Transformer for PSU

Ferrite core transformers are used in high frequency power supply. The main core parameters are core material and area of core cross-section. Area can be calculated using regular geometric formulas. The problem is to determine core material- they all look dark gray… But we can assume, that manufacturer used most common and cheapest core. So when cross section area and other physical dimensions are measured, we can adapt then to some datasheet.

Various ferrite cores

Another important parameter is air gap. The core can be ungapped and with some air gap. The gap size can be from some standard range, but sometimes it is specially made by rasping central core element.
Ungapped core is used in push-pull schematics- the core is force re-magnetized. Gapped core is used to prevent core saturation. The problem with the gap is, that all formulas and design software calculate core gap, ant there is NO software to calculate all other parameters when gap is known.

Gapped and Ungapped core

While making powerful transformers sometimes the wire is very thick. The diameter of either the primary or the secondary winding must not be larger than the recommended maximum (0.4 mm), for Skin Effect. Due to Skin Effect, the cross section of the wire may be under utilized, resulting in excessive heating of the windings and thermal related transformer failure. The thick wire is replaced with few smaller one winded at once, or using special multithread wire. If the bare conductor diameter of the wire is larger than that of the 27 AWG for 132 kHz or 25 AWG for 66 kHz, a parallel winding using multiple strands of thinner wire should be used to minimize skin effect.

In older and very powerful PSU I found copper foil used instead of wire.

Replacement of thick wire

Number of windings in transformer can be calculated using special formulas of special software. Here you can see the output of special transformer design software. It was downloaded form semiconductors manufacturer’s site for free.

Output of transformer design software