Designing a fixed-off-time PFC

AN3112

16/36

Doc ID 16820 Rev 3

Figure 9.

The effect of fixing off-time - boundary between DCM and CCM

The effect of fixing the off-time is to generate a continuous conduction mode in the center

region of the line half-cycle between the two transition angles. Close to the zero-crossing,

the system works in discontinuous conduction mode and in transition mode at the boundary.

The inductor core size is determined assuming a peak flux density Bx ~0.25 T (depending

on the ferrite grade selected and relevant specific losses) and calculating the maximum

current according to

Equation 15 as a function of the maximum current sense pin clamping

voltage and sense resistor value.

DC and AC copper losses and ferrite losses must also be calculated to determine the

maximum temperature rise of the inductor.

4.3.5

Power MOSFET selection and power dissipation calculation

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

output voltage

(4) 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 the heat sink dimensions into

account. 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

to correctly calculate the conduction losses at 100°C (typical MOSFET junction operating

DCM

TM

CCM

TOF F

Half Line Cycle

DCM

TM

CCM

TOF F

Half Line Cycle

rms

on

cond

)

VAC

(

ISW

RDS

)

VAC

(

P

⋅

=