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ML4800 データシート(PDF) 8 Page - Fairchild Semiconductor

部品番号 ML4800
部品情報  Power Factor Correction and PWM Controller Combo
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メーカー  FAIRCHILD [Fairchild Semiconductor]
ホームページ  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

ML4800 データシート(HTML) 8 Page - Fairchild Semiconductor

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ML4800
8
REV. 1.0.2 3/7/2001
The output of the gain modulator is a current signal, in the
form of a full wave rectified sinusoid at twice the line
frequency. This current is applied to the virtual-ground
(negative) input of the current error amplifier. In this way
the gain modulator forms the reference for the current
error loop, and ultimately controls the instantaneous
current draw of the PFC from the power line. The general
form for the output of the gain modulator is:
More exactly, the output current of the gain modulator is
given by:
where K is in units of V-1.
Note that the output current of the gain modulator is
limited to 500
µA.
Current Error Amplifier
The current error amplifier’s output controls the PFC duty
cycle to keep the average current through the boost
inductor a linear function of the line voltage. At the
inverting input to the current error amplifier, the output
current of the gain modulator is summed with a current
which results from a negative voltage being impressed
upon the ISENSE pin. The negative voltage on ISENSE
represents the sum of all currents flowing in the PFC
circuit, and is typically derived from a current sense
resistor in series with the negative terminal of the input
bridge rectifier. In higher power applications, two current
transformers are sometimes used, one to monitor the ID of
the boost MOSFET(s) and one to monitor the IF of the
boost diode. As stated above, the inverting input of the
current error amplifier is a virtual ground. Given this fact,
and the arrangement of the duty cycle modulator polarities
internal to the PFC, an increase in positive current from
the gain modulator will cause the output stage to increase
its duty cycle until the voltage on ISENSE is adequately
negative to cancel this increased current. Similarly, if the
gain modulator’s output decreases, the output duty cycle
will decrease, to achieve a less negative voltage on the
ISENSE pin.
Cycle-By-Cycle Current Limiter
The ISENSE pin, as well as being a part of the current
feedback loop, is a direct input to the cycle-by-cycle
current limiter for the PFC section. Should the input
voltage at this pin ever be more negative than -1V, the
output of the PFC will be disabled until the protection flip-
flop is reset by the clock pulse at the start of the next PFC
power cycle.
TriFault DetectTM
To improve power supply reliability, reduce system
component count, and simplify compliance to UL 1950
safety standards, the ML4800 includes TriFault Detect. This
feature monitors VFB (Pin 15) for certain PFC fault
conditions.
In the case of a feedback path failure, the output of the
PFC could go out of safe operating limits. With such a
failure, VFB will go outside of its normal operating area.
Should VFB go too low, too high, or open, TriFault Detect
senses the error and terminates the PFC output drive.
TriFault detect is an entirely internal circuit. It requires no
external components to serve its protective function.
Overvoltage Protection
The OVP comparator serves to protect the power circuit
from being subjected to excessive voltages if the load
should suddenly change. A resistor divider from the high
voltage DC output of the PFC is fed to VFB. When the
voltage on VFB exceeds 2.75V, the PFC output driver is
15
VEAO
IEAO
VFB
IAC
VRMS
ISENSE
RAMP 1
OSCILLATOR
OVP
PFC ILIMIT
TRI-FAULT
2.5V
+
+
16
2
4
3
VEA
7
+
IEA
1
+
+
PFC OUT
12
S
R
Q
Q
S
R
Q
Q
2.75V
–1V
+
0.5V
1.6k
1.6k
GAIN
MODULATOR
Figure 1. PFC Section Block Diagram
FUNCTIONAL DESCRIPTION (Continued)
I
IVEAO
V
V
GAINMOD
AC
RMS
=
×
×
2
1
(1)
More exactly, the output current of the gain modulator is
given by:
IK
VEAO
V
I
GAINMOD
AC
×
(.
)
0 625


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