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AN3112 Datasheet(PDF) 7 Page - STMicroelectronics

Part # AN3112
Description  Solution for designing a fixed off-time controlled PFC pre-regulator
PDF  36 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3112 Datasheet(HTML) 7 Page - STMicroelectronics

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AN3112
Implementing the line-modulated fixed-off-time
Doc ID 16820 Rev 3
7/36
Equation 3
and V''ZCD(t) decreases from Vmult+VBE to VZCDtrigger = 0.7 V (trigger 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 L6564 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 the
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, where TONmin is the minimum on-time of the
L6564 gate drive [
4]. This is a required condition in order to avoid line distortion [5].
()
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R
R
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R
t
BE
mult
0
ZCDclamp
0
BE
mult
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ZCDtrigger
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