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AN3142 Datasheet(PDF) 7 Page - STMicroelectronics

Part # AN3142
Description  Solution for designing a 400 W fixed-off-time controlled
PDF  44 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3142 Datasheet(HTML) 7 Page - STMicroelectronics

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AN3142
The circuit implementing the line-modulated fixed-off-time with the new L6563S
Doc ID 17005 Rev 3
7/44
Equation 3
and V''ZCD(t) decreases from Vmult+VBE to VZCDtrigger = 0.7 V level in the following time
period t'':
Equation 4
Figure 7 illustrates the signal on the ZCD pin with the two discharging time constants
depending on the two resistors R, R0 and the L6563S parameters, particularly the upper
clamp voltage and the triggering voltage of the ZCD pin.
Figure 7.
ZCD pin signal with the fixed off-time generator circuit
The sum of the two time periods is the off-time function:
Equation 5
In this way, once the multiplier operating point (that is, the Vmult / VAC ratio) is fixed, with a
proper selection of R and R0 it is possible to increase TOFF with the line voltage so that, at
maximum line voltage, it is always TON>TONmin = 450 ns for the L6563S [4]. This is a
condition needed in order to avoid line distortion [
5].
()
() ⎥⎥⎦
⎤
⎢
⎢
⎣
⎡
⋅
+
−
+
⋅
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+
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+
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−
=
′
R
V
V
)
R
R
(
V
R
V
V
ln
C
R
R
R
R
t
BE
mult
0
ZCDclamp
0
BE
mult
0
0
⎥
⎦
⎤
⎢
⎣
⎡
+
⋅
−
=
′′
BE
mult
ZCDtrigger
V
V
V
ln
RC
t
()
()
() ⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟⎠
⎞
⎜⎜⎝
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+
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⎣
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=
BE
mult
ZCDtrigger
BE
mult
0
ZCDclamp
0
BE
mult
0
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OFF
V
V
V
ln
R
V
V
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R
R
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R
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R
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RC
T



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