AN2782 datasheet(16/39 Pages) STMICROELECTRONICS

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Designing a fixed off-time PFC

AN2782

16/39

Doc ID 14763 Rev 2

4.3.5

Power MOSFET selection and the dissipation

The selection of the MOSFET concerns mainly its RDS(on), which depends on the output

power (10), since the breakdown voltage is fixed just by the output voltage (11), plus the

overvoltage allowed (15) and a safety margin (20%).

Thus, a voltage rating of 500 V (1.2 · Vout = 480 V) is selected. Using its current rating as a

rule of thumb, we can select a device having ~ 3 times the RMS switch current (29) but, the

power dissipation calculation gives the final confirmation that the selected device is the right

one for the circuit also taking into account the heat sink dimensions. In this 400 W TM PFC

application two parallel STP12NM50 MOSFETs have been selected in order to support the

high inductor current.

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

The conduction losses at maximum load and minimum input voltage are calculated by:

Because normally in the datasheets the RDS(on) is given at ambient temperature (25 °C) to

calculate correctly the conduction losses at 100°C (typical MOSFET junction operating

temperature), a factor of 1.75 to 2 should be taken into account. The exact factor can be

found in the device datasheet.

Now, the conduction losses referred to as 1

Ω R

DS(on) at ambient temperature as a function

of Pin and VAC can be calculated, combining equations (42) and (29):

The switching losses due to the MOSFET current-voltage IMOS, VMOS crossing occurs at

turn-on and turnoff because of the FOT operation and can be basically expressed by:

Because the switching frequency depends on the input line voltage and on the position on

the sinusoidal waveform, it can be demonstrated that from (44) the switching losses per 1 µs

of current rise and fall time can be written as:

From the selected MOSFET datasheet trise = tfall = 0.01 µs is the crossover time at turn-on

and off.

At turn-on the losses are due to the discharge of the total drain capacitance inside the

MOSFET itself.

(42)

(43)

(44)

(45)

(

rms

)

(

cond

)

VAC

(

ISW

)

VAC

(

⋅

out

rms

cond

)

VAC

(

)

VAC

(

))

VAC

(

ISW

(

)

VAC

(

⎟

⎠

⎞

⎜

⎝

⎛

⋅

−

⋅

′

)

VAC

(

)

VAC

(

fall

rise

MOS

switch

⋅

⎟

⎠

⎞

⎜

⎝

⎛

⋅

()

∫

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

′

max

out

switch

)

VAC

(

sin

)

VAC

(

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