FA5640 Datasheet, PDF (32/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
With the method shown in Fig. 32, the VH pin is connected
after the AC line voltage is rectified and smoothed. The
startup time of this method is the shortest of the three
connection methods. However, since the voltage charged in
the bulk capacitor is applied to the VH pin even if the AC
line voltage is interrupted after the IC enters the latch-off
mode, longer time is required to reset the latch-off mode.
Note that several minutes are required to reset the latch-off
mode after the AC line is interrupted, although the duration
depends on the operating conditions.
If power is turned on, the capacitor connected to the VCC
pin is charged due to the current supplied from the startup
circuit to the VCC pin via the VH pin, and the VCC voltage
increases. When the ON threshold voltage of 14 V of the
low-voltage malfunction prevention circuit (UVLO) is
exceeded, the internal supply is started to operate the IC. If
VCC is not supplied from the auxiliary winding, the startup
circuit is stopped. Meanwhile, if power is not supplied from
the auxiliary winding, the current supplied from the startup
circuit is used for the normal operation of the IC. If VCC is
supplied only from the startup circuit, without the supply
from the auxiliary winding, the standby power increases,
and the heating of the IC may increase. Consequently, to
keep the standby power at low level, it is desirable to supply
VCC from the auxiliary winding.
At the same time, if the startup circuit only is used for
startup, there is a limit in current to be supplied.
Consequently, the MOSFET to be driven must be selected
carefully.
The current fed from the VH pin to the VCC pin is
approximately 8 mA when VCC = 6.5 V. Note that when
VCC = 0 V, the current decreases to 0.7 mA to cope with
abnormal state such as short circuit between pins.
The threshold level for switching based on input voltage
include the pulse shutdown FB voltage, which is related to
the burst operation, and the maximum input threshold
voltage, which is the overload limit level of the IS pin.
VH ₈
起動回路
start
on/off信号
VCC
6
Fig.32 VH pin circuit (3)
Advice for designing
(1) Startup resistor
To prevent damage to the IC due to surge voltage of the AC
line, it is recommended to connect a startup resistor whose
resistance within the 2 k to 10 k range to the VH pin in
series.
Startup time or startup voltage cannot be adjusted using
this startup resistor. Note that a resistor having too large
resistance may result in inability to startup.
(2) To supply power directly to the VCC pin
To supply power directly to the VCC pin without using a VH
pin, short-circuit the VH and the VCC pins.
If the VH pin is connected to the GND or used in open state,
leakage current may be generated.
(ⅱ) The peak voltage of the line voltage is detected to
subject it to high/low line voltage compensation.
• Connection method
The same as the method of supplying startup current in (i)
• Operation
If voltage after rectification is input to the VH pin, each
threshold level is switched at 226 V when the VH pin
voltage is increasing, and 212 V when it is decreasing. If
half-wave and full-wave rectification waveforms are input, it
is switched at 160 Vrms. The input detection switching
delay time is 30 ms.
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
32
http://www.fujielectric.co.jp/products/semiconductor/