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AN3027

Designing a TM PFC

Doc ID 16134 Rev 4

7/41

Designing a TM PFC

The following section describes a design flowchart of a 100 W transition mode PFC, using

the L6563S. The same design procedure and formulas proposed can also be applied for

dimensioning a similar 100 W transition mode PFC, using the L6563H.

3.1

Input specifications

This section details the specifications of the operating conditions of the circuit that are

needed for the calculations given in

Section 3.2. In this example an L6563S, 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 mainly on the

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

higher than the input and thus, because Vin max is 265·1.414 = 374.7 Vpk, the output has

been set at 400 Vdc as typical value. If the input voltage is higher, as is 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 calculations of the operating conditions of the PFC in

Section 3.2. Of

course at high input voltage, the efficiency is higher.

Because of the narrow loop voltage bandwidth, the PFC output can face overvoltages at

startup or in the case of load transients. To prevent excessive output voltage that can

overstress the output components and the load, in the L6563S a pin of the device (PFC_OK,

pin #7) has been dedicated to monitor the output voltage with a separate resistor divider,

selected so that the voltage at the pin reaches 2.5 V if the output voltage exceeds a preset

●

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 =

fMAINS =

100

Pout =

400

out =

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