APR343 Datasheet, PDF (8/12 Pages) Diodes Incorporated – SECONDARY SIDE SYNCHRONOUS RECTIFICATION CONTROLLER

MOSFET Driver Operation Description (Cont.)
A Product Line of
Diodes Incorporated
APR343
The operation of the SR is described with timing diagram shown in Figure 2. APR343 monitors the MOSFET drain-source voltage. When the drain
voltage is lower than the turn-on threshold voltage VTHON, the IC outputs a positive drive voltage after a turn-on delay time (tDON). The MOSFET will
turn on and the current will transfer from the body diode into the MOSFET’s channel.
In the process of drain current decreasing linearly toward zero, the drain-source voltage rises synchronically. When it rises over the turn off
threshold voltage VTHOFF, APR343 pulls the drive signal down after a turn off delay (tDOFF).
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, APR343 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, APR343
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. APR343 has a parameter, Kqs, which converts RAREF value to volt-
second product.
Area2  R_AREF * Kqs
In general, Area1 and Area3 value 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.
Area3  R_AREF * Kqs  Area1
Area1=(V_DET-V_CC)*t_ONP
Area3
V_DET
V_CC
Area2=Kqs*R_AREF
Figure 3. AREF Function
SR Minimum Operating Voltage
APR343 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 APR343 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.
APR343
Document number: DS36884 Rev. 4 - 2
8 of 12
www.diodes.com
December 2014
© Diodes Incorporated