FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 22

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

9. Advice for designing
(1)Start up
To properly start or stop the power supply, a capacitor
having appropriate capacitance must be selected.
Fig.17 shows the VCC voltage at the time of startup when
an appropriate capacitor is connected.
When the power is turned on, the capacitor of the VCC is
charged with the current supplied from the startup circuit,
and the voltage increases.
When the VCC reaches the ON threshold voltage, the IC
starts operating. The IC is operated based on the voltage
supplied from the auxiliary winding. Note that during the
period immediately after startup until the voltage of the
auxiliary winding starts up, the VCC decreases. Select a
capacitor for the VCC that does not allow the VCC to
decrease down to the OFF threshold voltage.
Specifically, a VCC pin capacitor whose OFF threshold
voltage is 11V or higher is recommended.
If the capacitance of the VCC pin is too small, VCC
decreases to lower than the OFF threshold voltage before
the voltage of the auxiliary winding starts up as shown by
Fig.18. In this case, the VCC repeats up/down operation
between
consequently the power supply cannot be turned on.
AN-071E Rev.1.2
April-2011
Fuji Electric Co., Ltd.
ON
Fig.17 VCC pin voltage at startup
Fig.18 VCC pin voltage at startup (when
capacitance is too small)
and
OFF
threshold
Select a capacitor whose
capacitance does not allow VCC
voltage to decrease down to
VCCoff.
Auxiliary winding voltage
voltages,
Time t
and
FA5626
(2) VCC hold time
To prevent the VCC pin voltage from decreasing to lower
than the UVLO OFF threshold voltage due to sudden load
change and other reasons, it may be desirable that the
capacitance of the capacitor to be connected to the VCC
pin be made larger.
However, if the capacitance of the capacitor of the VCC pin
is increased, the startup time is made longer.
In such cases, the circuit shown in Fig.19 can balance the
capacitance and the startup time.
By setting C1 to less than C2, the startup time can be kept
short. Since current is supplied via C2 after startup, the
VCC pin voltage hold time can be kept long even under
sudden change conditions.
(3)
To adjust switching speed and prevent vibration of the gate
pin, a resistor is connected between the MOSFET gate pin
and the OUT pin of the IC in general.
In some cases, driving current for turning on the MOSFET
and that for turning it off are required to be determined
separately.
In this case, connect a gate drive circuit shown in Fig.20 or
21 between the gate pin of the MOSFET and the OUT pin.
In Fig.20, the current is limited by R1 and R2 when the
power is turned on, while the current is limited only by R2
when it is turned off.
In Fig.21, the current is limited only by R1 when the power
is turned on, while the current is limited by R1 and R2
connected in parallel when the power is turned off.
22
Gate drive circuit
Fig.21Gate drive circuit (2)
Fig.20 Gate drive circuit (1)
Fig.19 VCC circuit
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