STR-A6359 データシートの表示（PDF） - Sanken Electric co.,ltd.

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STR-A6359

Hybrid IC with a built-in power MOSFET and a controller IC

Sanken Electric co.,ltd. 

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

OCP circuit operates, due to the overload of the secondary side

output, is shown on Fig. 8. Where the output voltage drops with

出力電圧

Vout

入力補正により

No gAaCp高in低ACのby差coがmpなenくsaなtionる

the overload, drive winding voltage of the primary side also falls

proportionally, and the Vin terminal voltage falls below

AC低

AC Low

AC高High

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

itself re-operates intermittently at the operation start voltage.

However, when a transformer has many output windings and

出力電流 Iout

図８ 電源出力過負荷特性

Fig.8 Power supply output overload characteristics

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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