AN4501 datasheet(33/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the switching MOSFET

8.3.1

Conduction losses

Figure 15. Typical current waveform of M1 in CCM

The conduction losses of M1 are given by PM1,COND = RON,M1 x I²M1,RMS, where RON,M1 is

the Rdson of the MOSFET M1 and IM1,RMS is the root means square current (see

Section 8.5 for the calculation details):

Equation 25:

is in general negligible compared to 1 and the MOSFET RMS current is

dominated by the term

, which increases as D increases. The RMS current reaches

its maximum value at VIN,MIN:

Finally, the conduction losses are given by:

M1 R MS

OUT

–

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

------

LPP

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





 2







------

LPP

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





 2

–

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

M1 RMS M AX

OUT

MAX

–

------------------------- D

MAX

L,PP(VINMIN)

OUT

MAX

–

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





 2

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







M1 COND

ON M1

OUT

MAX

–

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





 2

MAX

L,PP(VINMIN)

OU T

MAX

–

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





 2

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







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