[105] Designing Custom Inductors

Inductor manufacturers have made great progress in the families of standard parts. The selection of off-the-shelf parts in this category have increased dramatically in the past 10 years.

However, more and more engineers are designing their own inductors and transformers from scratch. We hope that our design workshops [1] and design software [3] boosted this trend. Standard parts manufacturers cannot possibly provide exactly what is needed for every design, and customization is a great way to go if you have some expertise to tackle the design. One sign that this is becoming more commonplace is the availability of cores and bobbins. Some of the popular cores can be purchased from both Digikey and Mouser (Figure 1). The selection is still very limited, but it is a great start.

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Figure 1: Digikey and Mouser are Now Stocking a Range of Ferrite Cores

Choosing the Inductor Value

Many engineers believe that there is a single inductor value that should be used for a given design. They may have read an application note, or a text book, that sets the value of the current ripple in a buck converter, for example. I would caution becoming complacent in this technique. In reality, the range of inductors that can work in a converter is actually very large [2]. It can be as much as 40 to 1, corresponding to a ripple current to dc current ratio ranging from 5% to 200%. The 200% value may seem overly large, meaning that the buck converter is running in discontinuous mode. However, there are some soft-switching advantages that can be gained in this mode. Some of the most sophisticated and dense converters are designed that way. This is sometimes hard for inexperienced engineers to accept, but there is no “right” answer for the inductor value. Experimentation and testing is the best way to realize the optimal design for your application. Don’t be afraid of design iteration – just learn how to be efficient with your time.

Figure 2 shows the application for which we want to design an inductor—a 300 W buck converter running at 300 kHz

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Figure 2: Buck Power Converter Switching at 300 kHz

The inductor value determines the amount of ripple current in the converter. Figure 3 shows simulations with two different values of ripple, one at 20% of the load current, and one at 80% of the load current. With modern switching devices, and good quality output capacitors, either of the designs can work well. The small inductor with 80% ripple will have better efficiency in the magnetics, but the peak currents will be higher and the output voltage ripple will be greater. The true numbers for overall performance can be rapidly simulated as in [3], and verified by measuring the actual hardware for each case. When we design a converter, we make it easy to swap out magnetics parts so a variety of different designs can be quickly evaluated.

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Figure 3: Ripple Currents for Different Inductor Values

Choosing the right inductor value is an iterative process. The full implications of the choice do not become apparent until each value is realized in the core and winding design process, so it is important not to fix the value of the inductor until the magnetics iterations have been assessed.

The basic parts of the inductor to be designed are shown in Figure 4. We need to choose a core area, number of turns, core material, and gap to design an inductor. The first step that we take is selecting the area of the core.

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