"Drum core" is used to describe the shape of the ferrite core used in some power inductors, and it differentiates these inductors from other types such as toroidal, power bead, flat coil, or planar. There are two types of drum core inductors, unshielded and shielded. These devices are typically used in applications such as catalog power modules, computer equipment, telecom equipment, and consumer electronics such as game station consoles, digital cameras, PDAs, CD players and LCD displays and TVs.
The unshielded drum core has a narrow center rod with a flat disc on each end. The inductor wire is wound on the rod between the flat discs. An inductor is a device that stores energy within a magnetic field. Typically this field is located in an air gap in the flux path of the inductor.
For the unshielded drum core inductor, this air gap is between the two end discs. The flux path forms a closed loop through the center rod, outwards through one disc, through the air gap around to the outside of the other disc, and inwards through the second disc.
In the shielded drum core inductor, an additional ferrite cylinder is located around the outside of the two discs and guides the flux between the discs through the cylinder. The discrete air gap is the combination of the gaps between each end disc and the ferrite cylinder. Because the additional ferrite cylinder keeps the flux in a controlled path, shielded drum core inductors are better for applications where the stray flux of the unshielded drum core may interfere with nearby noise-sensitive electronics or where electromagnetic interference (EMI) is a concern.
The shielded drum core inductor has a higher inductance for the same number of wire turns than the unshielded inductor due to the smaller air gap and higher permeability of the ferrite cylinder. The unshielded drum core, however, is a lower cost technology and the larger energy storage air gap allows it to support higher peak currents without saturation effects.
Typically the application circuit (or topology) and application requirements (load current, ripple voltage, transient response) will dictate the inductance value, DC current and peak current that the inductor will need to support. The application will also dictate any size and height constraints, mounting method (surface mount or through-hole) and the need for an unshielded or shielded drum core.
Some inductor vendors, such as Pulse, provide inductance versus current graphs to allow the user to quickly identify an inductor that may meet their requirements. Typically, users are looking to identify the smallest inductor that will meet their inductance and current requirements. It is important, however, to look more closely at several parameters before finalizing the selection.
First, it is essential that the saturation current of the inductor be greater then the peak current in the application. Most magnetic vendors conservatively identify saturation current on their datasheets as the current at which the initial inductance of the part drops by 20% or 30%. Although true saturation, when the inductor core can no longer contain the magnetic field, occurs at a higher current it is not worth the risk of exceeding these listed values.
Secondly, it is important to consider the affects of the direct current resistance (DCR) of the inductor. The I2R losses associated with the DCR consume some portion of the input power, and therefore, decrease overall efficiency and contribute to the temperature rise of the inductor. In general a lower DCR is preferred, but typically lower DCR comes at the expense of having a larger physical part. It is up to the circuit designer to determine if the efficiency losses associated with the DCR are acceptable.
To determine if the DCR losses will cause the part to overheat it is essential to ensure that the temperature rise of the component added to the ambient temperature does not exceed the operating temperature of the inductor. To assist in this analysis some magnetic vendors provide a heating current specification that represents the root mean square (RMS) or dc current which will cause a 30°C to 40°C rise in the component temperature above ambient. Although application factors such as airflow and mounting will affect the temperature rise of the component, the heating current specifications provide an effective guideline.
Some drum core inductors, especially the unshielded drums with the large air gap, have significantly higher saturation current than heating current. These inductors are particularly suited to applications that allow a relatively large current ripple where the peak current will be higher than the RMS current.
Finally, thermal performance is another criterion to consider in selection. Both unshielded and shielded drum inductors in Pulse's new SMT inductor series have been designed to work in an ambient temperature range of -40°C to 125°C. The electrical performances given are for an ambient temperature of 25°C. This operating temperature range is important because in certain applications, such as portable products with small enclosures, the temperature at which the equipment is exposed can change significantly.
When it is not possible to achieve the required inductance at the rated current from the drum-core product range, the next step is to examine alternative technologies such as larger toroidal inductors, power bead, flat coil, or planar inductors. Pulse's Power Magnetics Catalog, which can be found on the Pulse website, can serve as a guide.