an8021l Panasonic Corporation of North America, an8021l Datasheet - Page 12

• Manufacturer Part Number: an8021l
• Description: Ac-dc Switching Power Supply Control Ic
• Manufacturer: Panasonic Corporation of North America

AN8021L, AN8021SB
12
[2] Operation descriptions (continued)
5. Soft start
Application Notes (continued)
rise under overload condition. Under this condition, the power supply output is low. For the normal PWM control,
attempt is made to limit the current by the pulse-by-pulse overcurrent protection so that the power supply output
can rise at maximum duty. However, pulses can not be made down to zero due to circuit delay. As a result, large
current flows in the mains switch (the power MOSFET) or in the diode in the secondary side, and in the worst case
these parts are damaged.
rises with gradually widening duty from the minimum one (0%) at the power supply start is adopted.
life of parts and raise the reliability of the power supply.
PWM, three voltages are input; the voltage to which the current feedback level is converted (hereinafter referred
to as V
inside the IC), and the triangular wave oscillation voltage (hereinafter referred to as V
input in the non-reverse input ( ) of the PWM comparator and V
signals of the non-reverse input, the lowest one is selected for input to the PWM comparator.
this capacitor is set to be sufficiently discharged by the transistor inside the IC.
the constant current source inside the IC. Therefore V
at the medium voltage of the triangular wave oscillation waveform as constant voltage. Therefore, at operation
starting, the V
oscillation waveform.
V
The soft start function works only up to that point and after that the normal control comes.
SS
At start of the power supply, the capacitor connected to the power supply output causes the power supply to
For this reason, soft start function in which the power supply output does not rise with maximum duty but
The use of this function requires more rise time of power supply output. However, it can extend the service
The soft start (SS) terminal is connected to the PWM input (hereinafter its voltage is referred to as V
The external capacitor (hereinafter referred to as C
When the supply voltage exceeds the start voltage to start the IC operation, charging is started in the C
On the other hand, the V
As the result, the output of the IC generates the pulse of duty which gradually becomes large with the rise of
However, when the V
from the minimum duty. (Refer to figure 7.)
FB
), the voltage determining the maximum duty (hereinafter referred to as V
SS
is input to the PWM comparator as the lowest voltage and is compared with the triangular wave
V
V
V
V
V
CT
SS
FB
DTC
OUT
0 V
0 V
SS
FB
exceeds the V
has high voltage because the power supply output is low. And, the V
Figure 7. Soft start operation waveform
V
V
FB
CT
FB
or V
DTC
SS
, the duty of the output pulse depends on the V
SS
) is connected to the SS terminal. In the pre-start condition,
gradually rises from 0 V.
CT
is input in the reverse input ( ). Among the three
DTC
V
V
V
Voltage Regulators
DTC
). This voltage is determined
FB
SS
CT
). V
SS
, V
DTC
FB
and V
is positioned
FB
SS
or V
). In the
DTC
SS
DTC
are
by
.