Planar technology reduces the size and weight of the transformers by incorporating flat windings.
Repeatability in PCB-based planar transformer manufacturing improves the predictability of parasitic elements such as leakage inductance, interwinding capacitance, etc.
In planar transformers, the winding board consists of copper traces routed in desired shapes in different layers of the PCB.
Planar transformer PCB design focuses on laying the copper windings on the PCB and inserting the core into the PCB
Transformers are used in power electronic circuits for step-up/step-down operation and isolation. In power electronic circuits, high-frequency transformers are used to minimize the size of the magnetics. However, magnetic size does not change significantly with high-frequency switching. To overcome the vertical length of the transformer preventing the compactness factor, planar transformers with windings on flat surfaces are developed. Planar windings can be implemented using various technologies such as stamped copper, flex circuits, etc. Planar transformer PCB design focuses on laying the copper windings on the PCB and inserting the core into the PCB.
Conventionally, transformers are built for power electronic applications using core, bobbin, and copper wires. Tape and wire insulation are included to prevent electrical shorts. A suitable core is selected for the application, and a bobbin is inserted. You must calculate the number of turns to wind over the bobbin. The windings can be made either manually or using a machine. However, the conventional transformers occupy more space, and the copper wire windings limit the compactness.
Conventional high-frequency transformer
Planar transformers can be used in power electronic circuits to minimize size and perform the same function as conventional transformers.
Planar magnetic technology replaces the tallest and heaviest conventional magnetics in power electronic circuits. Planar technology reduces the size and weight of transformers by incorporating flat windings. Instead of copper wires wound into primary and secondary coils, both windings are printed on a PCB. The form factor of the planar transformer is less than in conventional transformers but provides the same operation and enhanced efficiency.
Advantages of Planar Transformers
The windings of the planar transformers are laid flat on PCB surfaces to allow shorter winding windows.
Excellent temperature characteristics
The core can be of a large cross-sectional area. There can be a greater surface area on the top and bottom, which helps remove heat fast. The improved heat transfer offers excellent temperature characteristics for planar transformers.
Good fabrication consistency
The degree of consistency in PCB-based planar transformer fabrication is greater than in conventional transformers. The spacing between the turns and layers can be repeated.
Excellent repeatability of manufacturing
The PCB-based planar windings can be repeated. This repeatability improves the predictability of parasitic elements such as leakage inductance, interwinding capacitance, etc.
The insulation between the interlayers is greater in PCB-based planar transformers. The improved insulation increases the consistency in parasitic capacitance and decreases conduction losses.
Planar transformers improve the efficiency of the transformer action compared to conventional counterparts with increased heat dissipation, excellent creepage, clearance spacing, etc.
Types of Planar Transformer PCB Designs
Planar transformers can be constructed as:
- Stand-alone components - The planar transformer is just like a discrete component that can be connected to other circuits.
- Integrated components - The planar transformer is incorporated into a printed circuit board consisting of other circuit sub-assemblies. Slots are cut in the PCBs to insert the planar transformer cores.
Parts of Planar Transformer PCB Design
Main PCB board
Cut-outs are made in the PCB to insert the transformer core material. The cores are secured on both sides using screws or slips.
Usually, ferrite E or I cores are used as planar transformer cores.
Consists of copper traces routed in desired shapes in different layers of the PCB. Both the primary and secondary windings can be routed on the same winding layout board. Proper isolation should be provided between the primary and secondary windings and where the winding boards meet the main transformer board. The winding boards are connected to the main boards using I/0 pads.
Planar transformer PCB design can be repeated, and it is possible to get the same transformer parameters, efficiency, and other electrical characteristics for any number of replicas of the same design. Cadence OrCAD software can assist power electronic engineers in building planar transformers.
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