APR3415

Document number: DS36738 Rev. 7 - 2

9 of 13

www.diodes.com

June 2015

© Diodes Incorporated

APR3415

A Product Line of

Diodes Incorporated

Minimum On Time

MOSFET will keep on until the end of the minimum on time.

The Value and Meaning of AREF Resistor

As to DCM operation Flyback converter, after secondary rectifier stops conduction the primary MOSFET Drain-to-source ringing waveform is

resulted from the resonant of primary inductance and equivalent switch device output capacitance. This ringing waveform probably leads to

Synchronous Rectifier error conduction. To avoid this fault happening, APR3415 has a special function design by means of volt-second product

detecting. From the sensed voltage of VDET pin to see, the volt-second product of voltage above VCC at primary switch on time is much higher

second product.

Kqs

*

R

2

Area

AREF



In general, Area1 and Area3, the value of which should be test on system, depend on system design and always are fixed after system design

frozen. As to BCD PSR design, the Area1 value changes with primary peak current value and Area3 value generally keeps constant at all of

conditions. So the AREF resistor design should consider the worst case, the minimum primary peak current condition. Since of system design

parameter distribution, Area1 and Area3 have moderate tolerance. So Area2 should be designed between the middle of Area1 and Area3 to keep

enough design margin.

Area1

Kqs

*

R

3

Area

AREF





Area3

Figure 3. AREF Function

SR Minimum Operating Voltage

the SR driver will be disabled.

SR Turning off Timing Impact on PSR CV Sampling

As to synchronous rectification on Flyback power system, SR MOSFET need to turn off in advance of secondary side current decreasing to zero to

avoid current flowing reversely. When SR turns off in advance, the secondary current will flow through the body diode. The SR turning off time is

operation issue or the lower output voltage issue.