AN4149 datasheet(12/43 Pages) STMICROELECTRONICS

12 / 43 page

Designing a CCM FOT-controlled PFC

AN4149

12/43

DocID023523 Rev 2

The design will be done on the basis of a maximum admitted ripple factor Kr , that is the ratio

of the maximum peak-to-peak current ripple amplitude to the inductor peak current, at

minimum line voltage and rated load. In the continuous conduction mode converters, the

acceptable current ripple factor is typically fixed in a range between 20% and 35%. Low

values for the Kr keep the peak-to-peak ripple current and the input current distortion low,

but lead to larger inductor physical size. Choosing higher values for Kr , the inductor size

can be smaller, but the input current distortion is increased. For this design, the maximum

specified current ripple factor is chosen equal to 27%, as a trade-off between the inductor

size and the input current distortion.

Ripple factor:

Equation 24

In order to properly select the power components of the PFC and size the heat sinks, the

maximum operating ambient temperature around the PFC circuitry must be known. Please

note that this is not the maximum external operating temperature of the entire equipment,

but it is the local temperature at which the PFC components are working. The power

dissipation leading to a temperature rise combined with the ambient temperature must not

result in any temperature exceeding the operating temperature rating of the components.

Maximum ambient temperature ( °C):

Equation 25

2.2

Operating conditions

The first step is to define the main parameters of the circuit, using the specifications given in

Section 2.1.

Rated DC output current:

Equation 26

Maximum input power:

Equation 27

ambx

= 50

out

ou t

875

400

350

out

380

100

350

⋅

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