AN4149 datasheet(11/43 Pages) STMICROELECTRONICS

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AN4149

Designing a CCM FOT-controlled PFC

Expected power factor:

Equation 19

Because of the narrow loop voltage bandwidth of the E/A to reject the ripple at twice the line

frequency on the E/A output, the PFC output may experience overvoltage at startup or

during load transients. In order to protect the controller from excessive output voltage that

can overstress the output components and the load, in the L4984D a pin (PFC_OK, pin #6)

has been dedicated to monitor the output voltage with a separate resistor divider. The

divider is 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.

Maximum output voltage (Vdc):

Equation 20

The output voltage has a ripple Vout at twice the line frequency (2fL), and whose amplitude is

proportional to the load and depends on the impedance of the output capacitor. The ripple

amplitude determines the AC current flowing into the output capacitor and the ESR. The

value of the maximum output voltage ripple accepted should be below a certain level to

keep the output voltage far from the VOVP limit during normal operating conditions, to

increase hold-up time, and to reduce the AC losses on the output capacitor. The peak-to-

peak voltage ripple Vout is usually selected in the range between 2-8% of the output voltage.

Here a ratio of 5% is chosen for the output voltage ripple, that is 20 V.

Maximum output low frequency ripple (peak-to-peak):

Equation 21

The desired ripple on the output voltage is not the only parameter used to select the bulk

capacitor. The hold-up time, if requested, should also be taken into account. The hold-up

time is defined as the duration of time that a power supply's output will remain above a

minimum level in case of mains dips.

The hold-up time (t

Hold) is measured from the time when the line voltage disappears to the

time when the output voltage reaches the required minimum voltage value (Voutmin). A hold-

up time of 10-20 ms is often required for today’s offline power supplies. Values of 300 V and

15 ms for the minimum output voltage after line drop and hold-up time, respectively, are

chosen here:

Minimum output voltage after line drop (Vdc):

Equation 22

Hold-up time (ms):

Equation 23

OVP

430

out

300

min

Ho l d

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