Mastering Transformer Interleaving Techniques for Lower Leakage Inductance and EMI

---

The Challenge of Leakage Inductance (L_lk)

In an ideal transformer, 100% of the magnetic flux generated by the primary winding couples with the secondary winding. In reality, some flux escapes into the air or core gaps. This uncoupled flux is known as leakage inductance (L_lk).

In high-frequency switching power supplies (like flyback converters), leakage inductance is a massive problem. When the Mosfet switches off, the energy stored in the leakage inductance cannot be transferred to the secondary side. Instead, it creates a massive voltage spike that causes severe power loss, generates heat, and produces heavy EMI (Electromagnetic Interference).

---

What is an Interleaving Technique (Sandwich Winding)?

To minimize L_lk , engineers use interleaving techniques, most commonly referred to as “sandwich winding.”

Instead of winding the entire primary coil and then placing the secondary coil on top of it, the windings are split and layered:

1. Inner Layer: One-half of the primary winding (1/2 N_p ).
2. Middle Layer: The entire secondary winding (N_s ).
3. Outer Layer: The remaining half of the primary winding (1/2 N_p ).

By physically sandwiching the secondary winding between the primary layers, the magnetic coupling is significantly tightened. This technique can reduce leakage inductance by up to 50% to 70% compared to standard sequential winding.

---

EMI Mitigation and Proximity Effect

Beyond reducing voltage spikes, interleaving has two profound secondary benefits:

• Lower EMI: The sandwich structure naturally shields the secondary winding, reducing the parasitic capacitance between layers and significantly lowering common-mode noise, which makes passing EMI compliance testing much easier.
• Reducing Copper Loss: High-frequency transformers suffer from the “Proximity Effect,” where adjacent wire layers push current to the edges of the conductors, increasing AC resistance. Interleaving helps distribute the magnetic field more evenly. However, to fully reduce proximity effect and AC copper loss, engineers must combine this technique with the correct wire choice, such as Litz wire.

---

The Trade-off with Parasitic Capacitance

While interleaving is powerful, it is not a silver bullet. The “sandwich” structure significantly increases the contact surface area between the primary and secondary windings. This unavoidably increases the parasitic capacitance (C_ps ) between them. High C_ps can lead to its own set of high-frequency noise issues.

As an experienced manufacturer, we optimize this trade-off. We utilize high-quality electronic transformer bobbins with extended barrier tapes and specifically calculated insulation thicknesses (like Mylar tape layers) between the sandwich sections.

Furthermore, every interleaved transformer must pass rigorous comprehensive automated testing to ensure that while L_lk is minimized, the C_ps remains within safe design tolerances.

---

Conclusion

Implementing transformer interleaving techniques requires a delicate balance of electrical theory and manufacturing precision. By effectively utilizing sandwich winding to slash leakage inductance and control EMI, we empower engineers to design power supplies that run cooler, cleaner, and more efficiently.