參數(shù)資料
型號(hào)： FAN9612MX
廠商： Fairchild Semiconductor
文件頁(yè)數(shù)： 11/36頁(yè)
文件大?。?/td> 1561K
描述： IC CTLR PFC DUAL BCM 16SOICN
標(biāo)準(zhǔn)包裝： 1
模式： 臨界傳導(dǎo)（BCM）
頻率 - 開關(guān)： 16.5kHz ~ 525kHz
電流 - 啟動(dòng)： 80µA
電源電壓： 9 V ~ 18 V
工作溫度： -40°C ~ 125°C
安裝類型： 表面貼裝
封裝/外殼： 16-SOIC（0.154"，3.90mm 寬）
供應(yīng)商設(shè)備封裝： 16-SOIC N
包裝： 標(biāo)準(zhǔn)包裝
其它名稱： FAN9612MXFSDKR
 
?2008 Fairchild Semiconductor Corporation
 
www.fairchildsemi.com
FAN9612 " Rev. 1.1.7
11
8.   Power Limit
The architecture and operating principle of FAN9612
also provides inherent input power limiting capability.
 
Figure 14. On-Time vs. V
IN_RMS
 
When the slope of the PWM ramp is made proportional
to the square of the input RMS voltage, the maximum
on-time of the boost power switch becomes inversely
proportional to the square of V
IN,RMS
, as represented in
Figure 14. In boundary-conduction mode, the peak
current of the boost transistor is proportional to its on
time. Therefore, controlling the maximum pulse width of
the gate drive signal according to the curve shown is an
effective    method    to    implement    an    input-voltage
independent power limit for the boost PFC.
9.   Light-Load Operation (Phase Management)
One   of   the   parameters   determining   the   operating
frequency of a boundary conduction mode converter is
the output power. As the load decreases, lower peak
currents are commanded by the pulse width modulator
to maintain the output voltage at the desired set point.
Lower peak current means shorter on-time for the power
transistor and shorter time interval to ramp the inductor
current back to zero at any given input voltage. As a
result,   the   operating   frequency   of   the   converter
increases under light load condition.
As the operating frequency and corresponding switching
losses increase, conduction losses diminish at the same
time. Therefore, the power losses of the converter are
dominated   by   switching   losses   at   light   load.   This
phenomenon is especially evident in a BCM converter.
To improve light-load efficiency, FAN9612 disables one
of the two interleaved boost converters automatically
when the output power falls below approximately 13% of
the   maximum   power   limit   level.   By   managing   the
number of phases used at light load, the FAN9612 can
maintain high efficiency for a wider load range of the
power supply.
Normal   interleaved   operation   of   the   two   boost
converters   resumes   automatically   once   the   output
power exceeds approximately 18% of the maximum
power limit level of the converter.
By adjusting maximum on-time (using R
MOT
), the phase
management   thresholds   can   be   adjusted   upward,
described in the Adjusting the Phase-Management
Thresholds section of this datasheet.
 
 
Figure 15. Automatic Phase-Control Operation
10. Brownout Protection with Soft Recovery
An additional protection function usually offered by PFC
ICs is input brownout protection to prevent the converter
from operating below a user-defined minimum input
voltage level. For this function to work, the input voltage
of the converter is monitored. When the voltage falls
below the brownout protection threshold, the converter
stops working. The output voltage of the boost converter
falls until the load stops drawing current from the output
capacitor or until the input voltage gets back to its
nominal range and operation resumes.
As the output falls, the voltage at the feedback pin falls
proportionally, according to the feedback divider ratio.
To facilitate soft recovery after a brownout condition, the
soft-start capacitor  which is also the reference voltage
of the error amplifier  is pulled lower by the feedback
network.   This   effectively   pre-conditions   the   error
amplifier to provide closed-loop, soft-start-like behavior
during   the   converters   recovery   from   a   brownout
situation.   Once   the   input   voltage   goes   above   the
brownout protection threshold, the converter resumes
normal operation. The output voltage rises back to the
nominal   regulation   level   following   the   slowly   rising
voltage across the soft-start capacitor.
11. Soft Starting the Converter
During startup, the boost converter peak charges its
output capacitor to the peak value of the input voltage
waveform. The final voltage level, where the output is
regulated during normal operation, is reached after the
converter starts switching. There are two fundamentally
different approaches used in PWM controllers to control
the startup characteristics of a switched-mode power
supply. Both methods use some kind of soft-start
mechanism to reduce the potential overshoot of the
converters output after the desired output voltage level
is reached.
The first method is called open-loop soft-start and relies
on gradually increasing the current or power limit of the
converter during startup. In this case, the voltage error
amplifier is typically saturated, commanding maximum
current until the output voltage reaches its final value. At
that time, the voltage between the error amplifier inputs
changes polarity and the amplifier slowly comes out of
saturation. While the error amplifier recovers and before
it starts controlling the output voltage, the converter
operates with full power. Thus, output voltage overshoot
is unavoidable in converters utilizing the open-loop soft-
start scheme.
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