# SMPS Schematic – Transformer Calculator Guide

flybackswitch-mode-power-supplytransformer

I found this schematic for a simple SMPS.

I want to use it to build a 24V/2-3A power supply for a soldering station.

How are the turns on the transformer calculated?

For the primary I can use

$$N = \dfrac{V_{in} \cdot 10^ 8}{ 4 f \cdot B_{max} \cdot Ac}$$

and get like somewhere around 30 turns.

But for secondary I don't get it right. In the schematic it says 5 turns for 14.4V. I get about 1-2 turns.

$$240V \cdot \dfrac{1.4}{14.4V \cdot 0.98} \approx 23 \implies \frac{30}{23} \text{ turns} = 1.3 \text{ turns on the secondary}$$

What am I calculating wrong? Should I use a 50% duty ratio for this type of SMPS? Even then, it does not get close to 5 turns.

The flyback voltage of the primary is simply: $$V_{fb} = (V_{sec}=12.6V) \times \frac{T_{pri}=30}{T_{sec}=5} = 76V$$ This does not account for the extra spike due to leakage inductance. And be aware that as seen by the MOSFET, this is added on top of Vin, the rectified input voltage.