APR345 datasheet(7/11 Pages) DIODES

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APR345

Document number: DS38010 Rev. 5 - 2

7 of 11

www.diodes.com

June 2016

APR345

Synchronous Rectification Principle Description (Cont.)

VDET

VCC

Area3

Area1=(VDET-VCC)*tONP

Area2=Kqs*RAREF

Figure 3. AREF Function

SR Minimum Operating Voltage

APR345 sets a minimum SR operating voltage by comparing the difference between VDET and output voltage (VCC). The value of VDET–VCC must be

higher than its internal reference, then APR345 will begin to integrate the area of (VDET–VCC)*tONP. If not, the area integrating will not begin and 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

determined by the VTHOFF at a fixed system. When VTHOFF is more close to zero, the SR turning on time gets longer and body diode conduction time

gets shorter. Since of the different voltage drop between SR MOSFET and body diode, the PSR feedback signal VFB appears a voltage jump at the

time of SR MOSFET turning off. If the PSR CV sampling time tSAMPLE is close to even behind this voltage jump time, there will be system unstable

operation issue or the lower output voltage issue.

To ensure stable operating of system, it must be met:

tBODYDIODE<tONS- tSAMPLE

VFB

tSAMPLE

SR Operating

SR Turnoff,

Bodydiode operating

tBODYDIODE

tONS

Figure 4. SR Turning Off Timing Impact on PSR CV Sampling

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