an8027 Panasonic Corporation of North America, an8027 Datasheet - Page 22

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

AN8027, AN8037
22
[1] Operation descriptions (continued)
13. Overvoltage protection (OVP) circuit
Application Notes (continued)
1) If, as a result of an abnormality in the power supply output, the voltage input to the V
2) If the SD pin input voltage exceeds a threshold value of 3.9 V typical, the IC internal reference voltage is shut
3) OVP is reset in two methods. One is to decrease the output voltage on the secondary side so that the V
shuts down the power supply to protect the load if a voltage that is significantly and abnormally higher than the
normal output voltage occurs in the power supply output, due to, for example, a malfunction in the control system
or an abnormal voltage applied externally. (Refer to figure 22.)
monitored. Since the V
proportional to the secondary side output voltage. Thus the overvoltage protection circuit operates when an
overvoltage occurs in the secondary side output.
OVP stands for overvoltage protection. The overvoltage protection circuit is a self-diagnostic function that
Basically, the overvoltage protection circuit should be designed so that the V
threshold value (20.5 V typical), the IC internal reference voltage is shut down, and all control operation is
stopped. The IC then holds this state. If latching at a voltage lower than the threshold value is required,
connect a Zener diode between the V
voltage.
down, and all control operation is stopped. The IC then holds this state.
voltage is less than the OVP reset voltage, i.e., 8.1 V typical. The other is to decrease the SD pin input voltage
(V
V typical, however, the IC will be in remote operation to shut off the output.
The IC incorporates a circuit that discharges a constant current of approximately 25 mA from the capacitor
connected to the SD pin for the re-start operation. The operation of this circuit is stopped when the SD pin
voltage drops to 1.5 V or below. Refer to figure 21.
The following is a formula to allow the IC to be in OVP operation with the SD pin in the above method 2).
V
V
V
V7
OVP reset conditions
CC
SD
SD
th(OUT)
) to a value less than the SD reset voltage, i.e., 0.7 V typical, by external resetting. If V
OVP reset voltage
SD reset voltage
V
V
V
V
th(OUT)
TH-SD1
Z
TH-SD1
V
V
CC
CC
discharging conditions
V
(OUT)
: Secondary side output overvoltage threshold value
: SD operating threshold value
: Zener voltage (externally connected to OVP pin)
Timer capacitor
U.V.L.O. stop voltage
OVP reset voltage
OVP
V
Latch
Shut-
down
Z
CC
Secondary side output voltage under normal operation
pin voltage is normally supplied from the transformer bias winding, this voltage is
3.9 V typ.
V
V
CC
OVP-SD
AN8027, AN8037
pin voltage under normal operation
CC
and SD pins as shown in figure 22 and take method 2) below to set the
Timer capacitor
discharging
transistor
20.5 V typ.
V
OVP-VCC
Figure 21
On
V
GND
SD
CC
CC
V7
Voltage Regulators
pin voltage of the IC can be
Z
C
OVP
TIM
CC
SD
pin exceeds the
is less than 0.1
D
B
CC
NB
pin