# 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

Where:

- V
_{in(nom)}– Nominal input voltage. For 220VAC it will be 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 cm^{2}. This can be get from datasheets. A_{c}also referred as A_{e}. For ETD44, it is 1.75cm^{2}. For EI33, it is 1.19cm^{2}(from TDK datasheet)

#### Auxiliary winding calculation¶

Turn ratio for auxiliary

- 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.