AN4501 datasheet(36/48 Pages) STMICROELECTRONICS

36 / 48 page

Selection of the switching MOSFET

AN4501

36/48

DocID026394 Rev 1

Equation 26:

Where tM1,SWON = t2 + t3.

Switch-off phase

Figure 17 shows the waveforms during the switch-off of M1, which are relevant for the

switching losses. Prior to the phase t4, M1 is fully on and conducts the current IL,PEAK. The

diode is reverse biased.

Figure 17. Waveforms during the switch off of M1

At the beginning of the t4 phase, the voltage of the gate drive G1 drops and vM1,GS

decreases down to the sustaining voltage VM1,GS(ILPEAK) (minimum voltage required to

sustain the drain current IL,PEAK). There is no switching loss during this phase.

During t5, vM1,DS increases again up to ~ VOUT. As a consequence CGD is charged and

vM1,GS reaches again the plateau. The switching energy represented by the green area on

Figure 17 is estimated by the formula:

At the beginning of the t6 phase, vM1,DS is high enough to allow the freewheeling diode to

conduct. As the drain current of M1 decreases, so does vM1,GS.

M1,SWON

OUT

L VALLEY

M1,SWON

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

OUT

L PEAK

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