Powder Core and Gapped Ferrite Core Comparison

Although high grade ferrite core losses are lower than powder core losses, ferrite often requires low effective permeability to prevent saturation at high current levels. Ferrite, with its high initial permeability, requires a relatively large air gap to get a low effective permeability. This large air gap results in gap loss, a complex problem which is often overlooked when comparing material loss curves. Gap loss can drastically increase total losses due to fringing flux around the air gap (Figure 2). The fringing flux intersects the copper windings, creating excessive eddy currents in the wire. 

The soft-saturation characteristics of powder cores are designed to exploit the controlled, partial roll-off of permeability in the material while having more than twice the flux capacity of ferrite. This slow roll-off also has the added benefit of improved fault tolerance. Additionally, flux capacity in powder cores stays relatively constant with temperature in comparison to ferrite.

Gapped ferrite cores do have advantages over powder cores. Gapped ferrites typically have a ±3% tolerance on inductance compared to powder cores' ±8%. Gapped ferrites are available in a wider selection of shapes. Since ferrite material can have a higher gapped effective permeability it is well suited for relatively low bias applications, such as feed forward transformers and low biased inductors.

Advantages of Kool Mu compared with gapped ferrite solutions are:

• Soft Saturation: Ferrite must be designed in the safe, flat area of the rolloff curve. Powder cores like Kool Mu and XFlux are designed to exploit the controlled, partial roll-off in the material (Figure 3).
• Flux Capacity: With more than twice the flux capacity of ferrite at 50% inductance roll-off, powder cores can provide a reduction in required core size of up to 35%.
• Temperature: Flux capacity of ferrites decreases with temperature while powder cores stay relatively constant.
• Fault-tolerance: Powder core designs are inherently fault-tolerant with soft saturation curves, whereas gapped ferrite is not.
• Fringing Losses: Do not occur with powder cores, but can be excessive with gapped ferrites.