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參數(shù)資料
| 型號： | ML4801 |
| 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） |
| 文件頁數(shù)： | 9/14頁 |
| 文件大?。?/td> | 278K |
| 代理商： | ML4801 |
ML4801
operating frequency can typically be approximated by:
f
t
OSC
RAMP
=
1
(5)
EXAMPLE:
For the application circuit shown in the data sheet, with
the oscillator running at:
f
kHz
t
OSC
RAMP
=
=
100
1
t
R
C
RAMP
T
T
=
′
′
= ′
-
051
.
5
Solving for R
T
x C
T
yields 2 x 10
-4
. Selecting standard
components values, C
T
= 270pF, and R
T
= 36.5k
.
PWM SECTION
The PWM section of the ML4801 is straightforward, but
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
section of the device, and that the PWM stage is
optimized for current-mode operation. In the ML4801, the
operating frequency of the PFC section is fixed at 1/2 of
the PWM's operating frequency. This is done through the
use of a 2:1 digital frequency divider ("T" flip-flop) linking
the two functional sections of the IC.
No voltage error amplifier is included in the PWM stage
of the ML4801, as this function is generally performed on
the output side of the PWM’s isolation boundary. To
facilitate the design of optocoupler feedback circuitry, an
offset has been built into the PWM’s RAMP 2 input which
allows V
DC
to command a zero percent duty cycle for
input voltages below 1.25V.
PWM Current Limit
The RAMP 2 pin provides a direct input to the cycle-by-
cycle current limiter for the PWM section. Should the
input voltage at this pin ever exceed 1.5V, the output of
the PWM will be disabled until the output flip-flop is reset
by the clock pulse at the start of the next PWM power
cycle.
V
IN
OK Comparator
The V
IN
OK comparator monitors the DC output of the
PFC and inhibits the PWM if this voltage on V
FB
is less
than its nominal 2.5V. Once this voltage reaches 2.5V,
which corresponds to the PFC output capacitor being
charged to its rated boost voltage, the soft-start
commences.
PWM Control (RAMP 2)
In addition to its PWM current limit function, RAMP 2 is
used as the sampling point for a voltage representing the
current in the primary of the PWM’s output transformer.
This voltage may be derived either by a current sensing
resistor or a current transformer.
Soft Start
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 25μA
supplies the charging current for the capacitor, and start-
up of the PWM begins at 1.25V. Start-up delay can be
programmed by the following equation:
C
t
A
V
SS
DELAY
=
×
25
125
.
μ
(6)
where C
SS
is the required soft start capacitance, and
t
DELAY
is the desired start-up delay.
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
least 5ms.
Solving for the minimum value of C
SS
:
C
ms
5
A
V
nF
SS
=
×
=
25
.
100
μ
Generating V
CC
The ML4801 is a voltage-fed part. It requires an external
15V±10% or better Zener shunt voltage regulator, or some
other V
CC
regulator, to maintain the voltage supplied to
the part at 15V nominal. This allows a low power
dissipation while at the same time delivering 13V
nominal of gate drive at the PWM OUT and PFC OUT
outputs. If using a Zener diode, it is important to limit the
current through the Zener to avoid overheating or
destroying it. This can be easily done with a single resistor
in series with the Vcc pin, returned to a bias supply of
typically 18V to 20V. The resistor’s value must be chosen
to meet the operating current requirement of the ML4801
itself (8.5mA max.) plus the current required by the two
gate driver outputs.
EXAMPLE:
With a V
BIAS
of 20V, a V
CC
limit of 16.5V (max) and
driving a total gate charge of 110nC at 100kHz (1 IRF840
MOSFET and 2 IRF830 MOSFETs), the gate driver current
required is:
I
kHz
nC
mA
GATEDRIVE
=
′
=
100
110
11
R
V
V
mA
mA
BIAS
=
-
+
=
20
75
.
165
11
180
.
The ML4801 should be locally bypassed with a 10nF and
a 1
μ
F ceramic capacitor. In most applications, an
electrolytic capacitor of between 33μF and 100μF is also
required across the part, both for filtering and as part of
the start-up bootstrap circuitry.
FUNCTIONAL DESCRIPTION
(Continued)
REV. 1.0 10/10/2000
9
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| ML4802CP | PFC/PWM Controller Combo with Green Mode |
| ML4802CS | PFC/PWM Controller Combo with Green Mode |
| ML4802IP | PFC/PWM Controller Combo with Green Mode |
| ML4802IS | PFC/PWM Controller Combo with Green Mode |
| ML4802 | PFC/PWM Controller Combo with Green Mode(綠色模式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 |
|---|---|
| ML4801 WAF UP | 制造商:Fairchild Semiconductor Corporation 功能描述: |
| ML4801CP | 功能描述:功率因數(shù)校正 IC DIP-16 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 |
| ML4801CS | 功能描述:功率因數(shù)校正 IC SOIC-16 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 |
| ML4801CSX | 功能描述:功率因數(shù)校正 IC SOIC-16 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 |
| ML4801IP | 功能描述:功率因數(shù)校正 IC DIP-16 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 |
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