Side-by-side comparison
| Parameter | Buck-Boost | Cuk Converter |
|---|---|---|
| Output Voltage Polarity | Inverted — V_out = −D/(1−D)·V_in | Inverted — same polarity relationship |
| Voltage Conversion Ratio | M = −D/(1−D) | M = −D/(1−D) — same formula |
| Energy Transfer Mechanism | Inductor (single L) — energy via magnetic field | Capacitor C1 — energy via electric field (intermediate capacitor) |
| Number of Inductors | 1 inductor | 2 inductors (one on input, one on output) |
| Input Current | Pulsed — discontinuous | Continuous — smooth (L1 on input) |
| Output Current | Pulsed — discontinuous | Continuous — smooth (L2 on output) |
| EMI Performance | Poor — pulsed currents at both ports | Better — continuous currents reduce differential-mode EMI |
| Component Count | 1 L, 1 C, 1 switch, 1 diode | 2 L, 2 C, 1 switch, 1 diode |
| Ripple Cancellation | Not possible | Input and output ripple can be reduced by coupling inductors (zero ripple Cuk) |
| Typical Application | Low-cost inverting regulators, LED drivers | Precision instrumentation supply, audio pre-amp rails |
Key differences
Both converters invert output polarity with the same DC voltage ratio M = −D/(1−D), so they are analytically identical in DC terms. The critical difference is energy transfer: the buck-boost uses its single inductor directly, producing pulsed currents at both input and output and generating large differential-mode EMI. The Cuk uses an intermediate capacitor C1 to transfer energy between the two inductors, giving continuous (ripple only) currents at both ports. Coupled Cuk inductors (wound on the same core with opposite polarity) can achieve theoretically zero input or output ripple — a feature impossible in any single-inductor topology. The cost is two inductors, one extra capacitor, and more complex magnetics design.
When to use Buck-Boost
Use a buck-boost converter when you need an inverted output on a budget with minimal component count and EMI is not a critical constraint. Example: a single-chip MC34063A buck-boost generates −12 V from a +5 V USB supply for an op-amp negative rail in a low-cost handheld meter.
When to use Cuk Converter
Use a Cuk converter when low EMI and continuous input/output currents are mandatory, such as in precision analog or audio circuits. Example: an inverting Cuk converter (built around two coupled 47 µH inductors and a MOSFET gate driver IR2104) provides a clean −15 V rail for an instrumentation amplifier in a sensor interface board.
Recommendation
Choose the buck-boost when component count and cost dominate and some EMI is acceptable. Choose the Cuk when you need the same inverted voltage but cannot tolerate pulsed input or output currents — precision measurement supplies and audio equipment are the natural home of the Cuk topology. The two extra components are worth it whenever EMI filtering would cost more than the second inductor.
Exam tip: Examiners ask students to derive the voltage conversion ratio of the Cuk converter using volt-second balance on both inductors and charge balance on C1 — the result is the same as buck-boost: M = D/(1−D) in magnitude, inverted.
Interview tip: A placement interviewer at an analog or mixed-signal IC company will ask how the Cuk converter achieves zero input ripple — explain that by coupling L1 and L2 on the same core with appropriate turns ratio, the ripple currents cancel, leaving only DC at the input; this is the zero-ripple condition.