AN3973

Power factor section

Doc ID 022164 Rev 1

9/24

voltage value (VCOMP) is determined, it is the reference (in the multiplier characteristic

graph) for the “MULT” and “CS” signal. Taking this into account, the procedure to set the

operation point of the multiplier Pin3 (MULT) is suggested. First, the maximum peak value is

selected. This value, which occurs at maximum mains voltage, should be less 3 V in case of

single mains. Therefore:

●

Minimum peak value:

●

This value is then multiplied by the minimum guaranteed

and it gives

the maximum output voltage of multiplier:

.

●

In this way the divider resistor is:

.

●

Subsequently thanks to these relationships:

.

The divider resistor R3=15 k

Ω and R1/R2=2.7 MΩ and the sense resistors R9=0.39 Ω and

R10=1

Ω have been fixed.

In particular:

–

the reference for the current sense.

–

the typical multiplier gain

–

the voltage on the Pin2

–

the voltage on the Pin3

This experimental fine tuning guarantees a high value of both PF and THD.

Pin5 (ZCD).This pin is the zero current detector circuit. In this case (PFC in transition mode)

the ZCD pin is connected, through a resistor, to the auxiliary winding of the boost inductor.

In particular, the resistance value (R6=47 k

Ω) was tuned in order to minimize the switching

power losses at turn-on. In fact, this fine tuning allows the MOSFET to switch on when the

resonant phenomenon between boost inductor and drain-source capacitance occurs on the

valley of the drain voltage. In this way, the MOSFET device turns on with lower voltage

across drain source, this implies a reduced current peak and lower switch-on losses.

 

max

INRMS

min

INRMS

MULTpkx

min

MULTpk

V

V

V

V

=

1

.

1

V

V

MULT

CS

=

Δ

Δ

min

MULTpkx

CSpk

V

*

1

.

1

V

=

max

INRMS

MULTpkx

3

2

1

3

V

*

2

V

R

R

R

R

=

+

+

MULT

COMP

CS

V

)

V

5

.

2

V

(

k

V

−

=

 

peak

min

CS

S

IL

V

R

<