If an air gap needs to be introduced in a transformer core, why not simply adopt a core material with low magnetic permeability instead?
Conventional transformers built with ferrite cores require an air gap to avoid magnetic saturation. Why not instead design them directly with low-permeability magnetic cores such as metal powder cores, which would eliminate the need for an air gap entirely? This article systematically addresses this question.
The primary factor is cost. Despite decades of development in magnetic materials, ferrite remains the most cost-effective magnetic material available today. Many technical engineers underestimate the importance of cost, yet it directly determines the success or failure of numerous projects—especially amid the increasingly competitive power supply and transformer market. Ferrite cores feature mature manufacturing processes ideal for mass production, backed by abundant suppliers and relatively low prices. In contrast, metal powder cores carry a higher cost. Even iron-silicon-aluminum cores, the more affordable type among metal powder cores, are still pricier than ferrites.
The second key factor is power loss, particularly eddy current loss at high frequencies, which is substantially higher in metal magnetic cores than in ferrites. The formula for eddy current loss is provided below. From this formula, we can observe that eddy current loss P is inversely proportional to resistivity ρ. Power ferrites generally have a resistivity ranging from 4.5 to 6.5 ohm-meters, while metal powder cores exhibit extremely low resistivity—roughly one ten-thousandth to one millionth that of ferrites. Their particles are typically micron-sized. Overall, the eddy current loss of metal powder cores is 5 to 10 times greater than that of ferrites.
More critically, eddy current loss is proportional to the square of operating frequency. When the switching frequency exceeds 100 kHz, eddy current loss in metal powder cores surges sharply, even surpassing hysteresis loss to become the dominant source of power loss. Heat generated by such losses cannot dissipate promptly, leading to rapid temperature rise and potential burnout of the finished product.
Beyond cost and loss, flexibility in inductance tuning is another major consideration. Adjusting the air gap length of a ferrite core enables precise tuning of transformer inductance to suit diverse application requirements. For applications demanding high frequency, compact size and high efficiency, ferrite designs with air gaps deliver far more flexible solutions.
An air gap reduces the core’s effective permeability, making winding performance less reliant on the core material’s initial permeability. This allows tighter control over transformer inductance and improves inductance consistency. With a properly sized air gap, ferrite transformers can achieve inductance tolerances within 3%, or even tighter. Metal powder cores, however, deliver inductance tolerances of approximately 7% or wider.