Transformers have many winding layers, which generate distributed capacitance. When operating at high frequencies, this distributed capacitance can significantly impact transformer performance, such as poor EMC performance and overheating. Therefore, understanding these distributed capacitances is essential.

1. Inter-turn capacitance
Inter-turn capacitance is the capacitance between two adjacent windings of a transformer. Under high-voltage output conditions, inter-turn capacitance can alter the insulation strength between windings, especially in single-slot bobbins. In severe cases, it can cause inter-turn breakdown and short circuits.

To reduce inter-turn capacitance, choose enameled wire with a lower dielectric constant, increase the distance between winding turns, or use multi-slot bobbins for segmented winding. Overall, the impact of inter-turn capacitance is negligible compared to other distributed capacitances.

2. Winding Interlayer Capacitance
Interlayer capacitance refers to the capacitance between the individual layers of a transformer's windings. This capacitance is present only in transformers with two or more winding layers. Interlayer capacitance accounts for a significant portion of the transformer's total distributed capacitance, increasing voltage stress on the MOSFET and secondary diode, and degrading EMC.

There are several ways to reduce interlayer capacitance. You can increase the distance between winding layers, such as using triple-insulated wire; you can also choose a bobbin with a wider winding area; or you can use a cross-stack winding method to reduce interlayer capacitance. Each method has its advantages and disadvantages.

3. Winding Capacitance
Winding capacitance refers to the capacitance between windings, for example, the capacitance between the primary winding Np and the secondary winding Ns.

Interwinding capacitance is a key pathway for common-mode signal coupling, and therefore contributes significantly to EMI. It is typically reduced by increasing insulation thickness or adding a Faraday shield.