#73 🚀 How to Design a Transformer Winding
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Basic Winding Design Guide
One of the goals of this Newsletter is to help you design your magnetics. Maybe you don´t have any tool like Frenetic or Ansys to run proximity losses. Therefore, you can follow this essential guide for designing the winding of your transformer. In any case, I recommend simulating the Proximity effect losses before asking for a sample.
There are three main technologies for winding:
a) Round
Commonly used at low frequency (lower than 20 kHz), low power (>100W), or in those applications where the ac losses are not dominant (For example, low power DC inductors or CCM PFC inductors). It is by far the cheapest technology.
b) Litz
The most common technology in high-frequency transformers in the last 20 years. They exhibit great performance at high frequency, and the main drawback here is the cost of the wire and the customization needed.
c) Foil
Very common technology for high-turn ratio transformers, where one of the windings has very few turns and a lot of currents.
The selection of the technology is the first step. You can use your experience and previous designs or run calculations of winding losses for the three cases to choose the right technology.
Once we have chosen the technology, we need to distribute the available window and the position of the wires.
For Round or Litz, we need to define the following:
· Winding arrangement (Example: PS, PSP, PSPS...)
· Wire output diameter and number of wires in parallel
· Number of turns per layer
· Number of Layers
The proposed strategy in this article is to calculate the output diameter of the strand (in Litz) or the round according to skin effect and consider the high cost of tiny wires. Smaller wires of 0,05 mm are not recommended for production designs due to the logarithmic increase in cost.
Once we have the strand diameter, we must calculate the outer diameter and the number of strands. The window height and the number of turns will determine the output diameter. The maximum size depends on the complete component size, but as a guideline, one should avoid going bigger than 4-5 mm. If the required conduction area is too large, then consider adding parallels.
The number of turns per layer and the number of layers is a decision that also depends on the winding strategy. PS is the simplest winding, but it gives quite a bad performance. PSP allows a significant AC winding loss reduction and a good leakage inductance. In this type of winding arrangement, a symmetrical structure is recommended ( 1/2Pri / Sec /1/2Pri).
Until here, I have spoken about the AC losses due to proximity effects. These losses are the most complex to predict. In the current state of the art, the most interesting designs are decreasing the AC losses while keeping the core losses under control.
Bonus Point. If you are designing a resonant converter, you will need to obtain a specific Leakage inductance, which will be very high. You must use a different winding positioning for these cases to avoid increasing the proximity losses and a bad coupling.
Summarizing
1. Calculate the best current density to discard core options
2. Choose the winding technology within Litz, Round or Foil
3. Choose the round diameter or strand diameter in Litz with the Skin effect, considering cost and production limitations.
4. Calculate the wire diameter and construction considering the height of the window and the current density. Consider adding parallels.
5. Choose the number of turns per layer, according to the winding strategy (interleaving or not interleaving)
Remember to properly calculate the AC proximity losses and use two chamber configurations for high Leakage inductances.
I hope this will be useful!
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