Improving Planar Transformers for LLC Resonant Converters: Paired Layers Interleaving

With the increasing demand for flat displays, slim telecom/IT power supplies, and portable devices, the use of planar transformers (PTs) is becoming mandatory to achieve slim, compact converter designs. In particular, PTs present a significant opportunity to replace tall and bulky wire-wound transformers in industry standard LLC converters. However, the common-mode (CM) noise generated by PTs remains a significant obstacle to make LLCs effective. The CM noise issues are originated by large capacitances in overlapping primary and secondary layers of PTs, leading to serious electromagnetic interference issues. This paper develops a new winding method to overcome CM noise in LLC PTs. By minimizing noise, the proposed method prevents the use of bulky CM chokes, helping to keep a slim/flat form factor for LLC converters with high-power density. As well, the proposed winding method maintains low ac resistance and leakage inductance, two desirable features in high-efficiency transformers. The paper explains the root cause of CM noise in LLC PTs and develops a comprehensive method that targets all sources of CM noise in the transformer and is applicable to any type of PTs. The proposed winding method successfully shields problematic turns with high dv/dt and overlaps the turns with the same dv/dt. As a result, circulating currents between primary and secondary (which are known as CM noise) are prevented, which leads to significant reduction of the converter's noise. Experimental results show that the proposed PT generates 11-20 dBμV less CM noise than a traditional PT with the same parasitic capacitance, which is the result of using the proposed winding method.