Planar transformers are reshaping high-density power conversion. With a low-profile core and PCB-based flat windings, they deliver excellent thermal performance, predictable leakage inductance, and repeatable parasitics that traditional wire-wound transformers cannot match. This guide explains how planar transformers work, why designers choose them, and how to specify one for your OEM or ODM power project.
What Is a Planar Transformer?
A planar transformer is a high-frequency transformer that uses flat, PCB-embedded copper traces as windings instead of round magnet wire. The windings are typically formed as multi-layer printed circuit boards, stacked around a low-profile ferrite core such as E, ER, EER, or RM planar core shapes. Because the winding geometry is defined by the PCB artwork, planar transformers offer highly repeatable parasitic parameters, excellent current density, and low thermal resistance – properties that matter in server power supplies, EV onboard chargers, telecom rectifiers, and other space-constrained designs.
How Planar Transformers Work
Planar transformers transfer energy exactly like conventional transformers – a changing current in the primary PCB winding induces a magnetic flux in the ferrite core, which couples to the secondary winding. The difference is in the winding construction. Flat copper traces replace round wire, allowing windings to be interleaved precisely between core halves. This tight interleaving minimizes leakage inductance and AC resistance, while the large flat surface area spreads heat evenly across the winding layers.
The planar design also produces predictable, repeatable parasitics. Since the PCB pattern is identical from unit to unit, leakage inductance, inter-winding capacitance, and coupling are consistent in production – a decisive advantage for high-volume manufacturing and resonant converter designs that depend on precise inductance values.
Key Advantages of Planar Transformers
Low profile: Typical heights of 6-15 mm suit rack-mounted and card-based power modules.
Superior thermal management: Flat windings contact the core surface directly, conducting heat out efficiently and allowing higher power density.
Repeatable parasitics: PCB-defined geometry gives consistent leakage inductance and capacitance from unit to unit.
High frequency operation: Low AC resistance and minimal skin-effect losses support switching frequencies from 100 kHz to over 1 MHz.
Excellent interleaving: Precise winding layering reduces leakage inductance and improves cross-regulation in multi-output designs.
Design Considerations for Planar Transformers
Specify the operating frequency, power level, input and output voltages, isolation class, and mechanical height limit first. The number of PCB layers, copper thickness, and core geometry must then be chosen to balance winding loss, core loss, and thermal performance. Creepage and clearance distances across the PCB must meet the required isolation standard – especially for reinforced isolation in medical and EV applications. Thermal vias and embedded copper planes help move heat away from hot spots, and the core gap is typically designed into the magnetic structure to control inductance tolerance.
For custom planar transformer development, provide your schematic, switching frequency, power density target, and isolation requirements. Shaanxi SiRui Technology Co., Ltd supports the full development loop – core selection, PCB winding layout recommendations, prototype samples, and mass production under ISO 9001:2015 with RoHS and REACH compliant materials.
Applications of Planar Transformers
Server and telecom power supplies, EV onboard chargers and DC-DC converters, solar inverters, industrial power modules, LED drivers, medical isolation power, gate-drive transformers, and high-frequency bus converters. Planar transformers are also common in isolated current sensing and in power-over-Ethernet (PoE) injectors.
FAQ
Are planar transformers better than wire-wound transformers?
For high-frequency, high-power-density designs, planar transformers generally offer lower profile, better thermal performance, and more repeatable parasitics than wire-wound units. For very high power at low frequency, traditional wound designs may still be more cost-effective.
What is the typical frequency range of planar transformers?
Most planar transformers operate from about 100 kHz to 2 MHz, matching the switching frequencies of modern power converters. Higher-frequency designs depend on core material selection and PCB layout quality.
Can planar transformers handle high isolation voltages?
Yes. With proper PCB creepage and clearance design, planar transformers can meet reinforced isolation requirements of 3 kV to 5 kV or higher, subject to the insulation standard of your application.
Do you manufacture custom planar transformers?
Yes. Shaanxi SiRui Technology Co., Ltd designs and manufactures custom planar transformers – from core and winding layout to prototypes and volume production, with full quality documentation.
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