To address an ever-increasing demand for more power in less space, designers are turning to Planar Transformers as an attractive alternative to conventional core shapes where low-profile magnetic devices are required.
These devices provide functions critical to the effective operation of DC-DC converters and have a greater consistency of performance than traditionally wound devices.
A planar transformer is a flat, low profile transformer fabricated using PCB technology. The basic construction method of a planar transformer consists of creating a multi-layer printed circuit board. These windings are then placed in low profile, E/E or E/I ferrite core combinations.
Planar transformers provide the electrical isolation, voltage transformation and energy transfer functions of conventional wire wound transformers in a smaller surface mount device footprint.
Planar transformers fabricated on PCB comprise the following elements:
| Parameter | Planar Transformer | Wirewound Transformer |
|---|---|---|
| Construction | Windings etched as tracks on PCB | Windings made from insulated wires |
| Core | PCB dielectric material | Ferrite, iron alloys, etc. |
| Size | Extremely compact and low profile | Larger, significant height |
| Leakage Inductance | Very low due to tight coupling | Higher due to poorer coupling |
| Parasitic Capacitance | Low as limited overlapping area | High due to more overlapping wires |
| AC Resistance | Low as wider copper tracks | Higher due to thinner winding wires |
| Turns Ratio Range | Typical 1:1 to 1:5 | Wide – 10:1 or more |
| Power Levels | Lower – less than 10W usually | Higher power handling capacity |
| Cost | Lower due to PCB process | Higher labor for hand winding |
| Integration | Excellent, embed directly to PCB | Moderate, but requires mounting |
| Repeatability | Very consistent and controlled | Poorer tolerance and unit-unit variation |
| Efficiency | >90% in well designed planar transformers | >95% in conventional transformers |