FA5604 Datasheet, PDF (27/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(5)The correct overcurrent limiting protection
The output power at overvoltage is limited by the overcurrent limiting function of the IC. However, in general, the
characteristics that the voltage is reduced while the current is increased is shown as indicated in “(1) Not corrected”
in Fig. 27.
For this IC, as shown in Fig. 28, it is possible to correct the overcurrent limit in order to obtain the output
characteristics like (2) by smoothing the OUT terminal voltage by R4 and C6 and apply it to the VF terminal for the
forward circuit, or by applying the VCC voltage to the VF terminal for the flyback circuit. In addition, it is possible to
obtain the output characteristics like (3) by adding R5 and D3 to correct it
(1)Without correction
OUT
FA5604 ⁵
VCC
6
FA5605/06
(3)With correction (2)With correction
(+R5,D3)
7
VF
D3
R4
R5
7
VF
R4
C6
R6
C6
Output current
Fig.27
Fig.28
Note that if correcting the drooping characteristics at the power supply for which a hiccup operation is used, the
power supply may not start because a hiccup operation is started at the startup depending on the CS terminal
capacity (soft start) and the drooping correction constant (Fig. 29).
If the power supply starts normally
If the power supply cannot starts normally
Fig.29
Because this IC adopts the frequency reduction method in which only the off period is extended, if the frequency is
reduced at overload, the ON duty is also reduced, and therefore the output voltage is reduced. Consequently, the
load current is reduced at the point in which it enters the drooping characteristics at overload (see Fig. 5 on p. 16).
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
27
http://www.fujielectric.co.jp/products/semiconductor/