Denon DRA-F107 Service Manual page 40

6.5V
R
FB
FB
Optocoupler
Figure 11 PWM Controlling
3.3 Soft-Start
V
SoftS
5.6V
5.3V
T
Soft-Start
Gate Driver
Figure 12 Soft-Start Phase
The Soft-Start is realized by the internal pull-up resistor
R and the external Capacitor C
Soft-Start
Figure 5). The Soft-Start voltage V
charging the external capacitor C
Soft-Start Comparator
PWM-Latch
PWM Comparator
0.8V
PWM OP
Isense
x3.65
Improved
Current Mode
t
(see
Soft-Start
is generated by
SoftS
by the internal
Soft-Start
DRA-F107 / DRA-F107DAB
pull-up resistor R
compares the voltage at pin SoftS at the negative input
with the ramp signal of the PWM-OP at the positive
input. When Soft-Start voltage V
Feedback voltage V
the pulse width by resetting the PWM-Latch (see
Figure 12). In addition to Start-Up, Soft-Start is also
activated at each restart attempt during Auto Restart.
By means of the above mentioned C
Start can be defined by the user. The Soft-Start is
finished when V
SoftS
Protection Unit is activated by Comparator C4 and
senses the FB by Comparator C3 wether the voltage is
below 4.8V which means that the voltage on the
secondary side of the SMPS is settled. The internal
Zener Diode at SoftS has a clamp voltage of 5.6V to
prevent the internal circuit from saturation (see Figure
13).
6.5V
5.6V
R
SoftS
6.5V
5.3V
4.8V
R
FB
FB
Figure 13 Activation of Protection Unit
The Start-Up time T
voltage V is settled must be shorter than the Soft-
OUT
Start Phase T
Soft-Start
C = ------------------------------------ -
Soft Start –
t
R
Soft Start
By means of Soft-Start there is an effective
minimization of current and voltage stresses on the
integrated CoolMOS™, the clamp circuit and the output
overshoot and prevents saturation of the transformer
during Start-Up.
40
Functional Description
. The Soft-Start-Comparator
Soft-Start
is less than
SoftS
the Soft-Start-Comparator limits
FB
Soft-Start
exceeds 5.3V. At that time the
Power-Up Reset
Error-Latch
Soft-Start
R
C4
G2
S
R
C3
Clock
S
PWM-Latch
within the converter output
Start-Up
(see Figure 14).
T
Soft Start –
1.69
–
the Soft-
Q
Q
Q
Gate
Driver
Q