UM0670 datasheet(39/43 Pages) STMICROELECTRONICS

39 / 43 page

Design calculation

UM0670

39/43

Doc ID 15327 Rev 2

Equation 10

Power MOSFET conduction loss is:

Equation 11

Where the power MOSFET chosen is the STF9NM50N (see datasheet Section 11:

References and related materials: 3.) and its RDS(on) for 70 °C is:

Equation 12

Power MOSFET switching losses can be approximately calculated (turn OFF time was

measured 120 ns - see Figure 19):

Equation 13

The total power loss on the power MOSFET is 1.839 W, so the heat sink can be calculated

from following equation:

Equation 14

And maximum heat sink-to-ambient resistance is:

Equation 15

The heat sink used in the power MOSFET on the STEVAL-ILL013V1 has a thermal

resistance of 13.5 °C / W, and therefore this heat sink is optimized for this design.

The last power component remaining to be calculated is the power diode. The diode

conducts during the OFF time, and therefore its average current is:

Equation 16

RMS

AVR

--------

0.2 1

0.8

-----------

0.21

CON

RM S

DS ON

()

° C

()

0.21

0.756

0.159W

DS ON

() ° C

()

DS ON

() ° C

()

1.35

0.56

1.35

0.756

MAX

OFF SW

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

400

1.4

120

–

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

1.68W

TOT

JM AX MOSFET

–

thJC

th CH

thHA

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

thH A

JM AX MOSFET

–

TOT

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

thJC

–

th C H

–

1.839

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

–

0.5

16.25

° CW

⁄

–

AVR D

–

()

MAX

MIN

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

10.2

–

()

1.4

0.6

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

0.8A

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