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Designing a CCM FOT-controlled PFC

AN4149

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DocID023523 Rev 2

Designing a CCM FOT-controlled PFC

2.1

Input specification

The following is a possible design procedure for a CCM FOT-controlled PFC using the

L4984D. This first part is a detailed specification of the operating conditions of the circuit that

is needed for the calculations of the design steps in the following sections.

In this example a 350 W wide-input range mains PFC circuit has been considered. Some

design criteria are also given.

Mains voltage range (Vacrms) :

Equation 13

Minimum mains frequency:

Equation 14

Rated output power (W):

Equation 15

Typically the output for a boost PFC output voltage is 400 Vdc as it has to be higher than the

maximum rectified input voltage:

Equation 16

In cases where the maximum AC input voltage VACmax is higher than 265 V, as typical in

ballast applications, the output voltage must be set higher accordingly. As a rule of thumb

the output voltage must be set 6 or 7% higher than the maximum input voltage peak.

Regulated DC output voltage (Vdc):

Equation 17

The target efficiency and PF are set for the following calculations based on the operating

condition of the PFC, here at minimum input voltage and maximum load. An efficiency of

92%, at minimum input voltage and maximum load, could be a starting point for a typical

PFC.

Expected efficiency (%):

Equation 18

min

26 5

max

out

350

ma x

⋅

out

400

out

92%

η ==

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