The Zeta converter rounds out the family of fourth-order buck-boost topologies (alongside SEPIC and Cuk). Like SEPIC, it produces a non-inverted positive output that can be above or below the input voltage. But unlike SEPIC, the Zeta places the switch on the input side referenced to ground with the inductor feeding the load through a coupling capacitor — giving it a continuous output current (low output ripple) at the cost of discontinuous input current (noisy input).

Think of it as the topological dual of SEPIC. SEPIC has a clean input and noisy output; Zeta has a noisy input and clean output. That output-side cleanliness makes Zeta attractive when you're feeding sensitive analog loads or LED strings where ripple matters more than EMI on the supply side.

How it works: When the MOSFET is ON, L1 charges from Vin, and the coupling cap Cc (charged to Vin in steady state) drives L2 and the load. When the switch opens, both inductors freewheel through the catch diode, transferring stored energy to the output. The DC transfer ratio is the same as SEPIC and buck-boost:

Vout / Vin = D / (1 − D)

At D = 0.5, Vout = Vin. At D = 0.67, Vout = 2×Vin. At D = 0.33, Vout = 0.5×Vin.

Real-world example: Powering a 12 V LED driver from a 9–18 V automotive rail. A buck won't work at low battery, a boost won't work at high battery, and you need polarity-correct output. A Zeta gives you regulated 12 V across the entire range, and the L2-output topology means your LEDs see low-ripple current without needing a huge output capacitor.

Design rule of thumb: Size the coupling capacitor Cc so its ripple is under 5% of Vin:

Cc ≥ (Iout × D) / (0.05 × Vin × fsw)

For Iout = 1 A, D = 0.5, Vin = 12 V, fsw = 500 kHz: Cc ≥ 1 × 0.5 / (0.6 × 500k) ≈ 1.7 µF. Use a 2.2 µF X7R ceramic with adequate voltage derating (Cc sees Vin DC plus ripple).

Coupled inductors (L1 and L2 on a single core) are common in Zeta designs, just like SEPIC. They cut total inductance needed by roughly half and improve transient response — but require careful attention to polarity dots on the schematic.

Watch out for: the right-half-plane zero in continuous conduction mode limits achievable bandwidth, just like boost and SEPIC. Plan for crossover around fsw/10 or lower.