# Smps transformer calculation

#### arvinfx

Joined Nov 16, 2011
13
Hi,

I realy confused about transformer calculating !

I have a ETD39 core and I like to build a smps that 12v to 220v with push pull topology...

I have got this questions:

1.What is Bmax in push pull topology?
2.What is maximum power outpout for ETD39 on 100k and 150k?
3.How can I calculate second question? what is the formula?

Thanks

#### bountyhunter

Joined Sep 7, 2009
2,512
You have to calculate the primary current using the secondary current and turns ratio (you nned load current/voltage). Then using the maximum pulse width, you can calculate the flux density. The transformer core will have a maximum allowable flux density.

#### kubeek

Joined Sep 20, 2005
5,791
As for the maximum power, it depends only on allowed temperature rise of the transformer and related components. Temperature rise is caused by two main parts: losses in the core, which depend on frequency and flux density, and copper losses, which depend on copper crossection and frequency.
These all factors are related and calculating the transformer is not an easy job, but I can point you to some useful directions.

This document sums it nice, and there are more related application notes on that site.

#### arvinfx

Joined Nov 16, 2011
13
As for the maximum power, it depends only on allowed temperature rise of the transformer and related components. Temperature rise is caused by two main parts: losses in the core, which depend on frequency and flux density, and copper losses, which depend on copper crossection and frequency.
These all factors are related and calculating the transformer is not an easy job, but I can point you to some useful directions.

This document sums it nice, and there are more related application notes on that site.
Thank you, It was so useful!!!

Do you have complete document ? I couldn`t find it!

#### thatoneguy

Joined Feb 19, 2009
6,359

#### bountyhunter

Joined Sep 7, 2009
2,512
I believe Pulse Engineering and AIE used to make the transformers National bought for the simple switcher products.