AN3027 datasheet(8/41 Pages) STMICROELECTRONICS

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Designing a TM PFC

AN3027

8/41

Doc ID 16134 Rev 4

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

worst-case load/line transients.

The mains frequency generates a 2fMAINS 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 request for a certain hold-up capability can be sent to the PFC if mains dips

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

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

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

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

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

L6563S minimum internal startup period, given in the datasheet. On the other hand, if the

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

voltage and probably skips switching cycles not only at light load. The typical minimum

frequency range is 20 - 50 kHz for wide range operation.

In order to properly select the power components of the PFC and dimension the heatsinks in

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

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

temperature of the entire system, 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)

●

Maximum ambient temperature (°C):

(12)

430

OVP =

out =

∆

300

min

out

Hold =

kHz

min

ambx

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