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AN2761

Designing a TM PFC

Doc ID 14690 Rev 2

7/36

Designing a TM PFC

3.1

Input specification

The following is a possible design flowchart in reference to a transition mode PFC, using the

L6562A. This first part is a detailed specification of the operating conditions of the circuit that

is needed for the following calculation. In this example a 80 W, wide input range mains PFC

circuit has been considered. Some design criteria are also given.

Because the PFC is a boost topology the regulated output voltage depends strongly on the

maximum AC input voltage. In fact, for boost correct operation the output voltage must

always be higher than the input and thus, because the Vin max is

Vpk,

the output has been set at 400 Vdc as the typical value. If the input voltage is higher, as

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

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

The target efficiency and PF are set here at minimum input voltage and maximum load.

They are used for the following operating condition calculations of the PFC. Of course at

high input voltage the efficiency is higher.

●

Mains voltage range (Vac rms):

(1)

●

Minimum mains frequency:

(2)

●

Rated output power (W):

(3)

●

Regulated DC output voltage (Vdc):

(4)

●

Expected efficiency (%):

(5)

●

Expected power factor:

(6)

Vac

VACmin =

Vac

265

VACmax =

fl =

Pout =

VAC

max

374

⋅

400

out =

out

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