STCMB1 datasheet(19/59 Pages) STMICROELECTRONICS

19 / 59 page

DocID028958 Rev 4

19/59

STCMB1

Application information

Figure 7. Conventional COT controlled PFC

Since the slope of the inductor current during the on-time is proportional to the input voltage,

there is a perfect linear relation between the instantaneous AC voltage and the current

drawn by the circuit, which would ideally provide a perfect unity power factor and no

distortion when the on-time is kept constant along a line cycle.

Unfortunately, parasitic elements (essentially, the parasitic capacitance of the drain node,

Cdrain) cause a negative offset in the inductor current shape as can be seen in Figure 8. In

particular, the negative peak current ILvy can be calculated as follows:

Equation 1

Approximating the inductor current with a triangular waveshape, the commanded peak

current is:

Equation 2

and its average value in a switching cycle is:

Equation 3

(x)

The last term “(x)” in Equation 3 is non-sinusoidal (constant) and causes distortion in the

input current.



sin

drain

out

Lvy







Lvy

Lpk

sin









sin

drain

out

drain

Lpk

Lvy















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