FA5604 Datasheet, PDF (25/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(2)Determining the VCC capacitor value
To properly start the power supply, a certain value is required for the value is required for the capacitor connected to
the VCC pin.
Fig.20 shows the VCC voltage at startup when a proper value is given to the capacitor. When the input power is
turned on, the capacitor connected to the VCC pin is charged via the startup resistor and the voltage increases. The
IC is then in standby start and almost no current is consumed. (ICC < 2A) Thereafter, VCC reaches the OFF
threshold voltage of UVLO (VCCON) and the IC begins operation. When the IC begins operation to make output, the
IC operates based on the voltage from the auxiliary winding. When the IC is just starting up, however, it takes time
for the voltage from the auxiliary winding to rise enough, and VCC drops during this period.
Determine the VCC capacitor value so that VCC will not drop down to the ON threshold voltage of UVLO (VCCOFF)
during this period. When the VCC capacitor is too small, VCC will drop down to the ON threshold voltage of UVLO
before the auxiliary winding to rise. It becomes impossible to startup the power supply as VCC repeats the top and
bottom between the VCCOFF and VCCON in this case (Fig.21).
VCC
VCC
ON
VCC
OFF
VCC voltage at startup
With a adequate capacitor
VCC must not
Drop to VCCOFF
Auxiliary
Winding voltage
time
Fig.20
VCC
VCC
ON
VCC voltage at startup
With a inadequate capacitor
VCC
OFF
time
Fig.21
(3)The startup period
In the case of the circuit in Fig. 22 (R1: starting resistance), the starting time Tst [ms] until the IC is started after the
power is turned on is determined by the following approximate formula:
Tst

C
2

R1
ln_1

17.5


···················(7)
 ^V1 
In the case of the circuit in Fig. 23, the starting time Tst [ms] until the IC is started after the power is turned on is
determined by the following approximate equation:
Tst

C2 
R0

ln_1 
17.5


 ^Vvcc 
···················(8)
R0  ^{R1 R2}
R2
Vvcc 
V1
R1  R2
R1  R2
 2 Vac

   (If the starting resistor is before rectification)          (9)
V1  _ 


2 Vac    (If the starting resistor is afte rectification)              (10)
Where, R1: starting resistance [kΩ], C2: the capacitor between the VCC and the GND [uF], Vac: an effective value
of the AC input voltage [V]
R1 _D1
R1
D1
VCC
6
FA5604/05/05
C2
▽
VCC
C2
6
FA5604/05/06 ^{R2 ▽}
Fig.22
Fig.23
The above formulas are intended for approximate calculation of the starting time. Please note that the calculation
results may not actually be the same as measured values because of the startup current or the like.
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
25
http://www.fujielectric.co.jp/products/semiconductor/