AN2928

Design equations for the modified buck converter

13/21

Figure 5.

Real drain MOSFET current

The second part of the MOSFET losses is switching losses, which depend on the (on and

off) switching time, drain MOSFET current, drain-source voltage and the switching

frequency. The switching time, rise time and fall time is a function of the gate-to-drain “Miller”

of the MOSFET. As the correct calculation of switching power losses is complex due to non

linear behavior of the switch, it is not possible to obtain an exact equation for the calculation

of switching losses. Moreover, the switching behavior is also influenced by the performance

of the driver and layout design (leakage inductance and parasitic capacitors). To arrive at an

estimation of the switching power losses, Equation 27 can be used.

Equation 27

For example, switched off-time measured on the STEVAL-ILL013V1 using the STP9NM50N

power MOSFET (400 V input voltage) is 120 ns.

calculate:

Equation 28

insulating washer)

AM00371

i

IMIN

tOFF

tON

i(t)

IMAX

IPP

t

P

SW

V

IN

l

MAX

t

OFF SW

f

⋅⋅

⋅

2

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

=

P

TOT

T

JM AX

T

A

–

R

thJC

R

thCH

R

thH A

++

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

=