How to choose a surface-mount inductor for a DC/DC converter

When the designers have basic knowledge about the principles of the switch mode converters, selecting the surface mount inductors becomes easy. When selecting the DC/DC converter, the primary goal will be efficiency. Sometimes the design requirements include converting to the voltage of low supply from the battery voltage. Here are some factors considered when selecting the surface-mount inductor for DC to DC converters.

1. Test Conditions

Designers are required to provide special attention while testing the conditions. They require checking the wave shape and voltage and testing the frequency accordingly. Depending on the sinusoidal voltages, the inductance ratings are provided. A sinusoidal voltage is used to test the standard instrumentation condition. As the power indicators do not range frequency between a wide range between 20 to 500 kHz, The rating is generally measured with 100 kHz, which is effective and easily measurable. As the frequency level increases, the inductance level decreases. The ratings are therefore considered through the actual application’s frequencies as it keeps changing from 500 kHz to 1 Mhz and even more.

2. Current Rating 

While specifying a power indicator, the current rating of the surface mount inductor will be a bit difficult. Depending on the operation of the converter, the current through the DC/DC is converted through the switching cycle. The current through the converter indicator will change from one cycle to another, including spikes or temporary transients due to abrupt changes in the line or the loads.

3. Tolerance

The DC/DC converters do not require inductors with very tight tolerances to achieve the goals. The option is the most cost-effective if the standard tolerance parts are chosen. Most of the converter requirements also allow the standard tolerance components.

4. DC Resistance

The DC resistance depends strictly on the length and diameter of the wire. Though mentioned as ‘max’ in the catalog, it is also specified as nominal with the tolerance. The temperature should also be tested as DCR varies with the temperature difference. For example, the temperature resistance for copper is around +0.4%. So, DSR should have a rating corresponding to it.

5. Self Resonant Frequency

Besides the inductance, every inductor has some distributed capacitance, forming a resonant circuit with a natural self-resonant frequency. The inductors will operate best when they operate below the SRF frequency. The inductor data shows this value as the typical value.

6. Saturation Current 

Core saturation is one of the effects of the current through the inductor. The current wave shapes the DC component with the DC/DC converters. The core gets biased with the DC through the inductor and saturates with the magnetic flux. When this occurs, the inductance decreases, and the component may longer operate as inductors, and the designers need to understand it properly.

Conclusion

The applications of the Surface mount inductors should be properly understood by the designers so that the DC/DC converters are operated. Their conditions require proper testing. The tolerance and the inductance levels are also taken into consideration. The self-resonant frequency is considered the typical frequency value while converting the current.