ML4805
FUNCTIONAL DESCRIPTION (Continued)
Overvoltage Protection
the dominant factor in overall current loop response.
Therefore, this contouring is significantly less marked than
that of the voltage error amplifier.
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
For more information on compensating the current and
voltage control loops, see Application Notes 33, 34, and
55. Application Note 16 also contains valuable
information for the design of this class of PFC.
voltage DC output of the PFC is fed to V . When the
FB
voltage on V exceeds 2.75V, the PFC output driver is
FB
shut down. The PWM section will continue to operate. The
OVP comparator has 250mV of hysteresis, and the PFC
will not restart until the voltage at V drops below 2.5V.
Oscillator (R C )
T T
FB
The OVP trip level should be set at a level where the
active and passive external power components and the
ML4805 are within their safe operating voltages, but not
so low as to interfere with the regular operation of the
boost voltage regulation loop.
The oscillator frequency is determined by the values of R
T
and C , which determine the ramp and off-time of the
T
ML4805's master oscillator:
1
fOSC
=
(2)
(3)
Error Amplifier Compensation
tRAMP + tDEADTIME
The PWM loading of the PFC can be modeled as a
negative resistor; an increase in input voltage to the PWM
causes a decrease in the input current. This response
dictates the proper compensation of the two
transconductance error amplifiers. Figure 2 shows the
types of compensation networks most commonly used for
the voltage and current error amplifiers, along with their
respective return points. The current loop compensation is
The deadtime of the oscillator is derived from the
following equation:
FV - 1.25I
K
REF
= C ´R ´lnGV - 3.75J
tRAMP
T
T
H
REF
at V
= 7.5V:
REF
returned to V
to produce a soft-start characteristic on
tRAMP = CT ´ RT ´ 0.51
REF
the PFC: as the reference voltage comes up from zero
volts, it creates a differentiated voltage on IEAO which
prevents the PFC from immediately demanding a full duty
cycle on its boost converter.
The ramp of the oscillator may be determined using:
2.5V
tDEADTIME
=
´ CT = 455 ´ CT
(4)
5.5mA
There are two major concerns when compensating the
voltage loop error amplifier; stability and transient
response. Optimizing interaction between transient
response and stability requires that the error amplifier’s
open-loop crossover frequency should be 1/2 that of the
line frequency, or 23Hz for a 47Hz line (lowest
anticipated international power frequency). Rapid
perturbations in line or load conditions will cause the
The deadtime is so small (t
>> t
) that the
RAMP
DEADTIME
V
GND
REF
input to the voltage error amplifier (V ) to deviate from
FB
its 2.5V (nominal) value. If this happens, the
transconductance of the voltage error amplifier will
increase significantly. This increases the gain-bandwidth
product of the voltage loop, resulting in a much more
rapid voltage loop response to such perturbations than
would occur with a conventional linear gain
characteristic. The current amplifier compensation is
similar to that of the voltage error amplifier with the
exception of the choice of crossover frequency. The
crossover frequency of the current amplifier should be at
least 10 times that of the voltage amplifier, to prevent
interaction with the voltage loop. It should also be limited
to less than 1/6th that of the switching frequency, e.g.
16.7kHz for a 100kHz switching frequency.
PFC
OUTPUT
18
1
IEAO
VEAO
VEA
V
FB
IEA
17
-
+
-
+
-
2.5V
AC
+
I
2
4
3
GAIN
MODULATOR
V
RMS
I
SENSE
There is also a degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
perturbations. However, the boost inductor will usually be
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
