參數(shù)資料
型號(hào)： FAN9612MX
廠商： Fairchild Semiconductor
文件頁(yè)數(shù)： 10/36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(kāi)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
10
 
Figure 10. Input-Voltage Feedforward
When the PWM ramp is made proportional to the input
voltage squared, the system offers other noteworthy
benefits. The first is the input voltage-independent small
signal gain of the closed loop power supply, which
makes compensation of the voltage regulation loop
much easier. The second side benefit is that the output
of the error amplifier becomes directly proportional to
the input power of the converter. This phenomenon is
very   significant   and   it   is   re-visited   in   Section   9
describing light-load operation.
5.   Starting a PWM Cycle
The principle of boundary conduction mode calls for a
pulse width modulator able to operate with variable
frequency and initiate a switching period whenever the
current in the boost inductor reaches zero. Therefore,
BCM   controllers   cannot   utilize   a   fixed   frequency
oscillator circuit to control the operating frequency.
Instead, a zero current detector is used to sense the
inductor current and turn on the power switch once the
current in the boost inductor reaches zero. This process
is facilitated by an auxiliary winding on the boost
inductor. The voltage waveform of the auxiliary winding
can be used for indirect detection of the zero inductor
current condition of the boost inductor. Therefore it
should be connected to the zero current detect input, as
shown in Figure 11.
 
Figure 11. Simple Zero-Current Detection Method
The auxiliary winding can also be used to generate bias
for the PFC controller when an independent bias power
supply is not present in the system.
At startup condition and in the unlikely case of missing
zero current detection, the lack of an oscillator would
mean that the converter stops operating. To overcome
these situations, a restart timer is employed to kick start
the controller and provide the first turn-on command, as
shown in Figure 12.
 
Figure 12. PWM Cycle Start
6.   Terminating the Conduction Interval
Terminating   the   conduction   period   of   the   boost
transistor in boundary conduction mode controllers is
similar to any other pulse width modulator. During
normal operation, the PWM comparator turns off the
power transistor when the ramp waveform exceeds the
control voltage provided by the error amplifier. In the
FAN9612 and in similar voltage-mode PWMs, the ramp
is a linearly   rising   waveform at one input of the
comparator circuit.
 
Figure 13. Conduction Interval Termination
In addition to the PWM comparator, the current limit
circuit and a timer circuit limiting the maximum on-time
of the boost transistor can also terminate the gate drive
pulse   of   the   controller.   These   functions   provide
protection   for   the   power   switch   against   excessive
current stress.
7.   Protecting the Power Components
In   general,   power   converters   are   designed   with
adequate   margin   for   reliable   operation   under   all
operating conditions. However, it might be difficult to
predict dangerous conditions under transient or certain
fault   situations.   Therefore,   the   FAN9612   contains
dedicated protection circuits to monitor the individual
peak currents in the boost inductors and in the power
transistors. Furthermore, the boost output voltage is
sensed by two independent mechanisms to provide
over-voltage   protection   for   the   power   transistors,
rectifier diodes, and the output energy storage capacitor
of the converter.
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