APR34309CMPTRG1 datasheet(9/13 Pages) DIODES

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APR34309C

Document number: DS37492 Rev. 5 - 2

9 of 13

www.diodes.com

June 2015

APR34309C

A Product Line of

Diodes Incorporated

Operation Description (Cont.)

Minimum On Time

When the controlled MOSFET gate is turned on, some ringing noise is generated. The minimum on-time timer blanks the VTHOFF comparator,

keeping the controlled MOSFET on for at least the minimum on time. If VTHOFF falls below the threshold before minimum on time expires, the

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

The minimum on time is in direct proportion to the (VDET-VCC)*tONP. When (VDET-VCC)*tONP=5V*5μs, the minimum on time is about 1.8μs.

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, APR34309C 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 than the volt-second product of each cycle ringing voltage above VCC. Therefore, before every time Synchronous Rectifier turning on,

APR34309C judges if the detected volt-second product of VDET voltage above VCC is higher than a threshold and then turn on synchronous

Rectifier. The purpose of AREF resistor is to determine the volt-second product threshold. APR34309C has a parameter, Kqs, which converts

RAREF value to volt-second product,

Kqs

Area

AREF



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

Area

AREF



VDET

VCC

Area3

Area1=(VDET-VCC)*tONP

Area2=Kqs*RAREF

Figure 3. AREF Function

SR Minimum Operating Voltage

APR34309C 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 APR34309C 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.

APR34309CMPTRG1 数据表(PDF) 9 Page - Diodes Incorporated

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