AN3142 datasheet(17/44 Pages) STMICROELECTRONICS

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

17/44

4.3.5

Power MOSFET selection and power dissipation calculation

The selection of the MOSFET concerns mainly its RDS(on), basically proportional to the

output power. The MOSFET breakdown voltage is selected considering the PFC nominal

output voltage

(4) and adding some margin (20%) to guarantee reliable operation.

Therefore, 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

(

Equation 16) 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. For example in a 400 W PFC application two parallel STP12NM50FP

MOSFETs can be selected.

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:

Equation 29

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 on the device datasheet.

Now, combining

Equation 29 and Equation 16, the conduction losses referred to a 1

Ω

RDS(on), at ambient temperature as a function of Pin and VAC can be calculated:

Equation 30

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

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

Equation 31

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

the sinusoidal waveform, it can be demonstrated that from

Equation 31 the switching losses

per 1 µs of current rise and fall time can be written as:

Equation 32

From the STP12NM50FP 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. In general, the capacitive losses are given by:

()2

rms

cond

)

VAC

(

ISW

RDS

)

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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