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STCMB1 数据表(PDF) 21 Page - STMicroelectronics

部件名 STCMB1
功能描述  TM PFC with X-cap discharge and LLC resonant combo controller
PDF  59 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

STCMB1 数据表(HTML) 21 Page - STMicroelectronics

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DocID028958 Rev 4
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STCMB1
Application information
59
It is possible to prove that, when choosing:
Equation 6
the average value of the inductor current in a switching cycle will be:
Equation 7
(z)
The block diagram and the relevant waveforms of the ECOT-controlled TM PFC are shown
in Figure 9 and Figure 10.
On the one hand, a negative threshold is placed on the current sensing pin ISEN_PFC
(VISEN_PFC_Z = -25 mV, typ.): the on-time generator (ramp at pin TON) starts as the
(negative) voltage on the pin ISEN_PFC goes below VISEN_PFC_Z. This is equivalent to
having ILth = VISEN_PFC_Z/Rs, where Rs is the current sensing resistor.
On the other hand, when the threshold VISEN_PFC_Z is crossed by the rising signal on the
pin ISEN_PFC at the end of the off-time, an internal generator (Ios = 50
A typ.) is turned
on, which sources the current out of the pin ISEN_PFC. This becomes a positive offset
voltage on the pin itself, because of the external series resistor Ros (part of the RC filter of
the pin). In this way, the crossing of the threshold VISEN_PFC_Z by the negative-going signal
can be further delayed and the equivalent current threshold ILth set at:
Equation 8
The value of the series resistor Ros can be adjusted to find the optimum ILth. As a starting
point, one can select Ros based on the theoretical compensation condition:
Equation 9
Note in Figure 10 the effect of this offset in the region VISEN_PFC > VISENPFC_Z (green
portion of the inductor current waveform), compared to the unaffected waveform that would
occur with Ros = 0 (black portion of the same waveform in the previous cycle).
L
C
V
I
drain
out
Lth 

sin
V
L
C
T
L
I
I
I
pk
_
in
drain
C
_
ON
Lpk
Lvy
L
1
2
1
2
1
Rs
R
I
V
I
OS
OS
Z
PFC
ISEN
Lth
_
_
OS
Z
PFC
ISEN
drain
out
OS
I
V
L
C
V
Rs
R
_
_



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