Choosing the Right Surface‑Mount Inductor for Your DC‑DC Converter

Understanding the fundamentals of switch‑mode power conversion is essential for selecting the most suitable inductor for any DC‑DC application. This guide, written by Mitchell Rhine, Director of Engineering at Signal Transformer, explains the key concepts and practical steps.

Linear voltage regulators dissipate power as heat: the voltage dropped across the pass transistor multiplied by the load current equals wasted power. In contrast, a switch‑mode DC‑DC converter operates by rapidly turning the power transistor on and off with a duty cycle that, after filtering, delivers the desired output voltage. When the transistor is on, the voltage drop is negligible; when it is off, no current flows. This switching behavior can reduce power dissipation close to zero, enabling efficiencies of up to 95 %—far higher than the roughly 50 % typical of linear regulators. Additionally, the topology of switch‑mode converters allows them to function as buck (step‑down), boost (step‑up), or buck‑boost (inverting) circuits.

For clarity, we focus on the most widely used configuration: a fixed‑frequency buck converter operating in continuous‑mode operation.

Fig. 1: A simple switching DC‑DC converter.

A basic buck converter comprises a switch, an inductor, a capacitor, and a diode (see Fig. 1). Assuming an ideal switch and diode (V_sw = 0 and V_d = 0) simplifies the analysis, but real designs must account for non‑zero voltage drops.

During the ON period, the diode is reverse‑biased and the current ramps linearly from the input to the output through the inductor. When the switch turns OFF, the inductor’s stored energy forces the current to continue flowing; the diode becomes forward‑biased, allowing the current to decay back to zero before the next ON cycle. This ramping action stores energy in the magnetic core during the ON phase and releases it during the OFF phase.

Fig. 2: The inductor current waveform in a buck converter.

The current waveform consists of a DC component equal to the load current (I_load) and an AC component that ramps up and down periodically. Under steady‑state, the current at the end of a switching cycle equals the current at the start.

From the duty‑cycle control, the output voltage is given by

V_out = D · V_in   (Equation 1)

where D = t_on / (t_on + t_off) and the switching frequency is f_sw = 1 / (t_on + t_off), with t_on = D / f_sw.

Adding the voltage drops across the components during the ON period (and assuming V_sw = 0) yields

V_in – V_ind – V_out = 0   (Equation 2)

Substituting V_ind = L · di/dt, where di ≈ I_ramp and dt = t_on, we obtain

L · I_ramp = (V_in – V_out) · t_on   (Equation 3)

Equation 3 shows that for a fixed input‑output voltage difference, the product of inductance and current ramp is constant. Increasing L reduces I_ramp, while a smaller L enlarges the current ripple. If L is chosen too small, I_ramp can become large enough that the inductor current falls to zero for part of the cycle—entering discontinuous‑mode operation.

Large I_ramp not only increases output‑voltage ripple but also elevates AC core losses. As a rule of thumb, I_ramp should remain a small fraction of the maximum load current to keep ripple and losses acceptable.

The maximum peak current the inductor must withstand in steady‑state operation is

I_max = I_load, max + I_ramp/2   (Equation 4)

While I_ramp is independent of I_load, it depends on the input voltage. Considering the OFF period, with V_d = 0, we have

V_ind – V_out = 0   (Equation 5)

Using V_ind = L · di/dt with dt = t_off yields

I_ramp = V_out · t_off / L   (Equation 6)

Because V_out is constant, I_ramp reaches its maximum when t_off (and therefore V_in) is at its highest. This determines the worst‑case I_max via Equation 4.

With the inductance and peak‑current requirements established, the next step is to select an inductor that meets the electrical and mechanical constraints of the board. Shielded or low‑EMI inductors are ideal for dense, high‑performance designs—such as those found in IoT devices—because their magnetic flux is confined, reducing radiated interference.

For example, Signal Transformer’s SCRH series offers magnetically shielded parts with inductances from 1.0 µH to 180 µH, saturation currents from 0.15 A to 5.0 A, and package heights between 1.9 mm and 4 mm. If a larger inductance is needed, the SCxxxxC series provides values from 10 µH to 1 mH, saturation currents from 0.045 A to 8 A, and heights from 2.92 mm to 7.62 mm. Additional series are available for high‑current applications or for those requiring unshielded inductors that maximize efficiency and power handling in a low‑profile package.

Featured image source: Signal Transformer