How to select inductance in DC-DC circuit design 2018-07-25

The selection of inductors in DC-DC circuit design brings many challenges to engineers, not only to choose the inductor value, but also to consider the current that the inductor can withstand, the resistance of the winding, the mechanical size, and so on. Therefore, only by fully understanding the role of inductance in the circuit can a better design of DC-DC circuit.

Main reference parameters of DC-DC inductor selection:

Inductance L:

The larger L is, the stronger the energy storage capacity is, and the smaller the ripple is, the smaller the required filter capacitor is. However, the larger L is, the larger the inductor size is usually required.

Self-harmonic frequency f0:

Due to the parasitic capacitance in the inductor, the inductor has a self-resonant frequency. Above this F0, the inductance appears as a capacitive effect, and below this F0, the inductance appears as an inductive effect. (Impedance increases with increasing frequency)

Internal resistance DCR:

Refers to the DC impedance of the inductor. This internal resistance causes I2R energy loss, causing DC-DC to reduce efficiency, and it is also the main cause of inductance heating.

Saturation current Isat:

Usually refers to the corresponding DC current value when the inductance decreases by 30%.

Temperature rise current Irms:

Usually refers to the equivalent current value when the surface temperature of the inductor rises to 40 degrees.

DC-DC inductor selection steps

Calculate the minimum required inductance based on the input-output characteristics of DC-DC. (Note: For the calculation of inductance, each DC-DC chip manual has a clear calculation method, please refer to the manual, the following formula is only an example)

For Buck DC-DC, the calculation formula is as follows:

Lmin = [Vout * (1-Vout / Vinmax)] / Fsw * Irpp

among them:

Vinmax = maximum input voltage

Vout = output voltage

Fsw = switching frequency

Irpp = inductor peak-to-peak ripple current

Usually Irpp is controlled at 50% of the rated output current Irate. Then the above formula changes as follows:

Lmin = 2 * [Vout * (1-Vout / Vinmax)] / Fsw * Irate

The formula for calculating Lmin inductance of Boost DC-DC is as follows:

Lmin = 2 * [Vinmax * (1-Vinmax / Vout)] / Fsw * Irate

According to the accuracy of the inductance, the inductance value with a certain margin is calculated. For example: For an inductor with 20% accuracy, consider a design margin of 5%. Then the inductance required by DC-DC is L = 1.25 * Lmin

Determine that the inductance we need is a nominal inductance slightly larger than the calculated inductance L. For example: Buck-type DC-DC circuit design, its input is battery Vinmax = 4.2V, switching frequency Fsw = 1.2MHZ, output current Irate = 500mA, output power supply Vout = 1.2V.

Then the inductance required by its DC-DC:

Lmin = [2 * 1.2 * （1-1.2 / 4.2）] / (1.2 * 0.5) μH = 2.85μH

L = 2.86μH * 1.25 = 3.57μH

L = 2.86μH * 1.25 = 3.57μH

The nominal inductance closest to 3.57uH is 4.7uH, so 4.7uH inductance is used for the DC-DC external inductance.

Under the given nominal inductance, consider the following limiting factors to ultimately determine the choice of inductance:

1. The self-resonant frequency f0 of the inductor needs to be more than 10 times the switching frequency Fsw;

2. The lower of the saturation current Isat and the effective current Irms needs to be more than 1.3 times the DC-DC rated current input Irate;

3. The lower the DCR, the better;

4. Inductors with shielding are better than those without shielding. (Improve EMI)

In addition, the cost and volume of the inductor also need to be weighed. Shenzhen Kedajia Electronics Co., Ltd., as always, is committed to the research and development of materials and new products. Through continuous technological breakthroughs and innovations, we have developed many high-quality inductors for the design of various DC-DC conversion circuits in the market to help you create Higher market value!

Every tiny attempt embodies the wisdom and strength of the Kedajia people, and every innovation is only to give you a better experience!