FA5640 Datasheet, PDF (25/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
10. How to use each pin and advice for designing (The values that appear in the following description are typical
values, unless otherwise specified.)
(1) Pin No. 1 (ZCD pin)
• Connection method
Function
(ⅰ) Detects the timing that MOSFET is turned on.
The same as turn on timing in (ⅰ)
• Operation
(ⅱ)Performs latch-off protection by external signals.
If the output voltage (Vo) on the secondary side enters
(ⅲ)Performs latch-off protection in case overvoltage on overvoltage state, the auxiliary winding voltage and ZCD
the secondary side.
pin voltage also increase. This IC detects ZCD pin voltage
How to use
(ⅰ) Turn on timing detection
elapsed time of 4.5 s after MOSFET is turned off, and
when the ZCD pin voltage exceed 6.0V and this states
• Connection method
Connect the auxiliary winding of the transformer via the CR
continues for 60 s or longer, latch-off operation is
performed to stop switching (Fig. 15).
circuits, R1 and C1 (Fig. 11).
Once the latch operation is started, the VCC voltage is
Be careful the polarity of the auxiliary winding.
maintained by the startup circuit to continue the latch
• Operation
operation. Decrease the VCC to the OFF threshold voltage
If the voltage of the ZCD pin decreases to 60 mV or lower, of the UVLO or lower to reset the latch operation.
the MOSFET is turned on. The auxiliary winding voltage
fluctuates significantly in both positive and negative voltage
at the time of switching. To protect the IC from this voltage
fluctuation, a clamp circuit is integrated. If the auxiliary
winding voltage is positive, current is fed as shown in Fig.
12, and if it is negative, current is fed as shown in Fig. 13, to
clamp the voltage of the ZCD pin.
RZCD
30kΩ
Cd
7.5V
ZCD
1
R1
C1
In turning ON based on bottom detection of the ZCD pin is
not possible at the time of startup, for example, restart
Fig.11 ZCD pin circuit
operation is performed using timer to forcibly turn on the
MOSFET. If the OUT is Low (MOSFET is OFF) and the
voltage of the ZCD pin is below to input threshold voltage
(Vthzcd2) 150 mV or lower, the timer starts counting, and if
the time out time from the last trigger (7.6 s or 25 s), the
MOSFET is turned on.
30kΩ
ZCD
1
7.5V
Clamp
current
Fig.12 Clamp circuit (When auxiliary winding is in
(ⅱ) Latch-off protection by external signals
positive voltage.)
• Connection method
Pull up the ZCD pin by external signals.
Figure 14 is a typical connection showing the overvoltage
ZCD
1
on the primary side. (Constants are examples. Check the
operation with the actual power supply unit.)
30kΩ
-0.8V
Clamp
current
• Operation
If the voltage of the ZCD pin exceeds 6.0 V, and this state Fig.13 Clamp circuit (When auxiliary winding is in
continues for 60 s or longer, latch-off operation is
performed to stop output switching.
Once the latch-off operation is started, the VCC voltage is
maintained by the startup circuit to continue the latch-off
operation.
Decrease the VCC to the OFF threshold voltage or lower to
reset the latch operation.
negative voltage.)
6
VCC
0.47uF
2.2k
2.2k
8.2k
24V
ZCD
1
(ⅲ) Latch-off protection at overvoltage on the secondary
＊定数は一例で、動作を
保証する値ではありません。
side
Fig.14 Primary side overvoltage protection circuit
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
25
http://www.fujielectric.co.jp/products/semiconductor/