AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

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4

Designing a fixed-off-time PFC

4.1

Input specification

The following is a possible design procedure for a fixed-off-time mode PFC using the L6564.

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

needed for the calculations in

Section 4.2 on page 11. In this example a 400 W, wide-input

range mains PFC circuit is 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

400 Vdc as the typical value. 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 6/7% 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 there is higher efficiency.

Because of the narrow loop voltage bandwidth, the PFC output may experience

overvoltages at startup or in the case of load transients. To protect from excessive output

voltages that can overstress the output components and the load, in the L6564, 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) 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)

Vac

90

Vac

265

Hz

47

W

400

V

400

V

%

90

P

P

in

out

=

=

η

99

.

0

PF

=