Hello, Sotiris here! 

🚀 Today I’m going to design a 1kW DAB transformer for 24V battery charging application. The interesting part of this design is the use of copper foil instead of Litz wire, to address high currents.

🔥 Tempted to beat expectations from the get-go?

Personally, unless given specific parameters, I like using bigger transformer cores and coil formers to ensure a smooth winding process.

Bigger cores also mean playing on the safe side of things when temperature rise is expected to be significant.

All of this contradicts the power density, light & compact design that the industry calls for. But let’s be realistic: when you start a new design, better be safe than sorry.

Using Frenetic Online, you don’t have to go through endlessly reviewing design options. Simply finish a version of your magnetic component and ask for a verification, in case you’re not sure about something.

✅ Your assigned magnetics expert engineer will come back with suggestions.

✅ You can also create more dense (higher power density - kW/l) iterations of your initial design with little effort and ask for samples, once you are a customer, to see the performance in your test setup. That way you can quickly iterate through different designs, being efficient and well prepared for the next steps in your project.

⚠️​ Big wires create problems

From my personal experience with winding inductors and transformers, I realized that the bigger the wire gauges, the harder and less flexible a design is. Lots of time I prefer paralleling smaller windings in a transformer instead of using a bigger or thicker wire.

The issue came up recently when trying to design a winding that experiences a current of 50A rms. No matter the winding configuration, with Litz wire I couldn’t really fit everything in the bobbin/core material I had chosen.

☝️ The copper foil solution

Figure 1. A 0.1mm thick copper foil roll

A simple yet effective solution is the usage of simple copper foil. Typically, a 0.1mm thick foil is good enough to mitigate the skin effect phenomenon present near 100KHz switching frequency.

⚡ The 1kW foil DAB transformer

You can check here the transformer information: 1kW DAB Foil_202229140614

🔎 Specs:

Let’s have a look at a 1kW foil DAB transformer that employs a foil type winding. In Table 1 we can see the basic parameters of the transformer:

Tab 1. 1kW DAB specs

We designed a 28V to 400V step-up transformer using a 1/8 turns ratio at 1kW, with primary leakage inductance at 0.46μH. Looking from the secondary side, the leakage inductance would be 0.46μH/n²=29.44μH and the magnetizing inductance is 55μH/n²=3.52mH. The leakage inductance needed (if added in the secondary – 29.44μH) is high, so probably a series shim inductor must be employed, depending on the particular case. Frenetic Online will calculate the transformer leakage inductance and let us know what shim inductor is needed to achieve the total series inductance.

Remember, we can measure a total reflected leakage inductance on the primary winding by shorting the secondaries, or we can measure the secondary total reflected inductance on the secondary by shorting the primary windings. The one measurement is n² times the other measurement. It’s much easier to deal with low currents, for this reason the shim inductor in this project is placed at the HV side (secondary).

📌​ Core:

The selection of the core was decided based on the available coil formers, that favor the foil winding architecture. Reality is that, apart from specialized bobbins that might exist, “everyday use” bobbins have through-hole or SMD pins, requiring some engineering thinking to connect them to the copper foils.

Look what I found out:

Figure 2. E55 bobbin, easily adapted for foil windings

Of course, some preliminary tests were made in Frenetic Online to make sure that a range of core-bobbins are suitable for 1kW power range. In Figure 3 an E55/28/21-N87 was selected and simulated.

Figure 3. E55/28/21 core selection

A BpkT (peak flux density swing) of 84mT is safe enough. Core losses are at 2.33W for the N87 material. For the moment we’ll keep this core set.

​⚙️ Windings:

Figure 4. Windings selection

In Figure 4, the primary windings are shown.

▶️ A copper foil, of 0.1mm thickness is used.

▶️ The primary winding is a 4 parallel 3-turn, 15-layer setup.

▶️ All primary windings are connected in parallel at the pins of the bobbin.

▶️ The secondary winding is 660 strands, 0.05mm strand diameter, 24turns, 2-layer setup.

▶️ Both the primary and the secondary windings experience a current density value of less than 5A/mm², which is a good design choice.

▶️ The filling factor is kept below 70% to make sure the windings fit the available bobbin room.

Actually, for foil windings, usually extra layers of insulating tape are needed which will increase the actual filling factor, when dealing with multiple layers, as is the case in this project. Therefore, a mental note for me is to keep lower filling factors with foil windings, than I’d normally push for with all-Litz wire designs.

In Figure 5 the performance of the transformer is shown under full load conditions.

Figure 5. Performance indexes

🙌 Stay tuned for the upcoming Newsletters, where we’ll see how this transformer performs in reality.

😎​ Hope you liked it!

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Sotiris Zorbas, MSc 

Power Εlectronics Εngineer