Buck-Boost Converter Operating Principles and Basic Relationships with Discontinuous Current

(1) The switch tube V is turned on, and the inductor current iLf increases from zero to the maximum value ILf max;

(2) The switch tube V is turned off, the diode continues to flow, and the inductor current iLf drops from ILf max to zero;

(3) The switch tube V and the diode D are both turned off (cut off). During this period, the inductor current iLf is zero, and the load is powered by the output filter capacitor Cf.

The equivalent circuits of these three switching modes are shown in the figure.

During the on-time of the switch tube V, the inductor current iLf increases from zero, and the increase △iLf (+) is

After the switch V is turned off, the inductor current iLf decreases linearly from ILf max and drops to zero at the moment Tdis=TON+T'off, that is,

It can be seen from formula (3085) that when the inductor current is intermittent, the output voltage Ui is not only related to the input voltage Ui and the capacity ratio Du, but also to the load current Ii.

As can be seen from the figure, the maximum value of the switch tube V current Iv max is equal to the maximum value of the diode D current and the maximum value of the inductor current ID max, and is equal to the maximum value of the inductor current IL fmax, that is,

Where P0 -U0 I0 is the output power of the converter. This formula shows that the maximum current of the power device is determined only by the output power P0 when the inductor current is working discontinuously.