Inductor and transformer cores are made from soft magnetic materials. “Soft” magnetic materials are easily magnetized and demagnetized, and a magnetic field is not present until these cores are excited by changing electric current in the windings (or “turns”) wrapped around them, creating an electromagnetic field. The term soft denotes the magnetic field is not permanent and disappears when the current stops. This is different from what we commonly call magnets. “Permanent” magnets are typically used to pick up, or attach things to, iron-containing (ferrous) metals (e.g. refrigerator magnets) and have a permanent magnetic field without windings or outside stimulus.
There are many varieties of soft magnetic materials. Almost all are based on a compound of iron. This article will discuss some key differences between ferrite cores and pressed-powder core (sometimes called composite) materials.
Ferrite cores are crystalline ceramic iron-oxide compounds that are formed by high-temperature firing into hard, and sometimes brittle shapes. Ferrite cores are available in a wide variety of shapes, but the selection is somewhat limited because the cores are formed under very high pressure and temperature, making fine detail not feasible. Ferrite cores are used by winding wire on them to form transformers and inductors. Many inductors use ferrite cores, and they are particularly the material of choice for most transformer applications.
Ferrite cores
Pressed-powder cores are made from insulated ironoxide particles which are pressed together to form solid core shapes. Compared to ferrite, pressed-powder cores are lower permeability but generally support higher current without saturation. In addition, saturation is “softer” such that inductance does not drop precipitously as with ferrite. Like ferrite, pressed-powder cores are available in a limited variety of shapes, with pressed powder most common in toroid shapes.
Pressed-powder
A particular use of pressed powder has become very popular in recent times. Pressed powder requires much less temperature to form than ferrite cores and therefore can be formed directly over windings without melting the copper wire or the insulation. This allows core material to fill all the space that was previously wasted in inductor windings. This has enabled some of the industry’s most efficient and energy dense inductors. These inductors achieve very high current ratings and very low DC resistance (DCR) in a tiny overall size.
In addition to the efficient circuit performance, this method completely covers the windings with magnetic core material and achieves not only a robust and rugged design, but also one with excellent magnetic shielding to reduce the chance of radiated EMI emissions.
Pressed-powder Molded Inductors
Traditional winding on a ferrite or powder core
Pressed powder molded over a winding eliminates wasted air pockets, enabling reduced part size
Coilcraft’s inductor design tools are designed to find the top-performing parts for any set of operating conditions, enabling users to evaluate inductor performance without concern for the materials used or construction type. Those factors are reflected in the tool results so the user can focus on circuit performance while optimizing the use of off-the-shelf inductors. The following examples illustrate how pressed-powder composite inductors often, but not always, end up as the best solution from a size, saturation current, and low-loss / high-efficiency standpoint.
Example One:
For our first example, we’ll look at the results in our DC-DC Optimizer Tool for a buck regulator operating at 800 kHz with 12 V input, 9 V, 0.75 A output, at 85°C ambient. The results below show that the XGL3530-682 pressed-powder inductor is smallest, with a higher Isat rating, and total losses far below the ferrite inductors.
Example Two:
Our next example is a buck-boost regulator operating at 1 MHz with 9-12 V in and 5 V at 0.75 A output at 25°C ambient. Again, the composite XGL inductor is the smallest solution, with a higher Isat rating and total losses well below the larger ferrite inductors.
Example Three:
Our final example shows that there are exceptions to the idea that composite core (pressed-powder) inductors are always best. It is based on Texas Instruments reference design PMP21277 for a 12 V to 20.5 V / 7.5 A max continuous current boost application running at 250 kHz. While it is true that Coilcraft XAL pressed-powder parts give the smallest solution size, the total losses are higher than the ferrite core inductors in this case. The SER2918H ferrite-core inductor was selected by TI due to lower DCR, lower losses, and a much higher Isat rating.
Conclusion
There are a variety of core materials used in today’s inductors and each will continue to have their place. Understanding the major core types can be helpful in identifying the best inductor for a particular application, but even more important is that datasheets have evolved and users now can use Coilcraft’s suite of sophisticated tools like to better understand the available choices and optimize circuit performance.
Pressed-powder molded, magnetically-shielded inductors are often the popular choice because of many benefits, including small size, soft saturation, low loss, inherent robustness, low EMI, and minimized acoustical noise risk vs traditionally-wound, ferrite-based inductors. Using Coilcraft’s selection and analysis tools to compare and analyze inductors simplifies and optimizes selection in terms of critical circuit performance parameters.
