Design equations for the modified buck converter

AN2928

14/21

Finally, if the heat sink and its thermal resistance is known, it is possible to calculate

maximum static drain-source on resistance from Equation 26, Equation 27 and Equation 28

for easy power MOSFET selection.

Equation 29

2.5

Power diode selection

maximum peak current and total power losses. The power losses are lower for a larger duty

cycle and vice-versa, because the diode is opened (connected) during off-time.

power diode must be selected with some voltage margin. For example, if the input voltage is

higher.

Maximum peak diode current is selected in order to calculate the inductor size in

Equation 20. Also in this case, the power diode must be selected with some current margin.

Power losses are generally calculated with the following equation:

Equation 30

And assuming a constant voltage drop over the diode it is possible to approximately

calculate the power losses on the diode (switching losses are not included) with the

following equation:

Equation 31

R

DS ON

()

T

JMAX

T

A

–

R

th JC

R

th C H

R

th HA

++

() l

RM S

2

⋅

-------------------------------------------------------------------------------------

V

IN

l

MAX

t

OFF SW

f

⋅⋅

⋅

2l

RMS

2

⋅

--------------------------------------------------------------------

–

<

P

LOSS D

1

T

---

i

() u

D

⋅

t

() t

d

0

T

∫

=

P

LOSS D

I

AVR D

V

F

⋅

=