A good understanding of the desired in-circuit performance is required for a proper selection of an inductor. Fortunately, this relates to the information available in data sheets supplied by the manufacturer.
Any electronic equipment needs power to operate satisfactorily. Portable instruments invariably use dc-to-dc converters for generating the required voltages from power sources such as batteries. Additionally, as equipment size reduces, so does the need for optimizing inner parts, especially the power conversion solutions; it is no longer possible to stick on to the “one size fits all” approach. For example, manufacturers are producing low profile components, which are proving to be very popular.
Inductance is the primary functional parameter that defines an inductor. This value, calculated by the converter design equations, determines the ability of the inductor to handle the desired output power and control the ripple current in a dc-dc converter. However, specifying the inductance value alone is not sufficient for defining the required inductor. You also need to specify the DCR, SRF, Isat and Imms for a complete specification.
The DCR or the DC resistance of an inductor depends on the length and diameter of the wire used in manufacturing. The power dissipated by the inductor depends on the DCR. Therefore, if the inductor were to be used in an enclosed space, a lower value for the DCR would certainly be an advantage. Please note, resistance offered and hence power dissipated at high frequencies usually depends not on DCR, but on skin effect.
Another important factor is the SRF or the Self-Resonant Frequency of the inductor. At this frequency, an inductor begins to resonate naturally, using the distributed capacitance characteristic of the winding. In a dc-dc converter, SRF might cause unwanted oscillations and lead to instability of the output. The manufacturer specifies the SRF and it depends on the nature of the construction. The designer usually selects an inductor with an SRF that does lies outside the operating frequency range of the application.
Inductors with cores are liable to saturate. When in saturation, the inductance value falls drastically, upsetting the operation of the dc-dc converter. Isat or the Saturation Current, specified by the manufacturer is therefore, an important parameter, which states the amount of current that would cause saturation and subsequently a fall in the inductance value. Designers need to select an inductor that has an Isat value larger than the maximum currents expected in the converter.
Power dissipation in an inductor is dependent on two factors – the DCR and the Irms or the RMS Current flowing through it. Manufacturers specify the maximum rms currents that the inductor can handle causing a specified temperature rise. Therefore, the designer has to select an inductor whose Irms value is larger than the maximum rms currents that the converter is expected to deliver.
Fortunately, dc-dc converters are rather forgiving towards tolerances of inductors. These applications do not require inductors to have extremely tight tolerances to achieve their outputs. Standard tolerances indicated by manufacturers are +/-20%, which most converter designs accept as suitable.
