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參數(shù)資料
| 型號： | ML4800 |
| 廠商： | Fairchild Semiconductor Corporation |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Power Factor Correction and PWM Controller Combo(功率因數(shù)校正器和PWM控制器組合芯片) |
| 中文描述： | 功率因數(shù)校正和PWM控制器組合（功率因數(shù)校正器和脈寬調(diào)制控制器組合芯片） |
| 文件頁數(shù)： | 7/14頁 |
| 文件大?。?/td> | 270K |
| 代理商： | ML4800 |
ML4800
REV. 1.0 10/10/2000
7
FUNCTIONAL DESCRIPTION
The ML4800 consists of an average current controlled,
continuous boost Power Factor Corrector (PFC) front end
and a synchronized Pulse Width Modulator (PWM) back
end. The PWM can be used in either current or voltage
mode. In voltage mode, feedforward from the PFC output
buss can be used to improve the PWM
’
s line regulation. In
either mode, the PWM stage uses conventional trailing-
edge duty cycle modulation, while the PFC uses leading-
edge modulation. This patented leading/trailing edge
modulation technique results in a higher usable PFC error
amplifier bandwidth, and can significantly reduce the size
of the PFC DC buss capacitor.
The synchronization of the PWM with the PFC simplifies
the PWM compensation due to the controlled ripple on
the PFC output capacitor (the PWM input capacitor). The
PWM section of the ML4800 runs at the same frequency
as the PFC.
In addition to power factor correction, a number of
protection features have been built into the ML4800. These
include soft-start, PFC overvoltage protection, peak current
limiting, brownout protection, duty cycle limiting, and
under-voltage lockout.
POWER FACTOR CORRECTION
Power factor correction makes a nonlinear load look like a
resistive load to the AC line. For a resistor, the current
drawn from the line is in phase with and proportional to
the line voltage, so the power factor is unity (one). A
common class of nonlinear load is the input of most power
supplies, which use a bridge rectifier and capacitive input
filter fed from the line. The peak-charging effect, which
occurs on the input filter capacitor in these supplies,
causes brief high-amplitude pulses of current to flow from
the power line, rather than a sinusoidal current inphase
with the line voltage. Such supplies present a power factor
to the line of less than one (i.e. they cause significant
current harmonics of the power line frequency to appear
at their input). If the input current drawn by such a supply
(or any other nonlinear load) can be made to follow the
input voltage in instantaneous amplitude, it will appear
resistive to the AC line and a unity power factor will be
achieved.
To hold the input current draw of a device drawing power
from the AC line in phase with and proportional to the
input voltage, a way must be found to prevent that device
from loading the line except in proportion to the
instantaneous line voltage. The PFC section of the ML4800
uses a boost-mode DC-DC converter to accomplish this.
The input to the converter is the full wave rectified AC line
voltage. No bulk filtering is applied following the bridge
rectifier, so the input voltage to the boost converter ranges
(at twice line frequency) from zero volts to the peak value
of the AC input and back to zero. By forcing the boost
converter to meet two simultaneous conditions, it is
possible to ensure that the current drawn from the power
line is proportional to the input line voltage. One of these
conditions is that the output voltage of the boost converter
must be set higher than the peak value of the line voltage.
A commonly used value is 385VDC, to allow for a high
line of 270VAC
rms
. The other condition is that the current
drawn from the line at any given instant must be
proportional to the line voltage. Establishing a suitable
voltage control loop for the converter, which in turn drives
a current error amplifier and switching output driver
satisfies the first of these requirements. The second
requirement is met by using the rectified AC line voltage to
modulate the output of the voltage control loop. Such
modulation causes the current error amplifier to command
a power stage current that varies directly with the input
voltage. In order to prevent ripple, which will necessarily
appear at the output of the boost circuit (typically about
10VAC on a 385V DC level), from introducing distortion
back through the voltage error amplifier, the bandwidth of
the voltage loop is deliberately kept low. A final
refinement is to adjust the overall gain of the PFC such to
be proportional to 1/V
2
, which linearizes the transfer
function of the system as the AC input voltage varies.
Since the boost converter topology in the ML4800 PFC is
of the current-averaging type, no slope compensation is
required.
PFC SECTION
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4800. The gain modulator is the heart of the PFC, as it
is this circuit block which controls the response of the
current loop to line voltage waveform and frequency, rms
line voltage, and PFC output voltage. There are three
inputs to the gain modulator. These are:
1) A current representing the instantaneous input voltage
(amplitude and waveshape) to the PFC. The rectified AC
input sine wave is converted to a proportional current
via a resistor and is then fed into the gain modulator at
I
AC
. Sampling current in this way minimizes ground
noise, as is required in high power switching power
conversion environments. The gain modulator responds
linearly to this current.
2) A voltage proportional to the long-term RMS AC line
voltage, derived from the rectified line voltage after
scaling and filtering. This signal is presented to the gain
modulator at V
RMS
. The gain modulator
’
s output is
inversely proportional to V
RMS2
(except at unusually
low values of V
RMS
where special gain contouring takes
over, to limit power dissipation of the circuit
components under heavy brownout conditions). The
relationship between V
RMS
and gain is called K, and is
illustrated in the Typical Performance Characteristics.
3) The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| ML4801CP | Variable Feedforward PFC/PWM Controller Combo |
| ML4801CS | Variable Feedforward PFC/PWM Controller Combo |
| ML4801IP | Variable Feedforward PFC/PWM Controller Combo |
| ML4801IS | Variable Feedforward PFC/PWM Controller Combo |
| ML4801 | Variable Feedforward PFC/PWM Controller Combo(前饋可變PFC/PWM控制器組合芯片) |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ML4800_0109 | 制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:Power Factor Correction and PWM Controller Combo |
| ML4800CP | 功能描述:功率因數(shù)校正 IC PFC Controller PWM Combo RoHS:否 制造商:Fairchild Semiconductor 開關(guān)頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| ML4800CP | 制造商:Fairchild Semiconductor Corporation 功能描述:IC PFC/PWM COMBO 4800 DIP16 |
| ML4800CP_NL | 功能描述:功率因數(shù)校正 IC Power Factor PWM Contrl 0.2mA 81KHz RoHS:否 制造商:Fairchild Semiconductor 開關(guān)頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| ML4800CP_Q | 功能描述:功率因數(shù)校正 IC PFC Controller PWM Combo RoHS:否 制造商:Fairchild Semiconductor 開關(guān)頻率:300 KHz 最大功率耗散: 最大工作溫度:+ 125 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
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