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How to choose the power inductor?
Sep 10, 2018


How to choose the power inductor?


Power inductors are commonly used in DC-DC circuits to maintain a continuous current by accumulating and releasing energy. Speaking of integrated inductors, NR magnetic inductors, chip power inductors are all power inductors, then how to choose power inductors? The power inductor needs to be selected according to the selected DC-DC chip. Generally, the specifications of the DC-DC chip have recommended inductance values and the calculation of related parameters, which will not be described here. From the perspective of the inductor itself, how to select the type.



Inductance value


  Generally, the inductance value recommended by the DC-DC chip specification should be used; the larger the inductance value, the smaller the ripple, but the size will become larger; usually the switching frequency can be increased, and a small inductance can be used, but the increase of the switching frequency will increase the system loss and reduce effectiveness;





Rated current


  The power inductor generally has two rated currents, that is, temperature rise current and saturation current; when the inductor has current passing through, the temperature rises due to the heat generated by the inductor, and the larger the current, the larger the temperature rise; at the rated temperature Within the range, the maximum allowable current is the temperature rise current.


  Increasing the magnetic permeability of the magnetic core can increase the inductance value, and a ferromagnetic material is usually used as the magnetic core. The ferromagnetic material has a magnetic saturation phenomenon, that is, when the magnetic field strength exceeds a certain value, the magnetic induction strength does not increase, that is, the magnetic permeability decreases, that is, the inductance decreases. The maximum allowable current is the saturation current within the rated inductance range.


DC Resistance


  The DC resistance of the inductor generates heat loss, which causes temperature rise and reduces DC-DC efficiency; therefore, when sensitive to efficiency, an inductor with a low DC impedance, such as 15 milliohms, should be selected. There is also a requirement according to the application temperature of the product, whether it is required to meet the requirements of RoHS, automotive grade Q200, and PCB structure restrictions.


  In the application of high current, the leakage current of the inductor will be considerable, which will affect the surrounding circuits, such as the CPU. I have encountered the X86 CORE electric inductance leakage magnetic flux caused by the CPU can not start. Therefore, for high-current applications, you should choose a good shielding performance and pay attention to avoid key signals when Layout.




  Hysteresis loop


  Magnetic materials - - ferrite magnets, hydrometers, porous material density meters, liquid density meters, solid particle volume testers, magnetic material density meters.


  Usually for DC-DC circuit design, the peak (PEAK) current and the root mean square (RMS) current are calculated, and the calculation formula is usually given in the specification. The temperature rise current is an evaluation of the thermal effect of the inductor. According to Joule's law, the thermal effect needs to consider the integration of current over time for a period of time; when selecting an inductor, the designed RMS current cannot exceed the inductor temperature rise current. In order to ensure that the inductance value is stable within the design range, the design peak current cannot exceed the saturation current of the inductor. In order to improve reliability, derating design is a must, and it is generally recommended that the working value should be derated to not more than 80% of the rated value. Of course, the derating is too large to increase the cost of the General Assembly and needs to be considered comprehensively.



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