Power Electronics
The Ćuk converter (after its inventor, Slobodan Ćuk) is the "quiet" member of the buck-boost family. Like the buck-boost its output is inverted with the same ratio: Vo = −D/(1−D)·Vin. The difference is the architecture: there is an inductor at the input AND at the output, and energy moves between them through the coupling capacitor C1. Where the buck-boost carries energy in an inductor's magnetic field, the Ćuk carries it in a capacitor's electric field.
The two inductors' greatest gift is continuous current: both input and output currents flow without interruption as small triangles (in a buck-boost both are pulsed). That shrinks the input filter and slashes EMI — exactly why designers reach for the Ćuk. In steady state the coupling capacitor charges to VC1 = Vin + |Vo|: while the switch conducts, C1 discharges into the load through L2; while it is off, L1's current recharges C1 and diode D1 provides the return path.
The design equations are a blend of buck and boost: L1 sets the input ripple (ΔIL1 = Vin·D/(L1·fs)), L2 the output ripple; C1 carries the full load current every cycle, rippling by ΔVC1 = Io·D/(C1·fs) — high RMS current, so it demands a quality capacitor. Our ready-made circuit: 12 V input, D=40% → ideal −8 V; the diode drop and winding resistances pull the measurement to ~−7 V — see the gap between textbook and measurement for yourself.
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