How to choose inductors in DCDC circuits?

Choosing the right inductor is critical for the performance, efficiency, and stability of a DC-DC converter (Buck, Boost, Buck-Boost, etc.). It's not just about the inductance value.

Here is a structured guide on how to choose an inductor, moving from the essential calculations to the nuanced secondary parameters.

The 5 Key Steps to Selecting an Inductor

1. Determine the Topology and Key Parameters

First, you must know your circuit's requirements:

• Input Voltage Range (V_in_min, V_in_max)

• Output Voltage (V_out)

• Maximum Output Current (I_out_max)

• Switching Frequency (f_sw) (set by your DC-DC controller IC)

These are all defined by your design goals and the chosen IC.

2. Calculate the Required Inductance Value (L)

The fundamental formula for a Buck Converter is:

L = (V_out * (V_in - V_out)) / (V_in * f_sw * ΔI_L)

Where:

• ΔI_L is the inductor ripple current.

But what should ΔI_L be? A common rule of thumb is to set the ripple current to 20-40% of the maximum output current (I_out_max).
ΔI_L = (0.2 to 0.4) * I_out_max

Why this range?

• Too low ripple (e.g., <10%): Requires a larger, more expensive inductor. It can also make the control loop harder to stabilize.

• Too high ripple (e.g., >40%): Increases RMS currents (lowering efficiency), increases output voltage ripple, and may push the converter into Discontinuous Conduction Mode (DCM) at lower loads, which has its own complexities.

Pro Tip: Almost every modern DC-DC IC datasheet will provide a formula or even a lookup table for the recommended inductor value. ALWAYS START HERE. They have already done the calculations for their specific controller. Use their recommendation as a starting point.

3. Calculate the Peak Current Rating (I_sat)

The inductor must not saturate under worst-case conditions. Saturation causes a drastic drop in inductance, leading to high peak currents and potential converter failure.

The peak current through the inductor is:
I_peak = I_out_max + (ΔI_L / 2)

You must choose an inductor with a saturation current (I_sat) rating higher than this calculated I_peak. A good safety margin is 20-30%. For example, if I_peak = 3A, look for an inductor with an I_sat of at least 3.6A to 4A.

Critical:

• Rated Current vs. Saturation Current: Datasheets often list two current ratings:

  1. Saturation Current (I_sat): The current at which the inductance drops by a specified amount (usually 10% or 20%). This is the absolute limit for peak current.

  2. Thermal RMS Current (I_rms): The DC current that causes a specific temperature rise (usually 40°C) due to wire resistance (DCR). This is related to power loss and heating.

• You must check both. I_sat > your calculated I_peak, and I_rms > your I_out_max.

4. Consider DC Resistance (DCR)

The DCR is the parasitic resistance of the inductor's wire. It directly causes I²R power losses, reducing efficiency and causing self-heating.

• Lower DCR means higher efficiency, especially at high load currents.

• Lower DCR usually means a larger, more expensive physical component.

You must balance efficiency goals with size and cost constraints.

5. Choose the Type and Size

• Shielded (Closed Core) vs. Unshielded (Open Core):

  • Shielded (e.g., molded or drum core): Contains the magnetic field. Always prefer this. It prevents noise from coupling into nearby circuits and reduces EMI.

  • Unshielded (e.g., rod core): Cheaper but acts like a little antenna, radiating noise. Avoid them in noise-sensitive designs or dense PCBs.

• Package Size: Inductors come in standard package sizes (e.g., 0402, 0603, 0805, etc.). Larger sizes typically have lower DCR and higher current ratings.

Summary: A Practical Selection Checklist

1. Consult the IC Datasheet: Use their recommended inductance value and type.

2. Calculate Ripple Current: Verify ΔI_L is ~30% of I_out_max.

3. Calculate Peak Current: I_peak = I_out_max + (ΔI_L / 2)

4. Find Candidates: Search distributor websites using filters for:

   • Inductance Value: Your calculated value (e.g., 4.7µH).

   • Saturation Current (I_sat): > I_peak (with 20-30% margin).

   • RMS Current (I_rms): > I_out_max.

5. Narrow Down:

   • Choose a Shielded inductor.

   • Compare DCR values among finalists. Pick the lowest DCR that fits your size and budget.

   • Check the operating temperature range fits your application.

Example for a Buck Converter

• V_in = 12V

• V_out = 3.3V

• I_out_max = 2A

• f_sw = 500 kHz

1. Choose Ripple Current: ΔI_L = 0.3 * 2A = 0.6A

2. Calculate Inductance:
   L = (3.3V * (12V - 3.3V)) / (12V * 500,000 Hz * 0.6A)
L ≈ 7.9 µH -> A standard 8.2 µH value would be fine.

3. Calculate Peak Current:
   I_peak = 2A + (0.6A / 2) = 2.3A

4. Select Inductor:

   • Inductance: 8.2 µH

   • Saturation Current (I_sat): > 2.8A (2.3A + ~20% margin)

   • RMS Current (I_rms): > 2A

   • DCR: As low as possible (e.g., < 50mΩ)

   • Type: Shielded

A part like the Bourns SRR1280-8R2M or a similar Würth Elektronik or Coilcraft component would be a good candidate to evaluate.

By following these steps, you can systematically and confidently choose the right inductor for your DC-DC circuit.