APR343 datasheet(8/12 Pages) DIODES

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APR343

Document number: DS36884 Rev. 4 - 2

8 of 12

www.diodes.com

December 2014

APR343

A Product Line of

Diodes Incorporated

MOSFET Driver Operation Description (Cont.)

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.

Kqs

Area

AREF



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.

Area1

Kqs

Area

AREF



VDET

VCC

Area3

Area1=(VDET-VCC)*tONP

Area2=Kqs*RAREF

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.

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