Power Electronics
A buck only steps down, a boost only steps up. But what if your input sometimes sits above the output and sometimes below it? The SEPIC (Single-Ended Primary-Inductor Converter) is made for exactly this: with one topology it can both step down and step up, and unlike the classic buck-boost it does NOT invert the output (same polarity as the input). On top of that, a series coupling capacitor (Cs) breaks the DC path between input and output — even if the output shorts, no DC current flows back to the source, a natural safeguard.
The secret lies in the L1–Cs–L2 trio. In steady state the coupling capacitor Cs charges to an average of Vin. While the switch conducts, both inductors store energy; when the switch turns off, L1’s current flows through Cs and L2’s current flows through the diode to the output. Cs "floats" the two inductors, carrying energy from the input side to the output side.
The output ratio is the same as the buck-boost but positive: Vout = Vin·D/(1−D). At 50% duty, Vout ≈ Vin; below 50% it steps down, above it steps up. Producing a steady 3.3 V from a Li-ion cell that sags between 3.0–4.2 V, for instance, is a classic SEPIC job. The price is two inductors and a coupling capacitor.
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