Cuk Converter

The Cuk converter provides a regulated, inverted DC output with continuous input and output currents. It is highly valued in sensitive telecom equipment and EMI-sensitive LED drivers due to its low current ripple.

Core Concept

The Cuk converter relies on capacitive energy transfer instead of inductive transfer. A series capacitor couples the input and output stages.

When the switch is ON, the input inductor stores energy, and the coupling capacitor discharges into the output inductor and load.

When the switch is OFF, the input source charges the coupling capacitor, while the output inductor drives the load. This ensures continuous current at both terminals.

Key Formulas

Voltage ratio Vo / Vin = -D / (1 - D). Input current ripple dI1 = (Vin * D) / (L1 * fs). Output current ripple dI2 = (Vo * (1 - D)) / (L2 * fs).

Given:\nVin = 12 V, Duty cycle D = 0.75.\n\nFormula:\nVo = -Vin * (D / (1 - D))\n\nSteps:\nD / (1 - D) = 0.75 / 0.25 = 3\nVo = -12 * 3 = -36 V\n\nFinal Answer:\nThe output voltage is -36 V.

Exam Tip: GATE trap: Unlike the buck-boost converter, the Cuk converter uses a capacitor as the main energy storage element for transfer between input and output. It features continuous current at both the input and the output.

Performance Parameters

• Voltage Gain: Same magnitude as buck-boost, but continuous operation.
• Input Current: Continuous, virtually eliminating input EMI filters.
• Output Current: Continuous, requiring very small output filter capacitors.
• Coupling Capacitor: Must have very low equivalent series resistance to handle massive ripple current.
• Polarity: Output voltage is inverted.

Quick Revision

• Capacitive energy transfer topology.
• Inverted DC output.
• Uses two inductors and two capacitors.
• Both input and output currents are continuous.
• Excellent for reducing EMI.
• Exam trap: Assuming the coupling capacitor voltage is equal to Vin. The average voltage across the coupling capacitor is actually Vin + |Vo|.