Magnetic Component Design Depends on Core Selection

Early in the planning process of a magnetic component, designers select a core, which drives all of the component variables. The core then becomes an element in a series of calculations and is used to determine variables such as turns count, wire size, insulation system, temperature rise, and performance prediction. If the calculated performance prediction meets the specifications of the required component, physical testing can begin. However, it's rarely that simple. Typically, the process returns to core selection, and another series of calculations are performed. Such a loop continues until the calculated performance prediction exceeds the specifications the customer has set forth. Hence, the eventual core selection may not be determined until late in the component design process — a big problem considering the life of the design depends on this decision. Thus, performance features of the core are the most important qualifying considerations for the system engineer.

The main purpose of the core is to conduct flux in a closed magnetic circuit around the wound coil. A toroid shaped ring-core is one of the most popular styles within the spectrum of soft magnetic cores because of many advantages offered by the toroid shape alone, such as excellent stray field characteristics, no gap-produced fringing, rugged, and predictable performance.

Key System Benefits

The features in an ideal toroid include the following:

Mechanical strength, so it will not easily break;
Low magnetostriction, so inductance is insensitive to mechanical stress;
Permeability that does not change with temperature;
High saturation flux rating for lower turns count;
Permeability and loop-shape, which can be tailored for specific applications;
Low core loss over a wide frequency range; and
Repeatable quality from early prototypes through high volume production.

Each style of the toroid has its own advantages from this list. Sintered ferrites and pressed powders, for instance, offer low loss at high frequency, but are generally limited to modest saturation and permeability characteristics. Stripwound cores from rolled crystalline metal alloys, such as silicon-iron (SiFe) and nickel-iron (NiFe), can feature high permeability and excellent saturation characteristics, mechanical strength, and easy prototypes. Yet, SiFe and NiFe alloys are limited by modest performance at frequencies.

Cores wound using strip cast by rapid solidification technology offer a unique combination of the desirable features of ferrites and rolled crystalline alloys. This combination offers core loss similar to the best ferrites, but with saturation, permeability, and mechanical toughness competitively closer to the rolled crystalline stripwound toroids. VAC Magnetics Corp. uses a rapid solidification technology to manufacture cobalt and iron strip materials for its toroid cores. While the primary alloy ingredient of Vitrovac is cobalt at approximately 60% to 70%, the main alloy element in VitrOPERM is iron at about 70%.

Application Examples

The applications of wound magnetic components are across many fields within the power electronics discipline. Thus, the cores are annealed and magnetically tailored for specific applications. These include cores for common-mode chokes (CMC), current transformers (CT), signal and gate drive transformers, magnetic amplifiers (MagAmp), and switchmode power supply (SMPS) transformers.

An examination of two of the applications, namely CMC and magnetic amplifier, will help further detail the importance of magnetic materials as applied to wound magnetic component design. For instance, the primary function of a CMC is to reduce the magnitude of the undesirable high frequency common-mode current that causes EMI radiation without impeding the desired differential-mode working currents. This function is accomplished by the bucking winding topology generally used in CMC designs (Fig.1). The load current (blue) in winding 1-2 bucks the return load current (blue) in winding 3-4, causing an equal but opposite flux (blue) in the core. Since both windings have equal but opposite amp-turns, only a small leakage inductance between these two windings creates any impedance to the desired working current. On the other hand, the RF common mode current (red) is about equal and in phase at all of the cable conductors, so the CMC windings now behave as if connected parallel with like polarity. The effect is a significant impedance that attenuates the common-mode current.

Consider issues of winding capacitance, wire size, permeability, core saturation, and temperature rise when making a choice for the CMC core. Also, evaluate multiple solutions to assure arrival at an optimum design. In many instances, a nanocrystalline core delivers a better component due to higher saturation, higher permeability, and lower turns count.

Table 1, on page 39, shows benchmark calculations for a 1 mH choke, based on a variety of large toroid cores specifically identified by the respective manufacturer for CMC applications. The better features should be high Ibias, low turns, and large ID. The large ID allows for larger wire and/or lower winding capacitance.

These calculations show for the similar performing benchmark designs, the turns count ranges from 10 to 29, and the core ID range is from 38.8 mm to 59.5 mm. The three 10-turn designs also have the largest ID, and two of the three are nanocrystalline Vitroperm 500F, while the third is “W” ferrite. The combination of less turns and larger ID permits the designer either to increase the wire diameter for higher amp rating or to increase the spacing between the turns for reduced winding capacitance. The labor cost of additional turns needed by some of the designs must also be considered in the trade-off decisions.

The choke must provide enough impedance at undesired frequencies. Fig. 2, on page 40, illustrates impedance vs. frequency relationship for four similar designs. While the turns count affects the insertion loss, the 30-turn design appears to be optimal.

Temperature dependency of CMC impedance is illustrated in Fig. 3, on page 40. The solid lines represent a Vitroperm 500F design, and the dashed lines represent a ferrite design. The various colors indicate selected test temperatures across the -40°C to 120°C range. Vitroperm 500F is very stable; however, the performance of ferrite CMC decreases from high and to low temperatures. For low temperatures, the inductance breaks down, while for high temperatures the Q-values break down. Therefore, the sharpness of the self-resonance of the component is reduced and the maximum value of impedance goes down.

For MagAmp, the key properties of these materials are listed in Table 2. Nanocrystaline Vitroperm 500Z permits higher operating temperature, while amorphous Vitrovac 6025Z offers lower core loss for permitting higher frequency application.

The MagAmp can quickly and predictably move into saturation and is an effective “magnetic switch.” The MagAmp principle is a simple, reliable, efficient, and low-cost solution to independently regulate the multiple output voltages for SMPS designs.

Multiple output switching power supplies often use one main transformer with several secondary windings. The output of each secondary is rectified and filtered to provide an isolated dc output voltage. The output voltage of the secondary with the highest power load is typically regulated by single pulse-width modulation (PWM), but such a scheme can leave the auxiliary outputs unregulated. Independent regulation of the auxiliary outputs is possible with the MagAmp principle to provide a controlled delay to the power pulse applied to the buck regulator section of each auxiliary output. A MagAmp Choke uses “square-loop” core and behaves like a high-speed time delay switch. Due to the rectangular hysteresis loop, the switch is open, and its large inductance blocks voltage until saturation, when the inductance suddenly drops to near zero. A small reset current in the winding tightly controls the delay (Fig. 4). As the core saturates, the impedance of the choke rapidly changes by three to four orders of magnitude, thus closing the switch.

A new Excel routine known as “VAC MagAmp Calculator” performs most of the design arithmetic to select the proper core, wire size, and turns so the finished choke operates as desired ^[5].

References:

Dr. Joerg Petzold, “Advantages of Softmagnetic Nanocrystalline Materials for Modern Electronic Applications,” Vacuumschmelze GmbH, Journal of Magnetism and Magnetic Materials, 2002.
“Cores and Components Databook 2000,” VACUUMSCHMELZE GmbH & Co, September 2000.
Rodney Rodgers, “Inside the Three Phase Common Mode Choke for Variable Frequency Drive Applications,” VAC Magnetics Corp., Proceedings of the International Power Electronics Technology Conference, 2002.
Rodney Rodgers, “New Square Loop Nanocrystaline Core Material for MagAmp Inductors that Regulate/Control Multiple Outputs of Low-Loss SMPS,” VAC Magnetics Corp., Proceedings of the International Power Electronics Technology Conference, 2002.
MagAmp design software, “VAC MagAmp Calculator,” is available for free download at www.vacuumschmelze.com.