FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 25

• Manufacturer Part Number: FA5626
• Description: Fuji Switching Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

(9) Approximation of over current detection
intensity
The expression here explains how to calculate the over
current detection as below.
Case where only RC filter is connected (See Fig. 25)
On IS pin voltage, offset voltage Voffset1 by IS pin input
bias current of 45 A (IS = –0.5 V) appears at resistor R7.
In this case, the current limit voltage at resistor Rs is as
follows,
Rs voltage at OCP = VthIS1 + Voffset1
Fig.30 IS pin voltage when RC filter only is connected.
Example)
When AC input voltage Vin is a minimum, the primary
current will be a maximum.
However, in actual circuit, output current at OLP shows
tendency to be slightly larger than calculated current
because of the propagation delay in IC, etc. Please decide
Rs value after test in the actual circuit.
AN-071E Rev.1.2
April-2011
D
ILp
Example)Vin=85V,Np=28T,Ns=5T,Lp=340uH,fsw=65kHz,
η=0.9,Vo=19V,Po=100W,R7=1kΩ
Rs = 0.17  W has to be connected.
D
ILp
Rs



D : Duty , Vin :AC Input voltage (rms)
Np : Turn Number of primary winding
Ns : Turn Number of secondary winding
Vo : Output voltage
Po : Output power (overload detection power)
η : Efficiency
Fsw : Switching frequency
Lp: Primary side inductance

Fuji Electric Co., Ltd.



2
0
2

5 .
2
VthIS

2
Vin
Np

Ns

Vin

Po
85
28
(
5
85



1

45


Vo
Np
Ns
100


D
19
28
u
. 0
5
Voffset



47


Vo
k 1

19

/ )
0
2
1

9 .
2
. 3

. 0
/

Lp
24
ILp

47
Vin
2


2

fsw

. 0

340
D
85
168
u


. 0
65
47
k

. 3
24
FA5626
Case where input voltage dependency reducing
resistor R9 is connected (as in Fig. 27)
If an input voltage dependency reducing resistor is
connected, IS pin voltage changes as shown in Fig. 31.
In this case, the overload detection voltage level that
appears at IS pin is as follows,
Rs voltage at OCP =VthIS1 - Voffset2
Example 1)R7=1kΩ、R9=330kΩ、Vaux2=-20V
Voffset2=Voffset1-((Vaux2/R9)×R7)
Rs voltage at OCP= -0.5V - (15.6mV)= -0.516V
Example 2)R7=1kΩ、R9=680kΩ、Vaux2=-20V
Voffset2=(-45uA×1k) - ((-20V/680k)×1k)= -15.6mV
Rs voltage at OCP =- 0.5V –(-15.6mV)= -0.484V
Fig. 31 IS pin voltage when correction resistor R9 is
Case where R8 for input power improvement at light
load is connected (See Fig. 29)
If an input power improvement at light load is applied, the
waveform of IS pin voltage changes as shown in Fig. 32.
In this case, the current limit voltage at Rs is as follows,
Rs voltage at OCP =VthIS1 + Voffset1 + Voffset3
Example)R7=1kΩ、R8=1.0MΩ、VCC=18V
Voffset1=-45mV
Voffset3=-(18/1.0MΩ)×1k=-0.018V
Rs voltage at OCP = -0.5V - 45mV - 0.018V
Fig. 32 IS pin voltage when correcting resistor R8 is
=(-45uA×1k) - ((-20V/330k)×1k)= 15.6mV
25
connected.
connected.
http://www.fujielectric.co.jp/products/semiconductor/
= -0.563V