You already know the buck (step-down) and boost (step-up) topologies. But what if your input voltage straddles your output? A lithium battery at 4.2V full, 3.0V dead, powering a 3.3V rail. A buck won't go up; a boost won't go down. The classic inverting buck-boost works but flips polarity, which is a hassle. The SEPIC (Single-Ended Primary Inductor Converter) solves this elegantly: it can step up or step down while keeping the output the same polarity as the input.

The topology adds one capacitor and one inductor to a boost converter. The arrangement: input inductor L1, then a series coupling capacitor C1, then a second inductor L2 to ground, with the MOSFET tied between L1 and C1, and the diode feeding the output from the C1/L2 junction. The coupling capacitor blocks DC, so the output is isolated from the input rail in the DC sense — a key feature for short-circuit ruggedness.

How it works: When the switch closes, L1 charges from the input and L2 charges from C1 (which holds Vin in steady state). When the switch opens, both inductors dump their energy through the diode into the output cap. The conversion ratio is the classic buck-boost form:

• Vout/Vin = D/(1−D), where D is duty cycle
• D < 0.5 → step down; D = 0.5 → unity; D > 0.5 → step up

Real-world example: A GPS tracker running on a single Li-ion cell needs a stable 3.3V rail. With Vin sweeping 3.0V to 4.2V, a SEPIC running at D ≈ 0.5 ± 0.07 keeps the output rock-solid as the battery discharges. Same trick works in automotive 12V rails that dip during cranking — you want 5V out whether the input is 6V (cranking) or 14V (alternator).

Design rule of thumb: Use a coupled inductor (both windings on one core) instead of two discrete inductors. Coupling halves the required inductance and dramatically reduces input ripple. Pick L such that ripple current ΔI ≈ 30% of average inductor current: L = Vin × D / (ΔI × fsw). For C1, size it so its ripple voltage stays under ~5% of Vin; use a low-ESR ceramic rated for the full RMS switch current, which can be surprisingly high.

Gotchas: SEPIC has a right-half-plane zero like a boost, so loop bandwidth is limited (cross over well below the RHP zero frequency). The MOSFET sees Vin + Vout across it when off — choose a part with adequate Vds margin. And the coupling cap carries the full load current as ripple; cheap parts will cook.