AN3027 datasheet(14/41 Pages) STMICROELECTRONICS

14 / 41 page

Designing a TM PFC

AN3027

14/41

Doc ID 16134 Rev 4

values. The minimum switching frequency can be recalculated for the selected inductance

value, inverting the formula in

Equation 22 to the following:

Equation 24

From the comparison of fswmin(VACmin) and fswmin(VACmax) with L = 0.52 mH as the actual,

the calculated minimum switching frequency is 40.13 kHz, as expected.

The core size is determined, assuming a peak flux density Bx

≅ 0.25T (depending on the

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

according to

Equation 45 as a function of the maximum clamping voltage of the current

sense pin 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.

3.3.5

Power MOSFET selection and dissipation

The selection of the MOSFET concerns mainly its RDS(on), which depends on the output

power

(3), since the breakdown voltage is fixed just by the output voltage (4), plus the

overvoltage admitted

(7) and a safety margin (20%). Thus, a voltage rating of 500 V

(1.2 · Vout = 480 V) is selected. Regarding its current rating as a rule of thumb, we can

select a device having ~ 3 times the RMS switch current (see

Equation 7), but the power

dissipation calculation gives the final confirmation that the selected device is the right one

for the circuit. The heatsink dimensions must also be considered. In this L6563S TM PFC

application, an STF7NM50 MOSFET has been 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 25

Because normally in datasheets the RDS(on) is given at ambient temperature (25 °C) to

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

temperature) a factor of 1.75 to 2 should be taken into account. The correct factor can be

found in the device datasheet.

Now, the conduction losses referred to a 1

Ω RDS(on) at ambient temperature as a function

of Pin and VAC can be calculated, combining Equation 25 and Equation 7:

Equation 26

The switching losses in the MOSFET occur only at turn-off because of TM operation and

can be basically expressed by:

Equation 27

out

min

)

VAC

(

VAC

)

VAC

(

⋅

−

⋅

()2

rms

)

(

cond

)

VAC

(

ISW

)

VAC

(

⋅

out

rms

cond

VAC

))

VAC

(

ISW

(

)

VAC

(

⎟

⎠

⎞

⎜

⎝

⎛

⋅

−

⋅

′

)

VAC

(

)

VAC

(

fall

MOS

switch

⋅

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