DC_DC inductor Selection Guide

DC_DC inductor Selection Guide

One: Inductance main parameter meaning

The following parameters should be considered in the selection of inductor for DC-DC peripheral: Inductance L, self-resonant frequency F0, internal resistance DCR, saturation current ISAT, effective current irms.

The larger the inductance l:l, the stronger the energy storage capacity, the smaller the ripple, the smaller the required filter capacitance. But the larger the L, the more often the inductor size is required and the DCR increases. Results in a reduced DC-to efficiency. The corresponding inductance cost will also increase.

Self-resonant frequency F0: Due to the presence of parasitic capacitance in the inductor, there is a self resonance frequency in the inductor. More than this F0 is, inductance performance is capacitive effect, below this F0, inductance only shows as inductance effect (impedance increases with frequency increase).

Internal resistance DCR: refers to the DC impedance of the inductor. The internal resistance causes the energy loss of the I2R, on the one hand, it reduces the efficiency and is the main cause of the inductance heating.

Saturation current Isat: Usually refers to a DC current value corresponding to a 30% decrease in inductance.

Effective current irms: usually refers to the equivalent current value when the surface temperature of the inductor rises to 40 degrees.

Second: The DC-to inductance selection step

1, the minimum inductance required is calculated based on the input and output characteristics of the DC-to.

For Buck-DC, the formula is as follows

Lmin= "vout* (1-vout/vinmax)"/FSW*IRPP

Where: Vinmax = maximum input voltage Vout = Output voltage

FSW = Switching Frequency IRPP = inductor Peak-to-peak Ripple Current

The IRPP is typically controlled at a output rated current of 50% irate. The above formula changes as follows:

Lmin=2* "vout* (1-vout/vinmax)"/fsw*irate

The formula for the lmin inductance of the boost-DC is as follows:

Lmin=2* "vinmax* (1-vinmax/vout)"/fsw*irate

2, according to the accuracy of inductance, calculate a certain amount of headroom inductance value for example: 20% of the accuracy of the inductor, taking into account 5% of the design margin. Then the inductance required for the DC-L=1.25*lmin is

3, determine the inductance we need is a slightly larger nominal inductance than the calculated inductance L

For example: There is a mobile phone using buck-DC, the input is battery vinmax= =4.2v, switching frequency fsw=1.2mhz, output current irate=500ma, output power vout=1.2v

The inductor lmin= [2*1.2* (1-1.2/4.2)]/(1.2*0.5) Uh=2.85uh L=2.86uh*1.25=3.57uh.

One of the closest 3.57uH nominal inductance is 4.7uH, so the external inductor is selected from DC

4.7uH inductor.

4, under the given nominal inductance, consider the following limiting factors ultimately determine the inductor selection.

1), inductance self-harmonic frequency f0 need 10 times times more than the switching frequency FSW.

2), the lower one of the saturation current ISAT and the active current irms is more than 1.3 times times the DC rated current output irate.

3) The lower the DCR the better

4), multilayer inductance is better than winding inductance (low loss)

5), the shielded inductor is better than the inductor without shielding. (Improved EMI)

In addition, the cost and volume of the inductor also need to be weighed.