A transformer is a critical component in an LLC converter, and designing it will be challenging as there is no straightforward method for choosing the LLC transformer. At the end of this post, I have uploaded a Microsoft Excel worksheet that can be used to determine the size of the ferrite core of the LLC transformer.
In this transformer, we need to select a specific turn ratio while at the same time, the transformer also needs to have a particular inductance value. So to achieve that inductance we must implement a certain air gap into the transformer.
There are a few methods available now to determine the size of the transformer core. They are the Ap method and the Kg method. This time I am going to use the Ap method. Several papers discussed designing the LLC transformer using the Ap method.
I have made a Microsoft Excel sheet that contains all of the equations that will be needed for our calculation. I divided the calculations into three parts,
1. Electrical specification
2. Material specification
3. Core parameters
First, we just need to input the electrical specification of the Half-Bridge LLC converter
The temperature rise and the ambient temperature can be chosen based on our expectations. The second table which needs to be filled up is the date of the core material that we have chosen.
All of this data can be found in the core datasheet, books, and papers. After inputting all of these data we will have the Ap value. Using this value we can look up at the core geometry table. I use the table which is provided in the “Transformer and Inductor Handbook” by Colonel Wm. T. McLyman.
As can be seen from the table above the Ap value of the different sizes of RM cores are listed. We just need to choose the core which has the Ap value with the one that has been calculated using the Excel worksheet.
After choosing the core size, we need to input the data for the third table which is about the geometry specifications of the chosen core.
After inputting all of the geometry values of our chosen ferrite core, now we can calculate the number of winding turns, the length of the air gap, and also the wire size.
Below I put the link to download the Microsoft Excel Sheet that can be used to select the size of the ferrite core for a Half-Bridge LLC converter.
References:
1. Jun Zhang, William Gerard Hurley, Fellow, IEEE, and Werner HugoWölfle, “Gapped Transformer Design Methodology and Implementation for LLC Resonant Converters” DOI: 10.1109/TIA.2015.2465932 
2. W. G. Hurley, W. H.Wölfle, ” Transformers and Inductors for Power Electronics, Theory, Design and Applications”, John Wiley & Sons Ltd. 2013
3. Colonel Wm. T. McLyman, “Transformer and Inductor Handbook 4th Edition” CRC Press, 2011
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