APR345

Document number: DS38010 Rev. 5 - 2

6 of 11

www.diodes.com

June 2016

© Diodes Incorporated

APR345

t

t

0

0

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0

Figure 2. Typical Turn off Waveforms of APR345 in CCM

Minimum On Time

CCM comparator, keeping the controlled MOSFET on for at least the minimum on time. During the minimum on time, the turn off threshold (DCM

and CCM) is totally blanked.

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, APR345 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

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R

2

Area

AREF

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In general, Area1 and Area3 value depend on system design and are always fixed after system design frozen. As to Diodes 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.

Note: To keep the volt-second product threshold stable, a capacitor is suggested to parallel with AREF resistor. And the recommended value of

this capacitor is 20nF.

Area1

Kqs

*

R

3

Area

AREF

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