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

From https://talema.com/smps-transformer-design

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 http://tahmidmc.blogspot.com/2013/02/ferrite-transformer-turns-calculation_22.html

$$N_{pri} = \frac{V_{in(nom)} \cdot 10^8}{4 \cdot f \cdot B_{max} \cdot A_c}$$

Where:

• V_in(nom) – Nominal input voltage. For 220VAC it will be $220 \cdot \sqrt{2} = 311$VDC
• f – Switching frequency in Hertz.
• B_max – Maximum flux density in Gauss. Take B_max in range 1300G to 2000G. Too hight B_max will cause transformer to saturate and too low will be under utilizing the core.
• A_c – Effective cross-sectional area in cm². This can be get from datasheets. A_c also referred as A_e. For ETD44, it is 1.75cm². For EI33, it is 1.19cm² (from TDK datasheet)

Auxiliary winding calculation

Turn ratio for auxiliary $$N_A = \frac{N_{aux}}{N_{sec}} = \frac{V_{aux} + V_{d(aux)} }{ V_{sec} + V_{d(sec)} }$$

• N_aux – Number of turns in auxiliary.
• N_sec – Number of turns in secondary.
• V_aux – Voltage required from auxiliary.
• V_d(aux) – Forward drop in voltage of diode at auxiliary.
• V_sec – Voltage output of secondary.
• V_d(sec) – Forward drop in voltage of diode at secondary.