This app note from Bourns presents improvement in efficiency and reduce thermal rise using Litz wire for multi-output flyback transformers. Link here (PDF)
Flyback transformers are a robust, highly efficient and versatile option for AC-DC and DC-DC power supplies. Their wide input voltage range makes them an optimal choice for multiple designs while their use of minimal additional components makes them a cost-effective power conversion solution. Typically, flyback converter applications offer power levels up to 100 W allowing flyback transformers to be used for battery charger applications in EV (Electric Vehicle) and industrial ESS (Energy Storage System) solutions.
It is important that the size of the transformer be minimized in a power supply design as it is usually the biggest component on the PCB. However, minimizing the size of the transformer can lead to increased winding and core losses. A plus with flyback transformers is that they often offer multiple outputs, which can boost efficiency and provide more design flexibility. Conversely, this benefit can make it hard for the designer to minimize the size of the transformer while also balancing winding losses.
Litz wire is becoming more and more popular for flyback transformers as it can reduce power loss and boost power efficiencies. Litz wire is constructed using multiple fine wires that are woven together to replicate a typical copper wire. The benefit of Litz wire is that by having multiple conductors bunched together, the skin effect is greatly reduced. Reducing the skin effect has been shown to reduce AC resistance and boost the efficiency of power conversion designs.