AN4501 datasheet(37/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the switching MOSFET

The related switching energy is estimated by:

At the end of the t6 phase, vM1,GS reaches vM1,GS,TH. Therefore, the drain current of M1 is

zero and the diode conducts the inductor current IL,PEAK. The switching loss in this interval

is zero.

To sum up, the switching losses during the turn-off of M1 are equal to the total switching

energy during this phase, multiplied by the switching frequency.

Equation 27:

Where tM1,SWOFF = t5 + t6.

Total power switching losses of M1

From Equation 26 and Equation 27, we can estimate the total switching losses of M1:

Equation 28:

If the switch-on and switch-off times do not significantly differ, we can approximate the term

IL,VALLEY x tM1,SWON + IL,PEAK x tM1,SWOFF by IL (tM1,SWON + tM1,SWOFF):

Equation 29:

We see that the switching losses reach the maximum value at VIN,MIN, which corresponds to

IL,MAX.

8.3.2

Iterative calculation of the junction temperature of M1

The temperature dependence of the Rdson requires an iterative calculation. We propose

here a way how to proceed.

To have a first approximation of the junction temperature of M1, we consider that the

switching losses are temperature independent.

The Figure 18 sums up the proposed procedure:

OUT

L PEAK

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

M1,SWOFF

OUT

LPEAK

M1,SWOFF

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

M1,SW

M1,SWON

M1,SWOFF

OU T

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

L,VALLEY

M1,SWON

L,PEAK

M1,SWOFF

()

M1,SW

OUT

--------------------------------------------- t

M1,SWON

M1,SWOFF

()

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