AN2459 datasheet(32/35 Pages) STMICROELECTRONICS

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Components calculation

AN2459 - Application note

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A.4

Power MOSFET

The choice of MOSFET mainly concerns the RDSon, which depends on the output power.

The breakdown voltage is fixed by sum of the output voltage, the overvoltage and a safety

margin .

The MOSFET's power dissipation depends on conduction and switching losses.

The conduction losses are given by:

where:

Switching losses due to current-voltage cross occur only at turn-off because of the TM

operation:

where tfall is the crossover time at turn-off. At turn-on, loss is due to the discharge of the total

drain capacitance inside the MOSFET itself. In general, these losses are given by:

where Coss is the internal drain capacitance of the MOSFET (at VDS = 25V), Cd is the total

external drain parasitic capacitance and VDRAIN is the drain voltage at MOSFET turn-on. In

practice, it is possible to give only a rough estimate of the total switching losses because

both fsw and VDRAIN change along a given line half-cycle. VDRAIN, in particular, is affected

not only by the sinusoidal change of the input voltage but also by the drop due to the

resonance of the boost inductor with the total drain capacitance. At low mains voltage, this

causes VDRAIN to be zero during a significant portion of each line half-cycle. It is possible to

show that "Zero-Voltage-Switching" occurs as long as the instantaneous line voltage is less

than half the output voltage.

PON = IQrms • RDSon

PCROSS = VO • Irms • tfall • fsw

PCAP =

(3.3 • COSS • VDRAIN +

• Cd • VDRAIN

1.5

) • fsw

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