output inductor design and calculation for SMPS

In this article, you will learn how to design ferrite core inductor for switch mode power supplies designs? How to design inductor for dc to dc converter like buck converter, buck boost converter, boost converter, cuk converter and H bridge? After reading this article, you will be able to design output inductor for any SMPS topology. Output inductors are also used in pure sine wave inverter in LC filter to remove harmonics from sine wave.  Output inductor design and ferrite  transformer design are the two most important components in power electronics projects. because they are not easy to design. Professional winding machines are required for this purpose. But still you can design them with little bit effort.




Output inductor calculator method remains same for all power electronics topologies. Output inductor is used to store energy for load when input power is off. In electrical terms output inductor converts rectangular switching pulses into DC.  After inductor, capacitor produce clean dc output voltage. It is very easy to design output inductor. Usullay air gapped torroid core is used in output inductor.  output inductance calculation formula is given below:

L(min) = [Vin(max) – Vout] * Toff (est)/ 1.4 * Iout(min)

Where Vin(max) is maximum peak voltage of output rectifier. Vout is output voltage.  Toff (est) is the on time of converter at maximum input voltage. Iout(min) is the minimum load current. All above parameters should be known. So you can easily calculate inductance using above formula.L(min) is the minimum required inductance. Now we know the minimum inductance, now we need to calculate the number of turn for torroied core. So that we can design output inductor with required number of turns.  Formula to calculate required number of turns is given below:

AL = L/ N²

AL can be found from data sheet of core. Where L is the minimum inductance and N is the required number of turns. By change above formula in terms of number of turns it becomes:

 N= √(L/AL)




So above formula is used to calculate required number of turn after calculating minimum inductance from above given formula. There are many cases, when do not know the value of AL or you do not know about the number of torroid core. How can you design output inductor in that case? In this you can determine the value of AL with repeated experiments.

Wind few turn on torroid core and measure the inductance with the help of inductance meter. Then measure the inductance for different number of turns.  measure the inductance for 5, 10, 20, 30 and 40 turns and calculate value of AL each of them. Take the mean value of all the AL values. In this way you can calculate AL experimentally.

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