Designing a fixed-off-time PFC

AN3112

18/36

Doc ID 16820 Rev 3

The MOSFET total losses as a function of the input mains voltage is the sum of the three

previous losses from

Equation 30, Equation 32, and Equation 34, multiplied for relevant

MOSFET parameters:

Equation 35

From

Equation 35 using the data relevant to the MOSFET selected, the losses at Vitamin

and VACmax can be calculated and plotted like in

Figure 10. We can observe that the

maximum total losses is 9 W and it occurs at VACmin. From this number and the given

maximum ambient temperature

(13), the total maximum thermal resistance required to keep

the junction temperature below 125°C is:

Equation 36

If the result of

Equation 36 is lower than the junction-ambient thermal resistance given in the

MOSFET datasheet for the selected device package, a heat sink must be used.

Figure 10.

Conduction losses and total losses in the STP12NM50FP MOSFET

couples for the 400W FOT PFC

Figure 10 shows the trend of the total losses (Equation 35) versus the input line voltage for

two selected STP12NM50FP MOSFETs.

4.3.6

Boost diode selection

Following a similar criterion to that of the MOSFET, the output rectifier can also be selected.

A minimum breakdown voltage of 1.2·Vout

(4) and a current rating higher than 3·Iout

(

Equation 8) can be chosen for a rough, initial selection of the rectifier. The correct choice is

then confirmed by the thermal calculation. If the diode junction temperature works within

125°C the device has been correctly selected, otherwise a bigger device must be selected.

The switching losses can be significantly reduced if an ultra-fast diode is employed. Since

)

VAC

(

P

C

)

VAC

(

P

2

t

t

)

VAC

(

P

RDS

)

VAC

(

P

cap

d

sw

fall

rise

cond

on

loss

′

⋅

+

′

⋅

⎟

⎠

⎞

⎜

⎝

⎛

+

+

′

⋅

=

)

VAC

(

P

T

C

125

R

loss

ambx

th

−

°

=

W

C

1

.

8

W

9

C

50

C

125

R

th

°

=

°

−

°

=

MOSFETS total losses

0

5

10

15

20

25

85

110

135

160

185

210

235

260

285

Vi n_ac [Vr ms]

Plosses(Vi)

Range Limi ts