OPERATION PRINCIPLES

The equivalent circuit of dual switches coupled inductor buck-boost converter is given in Fig. 1, which is assembled from two power switches S 1 andS 2, three capacitors C 1,C 2, and Co , three diodesD 1, D 2 andD 3, an inductor L 1, and a coupled inductor. Coupled inductor combines with leakage inductorLk , magnetizing inductorLm and ideal transformer with primary windingLp and secondary windingLs . Two power switches operate synchronously. To simplify the calculation of steady-state analysis, several assumptions are considered as follow:
  1. The values of capacitors are large enough so that the voltage of each capacitor is regarded as constant in a switching cycle.
  2. The parasitic resistance and forward voltage drop of the components can be ignored.
  3. The turns ratio of coupled inductor can be defined asNp : Ns = 1 : n.
  4. The coupling coefficient k is defined as Lm / (Lm + Lk ).
Fig. Equivalent circuit.
Fig. 2 shows the voltage and current waveforms with main components of the converter which operates in continuous conduction mode (CCM). Current flow paths of three operating modes in a switching cycle are plotted in Fig. 3.
Mode I (t 0-t 1) [Fig. 3(a)]: The power switches S 1 andS 2 are turned on; diodesD 1 and D 3 are reverse biased, diode D 2 is on. The magnetizing inductorLm is charged by Vin , and capacitors C 1 and C 2release energy to inductor L 1.Co provides energy to the load.iL 1 starts to increase andiLk increases rapidly. This mode ends whileD 2 is off.
Mode II (t 1-t 2) [Fig. 3(b)]: The power switches S 1 andS 2 are still remain activated while all diodes are reverse biased. Other current flow paths are same as Mode I. Besides, the currents of Lm andL 1 increase. iLm starts to increase. When the signal of power switches becomes low, this mode ends. Some equations can be obtained in this mode as following.
()
()
()
()
Mode III (t 2-t 3) [Fig. 3(c)]: The power switches S 1 andS 2 are turned off simultaneously. DiodesD 1, D 2, andD 3 are forward biased. CapacitorC 1 is charged by the magnetizing inductorLm and Vin . Meanwhile, the magnetizing inductor Lm transfers energy to the secondary side of coupled inductor. Capacitor C 2stores the energy through the coupled inductor andD 2. The inductor L 1releases energy to the output capacitor Co and the load through D 3. The current values ofLk , Lm andL 1 are above zero and decrease. When the signal of power switches becomes high, this mode ends. Some equations can be obtained in this mode as follows:
()
()
()
()
()
(a) (b)
Fig. The key waveforms of the main components under (a) step-up mode; (b) step-down mode.
(a)
(b)
(c)
Fig. . The operating modes (a) Mode I, (b) Mode II, (c) Mode III.