AN3142 datasheet(9/44 Pages) STMICROELECTRONICS

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

9/44

Designing a fixed-off-time PFC

4.1

Input specification

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

needed for the following calculations in

Section 4.2 on page 11. In this example a 400 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 correct boost operation the output voltage must

always be higher than the input and therefore, because 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 calculation of the PFC. 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 case of load transients. To protect from excessive output voltage that can

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

#6) 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

value (Vovp), usually larger than the maximum Vout that can be expected, also including

worst-case load/line transients.

●

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)

●

Maximum. output voltage (Vdc):

(7)

Vac

VACmin =

Vac

265

VACmax =

fl =

400

Pout =

400

out =

out

430

OVP =

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