ml4827cs-2 Fairchild Semiconductor, ml4827cs-2 Datasheet - Page 9

• Manufacturer Part Number: ml4827cs-2
• Description: Fault-protected Pfc And Pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Error Amplifier Compensation
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 net-
works most commonly used for the voltage and current error
amplifiers, along with their respective return points. The cur-
rent loop compensation is returned to V
start characteristic on 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.
There are two major concerns when compensating the volt-
age loop error amplifier; stability and transient response.
Optimizing interaction between transient response and sta-
bility 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 fre-
quency). The gain vs. input voltage of the ML4827’s voltage
error amplifier has a specially shaped nonlinearity such that
under steady-state operating conditions the transconductance
of the error amplifier is at a local minimum. Rapid perturba-
tions in line or load conditions will cause the input to the
voltage error amplifier (V
nal) value. If this happens, the transconductance of the volt-
age error amplifier will increase significantly, as shown in the
Typical Performance Characteristics. This raises 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.
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop perturbations.
However, the boost inductor will usually be the dominant
factor in overall current loop response. Therefore, this con-
touring is significantly less marked than that of the voltage
error amplifier.
For more information on compensating the current and volt-
age control loops, see Application Notes 33 and 34. Applica-
tion Note 16 also contains valuable information for the
design of this class of PFC.
REV. 1.0.1 6/27/01
FB
) to deviate from its 2.5V (nomi-
REF
to produce a soft-
Oscillator (RAMP 1)
The oscillator frequency is determined by the values of R
and C
lator output clock:
The deadtime of the oscillator is derived from the following
equation:
at V
The deadtime of the oscillator may be determined using:
The deadtime is so small (t
ating frequency can typically be approximated by:
EXAMPLE:
For the application circuit shown in the data sheet, with the
oscillator running at:
Solving for R
ponents values, C
The deadtime of the oscillator adds to the Maximum PWM
Duty Cycle (it is an input to the Duty Cycle Limiter). With
zero oscillator deadtime, the Maximum PWM Duty Cycle is
typically 45% for the ML4827-1. In many applications of the
ML4827-1, care should be taken that C
large as to extend the Maximum Duty Cycle beyond 50%.
This can be accomplished by using a stable 470pF capacitor
for C
REF
T
T
.
, which determine the ramp and off-time of the oscil-
= 7.5V:
t
t
f
t
f
t
RAMP
DEADTIME
OSC
RAMP
RAMP
T
OSC
x C
f
OSC
T
=
=
T
=
= 470pF, and R
=
=
yields 2 x 10
--------------- -
t
C
100kHz
C
RAMP
C
=
T
T
1
T
=
×
-------------------------------------------------- -
t
×
×
RAMP
----------------- -
5.1mA
R
R
2.5V
RAMP
R
T
T
T
×
×
×
=
0.51
+
In
0.51
>> t
--------------- -
t
×
-4
RAMP
t
1


DEADTIME
. Selecting standard com-
T
V
------------------------------- -
V
C
1
= 41.2kΩ.
DEADTIME
REF
REF
T
=
=
1
T
490
×
not be made so
–
–
1.25
3.75
10
×
–
) that the oper-
5
C


T
ML4827
T
(2)
(3)
(4)
(5)
9