AN2870 datasheet(23/41 Pages) STMICROELECTRONICS

23 / 41 page

AN2870

Designing with L6585DE

23/41

MOSFET selection

PFC MOSFET is to be selected considering the maximum current flowing into the switch,

the maximum voltage between drain and source and the maximum allowed losses.

Maximum allowed losses depend on maximum allowed junction temperature; an ambient

temperature equal to 70 °C – 80 °C is usually considered.

Power losses can be summarized as follows:

●

Conduction losses: due to ohmic resistance of the MOSFET channel during its on

state; these losses are prevalent at minimum input voltage.

●

Switching losses: experienced only during turn off transitions.

●

Capacitive losses: experienced only during turn on transitions when the MOSFET has

to discharge the parasitic capacitance present at its drain. These losses are very high

at higher input voltages.

Conduction losses are related to RDS(on) and RMS value of the drain current:

Equation 11

Considering a maximum conduction losses less than PCOND,max, the maximum RDS(on),

measured at 100 °C, can be found as follows:

Equation 12

Switching losses are directly related to frequency, to output voltage, to input current and fall

time of drain of the MOSFET. The frequency should be averaged over the half period of the

mains and the fall time of the MOSFET can be found on MOSFET datasheet.

Equation 13

Equation 14

Capacitive losses are present only if instantaneous input voltage is greater than half the

output voltage. In fact when the inductor current becomes zero the parasitic capacitances

seen at the drain node starts to resonate with parasitic inductances causing a damped

oscillation whose peak to peak amplitude is equal to VOUT - Vin.

When Vin<VOUT/2 the drain voltage at MOSFET turn on is almost zero.

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

OUT

rms

RMS

MOS

RMS

MOS

max

COND

(max)

⎟

⎠

⎞

⎜

⎝

⎛

⋅

−

⋅

OUT

RMS

rms

OUT

rms

OUT

cross

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