STR-A6351 SanKen, STR-A6351 Datasheet - Page 9

• Manufacturer Part Number: STR-A6351
• Description: STR-A6300 Series
• Manufacturer: SanKen

STR-A6300 Application Notes
consumption shall be increased.
When the Vin terminal voltage drops lower than 10.1V(TYP.), the control operation stops by a Under Voltage
Lock Out(UVLO) function and returns to the start-up mode.
After the control circuit starts its operation, a power
supply is earned by rectifying and smoothing the voltage
of drive winding D.
Fig.3 shows the start-up voltage waveform at the
Vin terminal. The drive winding voltage does not
rise up to a set voltage immediately after the control
circuit starts falling, thanks to the operation stop
voltage set as low as 11.1V(MAX), the drive winding
voltage reaching stabilized voltage before falling to
the operation stop voltage, the control circuit continuing
its operation.
The number of windings is to be set at a normal
power supply operation so that both of the end volts
of C2 may be higher than the operation stop voltage
[Vin(OFF) 11.1V(MAX)] and lower than the OVP operating
voltage[Vin(OVP) 23.2V(MIN)].
The preferable drive winding D voltage for stabilization
is 18V approx.. However, in an actual power supply circuit,
there is a case that the Vin terminal voltage varies due to
the secondary output current as shown on Fig.4.
This is because the circuit current of STR-A6300 series
is small, the C2 being charged up to the peak value by
the surge voltage generated instantly after the MOSFET
is turned OFF. In order to prevent this, it is effective to add
a resistor having several to several tens of ohms(R8) in
series with the rectifier diode as shown on Fig.5.
determined in accordance with the specs of a
transformer since the Vin terminal voltage varies by the structural differences of the transformer.
Furthermore, the variation ratio of the Vin terminal voltage becomes worse due to an inaccurate coupling
between the primary and the secondary winding of the transformer(the coupling between the drive winding D
and the stabilized output winding for the constant voltage control circuitry). Thus, in designing the transformer,
the winding position of the drive winding D should also be studied carefully.
7.2
The oscillator, making use of charging and discharging of the capacitor incorporated in the IC, generates
pulse signals to determine OFF-time of the MOSFET. The control operation is made by the PRC (Pulse Ratio
Control) operation fixing OFF-time (≒15usec) and varying ON-time of the MOSFET.
The optimum resistance value of the R8 should be
OCP/FB Terminal (Pin 4), Oscillator and Constant Voltage Control Circuit
Page 9
図4 出力電流Iout ～Vin端子電圧
図5 出力電流Ioutの影響を受け難い補助電源回路
Vin
17.6V
(typ)
11.1V
(Max)
Vin(AC)→ON
Fig.5 Back-up Power Supply Circuit against
Vin
図3 起動時Vin端子電圧波形
Fig.3 Waveform of Vin at start-up
STR-A6300
Without R8
R8が無い場合
Control circuit operation start
制御回路動作開始点
Vin
Gnd
Output current(Io)
With R8
起動不良時
Abnormal start up
R8がある場合
Drive winding voltage
point
補助巻線電圧
Fig. 4
R8
（Ver.1.2）
Time
時間→
Iout