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APR3415MTRG1 Datasheet(PDF) 9 Page - Diodes Incorporated |
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APR3415MTRG1 Datasheet(HTML) 9 Page - Diodes Incorporated |
9 / 13 page APR3415 Document number: DS36738 Rev. 7 - 2 9 of 13 www.diodes.com June 2015 © Diodes Incorporated APR3415 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, 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 than the volt-second product of each cycle ringing voltage above VCC. Therefore, before every time Synchronous Rectifier turning on, APR3415 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. APR3415 has a parameter, Kqs, which converts RAREF value to volt- 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 VDET VCC Area3 Area1=(VDET-VCC)*tONP Area2=Kqs*RAREF Figure 3. AREF Function SR Minimum Operating Voltage APR3415 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 APR3415 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. |
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