APR340

Document number: DS43230 Rev. 3 - 2

6 of 9

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May 2021

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APR340

Synchronous Rectification Principle Description (continued)

The Value and Meaning of AREF Resistor

As to the DCM operation flyback converter

—after the secondary rectifier stops conducting, the resonance of the primary inductance and output

capacitance of the equivalent switch device creates the primary MOSFET drain-to-source ringing waveform. This ringing waveform may lead to

Synchronous Rectifier error conduction. To avoid this fault occurrence, the APR340 has a special function by means of volt-second product

detecting. Regarding the sensed voltage of the VDET pin, the volt-second product of a voltage above VCC at primary switch on-time is much

higher than that of each cycle ringing voltage above VCC. Therefore, before each time the Synchronous Rectifier turns on, the APR340 judges if

the detected volt-second product of the VDET voltage above VCC is higher than a threshold, and then turns on the Synchronous Rectifier. The

purpose of the AREF resistor is to calculate the volt-second product threshold. The APR340 has a parameter, Kqs, which converts the RAREF

value to the volt-second product.

Kqs

*

R

2

AREF



Area

In general, the Area1 and Area3 values depend on the system design and are always fixed if the system design is frozen. As to

Diodes’ PSR

design, the Area1 value changes with primary peak current value and Area3 value generally keeps constant in all conditions. So the AREF resistor

design should consider the worst case scenario and the minimum primary peak current condition. Because of the distribution of the system design

parameters, Area1 and Area3 may have moderate tolerance. So Area2 should be designed in the middle of Area1 and Area3 to ensure enough

design margins.

Area1

Kqs

*

R

3

Area

AREF





VDET

VCC

Area3

Area1=(VDET-VCC)*tONP

Area2=KQS*RAREF

Figure 2. AREF Function

SR Minimum Operating Voltage

The APR340 sets a minimum SR operating voltage by comparing the difference between VDET and the output voltage (VCC). When the value of

VDET–VCC is higher than its internal reference, the APR340 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.

Recommended Application Circuit Parameters

The CAREF is recommended to be parallel with the AREF resistor to keep the volt-second product threshold stable. The recommended value of the

CAREF is 20nF. The recommended value of the C24 is 100nF. The value of the VDD capacitor C23 is 4.7μF.

Green Mode at Light Load

When the system is running with light load, rectifier conduction loss no longer dominates the secondary-side power loss. In this condition, it is

recommended that the SR MOSFET stays off to save from driver loss.

The APR340 will sense the non-switching duration cycle-by-cycle. When the non-switching duration remains longer than the internal light load

timing tLL, the IC will shut down the gate driver, which will stay off for the next two cycles.