Power Sources and Supplies: World Class Designs

60

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Chapter 3

Applying the voltseconds law in CCM (or BCM), we also get

r

VD

ILf







OFF

L

(any topology, CCM or BCM only)

()1

(3-21)

a)

From the plots of r in Figure 3.4 , we see that both the buck and the buck-boost

have the highest value of r when D approaches zero—i.e., at maximum input

voltage. For these topologies, the equation for r (derivable from the more general

equation for r just given immediately above) is:

r

V

ILf

D





O

O

(buck)()1

(3-22)

r

V

ILf

D





O

O

(buck-boost)()1

2

(3-23)

So putting r  2 and D  0 (i.e., highest input voltage plus BCM), we get the limiting

condition

I

V

Lf

O

O

(buck and buck-boost) ...