias are part of the winding
Vias carry current between layers and can become local resistance or temperature bottlenecks. Define via diameter, plating thickness, count, spacing, current sharing, and connection geometry. Multiple vias should be placed so current enters them evenly rather than crowding into the nearest few.
Layer sequence controls leakage and capacitance
Interleaving primary and secondary layers improves coupling and reduces leakage, but increases overlapping electric-field area and primary-secondary capacitance. That capacitance carries common-mode current during fast switching. Sectioned layouts reduce overlap but may increase leakage, copper length, and voltage overshoot.
Insulation must include PCB manufacturing tolerances
Creepage and clearance depend on copper features, board edges, slots, core openings, vias, and assembly hardware. Solid insulation depends on laminate, prepreg thickness, resin content, layer registration, and applicable safety requirements. Solder mask should not be assumed to provide certified insulation unless the governing design and standard allow it.
Thermal spreading can help and mislead
Wide copper spreads heat, and planar cores offer a large surface area. However, inner layers can remain hot while outer copper appears cool. Core loss, via heating, rectifier proximity, and restricted airflow contribute to the final temperature. Use embedded sensors, calibrated resistance, or other methods to estimate internal hot spots.
Design for manufacturing ownership
The PCB fabricator controls stackup, copper thickness, plating, laminate, registration, and inspection. The transformer manufacturer controls core selection, assembly, gap, insulation barriers, clamping, terminals, and final electrical tests. The approved drawing should connect both sets of controls.
Prototype verification checklist
- Turns ratio, polarity, magnetizing inductance, and leakage inductance
- DC and AC winding resistance
- Via and internal winding temperature
- Primary-secondary capacitance and common-mode current
- Core loss and total temperature rise
- Creepage, clearance, solid insulation, and dielectric withstand
- PCB stackup and copper tolerance against the approved build
BaoHui Tech can review a high frequency transformer design using the converter waveform, PCB stackup, insulation requirement, mechanical envelope, and thermal target. Early coordination prevents the magnetic and PCB designs from evolving separately.
Frequently asked questions
Are planar transformers always more efficient?
No. They offer repeatable geometry and low profile, but AC copper loss, capacitance, vias, core choice, and cooling determine efficiency.
Can solder mask provide creepage insulation?
Do not assume so. Its role depends on material qualification, process control, the applicable standard, and the required insulation level.
Why can parallel PCB windings share current unevenly?
Different field exposure, path resistance, via placement, and layer geometry can give each parallel path a different impedance.