AN4501 datasheet(35/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the switching MOSFET

Figure 16. Waveforms during the switch-on of M1

During t2, vM1,GS is now above VM1,GS,TH ,and M1 starts to conduct, and overtakes partially

the diode current. As the diode still conducts, VM1,DS remains unchanged at

VOUT + VFDIODE ~ VOUT. This phase ends up, when the vM1,GS reaches the value which

allows M1 to drive the whole inductor current (called VM1,GS,ILVALLEY).

During t2, the switching energy is estimated by:

which represents the area of the red triangle on Figure 16.

At the beginning of t3, The diode stops conducting, allowing VM1,DS to drop. The gate-drain

capacitor of M1 (CGD) is being discharged. Apparently, the value of CGD increases, which

explains constant value of VM1,GS despite the current injected in the gate by the gate driver

G1.

The switching energy during on transition is represented by the green triangle on Figure 16

the:

After t3, the M1 is fully turned on. The power dissipation during this phase is considered as

conduction losses. CGS is charged by G1 and VM1,GS further increases to reach the gate

driver’s voltage.

The switch-on losses are the total switching energy multiplied by the switching frequency:

OU T

L VALLEY

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OU T

L VALLEY

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