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7-3. OCP/F.B terminal (pin 1) OCP circuit
This is a pulse-by-pulse overcurrent protection circuit
which detects the peak of the peak of the drain current of
the MOS FET in every pulse and reverses the oscillator
output.
Fig. 7 shows the overcurrent detection circuit.
The MOS FET drain current is detected by inputting the
voltage drop of R5 connected between the source
terminal(pin 2) of the MOS FET and the GND terminal
(pin 5) into the OCP/FB terminal.
Fig. 8 shows the output characteristics of the secondary side at the time when the overcurrent
protection circuit operates due to the overload of the secondary side.
When the output voltage drops in the overload condition, the drive winding voltage of the primary
side also goes down proportionally, and the V
to stop the operation. In this case, as the circuit current also decreases simultaneously, the V
terminal voltage increases again and the circuit operates intermittently by restoring the operation
start voltage. However, if there are a lot of output windings, even if overloaded output voltage drops,
the primary winding voltage may not drop so much and the operation may not be intermittent.
In STR-F6500 series, as the drain current of the MOS FET is detected for overcurrent protection,
the output power becomes high as the input voltage becomes high as shown in Fig. 8.
In order to prevent this high power, it is effective to add as shown if Fig. 9 a circuit by applying bias
which is in proportion with input voltage.
There are two advantages in providing this additional circuit.
(1) The drain current of the MOS FET can be suppressed to low level, when the input voltage
is high, therefore voltage stress to the MOS FET at start-up or overload can be reduced by
reducing surge voltage produced by the transformer.
(2) Current stress to the secondary side rectifier diode can be reduced by limiting the output
power in a certain degree.
Fig. 9 Compensation circuit of overcurrent protection by input voltage
Fig. 8 Power supply output
terminal voltage drops below the shutdown voltage
IN
RA = 680
RB =
33
overload characteristic
VIN(DC)MAX - VIN (DC)MIN
0.73
1Dpeak(MAX) - 1Dpeak(MIN)
1Dpeak(MAX)
RA (6.8 + 0.73)
VIN(DC)MIN - (6.8 + 0.73)
IN
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