Voltage Regulators
s
Application Notes (continued)
[1] Operation descriptions (continued)
AN8027, AN8037
4.
Power supply output control system (I
FB
: feedback)
Constant-voltage control of the power supply output is performed by changing the on- and off-period of the
power MOSFET. The on- and off-period are controlled with the photocoupler connected to the FB pin (Pin 9). The
photocoupler adjusts the current at the FB pin according to the signal that is output from the output voltage detection
circuit on the secondary side and changes the V
FB
voltage. Refer to figure 4.
The higher the AC input voltage is and the lower the load current is, the higher the current flow from FB pin,
the higher the V
FB
voltage, and the shorter the on-period (or longer the off-period in standby mode) will be.
Note that a current cancellation capacity of about 200
µA
is provided to compensate for the dark current of the
photocoupler. Refer to figure 5.
AN8027, AN8037
7.1 V
1:5
200
µA
typ.
V
FB
20 kΩ
Soft start current
is discharged
when the IC is
not in operation.
Power output on
secondary side
PC
A minimum of 1 mA
is required for biasing
the AN1431T/M.
9
FB
PC
Primary side
AN1431T/M
Secondary side
Figure 4. Power output control
V
FB
(V)
5
4
3
2
Dark current
1
0
0
−
0.2
−
0.4
−
0.6
−
0.8
−1.0
I
FB
(mA)
Figure 5. Feedback current vs. V
FB
characteristics
5.
Soft start
When a power supply is started, it starts up in an overload state due to the capacitor connected to the power
supply output. At that moment, the output voltage is low. Therefore, the IC in usual voltage regulation tries to start
up the power output circuit at a maximum duty cycle. Although the IC tries to limit the current by the pulse-by-
pulse overcurrent protection at the CLM pin, the pulses cannot be suppressed to zero due to delays caused by the
filter. As a result, high current flows to the main switch (i.e., power MOSFET) and to the diode on the secondary
side. Therefore, in the worst case, these components are totally broken. To prevent this, soft start is used to
suppress inrush currents at power supply startup.
13
PANASONIC [ PANASONIC SEMICONDUCTOR ]
