This article follows the prior Cuk topology article: Cuk Converter: Energy Flow Breakdown & Component Functions during Switch ON/OFF
1. Cuk Schematic Diagram
(Images sourced from Microchip)
Key Nodes & Components:
VIN: DC input voltage
L1: Input Inductor; L2: Output Inductor
C1: Flying Coupling Capacitor (core energy transfer component)
Q1: Main Switch (MOSFET / GaN, low-side ground-referenced switch)
VSW: Switch Node (Q1 Drain, right terminal of L1, left terminal of C1)
D1: Freewheeling Schottky Diode
VD: Diode Anode Node (right terminal of C1, D1 anode, left terminal of L2)
VOUT: Output voltage, inherently negative: VOUT < 0
Ideal CCM voltage transfer equation:
D = PWM duty cycle
2. Waveform Analysis (Continuous Conduction Mode, CCM)
Typical Waveforms
The diagram illustrates the correlation among VSW, VD, IL1, IL2 and PWM signals.
(Images sourced from Microchip)
Wave order from top to bottom: PWM, VSW, VD, IL1, IL2
PWM (Purple)
Gate drive signal for main switch Q1
High level → Q1 turns ON
Low level → Q1 turns OFF
VSW (Cyan, Switch Node Voltage)
When Q1 ON: VSW ≈ 0 V
When Q1 OFF: Node voltage jumps to VIN+|VOUT|
Engineering Conclusion: Steady-state drain-source voltage stress of Q1: VDS(max) = VIN+|VOUT|
VD (Yellow, Diode Anode Potential)
When Q1 ON: Diode is reverse biased; potential stays at −(VIN+|VOUT|)
When Q1 OFF: Diode conducts forward; VD ≈ 0 V
Engineering Conclusion: Steady-state reverse voltage rating of D1: VR(max) = VIN+|VOUT|, identical to the voltage stress of the main switch.
IL1 (Blue, Input Inductor Current)
Current remains positive throughout the cycle, operating in CCM:
Q1 ON: L1 stores energy, current rises linearly
Q1 OFF: L1 releases energy, current falls linearly
The continuous input current yields low input ripple current, delivering superior EMI performance compared to SEPIC and conventional Buck-Boost topologies.
IL2 (Green, Output Inductor Current, series-connected with load)
Q1 ON: Coupling capacitor C1 discharges; IL2 rises linearly
Q1 OFF: L2 freewheels and supplies power to the load; current falls linearly
Engineering Tip: Continuous load current with low ripple is the core advantage making the Ćuk topology suitable for LED drivers, capable of suppressing light flicker.
Related Products
Additional References
More Cuk Converter Articles
Cuk Applications
- Cuk Converter: Analysis of Cuk LED Driver Implementation (MOSFET Version)
- Cuk Converter: Analysis of Cuk LED Driver Implementation (GaN Version)
- Cuk Converter: How to Resolve Current Overshoot During Cuk PWM Dimming?
Cuk Operating Principles
- Cuk Converter: Energy Flow Breakdown & Component Functions during Switch ON/OFF
- Cuk Converter: Comprehensive Analysis of Typical Operating Waveforms
- Cuk Converter: Derivation of Negative‑Output‑Voltage Formula
- Cuk Converter: Cuk vs SEPIC Topology – Pros, Cons and Differences in Coupling Capacitor Operation