If the topology of the buck (Buck) conversion circuit and the boost (Boost) conversion circuit are dually transformed, that is, the Boost conversion circuit and the Buck conversion circuit are connected in series to obtain a new circuit topology - Chuuk (CUK) Transformation circuit, as shown in Figure 1.
As mentioned earlier, in the Buck-Boost conversion circuit, both the input current and the output voltage are pulsating. The input and output of the Cuk conversion circuit both have inductance. Increasing the value of the inductance can make the value of the AC ripple current arbitrarily small, but it may be difficult to achieve in practice. There is no coupling between the two inductors in the Cuk circuit, or there may be coupling, and the coupled inductor can further reduce the amount of current ripple. , theoretically can achieve "zero ripple". This is the characteristic of the DC/DC Cuk (Cuk) conversion circuit.
By controlling the duty cycle of the switching tube T, the output voltage can be higher than, equal to or lower than the input voltage, which is the same as the Buck-Boost conversion circuit.
Regardless of whether the switching tube T is turned on or not, there are two loops in the Cuk conversion circuit, which also store and transfer energy, as shown in Figure 2 and Figure 3 .
During the conduction period of the switch tube T, the input and output loop is shown in Figure 2, the diode D is cut off due to the reverse voltage, the input loop current i1 stores energy for the inductor L1, and the capacitor C1 discharges to store energy for the inductor L2, and supplies Load resistor R. The sum of the current flowing through the switch tube is iT=i1+i2.
During the cut-off period of the switch tube T, the diode D bears the forward voltage and conducts, and the input and output loop is shown in FIG. 3 . The power supply and the inductor L1 release energy to charge the capacitor C1, while the inductor L2 releases the current i2 to maintain the load. At this time, the current flowing through the diode D is the sum of the input and output currents, that is, iD=i1+i2.
In summary, no matter whether the switch tube is turned on or not, the Cuk conversion circuit transfers power from the input terminal to the output terminal. During the T off period, the capacitor C1 is charged, and during the T on period, the capacitor C1 discharges to the load. It can be seen that the capacitor C1 plays the role of transferring energy.
The Cuk conversion circuit also has three working states: continuous conduction state, discontinuous conduction state and critical state. Discontinuous conduction can be understood as the interruption of the current flowing through the diode. When analyzing the continuous conduction state, the required assumptions are consistent with the Buck circuit and the Boost circuit, and it is also necessary to assume that the capacity of the capacitor C1 is large. When the conversion circuit works in steady state, the voltage ripple on capacitor C1 is ignored, and its voltage is considered to be Uc1, and the average voltage of inductor L1 and inductor L2 in steady state is 0, then there is Uc1=Us+Uo. The following is a detailed description of the conduction and cut-off of the switch tube
When the switch tube is turned on, the increase of the inductance L1 in the loop 1
Increase of inductance L2 in loop 2
When the switch tube is cut off, the reduction of inductance L1 in loop 1
Reduction of inductance L2 in loop 2
Because in the steady state of the current, the average value of the inductor current fluctuation is zero, that is,
The voltage gain expression of the Cuk conversion circuit is obtained as
Analysis: When the duty cycle D=0.5, the output voltage Uo is equal to the input voltage Us;
When the duty ratio D<0.5, the output voltage Uo is less than the input voltage Us, which is actually a step-down device;
When the duty ratio D>0.5, the output voltage Uo is greater than the input voltage Us, which is actually a booster.
Assuming that the input power and output power are equal, then we have
Calculate the inductance value
When the working state of the circuit is in a critical state, there are
Simultaneous equations
The expressions of inductance L1 and inductance L2 are
Note: Compared with the Buck-Boost conversion circuit, the Cuk conversion circuit has the advantage that the input and output currents are ripple-free, which reduces the requirements for external filters, but it also has a disadvantage, that is, it needs to have a large enough energy storage Capacitor C1.
Experimental simulation
Technical indicators: The input voltage is 20V, and the output voltage is between 5V and 30V.
1) Output voltage 5V
2) Output voltage 30V
By analyzing the simulation experiment results of the Cuk transformation circuit, it can be seen that the Cuk transformation circuit can realize both the step-down effect and the step-up effect, and the most important thing is that the generated ripple voltage is very small.