# SMPS Push-Pull Transformer Calculator

## Another Transformer Calculator

Many times I've had to go through old documentation to figure out how to wind a transformer. I decided to put the calculations in a web-based calculator. It is a simple calculator - it just calculates the number of turns. You have to figure out how big the wire needs to be and if it will fit on the bobbin.

The two things you need to know about the transformer are the Bmax, which you can generally guess at without
too much trouble, and the cross-sectional area in cm^{2}. Bmax is the maximum flux
density you want in the core. 1500G with a 3622-77 pot core at 25kHz will produce 0.68W in core losses.
Lower the Bmax as the frequency increases. Don't use type 77 over 100kHz.
For type 77 it looks like you can't saturate the core if you keep Bmax below 3000, but then those core losses
will get you. One-half to one-third of that is more appropriate. Your turns
count will increase as you decrease the Bmax. So will your winding losses. Ae is the cross-sectional area,
and that is always in the datasheet. If it is given in mm^{2} divide by 100 to get
cm^{2}.

**Pay attention to the switching frequency.** If you are using something like an LM3524D, the frequency it runs is twice
the actual transformer frequency. If it has a clock frequency of 50kHz, the transformer is only running at 25kHz.
If you don't take that into account you will design a transformer that is very much too small for the frequency.

When I get a fractional turn on the primary, I round up. It is safer than rounding down. Higher turns on the primary means more inductance and less current at the same frequency. If you round down, you may increase B to the point that the core saturates or overheats. You'll also have to decide if you need to round up any secondaries, since rounding the primary changes the number of turns required on the secondary.

The schematic shows the transformer for a push-pull forward converter. If you are doing a flyback, all bets are off. The transformer calculations aren't the same.

Vin is the voltage at the center tap of the primary. For a 12V input, there are two 12V primaries. Each supplies one half of the cycle. The voltage on the driving transistor wants to go to 2x Vin when the transistor turns off. The inductive spike can go quite a bit higher, so I generally use 51V Transorbs to protect the transistors.