ZETA Converter Calculator
Design and analyze ZETA (non-inverting buck-boost) DC-DC converters for power electronics.
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Given: Vin=12V, Vout=24V, Pout=48W, Ripple=10%, fsw=100kHz
Given: Vin=24V, Vout=12V, Pout=60W, Ripple=15%, fsw=75kHz
Given: Vin=20V, Vout=60V, Pout=72W, Ripple=10%, fsw=50kHz
A ZETA converter is a DC-DC converter that can step voltage up or down while producing a positive, non-inverted output. It is closely related to the SEPIC — both use two inductors and a coupling capacitor — but the ZETA is arranged as the “dual” of the SEPIC, with the active switch on the input side referenced to the input and the output taken through the second inductor. The practical benefit is a well-filtered output with low voltage ripple.
During the switch-on time, energy is drawn from the input and stored in the inductors, and the coupling capacitor is charged. When the switch turns off, the coupling capacitor and the output inductor deliver energy to the load through the freewheeling path, maintaining a smooth output. The steady-state relationship is Vout = Vin × D / (1 − D), identical in form to the SEPIC: below 50% duty it bucks, above 50% it boosts.
Because the output inductor sits in series with the load, the ZETA delivers continuous, low-ripple output current, which is attractive for noise-sensitive loads and LED strings. The SEPIC, by contrast, has continuous input current (easier on the source and for PFC). Choosing between them usually comes down to whether you care more about clean output current (ZETA) or clean input current (SEPIC). The ZETA typically needs a robust input capacitor because its input current is pulsating.
LED lighting drivers, battery-powered instruments needing a quiet regulated rail, sensor and audio supplies where output ripple must be small, and automotive electronics that must hold a fixed positive voltage across a wide, fluctuating battery range.
Is the ZETA output inverted? No — like the SEPIC it keeps the output positive, unlike the classic buck-boost.
Can the two inductors share a core? Yes. As with the SEPIC, coupled inductors on one core are common and can reduce ripple, provided they are wound for the correct turns ratio and polarity.
A ZETA converter steps up or down with a non-inverted output, using the same duty-cycle relationship as the SEPIC: D = Vout/(Vin + Vout). It is often described as the SEPIC’s dual, and the comparison is the clearest way to understand it.
Where a SEPIC places its inductor at the input and so draws continuous input current with a pulsed output, the ZETA does the reverse: it has an inductor at the output, giving continuous output current and low output ripple, while its input current is pulsed.
That makes the choice between them a question of which port matters more. If input EMI is the constraint, the SEPIC is better; if output ripple must be low without a large output capacitor, the ZETA is. Both need a coupling capacitor rated for full throughput ripple current, and both need two inductors.
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.
They have the same duty-cycle relationship and both give a non-inverted output, but their inductors are placed oppositely. A SEPIC has continuous input current and pulsed output; a ZETA has continuous output current and pulsed input.
When low output ripple matters more than input EMI. The ZETA's output inductor gives continuous output current, so the output capacitor has an easier job.
No. Like the SEPIC it keeps the output at the same polarity as the input, which is its main advantage over a conventional buck-boost.
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.
Written and reviewed by
Lead Electronic Engineer (R&D) · Power Electronics Specialist · M.E. Power Electronics
Founder of Power4All. Works in power-electronics R&D and writes the converter guides, calculators and simulators on this site.
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