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SMPS Transformer Design

Switching Mode Power Supplies user ferrite core transformers and they come in various sizes. Its winding turns are required to be calculated.

Core Size to Output Power Level Recommendation


Output power level (watts) Recommended core types
0–10 EFD15, EF16, EE19, EFD20, EFD25
10–20 EE19, EFD20, EF20, EI22, EFD25
20–30 EI25, EFD25, EFD30, ETD29, EER28(L)
30–50 EI28, EER28(L), ETD29, EFD30, EER35
50–70 EER28L, ETD34, EER35, ETD39
70–100 ETD34, EER35, ETD39, EER40
100–150 EI50, EE40, EER42
150–200 EI60, EE50, EE60, EER49
200–500 ETD44, ETD49, E55
> 500 ETD59, E65, E70, E80

Calculate Primary Turns

For Half Bridge Topology

Equation from Tahmid's blog


  • Vin(nom) – Nominal input voltage. For 220VAC it will be VDC
  • f – Switching frequency in Hertz.
  • Bmax – Maximum flux density in Gauss. Take Bmax in range 1300G to 2000G. Too hight Bmax will cause transformer to saturate and too low will be under utilizing the core.
  • Ac – Effective cross-sectional area in cm2. This can be get from datasheets. Ac also referred as Ae. For ETD44, it is 1.75cm2. For EI33, it is 1.19cm2 (from TDK datasheet)

Auxiliary winding calculation

Turn ratio for auxiliary

  • Naux – Number of turns in auxiliary.
  • Nsec – Number of turns in secondary.
  • Vaux – Voltage required from auxiliary.
  • Vd(aux) – Forward drop in voltage of diode at auxiliary.
  • Vsec – Voltage output of secondary.
  • Vd(sec) – Forward drop in voltage of diode at secondary.