Switching Power Supplies A - Z

Chapter 1

t

OFF

+ t

ON

t

ON

=

V

IN

− V

SW

+ V

O

+ V

D

− V

IN

V

O

+ V

D

− V

IN

Finally, the ‘duty cycle’ of the converter D, which is defined as

D =

t

ON

T

(any topology)

is the reciprocal of the preceding equation. So

D =

V

O

+ V

D

− V

IN

V

O

+ V

D

− V

SW

(Boost)

We have thus derived the classical dc transfer function of a boost converter.

If the switch and diode drops are small as compared to the input and output rails, we can just

write

D ≈

V

O

− V

IN

V

O

(Boost)

We can also write the relationship between the input and output as follows

V

O

= V

IN

×

1

1 − D

(Boost)

The Buck Topology

In Figure 1-16 we had also presented the schematic of the buck topology. The direct and the

freewheeling paths are indicated ...