AN2870 datasheet(25/41 Pages) STMICROELECTRONICS

25 / 41 page

AN2870

Designing with L6585DE

25/41

Boost diode selection

Boost diode experiences a maximum current equal to maximum boost inductor current, an

average current equal to POUT/VOUT and a RMS current equal to:

Equation 18

The maximum reverse voltage must be greater or equal to VOUT and a fast Schottky diode is

suggested.

Diode power losses can be calculated using the formula reported in diode datasheet:

Equation 19

Bulk capacitor selection

Output voltage ripple is due to capacitance value and equivalent series resistor (ESR) of

bulk capacitor. ESR value is a function of frequency: higher the frequency, lower the ESR

value. The worst ESR will be measured in correspondence of the peak of the input voltage,

when the PFC frequency reaches the minimum frequency.

Equation 20

Rms value of the bulk capacitor current is:

Equation 21

Multiplier biasing and PFC current sense resistor selection

The multiplier biasing proceeds as follows (Figure 14):

Calculate the range of the peak of the input voltage.

Consider the characteristic curve that exploits the maximum slope and the point that

guarantees, on this curve, a linear behavior. Indicate this point as (Vmult,1,VCS,1)

rms

OUT

RMS

MOS

rms

−

rms

⋅

RMS

out

OUT

mains

OUT

out

min

PFC

ESR

⋅

OUT

rms

RMS

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

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