Below are step by step guide on how to select inductor for boost converter. This is proven in many products released to the market.
1. Select Inductance Value
a. Define the switching frequency for the boost converter
Example: Fsw = 300kHz
b. Define the input and output voltage
Example: Vin = 12V, Vout = 24V
c. Determine the duty cycle
Dutycycle = 1 – (Vin / Vout)
Example:
Dutycycle = 1 – (Vin / Vout) = 1 – (12V / 24V) = 50%
d. Know the Maximum Load Current, (Iload)
Example: Iload_max = 10A
e. Know the Input Current (Iin)
Input current = Vout X Iload_max / Vin
Example:
Input current = Vout X Iload_max / Vin = 24V X 10A / 12V = 20A
Above equation is just a quick method; not that accurate but more than enough to do the job.
f. Assume Ripple Current, (ripple)
To determine the inductance size, you need to assume a ripple current.
The rule of thumb is 10%-30% of the input current.
Example: ripple = 10% X Iin = 0.1 X 20A = 2A
g. Compute the Inductance Value using Below Equation
L = [ (Dutycycle / Fsw) X Vin ] / [ ripple ]
Example:
L = [ (0.5 / 300kHz) X 12V ] / [ 2A ] = 10uH
Where;
Dutycycle – boost converter duty cycle
Fsw – Boost converter switching frequency
Vin – Boost converter input voltage
Vsw_drop – voltage drop of the switch (can be assumed 0 for ideal)
Ripple – ripple current of the inductor
Vout – Output voltage of the boost converter
2. Selecting the Inductance Value Based on Catalogue Part and Recompute the Ripple Current
In case the resulting inductance value is not a standard value, you need to select a standard one. Then, rewrite the inductance equation to get the corresponding ripple current based on the selected inductance.
The computed inductance above is already a standard value.
3. Compute the Peak Current (Ipeak)
The peak current will be the basis of the inductor saturation current rating. The actual inductor must have a saturation current rating higher than the computed peak current.
Ipeak = [ Dutycycle X ripple – 2 X Iload – ripple ] / [ 2 X (Dutycycle – 1) ]
Example:
Ipeak = [ 0.5 X 2A – 2 X 10A – 2A ] / [ 2 X (0.5 – 1) ] = 21A
4. Compute the RMS Current
The RMS current is needed to size accordingly the continuous current rating of the inductor.
Irms = [ ripple / sqrt(3) ] + Ipeak – ripple
Exmple:
Irms = [ ripple / sqrt(3) ] + Ipeak – ripple = [ 2A / sqrt(3) ] + 21A – 2A = 20.15A
5. Final Part Selection
Select an inductor with inductance of 10uH. Select the one that has the smallest tolerance. The inductor rms current must be higher than 20.15A. Consider a maximum stress of 75%. The inductor saturation current rating must be higher than 21A. The 75% maximum stress will do.