Buck-Boost Converter

A buck-boost regulator provides an output voltage that may be less than or greater than the input voltage hence the name ‘buck-boost’. The output voltage polarity is opposite to that of the input voltage. The regulator is also known as inverting regulator.

Buck-boost regulator circuit diagram is shown below.

Assumptions :-

(1) The value of capacitance be large enough that the output voltage V_o remains almost constant.

(2) Lossless circuit.

(3) The load current remains constant at I_o .

BUCK-BOOST CONVERTER WORKING

The buck-boost converter operation can be divided into two modes.

Mode-1 :- ( 0 ≤ t ≤ T_ON )

During this mode, chopper remains ON. With chopper ON, the diode is reverse biased and acts like an open circuit.

The circuit diagram during this mode looks like

INDUCTOR CURRENT AND VOLTAGE :-

CAPACITOR CURRENT :-

In loop-2, the capacitor discharges through the load.

As, i_c = - I_o and I_o= constant , therefore i_c is constant with negative sign.

CAPACITOR VOLTAGE :-

Capacitor voltage variation is explained by the basic capacitor equation

Since capacitor current i_c is a constant, therefore its integral will be linear and hence the capacitor voltage decreases (capacitor is discharging) linearly.

BUCK-BOOST CONVERTER WAVEFORMS

Mode-2 :- (T_ON ≤ t ≤ T )

During this mode, chopper is swithed OFF. But since i_L cannot become zero immediately therefore, i_L would flow through L,D,C and the load. I_L decreases linearly from I_maxto I_min . V_L polarity is reversed.

The energy stored in inductor L would be transferred to the load and the inductor current would fall until chopper is switched ON again in the next cycle.

The circuit during this mode looks like

CAPACITOR CURRENT :-

i_c = i_L – I_o from the circuit.

Since i_L decreases linearly from I_max to I_min and I_o is constant. Therefore, i_c decreases linearly from ( I_max- I_o)to ( I_min - I_o ) in T_off time.

AVERAGE INDUCTOR VOLTAGE OVER FULL SWITCHING CYCLE