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AN-6961 Schematic ( PDF Datasheet ) - Fairchild Semiconductor

Teilenummer AN-6961
Beschreibung Critical Conduction Mode PFC Controller
Hersteller Fairchild Semiconductor
Logo Fairchild Semiconductor Logo 




Gesamt 8 Seiten
AN-6961 Datasheet, Funktion
www.fairchildsemi.com
AN-6961
Critical Conduction Mode PFC Controller
Description
This application note describes a power factor correction
(PFC) circuit using the FAN6961. Both the features of this
controller, as well as the operation of the power factor
correction circuit, are presented in detail. Based on the
proposed design guideline, a design example with detailed
parameters demonstrates the performance of the controller.
Introduction
The FAN6961 PFC controller is an 8-pin Boundary Current
Mode (BCM) IC intended for controlling PFC pre-regulators.
The FAN6961 provides a controlled on-time to regulate the
output DC voltage and achieve natural power factor
correction. The maximum on-time of the switch is
programmable to ensure safe operation during AC
brownouts. An innovative multi-vector error amplifier is built
in to provide rapid transient response and precise output
voltage clamping. Once the output feedback loop is opened,
the output driver (GD) is disabled to provide protection of
the system. The start-up current is lower than 20µA and the
operating current has been reduced to 5mA. The supply
voltage can be operated up to 25V, maximizing application
flexibility. The FAN6961 also enables cycle-by-cycle current
limiting protection for the external power MOSFET.
Figure 1. Power Factor Correction Circuit
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.2 • 4/8/09
www.fairchildsemi.com






AN-6961 Datasheet, Funktion
AN-6961
Determine Current-Sense Resistor
The MOSFET on-time and the input current increase with
the decreasing AC input voltage or increasing load. The
FAN6961 can establish the maximum on-time limit (25µs is
recommended) of power MOSFET. Once the voltage on
current-sense pin reaches the internal limit VCS, 0.82V
typically, the FAN6961 stops the PWM output immediately.
Thus, the maximum output power can be designed by the
current-sense resistor and maximum on-time limit. In
general operation, the maximum on-time occurs at minimum
AC input voltage and maximum loading conditions.
When the output power increases from full load to
maximum load, the on-time is restricted to the maximum on-
time limit first, then the current limit. In the design example,
the voltage on the current-sense pin is set to 0.57V at full
load and minimum input voltage conditions. At this condition,
the maximum power is about 156% of full load at minimum
input voltage condition. The current-sense resistor can be
calculated from Equation 7. The calculated curve of the
MOSFET turn-on time at different loading conditions are
shown in Figure 16. The calculated waveforms of the PFC
inductor current at two kinds of current limit are shown in
Figure 17.
2.60E-05
2.40E-05
2.20E-05
2.00E-05
1.80E-05
1.60E-05
1.40E-05
1.20E-05
1.00E-05
0
Full load Pin=105.9W
120% Full load Pin=127W
Max. Pin=164.7W
110% Full load Pin=116.46W
150% Full load Pin=158.8W
Current Sense Limit(0.82V/4.55A) Max. On -Time Limit(25uS )
35 70 105 140 175 210 245 280 315
Time(S/20000)
APPLICATION NOTE
The FAN6961 current-sense limit, VCS, is 0.82V typically.
From Faraday’s law, the number of turns for PFC inductor
can be obtained by:
Nb
=
Lb IL,pk
Bmax Ae
106
where:
Ae is the effective area of the core-section,
Bmax is saturation magnetic flux density.
(8)
Determine the Auxiliary Winding
The FAN6961 can perform zero-current detection by
sensing the information on an auxiliary winding of the PFC
inductor. As discussed previously, when the ZCD voltage is
lower than the threshold voltage (1.75V typical), the PWM
output is high again and initiates a new switching cycle.
However, there is a prerequisite: the zero-current detector
voltage must exceed the rising-edge threshold voltage (2.1V
typical) before it falls below 1.75V. The minimum rising-
edge voltage of zero-current detector input occurs at the
peak of the highest AC line voltage, which is equal to
VO - 2 Vrms,max / n and must be larger than the ZCD input
rising-edge threshold voltage (2.1V typical). The ZCD
voltage VZCD should be established as high as 120% of 2.3V
to have a safe margin; therefore, the number of turns for
auxiliary winding is obtained as:
Naux
=
Vo
Vzcd 1.2
2 Vrms.max
Nb
(9)
where Vrms,max is the maximum input line rms voltage.
VZCD is the rising-edge voltage of zero-current detector
input.
Figure 16.
Calculated Curve of the MOSFET
Turn-on Time at Different Loading Conditions
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
Max. Pin=164.7W,Current Limit=4.55A
Max. Pin=140.4W,Current Limit=3.727A
150% Full load Pin=158.8W, Current Limit=4.55A
Current Limit(4.55A)
Full load Vcs = 0.57V
Max. Load 156% Full load
Current Limit(3.727A)
Full load Vcs = 0.7V
Max. Load 133% Full Load
35 70 105 140 175 210 245 280 315
Time(S/20000)
Figure 17.
Calculated Waveforms of the PFC
Inductor Current at Two Current Limits
RS
=
0.57
IL,PK 95%
(7)
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.2 • 4/8/09
Figure 18.
Simplified Power Stage
Calculate On-Time ton.fix
The fixed on-time for the specific output power, inductor,
and input voltage can be calculated by:
ton.fix
=
2 Po Lb
Vrms2 η
where:
(10)
Lb is the PFC inductor,
η is conversion efficiency,
PO is the maximum rated output power,
Vrms is the input line rms voltage.
6
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