AN3112 Datasheet, PDF (18/36 Pages) STMicroelectronics – Solution for designing a fixed off-time controlled PFC pre-regulator

Designing a fixed-off-time PFC
AN3112
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
P_loss
(VAC)
=
RDS_on
⋅ P_c′_ond (VAC)
+
⎜⎛
⎝
t_rise
+
2
t _fall
⎟⎞
⎠
⋅ P_s′_w
(VAC)
+
C_d
⋅
P_c′_ap
(VAC)
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
R_th
=
125°C − T_ambx
P_loss (VAC)
R_th
=
125°C − 50°C
9W
=
8.1^°C
W
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
MOSFETS total losses
25
P los se s( V i)
Range Limi ts
20
15
10
5
4.3.6
0
85
110
135
160
185
210
235
26 0
285
Vin_ac [Vr ms]
Figure 10 shows the trend of the total losses (Equation 35) versus the input line voltage for
two selected STP12NM50FP MOSFETs.
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
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