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How to choose low-power power inductor

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Ultra-low power or ultra-high power switching power supply | Regulator inductance, not as easy as the general switching power supply options. Conventional inductors are currently manufactured for some mainstream designs and do not adequately meet some special designs. This article focuses on the ultra-low power, ultra-high efficiency Buck circuit inductance selection problem. Typical application examples are small-size battery power supply equipment for a long time. In this circuit, the problem that makes engineers feel difficult is the contradiction between battery capacity (cost and volume) and Buck circuit size and efficiency. In order to reduce the size of the switching power supply, it is best to choose the highest possible switching frequency. However, the switching losses and the losses of the output inductors increase with the switching frequency, and are likely to become the main factors affecting the efficiency. It is these contradictions that greatly increase the circuit design difficulty.
Buck circuit inductance requirements

For engineers, ferromagnetic components (inductors) may be the earliest contact nonlinear devices. However, according to the data provided by the manufacturer, it is difficult to predict the inductor loss at high frequencies. Because manufacturers usually only provide such as open-circuit inductance, operating current, saturation current, DC resistance and self-excited frequency and other parameters. These parameters are sufficient for most switching power supply designs and it is easy to select the right inductor based on these parameters. However, for ultra-low current, ultra-high frequency switching power supplies, the nonlinearity of the inductor core is very frequency sensitive and, secondly, the frequency also determines the coil loss.

For a typical switching power supply, the core loss is almost negligible relative to the DC I2R loss. So, in general, there are few other frequency dependent parameters for the inductor, except for the frequency-dependent parameter, "self-excited frequency." However, these high frequency losses (core loss and coil loss) are usually much greater than dc losses for ultra-low power, ultra-high frequency systems (battery-powered devices).

Coil losses include DC I2R loss and AC loss. Among them, the exchange loss is mainly due to the skin effect and the proximity effect. Skin effect is that as the frequency increases the moving charge more and more tends to flow on the conductor surface, which is equivalent to reducing the cross-sectional area of conductor conductivity and improve the AC impedance. For example: at 2MHz frequency, conductor conduction depth (down from the conductor surface down) is only about 0.00464 cm. This leads to the current density is reduced to the original 1 / e (about 0.37). Proximity effect means the magnetic field generated by the current in the adjacent conductors of the inductor will affect each other, resulting in the so-called "crowded current" and also the AC impedance. For the skin effect, through the multi-core wire (the same wire with multiple fine wires) moderately alleviate. For those circuits where the AC current ripple is much smaller than the DC current, the multi-core wire can effectively reduce the total inductance loss.

Core loss is mainly due to hysteresis and magnetic core internal conductivity or other nonlinear parameters of the mutual inductance. In the Buck topology, the B-H hysteresis loop in the first quadrant has the greatest effect on the core loss. In the first quadrant of this partial graph, the hysteresis loop shows how the inductor transitions from the initial inductance to the peak inductance and back to the initial inductance. If the switching power supply is stable in a discontinuous state, the hysteresis loop will transition from the remaining inductance (Br) to the peak inductance (refer to Figure 1). If the switching power supply is in a continuous state, the hysteresis loop will rise from the DC bias point to the peak of the curve and return to the DC bias point. The exact shape of the hysteresis loop (basically an elliptic curve) can be determined experimentally.
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