SOLUTION

The core used in the high-frequency transformer is usually Air core.

Transformer core

Air core transformer- This transformer has no iron core. It consists of simply two insulated primary and secondary coils placed closely one to the other.

Open core transformer:- In this transformer, the primary and secondary coils are each separately wrapped around their own ferromagnetic cores. Again both coils+cores are placed close to each other. Some auto- transformers are open-core type transformers (both insulated coils are wrapped around a single iron core).

Closed core transformer (most often used):- In this transformer, the primary and secondary coils are wrapped around an iron core with closed geometry (either a ring or square shape).

• From the application parameters point of view, the type of a transformer core depends on operational factors such as applied voltage, current, and frequency.
• In addition to these, size limitations and construction costs are further factors to be considered. Conventionally applied core materials are air, soft iron, and steel.
• Generally, air-core transformers are used when the voltage source has a  higher frequency above 20kHz
• Iron-core transformers are usually used when the source frequency is below 20kHz.
• Transformer cores composed of laminated steel sheets isolated with a nonconducting material provide the highest efficiency among conventional core materials primarily due to better dissipation of heat generated during the process.
• Iron becomes less effective as a magnetic circuit because at high frequencies, the fluX penetrates to lesser extent in the core