FA5640 Datasheet, PDF (34/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
11. Precautions for pattern design
(1) Precautions for pattern design
In order to prevent the malfunction of the control IC (unstable voltage, unstable waveform, latch-off, etc.) caused by the surge
voltage (noise) when a current is applied to the pattern on the minus side because of a principal current, a lightning surge test,
an AC line surge test, and a static electricity test, consider the following contents when designing the pattern.
The power supply has the following current paths:
1) A principal current applied from the electrolytic capacitor to the primary winding of the transformer, the MOSFET, and the
current sensing resistor after AC power supply rectification
2) A rectified current applied from the auxiliary winding of the transformer to the electrolytic capacitor; a drive current applied
from the electrolytic capacitor to the control IC and the MOSFET gate.
3) A control current of the control IC for output feedback or the like
4) Filter and surge currents applied between the primary and secondary sides
▪ Separate the patterns on the minus side in 1) to 4) to avoid interference from each other.
▪ To reduce the surge voltage of the MOSFET, minimize the loop of the principal current path.
▪ Install the electrolytic and film capacitors between the VCC pin and the GND in a closest position to each pin in order to
connect them at the shortest distance.
▪ Install the filter capacitors for the FB, IS, and ZCD pins and the like in a closest position to each pin in order to connect them
at the shortest distance. Especially, connect the patterns on the negative side of the FB and IS pins to the GND pin of the IC,
separately from other patterns, keeping the wiring as short as possible.
▪ Avoid installing the control circuit and pattern with high impedance directly below the transformer.
AC Input
Principal current
1 _Output
4
CN2
Drive current
6
8
7
6
5
VH (NC) VCC OUT
5
LAT FB
1
2
IS GND
3
4
Control current
Filter and surge
current
Fig.34 Pattern design image
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
34
http://www.fujielectric.co.jp/products/semiconductor/