In a step-down switching converter, the theoretical peak current that the power inductor can withstand is IL,PK=(1+r/2)×IOUT. This is the basis for the minimum value of the rated current or saturation current of the power inductor. The actual selection value needs to be greater than the theoretical value of the peak current.
Once the inductance value and other conditions of the power inductor are determined, the actual peak current of the inductor is Ipeak = Iout,max + ∆I_L/2 (where ∆I_L is the actual ripple current on the inductor after the inductance value is determined).
The formula for calculating the minimum value of the power inductor in a BUCK circuit is as follows:
Where VIN,MAX is the maximum input voltage that the BUCK circuit needs to support, VOUT is the typical output voltage, FSW is the switching frequency, IOUT,MAX is the maximum load current that needs to be supported, and r is the ripple current coefficient, which is typically set to 0.3~0.5.
The above formula is the basis for determining the minimum power inductance value in the BUCK buck converter. The actual value must not be less than the theoretically calculated value; otherwise, the actual ripple current and other parameters in the circuit will not meet the target values in the design requirements.
By transforming the above formula, we obtain the formula for calculating the minimum power inductance value, as shown below:
Therefore, it can be seen that the input voltage VIN is directly proportional to the inductance L; the larger the input voltage (the maximum input voltage that needs to be supported is VIN,MAX), the larger the required inductance.
If the designed BUCK circuit only needs to support the typical input voltage value VIN,TYP, that is, the input voltage is a fixed value, then the calculation of the minimum power inductance mentioned above can also use VIN,TYP.
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