Output Inductor Design and Calculation for SMPS

In this article, you will learn how to design a ferrite core inductor for switch-mode power supply designs. How to design an inductor for a DC-to-DC converter like a buck converter, buck-boost converter, boost converter, cuk converter, and H-bridge? After reading this article, you will be able to design an output inductor for any SMPS topology. Output inductors are also used in pure sine wave inverters in an LC filter to remove harmonics from the 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 a little bit of effort.

What is the Output Inductor in SMPS?

The output inductor in a Switch-Mode Power Supply (SMPS) is an essential component that plays a crucial role in converting rectangular switching pulses into a clean DC output voltage. It is responsible for storing energy for the load when the input power is off. The output inductor, in conjunction with the output capacitor, helps to smoothen and regulate the output voltage.

The output inductor is typically designed using an air-gapped toroidal core. It is important to calculate the minimum required inductance (L(min)) for the specific SMPS topology.

Output Inductor Value Calculation

The output inductor calculator method remains the same for all power electronics topologies. The output inductor is used to store energy for the load when the input power is off. In electrical terms, the output inductor converts rectangular switching pulses into DC. After the inductor, the capacitor produces a clean DC output voltage. It is very easy to design an output inductor. Usually, an air-gapped toroidal core is used in the output inductor.

Inductance Formula

The output inductance calculation formula is given below:

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

The formula allows you to calculate the inductance by knowing the maximum peak voltage of the output rectifier, the output voltage, the on-time of the converter at the maximum input voltage, and the minimum load current. You can calculate the minimum inductance using the above formula. The minimum required inductance is represented by L(min). Once you have the minimum inductance, you can calculate the number of turns for the toroidal core in order to design the output inductor with the required number of turns.

Formula to Find the Number of Turns

The formula to calculate the required number of turns is given below:

AL = L/ NĀ²

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

 N= āˆš(L/AL)

So the above formula is used to calculate the required number of turns after calculating the minimum inductance from the given formula. There are many cases where you may not know the value of AL or you do not know about the number of toroid cores. How can you design the output inductor in that case? In this scenario, you can determine the value of AL through repeated experiments.

Wind a few turns on a toroidal core and measure the inductance with the help of an inductance meter. Then measure the inductance for different numbers of turns: 5, 10, 20, 30, and 40. Calculate the value of AL for each of them. Take the mean value of all the AL values. This way, you can calculate AL experimentally.


In conclusion, designing an output inductor for switch-mode power supply (SMPS) designs is a critical aspect of power electronics projects. The output inductor, along with the output capacitor, plays a significant role in converting rectangular switching pulses into a clean DC output voltage. By calculating the minimum required inductance (L(min)) using the appropriate formula, and determining the number of turns for the toroidal core, one can design an output inductor that meets the specific SMPS topology requirements. Additionally, experimental methods can be employed to determine the value of AL, the inductance factor, when specific information is unavailable. With the right knowledge and effort, designing output inductors for various SMPS topologies becomes achievable, contributing to the successful implementation of power electronics projects.

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1 thought on “Output Inductor Design and Calculation for SMPS”

  1. Thank you very much for your explanation, but i have a question
    . What about turns that was made without a core, how do you calculate that?


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