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AN3027 Datasheet(PDF) 27 Page - STMicroelectronics

Part # AN3027
Description  How to design a transition-mode PFC pre-regulator
PDF  41 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3027 Datasheet(HTML) 27 Page - STMicroelectronics

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AN3027
Designing a TM PFC
Doc ID 16134 Rev 4
27/41
during the MOSFET's off-time is sufficient to arm the ZCD circuit. A safe margin of 15% is
added.
Equation 59
If the winding is also used for supplying the IC, the above criterion may not be compatible
with the Vcc voltage range. To solve this incompatibility the self-supply network shown in the
schematic of
Figure 28, Figure 29 can be used.
The minimum value of the limiting resistor can be found considering the maximum voltage
across the auxiliary winding with a selected turn ratio of 10 and assuming 0.6 mA current
through the pin.
Equation 60
Equation 61
VZCDH = 5.7 V and VZCDL = 0 V are the upper and lower ZCD clamp voltages of the L6563S.
Considering the higher value between the two calculated, an RZCD = 68 kΩ has been
selected as the limiting resistor.
The actual value can then be fine-tuned in order to make the turn-on of the MOSFET occur
just on the valley of the drain voltage (which is resonating because the boost inductor has
run out of energy, (
Figure 15).This minimizes the power dissipation at turn-on.
Figure 15.
Optimum MOSFET turn-on
Pin 6 (TBO): In some applications it may be advantageous to regulate the output voltage of
the PFC preregulator so that it tracks the RMS input voltage rather than at a fixed value like
in conventional boost pre-regulators. This is commonly referred to as a “tracking boost” or
“follower boost” approach.
15
.
1
V
4
.
1
VAC
2
V
n
n
max
n
max
out
auxiliary
primary
⋅
⋅
−
=
=
71
.
15
15
.
1
V
4
.
1
Vac
265
2
V
400
max
n
=
⋅
⋅
−
=
Ω
=
−
=
k
16
.
57
mA
6
.
0
V
7
.
5
10
V
400
R
1
mA
6
.
0
V
n
V
R
ZCDH
aux
out
1
−
=
mA
6
.
0
V
n
VAC
2
R
ZCDL
aux
max
2
−
⋅
=
Ω
=
−
⋅
=
k
4
.
62
mA
6
.
0
V
0
10
Vac
265
2
R
2



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