R&D engineers design quickly various types of “Airgap-less” ferrite core power transformers for Push-Pull / Forward topologies as used in modern switch mode power supply manufacturer or SMPS. It takes the boredom out of repetitive calculations with variations in design parameters, vital for drilling down to a final design. The users can easily understand effects of changing a certain parameter and observe its impact on the overall design. The compact presentation of Input data and Results side by side, is intended to make modifications easy, and arrive at the desired final working design in minimum time.
The Push-Pull / Forward ferrite core transformers are the most widely used high power transformers in the power electronic industry. They are found inevitably in all high power SMPS and popular power conversion equipment such as solar charge controllers, dc-dc converters, solar hybrid inverters, etc. Therefore, a special treatment is justified for this type alone.
Why have a tool like this when so much else is there to help?
For the most part, a real working design is extremely hard to predict using CAD alone, and achieving the desired performance and its verification, can come only after making and testing a few prototypes. No matter how sophisticated a software package be, the only way to arrive at a successful working design is by actual physical construction and subsequent testing. There is just no work-around to this design cycle. And the sooner you get down to it, the earlier you’ll have your design.
Secondly, it allows users to quickly vary the most critical design parameters, and immediately observe what changes occur in the calculated values of other parameters. e.g. number or turns in primary/secondary, turns per volt, power handling capacity, core area, flux density, etc.
The Transformer Designer brings the experience of “live-design” to the user, and is mainly divided into 3 functional areas namely Turns, Bobbin, Power & Wire Sizing. The following pages contain detailed info on all three sections, along with reference figures in the Excel workbook. The spread-sheet tabulates the design data and results, side by side. The design data is entered sequentially, so that the design proceeds in a step by step easy to understand, logical fashion.
Inverter / Converter Transformer Topologies
Your desired transformer will be a mix of Primary and Secondary configurations. Only use “gap-less” type ferrite or other power cores, otherwise damage can occur. Select your desired transformer topology from choices and refer to the appropriate Primary / Secondary number.
Make sure to always read the correct number of turns for Np and Ns from highlighted Results-Section against your desired Fig-number, mentioned alongside each instance of Np and Ns. Similarly, the required wire area halves and turns are doubled in all Center-Tapped windings because only each half of CT winding is conducting on 50% of the duty cycle. This is also highlighted in wire area selection at the end of the Designer table.
Important Physical Design Factors (K, S, Wf)
In order to get the correct design data from the designer worksheet in Results Output column, there are 3 very important factors (K, S, Wf) which must be well understood, as their data input values will greatly alter the transformer’s performance. These are defined as under:
1. Waveform Factor K
This is a measure of the peak to average values of a given waveform and should be chosen according to the desired waveform used.
K=4 for square waves or rectangular pulses in SMPS.
K=4.44 for sine waves as in mains power transformers, etc.
2. Core Stacking Factor S
Stacking factor represents the compactness of the core laminations within the bobbin. The effect of tight stacking is to increase the core effective area. It’s mainly crucial for laminated cores where saturation can occur if not enough iron core is present. Ferrite cores are not compressible and always present the same area as measured, so for ferrites
S=1 always. The saturation of ferrites is more serious and is covered by choosing safe values of maximum flux densities, as mentioned above.
S=1 for ferrite cores
S=0.95 or less for laminated cores
3. Winding Factor Wf
This is the most critical design parameter to choose due to its direct impact on the transformer’s power output, temperature rise, and winding technique (choice of wire diameters of pri /sec).
Directly, it’s the ratio of the copper cross-section of the winding to the core winding window area. The winding factor represents how effectively a core’s winding window area will be filled up by the wire. It is impossible to fill a core’s window 100% with copper, as some space is always lost to the bobbin, insulating tape/paper, wire insulation and round wire geometry.
Since turns of round wire wound side by side will always leave small triangular inter-turn space inside a rectangular window, it’s impossible to achieve the theoretical maximum of Wf =1 (or 100%). Moreover, as the windings are layered and some insulation is always used between layers of same winding and also between the primary and secondary, the additional winding space taken by the insulating materials further lowers Wf. So, it’s important to choose Wf with a fair bit of accuracy in order to get meaningful results from the Transformer Designer worksheet. Wf is close to 1 for large transformers, and less for smaller ones.
As a good starting point take Wf = 0.8 for small to medium power designs, and perform few iterations of winding with smaller wires in parallel until you reach your desired wire area in C.M. (circular mils) and power output / temperature rise, etc.
The Design Process
The design process involves entering a total number of 16 data items in the Data Input column of the accompanying Excel worksheet “Transformer Designer”. The Results Output immediately displays the design results for use in constructing a real working prototype transformer.
The worksheet is divided into 3 sections to facilitate clarity of workflow throughout the design process. These sections are labelled as:
– Turns Pri / Sec (Data inputs 1-8, and Results outputs 1-8. Topology is also discussed)
– Bobbin (Data inputs 9-13, and Results outputs 9-18)
– Power & Wire Sizing (Data inputs 14-16, and Results outputs 19-28)
A detailed description of method and interpretation of results is given on the following pages. Each section above is explained in more detail with reference to the drawings and figures relevant at each step of the design.