AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
38 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
SNSAUXPFC detects the valleys. An auxiliary winding on the PFC coil provides a
measurement signal on SNSAUXPFC. It gives a reduced and inverted copy of the
MOSFET drain voltage. When a valley of the drain voltage (top at SNSAUXPFC voltage)
is detected, the MOSFET is switched on.
If no top (valley at the drain) is detected on SNSAUXPFC within 4
s after
demagnetization is detected, the MOSFET is forced to switch on.
7.3.1 PFC auxiliary sensing circuit
Add a 5 k
series resistor to SNSAUXPFC to protect the internal circuit of the IC against
excessive voltage, for example during lightning surges. In the PCB layout, place this
resistor close to the IC to prevent disturbances causing incorrect switching.
It is important to maintain valley detection even at low ringing amplitudes. Set the voltage
at the SNSAUXPFC as high as possible, while taking into account its absolute maximum
rating of
25 V.
The number of turns of the auxiliary winding on the PFC coil can be calculated using the
following equation:
(11)
Fig 19. PFC demagnetization and valley sensing
demagnetized
V
Rect
/N
V
Rect
V
Boost
(V
Boost
−
V
Rect
)/N
V
aux,demag
0
0
magnetized
Demagnetization
l
TrPfc
AuxPfc
DrPfc
GatePfc
Valley
(= top for detection)
0
off
on
t
001aal029
N
aux max
V
SNSAUXPFC
V
Lmax
-------------------------------
N
p
25
V
415
-----------
52
3.13
3
turns
=
=
=
