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參數(shù)資料
| 型號(hào)： | ML4841 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | Variable Feedforward PFC/PWM Controller Combo(前饋可變PFC/PWM控制器組合芯片) |
| 中文描述： | 可變前饋式PFC / PWM控制器組合（前饋可變式PFC /脈寬調(diào)制控制器組合芯片） |
| 文件頁(yè)數(shù)： | 7/15頁(yè) |
| 文件大?。?/td> | 149K |
| 代理商： | ML4841 |
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ML4841
REV. 1.0 10/12/2000
7
FUNCTIONAL DESCRIPTION
The ML4841 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 section 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 useable 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 ML4841 runs at twice the frequency
of the PFC, which allows the use of smaller PWM output
magnetics and filter capacitors while holding down the
losses in the PFC stage power components.
In addition to power factor correction, a number of
protection features have been built into the ML4841.
These include soft-start, PFC over-voltage protection, peak
current limiting, brown-out protection, duty cycle limit,
and under-voltage lockout.
POWER FACTOR CORRECTION
Power factor correction makes a non-linear 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 non-linear load is the input of a 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 such a supply causes brief high-amplitude pulses of
current to flow from the power line, rather than a
sinusoidal current in phase with the line voltage. Such a
supply presents a power factor to the line of less than one
(another way to state this is that it causes significant
current harmonics to appear at its input). If the input
current drawn by such a supply (or any other non-linear
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
ML4841 uses a boost-mode DC-DC converter to
accomplish this. The input to the converter is the full
wave rectified AC line voltage. No 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 which
the converter draws from the power line agrees with the
instantaneous 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
which the converter is allowed to draw from the line at
any given instant must be proportional to the line voltage.
The first of these requirements is satisfied by establishing
a suitable voltage control loop for the converter, which in
turn drives a current error amplifier and switching output
driver. 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 which
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
which linearizes the transfer function of the system as the
AC input voltage varies.
2
,
Since the boost converter topology in the ML4841 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
ML4841. 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 designated as K,
相關(guān)PDF資料 |
PDF描述 |
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| ML4851 | Low Current, Voltage Boost Regulator(小電流升壓穩(wěn)壓器) |
| ML4854IT | Adjustable, Low-Current, 2-Cell Boost Regulator with Shutdown and Low Battery Detect |
| ML4854 | Adjustable, Low-Current, 2-Cell Boost Regulator with Shutdown and Low Battery Detect |
| ML4861 | Low Voltage Drop Boost Regulator(低壓差升壓穩(wěn)壓器) |
| ML4863 | High Efficiency Flyback Controller(高效回轉(zhuǎn)電源控制器) |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ML4841CP | 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(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| ML4841CP_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(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Reel |
| ML4841CS | 制造商:MICRO-LINEAR 制造商全稱:MICRO-LINEAR 功能描述:Variable Feedforward PFC/PWM Controller Combo |
| ML4841IP | 制造商:MICRO-LINEAR 制造商全稱:MICRO-LINEAR 功能描述:Variable Feedforward PFC/PWM Controller Combo |
| ML4841IS | 制造商:MICRO-LINEAR 制造商全稱:MICRO-LINEAR 功能描述:Variable Feedforward PFC/PWM Controller Combo |
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