SMPS Inductor Calculator

Understanding SMPS Inductors & Topologies

Switched-Mode Power Supplies (SMPS) efficiently convert electrical power using switching devices and energy storage components like inductors and capacitors. The inductor plays a crucial role in storing and transferring energy, smoothing current, and contributing to the output voltage regulation.

1. Buck Converter (Step-Down)

A **Buck converter** produces a DC output voltage lower than its DC input voltage. The inductor in a Buck converter is placed in series with the load and acts as a current filter. When the switch is on, the inductor stores energy. When the switch is off, the inductor releases this energy to the load, along with the output capacitor, ensuring a continuous current flow.
**Key relationships (Ideal, CCM):**

• Duty Cycle ($D$) = $V_{out} / V_{in}$
• Average Inductor Current ($I_L$) = $I_{out}$
• Inductor ($L$) = $\frac{(V_{in} – V_{out}) \cdot D}{f \cdot \Delta I_L}$

2. Boost Converter (Step-Up)

A **Boost converter** produces a DC output voltage higher than its DC input voltage. Here, the inductor is placed in series with the input voltage source. When the switch is on, the inductor stores energy from the input. When the switch is off, the stored energy is transferred to the output, adding to the input voltage, thus boosting the output. The inductor’s continuous current ensures smoother input current.
**Key relationships (Ideal, CCM):**

• Duty Cycle ($D$) = $1 – (V_{in} / V_{out})$
• Average Inductor Current ($I_L$) = $I_{out} / (1 – D)$
• Inductor ($L$) = $\frac{V_{in} \cdot D}{f \cdot \Delta I_L}$

3. Buck-Boost Converter (Inverting)

A **Buck-Boost converter** provides an output voltage that can be either higher or lower than the input voltage, but it is always of opposite polarity (inverting). The inductor here is critical for energy transfer. During the ON state, it stores energy from the input. During the OFF state, it transfers this energy to the output capacitor and load.
**Key relationships (Ideal, CCM):**

• Duty Cycle ($D$) = $V_{out} / (V_{in} + V_{out})$ (magnitude of output voltage)
• Average Inductor Current ($I_L$) = $I_{out} / (1 – D)$
• Inductor ($L$) = $\frac{V_{in} \cdot D}{f \cdot \Delta I_L}$

Note: $\Delta I_L$ is the peak-to-peak inductor ripple current. For design purposes, it’s often set as a percentage (e.g., 20-40%) of the average inductor current. The formulas provided assume ideal components and Continuous Conduction Mode (CCM). Actual designs require considering component losses, efficiency, and transient responses.