AN4149 datasheet(19/43 Pages) STMICROELECTRONICS

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19/43

AN4149

Designing a CCM FOT-controlled PFC

The conduction losses at maximum load and minimum input voltage are calculated by:

Equation 52

Because normally in a MOSFET datasheet RDS(on) is given at ambient temperature (25 °C),

in order to properly calculate the conduction losses at 100 °C (typical MOSFET junction

operating temperature), a factor KTEMP between 1.5 to 2, which can be found in the device

datasheet, should be taken into account. In the case of the STF21N65M5, looking at the

normalized ON resistance vs. temperature graph, a factor of 1.7 should be considered at

100 °C.

Equation 53

where the RDSon value is divided by two since two MOSFETs are placed in parallel. The

maximum RMS switching current, at minimum VAC, has been found from equation 33. Now,

from equation 52 and equation 53, and considering that two MOSFETs in parallel have been

used, the maximum conduction losses at low line and full load can be calculated as:

Equation 54

The switching losses are difficult to predict as they depend on the particular switching

waveform, determined by many factors (driving current, gate resistors, MOSFET gate

internal resistance, Vth, gate charge, total capacitance on the drain node including parasitic

capacitances etc.). A good approximation to determine the generic switching losses due to

the MOSFET commutation occurring at turn-on and turnoff can be basically expressed by:

Equation 55

where VDS is the drain-to-source of the MOSFET, ID the average drain current and trise and

tfall refer to the rising and the falling edge of VDS . To estimate the rising and falling times of

the drain voltage, datasheet values of the switching performance of the MOSFET can be

used.

First the average rising times of the drain voltage can be calculated considering the total

drain node capacitance and the average value of the peak current flowing through the

inductor.

()2

)

(

)

(

VAC

ISW

RDS

VAC

rms

cond

⋅

152

179

100

TEMP

DSon

MOSFETs

paralleled

Num

RDS

()

(

100

)

(VAC

ISW

VAC

rms

DSon

cond

⋅

()

(

)

cond

152

2 =

⋅

()

fall

rise

VAC

⋅

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