AN4149 datasheet(18/43 Pages) STMICROELECTRONICS

18 / 43 page

Designing a CCM FOT-controlled PFC

AN4149

18/43

DocID023523 Rev 2

The switching frequency chosen for this design is around 70 kHz, so the capacitor on the

TIMER pin needed to obtain the desired frequency is (equation 47):

Equation 49

A commercial value of 680 pF has been selected. An NP0 capacitor has to be used.

Now TOFF can be finally calculated. Along a line half-cycle, TOFF varies from nearly zero to

the maximum value, occurring at the MULT peak voltage. To calculate the boost inductor the

value at the MULT peak should be considered, found simply from the product of the

maximum required divider ratio kp and the peak of the rectified input voltage at low mains

voltage (Vpkmin).

The maximum OFF-time at VACmin is then:

Equation 50

The value of the inductance L required for the boost inductor at VACmin can now be

calculated with equation 46.

Equation 51

The value chosen for the inductor is 700

μH.

2.3.5

Power MOSFET selection and power dissipation calculation

The selection of the MOSFET concerns mainly RDS(on), that should be low in order to

minimize conduction losses, without increasing the switching losses due to the MOSFET 's

equivalent output capacitance Coss. To achieve high efficiency both RDS(on) and Coss have

to be taken into account, and the trade-off between cost vs. performance must also be

considered.

The MOSFET breakdown voltage is needed, considering the PFC nominal output voltage

and adding some margin (20%) to guarantee reliable operation. Therefore, a minimum

voltage rating of 500 V (1.2 · Vout = 480 V) is selected.

In this 350 W CCM PFC application, two STF21N65M5 (placed in parallel) have been

chosen, to improve robustness against surges and burst tests, and 650 V MOSFETs have

been chosen, having a good balance between RDSon and Coss. In order to calculate the

contribution of the MOSFETs to the total efficiency of the system, the power losses have

been calculated, which are mainly the sum of the conduction, switching and capacitive

losses.

kH z

695

40 0

156

⋅

−

OFF

156

680

)

(

min

⋅

−

)

(

)

(

)

(

min

OFF

out

VAC

⋅

−

VAC

654

400

)

(

min

⋅

−

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