SEPIC Converter Calculator
Design a SEPIC (Single-Ended Primary-Inductor Converter) for buck-boost operation. Solves duty cycle, coupled-inductor value, and coupling capacitor.
How to Use
- Enter input voltage range (min/max), output voltage, output current, and switching frequency.
- Tool computes duty cycle, inductor value, and coupling capacitor.
Show Work
Formulas
History of SEPIC
SEPIC (Single-Ended Primary-Inductor Converter) was described in a 1977 paper but did not become widely used until cheap coupled inductors made the topology practical in the 1990s. Linear Technology (now ADI) championed SEPIC with the LT1513 and LT1619 controllers, emphasizing automotive use where input voltage swings dramatically (cold crank to load dump) around a fixed output target. Modern SEPIC controllers (LT3757, LM5155) support coupled inductors, synchronous rectification, and spread-spectrum modulation for EMI compliance.
About This Calculator
Enter input voltage range (min and max), target output, load current, and switching frequency. The tool uses the worst-case V_in_min for duty cycle and inductor sizing. SEPIC requires two equal inductors plus a coupling capacitor between them.
For production design, use a dedicated SEPIC controller (TI LT3757, LM5155; ADI LT3580). Coupled inductors from Coilcraft/Wurth (e.g., MSD7342) save board space vs two discrete inductors. Everything runs client-side.
Frequently Asked Questions
Why SEPIC?
Buck-boost with non-inverting output. V_out can be above OR below V_in without sign inversion. Ideal for battery-powered devices where input voltage range spans the target output (e.g., 3.3V regulator from 2-5V Li-ion).
Two inductors?
L1 and L2 can be separate or coupled on a common core. Coupled inductors need lower individual value (Lc = L/2) and give better transient response. LT3757 and similar SEPIC controllers work with either topology.
Coupling cap?
Cs blocks DC between L1 and L2. Must be rated for full V_in+V_out and handle RMS ripple current I_out·√(D/(1−D)). Use low-ESR ceramics (X5R/X7R) or polymer caps.
Common Use Cases
Lead-Acid LED Light
Fully-charged 14 V → 12 V LED strip; discharged 10 V → 12 V LED strip. SEPIC maintains regulation.
Solar Battery Charger
Panel voltage (5-18 V) charging 12 V SLA battery (13.6 V float): SEPIC handles both up and down.
Automotive 12 V Regulator
Cold-crank 7 V to load-dump 42 V input; 12 V output across the range.
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