2026-06-06
The Cuk converter (pronounced "chook," after Slobodan Ćuk) is a switching topology that steps voltage up or down and inverts polarity — like a buck-boost — but with a critical advantage: both input and output currents are continuous. That makes it the quietest buck-boost variant for EMI-sensitive applications.
The trick is energy transfer through a series capacitor instead of an inductor. The topology uses two inductors (L1 on input, L2 on output), one switch, one diode, and a coupling capacitor C1 that shuttles energy from input to output side each cycle.
How it works:
Because L1 is always in series with the input and L2 is always in series with the output, neither current ever pulses to zero — unlike a standard buck-boost where the input current is brutally choppy. Input ripple current is small, set by L1's inductance and switching frequency, not by load current pulses.
Conversion ratio (same as buck-boost, inverted output):
Vout/Vin = −D/(1−D)
where D is duty cycle. At D = 0.5, |Vout| = Vin. At D = 0.67, |Vout| = 2·Vin.
Real-world example: Powering a sensitive RF receiver front-end from a 12 V battery that needs −9 V at 200 mA. A standard inverting buck-boost would inject sharp input current spikes into your battery wiring, radiating EMI into your antenna. A Cuk converter with L1 = L2 = 100 µH at 200 kHz gives you inductor ripple under 50 mA peak-to-peak on both sides, dramatically reducing conducted EMI without needing a massive input LC filter.
Rule of thumb for C1: Size the coupling capacitor for under 10% voltage ripple. C1 ≥ Iout·D / (fsw·ΔVc1). For the example above: 0.2·0.43 / (200k·2 V) ≈ 0.22 µF. Use a low-ESR ceramic or film cap — C1 carries the full load current as AC ripple, so ESR losses matter.
Gotchas: Output is inverted (watch your ground reference). C1 must handle high RMS current. Coupling the two inductors on a single core (integrated magnetics) can drive ripple on one side to nearly zero — a favorite trick in audio and instrumentation supplies.