Reduced Loss Tristate SEPIC

The Sepic is a very well-known DC/DC converter topology. A tristate converter extends the circuit by an additional electronic switch and a diode. Three modes follow each other within one switching period. During the first mode M1, both transistors are on and both diodes are off. In the second mode M2 only the second switch is on and the first diode is conducting, and in mode M3 only the second diode is conducting. The voltage transformation ratio is a function of the duty cycles of the two electronic switches. In a typical tristate converter, the current flows through the second switch during the first two modes. In the converter treated here, the current is flowing through the second switch only during the second mode, so the losses are reduced compared to the normal tristate converter. The function of the converter is described in the steady state by the voltages across and the currents through the components. The large and the small signal models are derived. When the duty cycle of the second switch is held constant and the duty cycle of the first switch is used as a variable, the voltage transformation ratio is linearized, and additionally, the converter operates as a phase minimum system. The calculation of the transfer functions is explained and further possible modifications are shown. The start-up and the reaction of a short-circuit are investigated. The considerations are proved by simulations with the help of LTSpice.