# What parameters need to be considered in the selection of power inductors?

28/09/2022

1. Nominal inductance value

Measure the inductance value at a frequency of 100KHz. The test frequency is not fixed, and the test frequency of the nominal inductance value of different products is also different, and the manufacturer will indicate it in the product catalog.

2. Self-resonance frequency

Depending on the frequency at which the inductance and parasitic capacitance resonate, the critical frequency that acts as an inductor. The impedance of an ideal inductor increases as the frequency increases. However, due to the existence of parasitic capacitance and parasitic resistance, the actual inductance is inductive at a certain frequency, and capacitive when it exceeds a certain frequency. On the contrary, the impedance decreases with the increase of frequency. This frequency is the critical frequency.

3. DC resistance

The resistance when applying direct current is mainly the resistance of the winding. Due to this impedance value, power loss occurs due to heat generation, so the smaller the DC impedance, the less the loss. However, there is a trade-off relationship between reducing Rdc, DC superposition characteristics, and size reduction. It is sufficient to select a product with a smaller Rdc from among the above-mentioned inductors that satisfy the necessary characteristics such as inductance and rated current.

4. Rated current

The rated current specifies the upper limit of the current value, and when the passing DC current exceeds this value, the quality cannot be guaranteed. There are two rated currents for power inductors, one is the allowable current for DC overlap, and the other is the allowable current for temperature rise. Each has important meanings, and technical specifications are often recorded separately. If only one rated current is specified, it is generally specified according to the smaller of the DC overlap and the temperature rise.

5. DC overlap allowable current

When a current flows through an inductor, the magnetic saturation of the magnetic material occurs, and the inductance decreases. This characteristic is called the DC superposition characteristic. The DC superimposed allowable current, also known as the saturation allowable current, specifies the current value when the inductance decreases by a certain percentage relative to the initial characteristics of the non-superimposed current, generally by 10% to 30%, depending on the product specification.

6. Allowable current for temperature rise

The rated current is specified with the heating of the components as the index. Use beyond this range will cause damage to the components or failure of the components. Generally, it is specified according to the current value at which the temperature rises by 20~40°C (depending on the product specification) when a DC current is applied.

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