AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
39 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
Where:
•
V
SNSAUXPFC
is the absolute maximum voltage rating
•
V
Lmax
is the maximum voltage across the PFC primary winding
•
N
P
is the number of turns on the PFC coil (for this example, a value of 52 is used)
The boost output voltage at OverVoltage Protection (OVP) determines the maximum
voltage across the PFC primary winding and can be calculated using the following
equation:
(12)
In this example, a design value of 394 V is used for nominal
V
BOOST
.
When a PFC coil with a higher number of auxiliary turns is used, a resistor voltage divider
can be placed between the auxiliary winding and SNSAUXPFC. The total resistive value
of the divider must be less than 10 k
to prevent delay of the valley detection in
combination with parasitic capacitances.
7.3.2 PFC frequency limit
The switching frequency is limited to 125 kHz to minimize the switching losses. If the
frequency for quasi-resonant operation is above the 125 kHz limit, the system switches
over to Discontinuous conduction mode. The PFC MOSFET is only switched on at a
minimum voltage across the switch (valley switching). One or more valleys are skipped,
when necessary, to keep the frequency below 125 kHz (valley skipping).
The minimum off-time is limited to 50
s after the last PFC gate signal to ensure proper
control of the PFC MOSFET under all circumstances.
7.4 PFC OverCurrent Regulation/Protection (OCR/OCP)
The maximum peak current, switched by the external MOSFET, is limited cycle-by-cycle
by sensing the voltage across a measurement resistor R
SENSE(PFC)
in the source of the
MOSFET. SNSCURPFC measures the voltage, which is limited to 0.5 V. At this voltage
level the MOSFET is switched off.
Take a small voltage margin into account to avoid false triggering of the OCP.
The value of the measurement resistor R
SENSE(PFC)
can be calculated with
(13)
The SNSCURPFC voltage senses an initial voltage peak at the moment the PFC
MOSFET switches on, because its (parasitic) capacitances are discharged. SNSCURPFC
has a leading edge blanking of 310 ns to mask this event, so it does not react to this initial
peak.
V
Lmax
V
OVP SNSBOOST
V
reg SNSBOOST
-----------------------------------------
V
BOOST
2.63
V
2.5
V
----------------
394
V
415
V
=
=
=
R
SENSE PFC
V
OCR SNSCURPFC
V
–
m
in
arg
I
P
max
-----------------------------------------------------------------------
0.52
V
0.1
V
–
8.73
A
-------------------------------------
48
m
=
=
=