AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

17/36

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 equations

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

1

Equation 30

The generic switching losses due to the MOSFET commutation occurring at turn-on and

turn-off can be basically expressed by:

Equation 31

Because the switching frequency depends on the input line voltage and phase angle 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:

Equation 33

Where Cd is the total drain capacitance including the MOSFET and the other parasitic

turn-on.

Taking into account the frequency variation with the input line voltage and the phase angle,

the capacitive losses per 1 nF of total drain capacitance can be calculated as:

Equation 34

The total drain capacitance (Cd) of the two parallel MOSFETs is 0.36 nF, not including the

other component contributions, Vout is the drain voltage at MOSFET turn-on.

2

out

in

2

rms

cond

3

)

VAC

(

k

16

2

V

)

VAC

(

k

P

2

))

VAC

(

ISW

(

2

)

VAC

(

P

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π

⋅

−

⋅

⋅

⋅

=

⋅

=

′

)

VAC

(

f

2

t

t

I

V

)

VAC

(

P

sw

fall

rise

MOS

MOS

switch

⋅

⎟

⎠

⎞

⎜

⎝

⎛

+

⋅

⋅

=

()

∫

π

ϑ

⋅

θ

⋅

ϑ

π

⋅

⎟⎟⎠

⎞

⎜⎜⎝

⎛

∆

−

⋅

=

′

0

sw

2

pk

max

pk

out

switch

d

)

,

VAC

(

f

sin

1

2

IL

IL

V

)

VAC

(

P

)

VAC

(

f

V

C

2

1

)

VAC

(

P

sw

MOS

2

d

cap

⋅

⋅

⋅

=

()

ϑ

⋅

ϑ

π

⋅

=

′

∫

π

d

)

,

VAC

(

f

V

1

2

1

)

VAC

(

P

0

sw

2

out

cap