# Learn More About Bobbin Cores

Bobbin Core Design

Basic properties of a bobbin core are its size, material type and thickness, and its flux capacity. The size determines the maximum number of turns of wire that can be wound on the core and the DC winding resistance. The operating frequency and the losses that can be tolerated in the circuit determine the type of material selected and the tape thickness. The flux capacity, or volt second area, of the core determines its output per turn of wire and the voltage the core can support. Bobbin cores were designed for pulse applications. It is for this reason that the test conditions and measured characteristics supply information about Ts, switching time, Core One Flux, the amount of flux switched in one cycle, and squareness.

Flux capacities in Maxwells for each core are shown in the Bobbin Core Sizes Table. Nomograms related to core selection have been developed. For power applications a graph of Power handling vs Window Area Flux Product allows the designer to select a core based upon operating frequency and output power. Another graph illustrates switching time vs. H in Oersteds for switching applications. Core loss curves for the material will allow the designer to calculate core losses. Please contact Magnetics for additional bobbin core design information and to receive the families of curves.

### Select the bobbin core best suited for your application:

Select the material type and thickness. Based on operation at or near saturation flux density, the following is a guide in selecting the proper thickness of materials for various frequency ranges:

**If operating flux density is reduced, frequencies can be extended upwards from those listed. Square Permalloy has lower losses. Square Orthonol has greater flux capacity.*

## Bobbin Core Testing

### Integrated One Flux (ø1)

The integrated one flux is the value in Maxwells of the response produced when the one output voltage is passed through a calibrated integrator. It is the area under the one output voltage waveform, and is the flux switched when the core is driven from positive residual to negative saturation. Reference Figure #2.

### Squareness (Br/Bm)

The squareness is the ratio of the residual flux of a core to the saturation flux of a core.

### Switching Time (Ts)

The switching time is that time interval between the point where the core output has risen to 10% of the core one output voltage and the point where the core output has decreased to 10% of the one output voltage. Reference Figure #3.

### Noise to Signal Ratio (ø0/ø1)

The integrated zero flux, ø0, measured in Maxwells is the integral of the area under the Open circuit zero waveform when the flux is switched from negative residual to negative saturation. Divide this value by ø1 to obtain ø0/ø1

Please visit the Technical Documents library for the Tape Wound Core Design Manual. Other reference materials including technical bulletins, application notes and white papers are also available for download.