FA5546 Datasheet, PDF (14/27 Pages) Fuji Electric – Switching Power Supply Control IC

FA5546/47
8. Operation of each block
(1) Startup circuit
The IC integrates a startup circuit having withstand voltage
of 500V to achieve low power consumption.
Fig.1 to Fig.3 show connections.
Turning on the power, capacitor C2 connected to the VCC
terminal is charged and the voltage increases due to the
current fed from the startup circuit to the VCC terminal. If
the ON threshold voltage (Vcc = 18V typ) of the
under-voltage lockout circuit (UVLO) is exceeded, the
power for internal operation is turned on, and the IC starts
operating.
The current supplied from the VH terminal to the VCC
terminal is approximately 6mA when VCC = 0V. As the VCC
voltage increases, the supply current decreases and
reaches 4.9mA at the startup voltage.
After IC operation is started, the startup circuit is shut down
to minimize power loss. The current flowing into the VH
terminal is 130A (typ) at VH = 450Vdc.
A resistor, RVH (2k to 10k), is connected in series to the
VH terminal to prevent the IC from being damaged by the
surge voltage of the AC line.
Fig.1 shows a typical connection where the VH terminal is
connected to the half-wave rectifier circuit of AC input
voltage.
The startup time of this connection is the longest in 3 types
of connection. However, if AC input voltage is shut down
after the IC enters the latch mode, the supply of current
from the VH terminal is interrupted by overload or
overvoltage protection, which allows the latch mode to be
reset in a time as short as several seconds.
Fig.2 shows the connection where the VH terminal is
connected to the full-wave rectifier circuit of AC input
voltage. The startup time of this connection is
approximately half of the connection shown in Fig.1, and its
latch mode reset time can be made as short as that of
connection shown in Fig.1 by interrupting the AC input
voltage.
Fig.3 shows the connection where the VH terminal is
connected to the back of rectification and smoothing of AC
input voltage. The startup time of this connection is the
shortest in 3 types. In this connection, however, even if the
AC input voltage is shut down after the IC enters the latch
mode, the voltage charged in C1 is kept impressed to the
VH terminal, requiring much time for the latch mode to be
reset. It takes approximately several minutes to reset the
latch mode, although the time varies depending on
conditions.
After the startup, the IC goes into switching operation, and
is operated with the power supplied from the auxiliary
winding. While the brownout function is working in a state in
which the AC input voltage is low, the VCC voltage is kept
within the 15.5V to 18V (typ) range by the ON/OFF control
of the startup circuit. If the overload or overvoltage
Fuji Electric Co., Ltd.
AN-027E Rev.0.4
April -2011
protection is actuated, causing the IC to enter the latch
mode, the one time clamp circuit discharges the VCC
voltage immediately down to 14.5V (typ). This function
interrupts the AC input voltage and shortens the latch mode
reset (by the UVLO reset function) time. Then the startup
circuit is subjected to ON/OFF control to maintain the VCC
voltage within the 12V to 13V (typ) range. The VCC voltage
of the auto recovery type is maintained within the 15.5V to
18V (typ) range by the ON/OFF control of the startup circuit
during the auto reset wait time after the overload shutdown.
C1
Startup
circuit
current
RVH
VH
8
Startup
circuit
control
signal
start
VCC
6
C2
Fig.1 Startup circuit 1 (Half-wave)
Fig.2 Startup circuit 2 (Full-wave)
Fig.3 Startup circuit 3 (Rectification)
14
http://www.fujielectric.co.jp/products/semiconductor/