Series STR-F6600

OFF-LINE

QUASI-RESONANT FLYBACK

SWITCHING REGULATORS

7

Functional Description and Operation (cont’d)

Figure 6 – Series STR-F6600 Typical Application

+

Dwg. EK-003-5A

0.73 V

–

–

TSD

OVER-VOLT.

PROTECT

R

SQ

REF.

FAULT

LATCH

4

3

2

1.45 V

UVLO

+

1

+

OSC

DRIVE

REG.

+

VOLTAGE

SENSE

+ OUTPUT

– OUTPUT

+

5

designs that embody these components must conform with applicable safety requirements. Precau-

tions must be taken to prevent accidental contact with power-line potentials. Do not connect

grounded test equipment.

The use of an isolation transformer is recommended during circuit development and breadboarding.

Soft Start with Voltage Feedback (refer to figure 7)

Output voltage control is achieved by sensing the opto-

coupled feedback current (proportional to the output

voltage error signal) across resistor R4 and summing this

with the drain current ramp on R5. The signal on pin 1 is

therefore the opposite of the output voltage error signal

and the drain current ramp. The dc bias signal across R4 is

thus a function of the load. Consequently at light load, the

bias signal on R4 is closer to the threshold voltage of the

comparator.

To eliminate the possibility of false shutdown at

MOSFET turn on (when there is a current spike due to the

discharge of primary capacitance), a constant-current sink

of 1.35 mA is turned on, effectively lowering the input

impedance on pin 1, and momentarily increasing the

shutdown threshold.