PARASITIC CAPACITANCE IN PLANAR TRANSFORMERS
The use of planar transformers throughout the industry is growing fast due to their advantages as low profile, higher efficiency, manufacturing repeatability, and reduced electromagnetic interferences. One of the characteristics of planar transformers is the possibility of obtaining a very good coupling between windings, but the drawback is the increment of the parasitic capacitance. Consequently, obtaining the value before manufacturing the transformer is crucial for avoiding potential problems.
As an example, we will use a 2 layers PCB on primary and two layers PCB on secondary transformer.
The values needed are obtained, with a measurement from the equipment Bode 100 and processing this data with the equations that follow.
The theoretical models from (Ziwei Ouyang, Ole C.Thomsen, Majid Pahlevaninezhad, Djilali Hamza, Amish Servansing) are used for a theoretical estimation of the parasitic capacitance.
S : overlapping surface area of the layers
εo : permittivity of free air space
εr : permittivity of the material
hΔ : distances between the layers
n : number of turns
m : number of layers
C0 : capacitance between two plates
Cd : capacitance of the same winding
C' d : capacitance in all layers
In planar technology, the PCB windings implementation causes higher parasitic capacitance than other technologies. The capacitance affects the resonance frequency and operation of the transformer, limiting the operating region of the transformer, therefore, obtaining a very accurate estimation will avoid future problems.