A Ćuk converter transforms a DC voltage at the input to a DC voltage at the output with reversed polarity. Compared to the Buck, Boost and Buck-Boost converters the Ćuk converter uses an additional inductor and capacitor to store energy. Consider the following sequence of the MOSFET conduction state:
- 1. On-state: The current through the inductor L1 increases linearly and the diode blocks.
- 2. Off-state: Since the current through the inductor L1 can not abruptly change the diode must carry the current so it commutates and begins conducting. Energy is transferred from the inductor L1 to the middle capacitor C2 resulting in a decreasing inductor current.
- 3. On-state: The current through the inductor L1 again increases linearly and the diode blocks. The middle capacitor discharges and supplies the RC load through the inductor L2. The induced voltage across the resistor R has the opposite polarity of the input voltage.
The circuit has two limits of operation. For a PWM duty cycle D ➝ 0 the output voltage equals zero, and for D ➝ 1 the output voltage grows toward negative infinity. In between those limits the output voltage in continuous conduction mode is given by: Vout = -D/(1-D) · Vin. The combination of inductors and capacitors acts as a second order low pass filter reducing the voltage ripple at the output.
Compared to the previous DC to DC voltage converters (Buck, Boost and Buck-Boost) the Ćuk converter always allows continuous current flow through the inductors, and therefore, no discontinuous conduction mode is possible.
- Change the PWM duty cycle of the converter from 0.5 to 0.4 and 0.6 and observe how the average output voltage changes to 16 V and 36 V, respectively.
- Change the DC capacitor value from 10 μF to 100 μF and observe how the ripple of the output voltage is reduced.