STR-A6351 SanKen, STR-A6351 Datasheet - Page 12

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

STR-A6300 Application Notes
（Ver.1.2）
7.3 OCP/FB Terminal(Pin 4)/OCP Circuit
This is a pulse-by-pulse overcurrent protection circuit that detects peak value of drain current of the MOSFET
in every pulse and reverses the oscillator output. OCP(Overcurrent Protection) circuit is shown on Fig. 9.
MOSFET drain current is detected by puttig the voltage drop of R5 into OCP/FB terminal. <R5 is connected
between Source terminal(Pin 1) and GND terminal(Pin 2 and 5)>
Threshold voltage, Vth(1) of the OCP terminal is set at around 0.76V against GND terminal. External
components, R4 and C5, function as a filter circuit to prevent malfunction caused by surge current when the
MOSFET is turned ON.
Output characteristics of the secondary side at the time the
出力電圧
Vout
OCP circuit operates, due to the overload of the secondary side
入力補正により
No gap in AC by compensation
AC高低の差がなくなる
output, is shown on Fig. 8. Where the output voltage drops with
the overload, drive winding voltage of the primary side also falls
AC高
High
AC低
proportionally, and the Vin terminal voltage falls below
AC
Low
shutdown voltage to stop the operation. In this case, as circuit
current also decreases simultaneously, the Vin terminal voltage
rises again by the charged current through Rs, and the circuit
出力電流Iout
itself re-operates intermittently at the operation start voltage.
図８ 電源出力過負荷特性
Fig.8 Power supply output overload characteristics
However, when a transformer has many output windings and
coupling is not sufficient, primary side winding voltage would not drop, and operation may not become
intermittent accordingly, even though the secondary side output voltage drops in overload status.
As the OCP circuit is operated by detecting drain current of the MOSFET, output power becomes high when
the protection circuit operates with high input voltage as shown on Fig. 8. In order to prevent this, it is
effective to add a bias circuit(using two resistors and a Zener diode) which should be in proportional to the
input voltage as displayed on Fig.9.
There are two merits in adding the bias circuit;
(1) When input voltage is high, the drain current of the MOSFET is kept at low level; thus, the voltage
stresses to the MOSFET both at start up and at light load are also reduced by lowering the surge voltage
generated by the transformer.
(2) Current stresses to the diodes at the secondary side are reduced by the constant control of the output
power.
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