Inductor Selection in DC-DC Switching Power Supply

13 Jan 2024

1、 Introduction

Inductors are widely used in circuit design, and their main functions in circuits include

energy storage, filtering, choking, resonance, matching, etc. The main parameters of

inductance are as follows:

Table 1 Key parameters of inductance

NO.	Name	Significance
1	Inductance L	The larger the L, the stronger the energy storage capacity, the smaller the ripple, and the smaller the required filtering capacitance. However, the larger L, the larger the inductance size is usually required, resulting in an increase in DCR and a decrease in DC-DC efficiency, which in turn increases the cost of inductance.
2	Self harmonic frequency f0	Due to the presence of parasitic capacitance in the inductor, there exists a self resonant frequency. Beyond this f0, inductance exhibits capacitance effect, while below this f0, inductance exhibits inductance effect (impedance increases with frequency).
3	DC resistance DCR	The total DC resistance of the enameled wire used between the product electrodes. According to W=I2R, DCR can cause energy loss, reduce DC-DC efficiency, and is also the main cause of inductance heating.
4	AC resistor RAC	The resistance value of an inductor at a specified frequency, mainly composed of the DC resistance of the inductor coil (skin effect under AC), magnetic core loss, and dielectric loss. The larger the RAC, the higher the Q value.
5	Saturation current Isat	Usually refers to the DC current value corresponding to a 30% decrease in inductance.

2、Calculate the minimum inductance

Calculate the minimum inductance and consider dividing the DC-DC conversion circuit into Buck circuit and Boost circuit.

Calculation of the minimum inductance value for a Buck circuit

The formula for calculating the minimum inductance of a Buck conversion circuit is as follows:

wherein：

Table 2 Parameter Significance

Name	Significance
Vin	Input Voltage
Vout	Output voltage
Fs	switching frequency
ΔI	Inductive peak current

usually Δ If I take "1/2" of the rated current Irat, then the above formula changes as follows：

Calculation of minimum inductance value for 2 Boost circuits

The formula for calculating the minimum inductance of the Boost conversion circuit is as follows：

The maximum current flowing through the DC-DC conversion circuit, Imax, according to Δ I take 50% of the rated current Irat, and the calculation formula is as follows：

3、Selection of inductance

1. Calculate inductance value

It is necessary to calculate the inductance value based on the accuracy of the inductance, and leave a certain margin.

For inductors with an accuracy of 20%, if the designed margin is 5%, the required inductance value to be designed is：

L = （1+20%+5%）*Lmin

2、Calculation of nominal inductance

The actual inductance used will be slightly larger than the inductance we calculated, and we call it the nominal inductance.

Taking the Boost circuit as an example, with an input voltage of 4.2V, a switching frequency of 1.2MHz, an output current of 500mA, and an output voltage of 5V, the required inductance is：

So: L=2.24 μ H × 1.25=2.8 μ H. Compared to 2.8 μ The nominal inductance for slightly larger H is 3.3 μ H. So the external inductance of DC-DC is selected as 3.3 μ H inductance.

3、Current calculation

Confirm that the saturation current Isat of the selected inductor should be greater than Imax, and the temperature rise current should be greater than the output current. Current requirements：

Imax=Iout+1/2×ΔI=Iout+1/4×Iout=1.25×Iout=1.25×0.5A=0.625A

4、NOTE

a. The self resonant frequency f0 of the inductor should be more than 10 times the switching frequency Fs.

b. Set the smaller value of the saturation current Isat and the temperature rise current Irms.

c. In the high current region, DCR plays a decisive role, and the larger the DCR, the lower the conversion efficiency.

d. Stacked inductors have better heat dissipation, lower ESR value, and lower cost than wound inductors, but they have lower current resistance than wound inductors.

e. Choosing inductors with magnetic shielding (shield or magnetic adhesive coating) can effectively improve EMI.f. The size of inductance: With the same nominal inductance, the size directly affects the

cost of material selection.

g. Inductance saturation can cause a sharp increase in current, raise the temperature of the inductor, and affect the lifespan of other components.

The AC parameters of inductors are measured at a frequency of 100K sine wave, and in practical applications, it is necessary to confirm whether the inductance is suitable through actual measurement

tags :