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AN2761 Datasheet(PDF) 23 Page - STMicroelectronics

Part # AN2761
Description  Solution for designing a transition mode
PDF  36 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2761 Datasheet(HTML) 23 Page - STMicroelectronics

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AN2761
Designing a TM PFC
Doc ID 14690 Rev 2
23/36
Supposing a 200 µA current flowing into the multiplier divider, the lower resistor value can
be calculated:
In this application example RmultH = 2 MΩ and RmultL = 15 kΩ have been selected. Please
note that for RmultH a resistor with a suitable voltage rating (>400 V) is needed, or more
resistors in series must be used.
The voltage on the multiplier pin with the selected component values recalculated is 0.89 V
at minimum line voltage and is 2.8 V at maximum line voltage. The multiplier works correctly
within its linear region.
●
Pin 5 (ZCD): Pin #5 is the input of the zero current detector circuit. In transition mode
PFC, the ZCD pin is connected, through a limiting resistor, to the auxiliary winding of
the boost inductor. The ZCD circuit is negative-going edge triggered. When the voltage
on the pin falls below 0.7 V, it sets the PWM latch and the MOSFET is turned on. To do
so the circuit must first be armed. Prior to falling below 0.7 V, the voltage on pin 5 must
experience a positive-going edge exceeding 1.4 V (due to the MOSFET's turnoff). The
maximum main-to-auxiliary winding turn ratio, nmax, has to ensure that the voltage
delivered to the pin during the MOSFET's OFF-time is sufficient to arm the ZCD circuit.
A safe margin of 15% is added.
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 18 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 = 10 and assuming 0.8 mA current
through the pin.
VZCDH = 5.7 V and VZCDL = 0 V are the upper and lower ZCD clamp voltages of the L6562A.
Considering the higher value between the two calculated, RZCD = 47 kΩ has been selected
as the limiting resistor.
(64)
(65)
(66)
(67)
Ω
=
Ω
⋅
⋅
−
=
−
=
−
−
M
85
.
1
k
15
10
16
.
8
10
16
.
8
1
R
k
k
1
R
3
3
multL
p
p
multH
15
.
1
V
4
.
1
VAC
2
V
n
n
max
n
max
out
auxiliary
primary
⋅
⋅
−
=
=
7
.
15
15
.
1
V
4
.
1
Vac
265
2
V
400
max
n
=
⋅
⋅
−
=
mA
8
.
0
V
n
V
R
ZCDH
aux
out
1
−
=
Ω
=
−
=
k
9
.
42
mA
8
.
0
V
7
.
5
10
V
400
R1
mA
8
.
0
V
n
VAC
2
R
ZCDL
aux
max
2
−
⋅
=
Ω
=
−
⋅
=
k
8
.
46
mA
8
.
0
V
0
10
Vac
265
2
R2



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