AN3112 datasheet(10/36 Pages) STMICROELECTRONICS

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Designing a fixed-off-time PFC

AN3112

10/36

Doc ID 16820 Rev 3

The mains frequency generates a 2fL voltage ripple on the output voltage at full load. The

ripple amplitude determines the current flowing into the output capacitor and the ESR.

Additionally, a certain hold-up capability in case of mains dips can be requested from the

PFC in which case the output capacitor must also be dimensioned, taking into account the

required minimum voltage value (Vout min) after the elapsed hold-up time (tHold).

The PFC minimum switching frequency is one of the main parameters used to dimension

the boost inductor. Here we consider the switching frequency at low mains on top of the

sinusoid and at full load conditions. As a rule of thumb, it must be higher than the audio

bandwidth in order to avoid audible noise and additionally it must not interfere with the

L6564 minimum internal starter period, as given in the datasheet. Alternatively, if the

minimum frequency is set too high the circuit shows excessive losses at higher input voltage

and probably operates skipping switching cycles not only at light load. The typical minimum

frequency range is 55÷95 kHz for wide range operation.

The design is done on the basis of a ripple factor (the ratio of the maximum current ripple

amplitude to the inductor peak current at minimum line voltage) kr=0.34.

In order to properly select the power components of the PFC and dimension the heat sinks

in case they are needed, the maximum operating ambient temperature around the PFC

circuit must be known. Please note that this is not the maximum external operating

temperature of the entire equipment, but it is the local temperature at which the PFC

components are working.

●

Maximum. output voltage (Vdc):

(7)

●

Maximum output low frequency ripple:

(8)

●

Minimum output voltage after line drop (Vdc):

(9)

●

Hold-up capability

(ms):

(10)

●

Minimum switching frequency (kHz)

(11)

●

Ripple factor

(12)

●

Maximum ambient temperature (°C):

(13)

430

OVP =

out =

∆

300

min

out

Hold =

kHz

min

r =

ambx

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