Design of Buck Converter

The basic topology of DC-DC buck, or step down converter is shown in Figure 1. Buck Converter operates in two modes, continuous and discontinuous mode. In discontinuous mode, the amount of energy required by the load is too small. Therefore, the current through the inductor falls to zero during that period. The inductor is completely discharged at the end of the commutation cycle.

Figure 1

A buck converter operates in continuous conduction mode, if the current in the conductor never falls to zero during commutation. The operating principle is described below:

• When the switch is closed (i.e. ON state) as shown in figure 2, the voltage across inductor is V(L) = Vi - Vo. As the diode is reverse biased by the voltage source V, no current flows through it.

Figure 2 - Buck Converter when switch is closed (ON State)

Eq. 1

• When the switch is opened (i.e. OFF state) as shown in figure 3, the diode is forward biased and the voltage across the inductor V(L) = -V.

Figure 3 - Buck converter when switch is open (OFF State)

Eq. 2

Here D is the Duty cycle and T is time period of the buck operation.

Increase in current when the switch is turned ON, must be equal to decrease in current when the switch is turned OFF, as the net change in flux in the inductor must be zero.

From Eq. 1:

From Eq. 2:

Equating the above equations:

Calculation of ripples:

From the equation of current ripple the maximum and minimum value of the inductor can be calculated.

Assuming 100% efficiency, in an ideal converter Pin = Pout. This gives the relation between input and output current as Iout = Iin/D . The value of D is always less than 1 therefore, the value of Iout will be greater than the value of Iin. While the buck converter steps down the voltage it steps up the current to transfer power with minimum loss.