ML4804
FUNCTIONAL DESCRIPTION
(Continued)
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
I
SENSE
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 I
SENSE
pin.
Cycle-By-Cycle Current Limiter
The I
SENSE
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 Detect
TM
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 I
SENSE
pin. The negative voltage on I
SENSE
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 I
D
of
the boost MOSFET(s) and one to monitor the I
F
of the
16
VEAO
IEAO
OVP
+
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:
I
GAINMOD
=
I
AC
×
VEAO
×
1V
V
RMS 2
(1)
More exactly, the output current of the gain modulator is
given by:
I
GAINMOD
=
K
×
(VEAO
−
0.625V )
×
I
AC
where K is in units of V
-1
.
Note that the output current of the gain modulator is
limited to 500µA.
To improve power supply reliability, reduce system
component count, and simplify compliance to UL 1950
safety standards, the ML4800 (ML4804) 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.
1
TRI-FAULT
0.5V
+
–
2.75V
–
VCCOVP
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
RAMP 1
7
OSCILLATOR
R
Q
GAIN
MODULATOR
1.6kΩ
VEA
–
+
–
–
VCC
1.6kΩ
+
+
+
–
IEA
16.4V
S
–1V
+
–
Q
Q
PFC OUT
Q
12
R
S
PFC ILIMIT
Figure 1. PFC Section Block Diagram
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
