Given: Vs=325V, Vo=56V, Pout=840W, Ripple=10%, fsw=50kHz
Given: Vs=12V, Vo=24V, Pout=48W, Ripple=20%, fsw=100kHz
Given: Vs=48V, Vo=24V, Pout=120W, Ripple=15%, fsw=75kHz
A SEPIC (Single-Ended Primary-Inductor Converter) is a DC-DC converter that can step voltage up or down while keeping the output the same polarity as the input — unlike the classic buck-boost, which inverts. This positive, wide-range output is what makes SEPIC so popular for battery and automotive supplies.
The SEPIC uses two inductors (often wound on a single core) linked by a coupling capacitor that blocks DC and transfers energy from input to output. When the switch is on, both inductors store energy; when it turns off, the coupling capacitor and the second inductor deliver that energy through the diode to the output. The DC relationship is Vout = Vin × D / (1 − D), so D < 0.5 steps down, D > 0.5 steps up, and D = 0.5 gives unity.
Its standout feature is regulating cleanly through the point where input equals output. A 12 V rail from a car battery (which sags to 6 V on cranking and spikes to 15 V) is a textbook SEPIC job. The series coupling capacitor also provides natural input-to-output isolation for shorts and true output disconnect when the switch is off — something a boost cannot offer. The cost is more components (two inductors, a coupling capacitor) and higher ripple current in that capacitor, which must be rated for it.
Automotive 12 V accessory supplies, LED drivers that must run from a varying battery, portable and handheld devices, and any application that needs a fixed positive output from an input that swings above and below it.
SEPIC vs boost? A boost can only step up; a SEPIC can step both up and down, and it disconnects the output when off. Choose SEPIC when the input can fall below the target.
SEPIC vs ZETA? Both give a positive step-up/step-down output. SEPIC places the switch at the input side (good for input-current control), while ZETA places it at the output side and generally gives lower output-voltage ripple.
A SEPIC — single-ended primary-inductor converter — steps up or down while keeping the output the same polarity as the input, which is its main advantage over the buck-boost. Its duty cycle is D = Vout/(Vin + Vout).
This makes it the natural choice when the input can be above or below the output — a battery that starts at 12.6 V and sags to 9 V while the load needs a steady 12 V, for example, where neither a buck nor a boost alone can cover the whole range.
Like the Cuk, it transfers energy through a series coupling capacitor, which must be rated for the full throughput ripple current rather than merely chosen for capacitance. The input inductor gives continuous input current and therefore low input EMI; the output, however, is pulsed, so the output capacitor works hard.
The duty-cycle formula above assumes continuous conduction — inductor current never reaches zero. At light load the current can fall to zero before the cycle ends, entering discontinuous conduction, where the relationship changes and output voltage rises above the predicted value unless the control loop corrects it.
This is why a converter that regulates perfectly at full load can misbehave at very light load, and why synchronous designs often deliberately emulate diode behaviour at low current rather than allowing reverse inductor current.
Switch on-resistance, inductor winding resistance, core loss, and the forward drop of the diode or synchronous rectifier all consume part of the input. Real duty cycle therefore runs a little higher than the ideal value, and the gap widens at high current — which is itself a useful diagnostic: a duty cycle well above prediction usually points to more loss than expected somewhere in the power path.
D = D = Vout/(Vin + Vout) in continuous conduction. It assumes ideal components, so a real converter runs at a slightly higher duty cycle.
It steps up or down while keeping the output polarity the same as the input. That makes it suitable where the input voltage can be either above or below the desired output.
When the input range spans the output voltage — a battery sagging from 12.6 V to 9 V while the load needs a steady 12 V, for example. Neither a buck nor a boost alone can cover that.
Size the capacitance from the allowed ripple, but select the part on RMS ripple-current rating, since it carries the full throughput. Low-ESR film or ceramic types are usual; ordinary electrolytics overheat.
The condition where inductor current falls to zero before the switching cycle ends, which happens at light load. The voltage relationship changes and the output tends to rise unless the control loop compensates.
Because the ideal formula ignores losses. Switch resistance, inductor resistance and rectifier drop all consume input voltage, and the gap grows with load current.
Higher frequency shrinks the inductor and capacitors, since both scale inversely with it, but increases switching and gate-drive losses. The choice is a direct trade between magnetic size and efficiency.