an8029 Panasonic Corporation of North America, an8029 Datasheet - Page 12

• Manufacturer Part Number: an8029
• Description: Self-excited Rcc Pseudo-resonance Type Ac-dc Switching Power Supply Control Ic
• Manufacturer: Panasonic Corporation of North America

AN8029
12
[2] Design reference data
Application Notes (continued)
How to start the soft start function by external parts
In this condition, since the voltage of the power supply output is low, the normal constant voltage control attempts
to rise the power supply output at the maximum duty. The control uses the pulse-by-pulse overcurrent protection
(CLM) , attempting to limit the current. However, the pulse can not be brought down to zero due to delay of filter,
etc. As a result, a large current flows into the main switch (the power MOSFET) or the diode in the secondary
side, and in the worst case these parts are damaged. For this reason, the soft start function is used to suppress the
rush current at start of the power supply.
(pin 5) to use the soft start function. When the supply voltage of the IC reaches the start voltage and the start
circuit begins operation, the open bias is outputted to the FB terminal.
become flowing-out current of the FB terminal (I
output control begins under the condition in which T
period is short. The I
since it changes proportionally to the charge current of the
C4. The CLM circuit operates, depending on the sum of
the R7 end-to-end voltage by the current which flows in
the power MOSFET at power-on and the R6 end-to-end
voltage by the charge current of the C4.
MOSFET at power-on gradually increases, the rush cur-
rent can be suppressed.
The power supply rises under overload condition due to the capacitor connected to the power supply output.
As shown in figure 12, the R3 and C4 are connected between the FB terminal (pin 9) and the GND terminal
By this voltage, the charge current flows in the C4 to
Therefore, since the current which flows in the power
FB
decreases with elapse of time,
FB
) and
ON
After AC rectification
GND
V
CLM
V
OUT
FB
CC
Start resistor
Voltage Regulators
R8
Figure 12
R1
R3
C4
C6
C8
R6
R7