Free SEPIC Converter Sizing Calculator
Size power inductors (L1, L2), AC series coupling capacitor (Cp), peak MOSFET current, and diode reverse voltage for non-inverting buck-boost SEPIC power supplies.
⚡ SEPIC Converter Specifications
Series Coupling Cap (C_p): ≥ 4.7 μF (X7R Ceramic)
• Coupling Cap RMS Current: 2.88 A RMS (Use low-ESR ceramic).
• Recommended MOSFET Breakdown: ≥ 40V to 50V (Must include 1.5× safety margin).
The Non-Inverting Advantage: Unlike inverting buck-boost regulators that produce negative voltage rails, the SEPIC topology produces a clean, positive, non-inverted output voltage while naturally blocking DC current from input to output when disabled.
Recommended Tools & Equipment
Tested hardware and components for high reliability
Principles of the SEPIC Converter
The Single-Ended Primary-Inductor Converter (SEPIC) is an essential power topology when the input voltage swings both above and below the regulated output rail (e.g. 12V output powered by an automotive battery that varies from 6V during engine cranking to 16V during alternator charging).
Governing Mathematical Formulas
Minimum Duty Cycle: D_min = (V_out + V_F) / (V_in,max + V_out + V_F)
Input Current (Avg): I_in = (V_out × I_out) / (V_in,min × η)
Inductor Ripple I: ΔI_L = I_in × Ripple_Ratio
Separate Inductance: L_1 = L_2 = (V_in,min × D_max) / (ΔI_L × f_sw)
Coupled Inductance: L_coupled = L_separate / 2
Coupling Cap RMS I: I_Cp,rms = I_out × √((V_out + V_F) / V_in,min)
Switch Voltage Stress: V_switch = V_in,max + V_out + V_F
Coupled vs. Separate Inductors
Winding both $L_1$ and $L_2$ onto a single magnetic core with 1:1 coupling offers massive advantages:
- Ripple Cancellation: The mutual inductance forces AC ripple currents to cancel in one winding, cutting required board space in half.
- Reduced Component Count: Requires only half the nominal inductance value compared to two separate uncoupled inductors.
Frequently Asked Questions
Why does a SEPIC converter have true DC isolation when shut down?
Unlike a standard boost converter where a parasitic path exists through the inductor and diode directly to the load when the switch is OFF, the SEPIC features an AC coupling series capacitor Cp that physically breaks DC continuity between input and output, drawing zero quiescent current in shutdown.
What type of capacitor must be used for the SEPIC coupling capacitor Cp?
Always use high-ripple-current Class-2 X7R ceramic capacitors or specialized low-ESR polymer film capacitors. Electrolytic and tantalum capacitors will overheat and fail catastrophically due to high high-frequency circulating RMS ripple currents.
What voltage rating is required for the coupling capacitor Cp?
In steady-state operation, the DC voltage across Cp equals the input voltage Vin. Therefore, Cp must be rated to comfortably exceed the maximum input voltage Vin,max (including line transients).