an8027 Panasonic Corporation of North America, an8027 Datasheet - Page 15

• Manufacturer Part Number: an8027
• Description: Ac-dc Switching Power Supply Control Ic With Standby Mode
• Manufacturer: Panasonic Corporation of North America

Voltage Regulators
[1] Operation descriptions (continued)
Application Notes (continued)
Soft start (continued)
Notes on the feedback control
negative voltages lower than the ground level are applied to
their pins. (Except for special applications.) This is because
parasitic device operations may be induced when negative volt-
ages are applied due to the structure of ICs themselves.
is turned off, the power MOSFET drain-to-source voltage,
V
chattering that occurs at this time is superposed on V
through the parasitic capacitance Cgd between the power
MOSFET gate and drain, and generates a negative voltage
with respect to the pin. No problems occur if the peak volt-
age, Vex, of this negative voltage does not exceed the para-
sitic device conduction voltage (about
higher input voltages and for larger leakage inductance in the
transformer used. Also, the influence of this phenomenon be-
comes more noticeable for the larger Cgd of the power
MOSFET used, and the Vex peak value also increases. If the
parasitic device conduction voltage is exceeded, then, in this
IC, the parts of the circuit around the feedback circuit (FB)
(in particular, the FB discharge circuit) are influenced. This
can cause momentary drops in the FB pin voltage (the control
voltage), and as a result increase the FB current I
does not allow the drive pulse on-period T
It may also prevent stabilization of the circuit. These are symp-
toms of the case described here. (Refer to figures 9 and 10.)
DS
Effective period for the soft start
calculated by the following formula.
[Approximate formula]
constant ( R3 C4), resulting in decrease of the charge current I
feedback by the photocoupler.
Soft start at the re-startup
the FB pin in order to ensure the soft start at the re-startup.
If the IC output pin (pin 6) falls to a negative voltage lower than that of the GND pin,
the startup operation may fail or the output oscillation may become unstable.
ICs in general, not just this IC, do not respond well when
In the case mentioned above, when the IC output (V
However, the amplitude of the chattering is larger for
, jumps from a low voltage to a high voltage. The voltage
The effective period for the soft start with the connection of R3 and C4 to the FB pin is approximately
t
It is thought that the effectiveness of the above formula is weakened because of the following reason:
The voltage difference between both ends of C4 rises up to 63% of the voltage at the FB pin within the time
Note that if you increase the capacitance value of C4 unnecessarily, it would decrease the sensitivity of the
This IC includes a discharge circuit to instantly discharge charged electrons in the capacitor connected to
Conditions for the operation of the discharge circuit are as follows:
1) V
2) V
SS
R3
CC
CC
has become the stop voltage or below. (at the normal operation)
has become the OVP reset threshold voltage or below. (at the OVP operation)
C4 (s)
0.7 V).
ON
to be increased.
FB
and thus
OUT
OUT
)
V
V
FB
OUT
DS
Rectified AC
FB
, although it depends on the value of R3.
GND
FB
Out
Figure 10
Figure 9
AN8027, AN8037
Cgd
Vex
GND
Control
voltage
V
DS
15