X

8

Q

© 2010 Exar Corporation

25/27

Rev. 1.0.3

ESR

Cin

Tot

V

V

V

Δ

+

Δ

=

Δ

Power MOSFETs Selection

Selecting MOSFETs with lower Rdson reduces conduction losses at the expense of increased

switching losses. A simplified expression for conduction losses is given by:

in

out

dson

out

cond

V

V

R

I

P

⋅

⋅

=

2

MOSFET’s junction temperature can be estimated from:

ambient

thja

cond

j

T

R

P

T

+

= 2

This assumes that the switching loss is the same as the conduction loss.

thja

R

is the total MOSFET

thermal resistance from junction to ambient.